FreeBSD Developers` Handbook

FreeBSD Developers` Handbook
FreeBSD Developers' Handbook
FreeBSD Developers' Handbook
Revision: 50284
2017-05-25 16:22:12 by gjb.
Copyright © 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016,
2017 The FreeBSD Documentation Project
Welcome to the Developers' Handbook. This manual is a work in progress and is the work of many individuals.
Many sections do not yet exist and some of those that do exist need to be updated. If you are interested in
helping with this project, send email to the FreeBSD documentation project mailing list.
The latest version of this document is always available from the FreeBSD World Wide Web server. It may also
be downloaded in a variety of formats and compression options from the FreeBSD FTP server or one of the
numerous mirror sites.
Redistribution and use in source (XML DocBook) and 'compiled' forms (XML, HTML, PDF, PostScript, RTF and so forth) with or without
modification, are permitted provided that the following conditions are met:
1. Redistributions of source code (XML DocBook) must retain the above copyright notice, this list of conditions and the following disclaimer as the rst lines of this le unmodified.
2. Redistributions in compiled form (transformed to other DTDs, converted to PDF, PostScript, RTF and other formats) must reproduce
the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided
with the distribution.
FreeBSD is a registered trademark of the FreeBSD Foundation.
Apple, AirPort, FireWire, iMac, iPhone, iPad, Mac, Macintosh, Mac OS, Quicktime, and TrueType are trademarks of Apple Inc., registered
in the U.S. and other countries.
IBM, AIX, OS/2, PowerPC, PS/2, S/390, and ThinkPad are trademarks of International Business Machines Corporation in the United
States, other countries, or both.
IEEE, POSIX, and 802 are registered trademarks of Institute of Electrical and Electronics Engineers, Inc. in the United States.
Intel, Celeron, Centrino, Core, EtherExpress, i386, i486, Itanium, Pentium, and Xeon are trademarks or registered trademarks of Intel
Corporation or its subsidiaries in the United States and other countries.
Linux is a registered trademark of Linus Torvalds.
Microsoft, IntelliMouse, MS-DOS, Outlook, Windows, Windows Media and Windows NT are either registered trademarks or trademarks
of Microsoft Corporation in the United States and/or other countries.
Motif, OSF/1, and UNIX are registered trademarks and IT DialTone and The Open Group are trademarks of The Open Group in the United
States and other countries.
Sun, Sun Microsystems, Java, Java Virtual Machine, JDK, JRE, JSP, JVM, Netra, OpenJDK, Solaris, StarOffice, SunOS and VirtualBox are
trademarks or registered trademarks of Sun Microsystems, Inc. in the United States and other countries.
Many of the designations used by manufacturers and sellers to distinguish their products are claimed as trademarks. Where those
designations appear in this document, and the FreeBSD Project was aware of the trademark claim, the designations have been followed
by the “™” or the “®” symbol.
Table of Contents
I. Basics ..................................................................................................................................... 1
1. Introduction ................................................................................................................... 5
1.1. Developing on FreeBSD ........................................................................................... 5
1.2. The BSD Vision ..................................................................................................... 5
1.3. Architectural Guidelines .......................................................................................... 5
1.4. The Layout of /usr/src ....................................................................................... 5
2. Programming Tools .......................................................................................................... 7
2.1. Synopsis .............................................................................................................. 7
2.2. Introduction ......................................................................................................... 7
2.3. Introduction to Programming ................................................................................... 7
2.4. Compiling with cc ................................................................................................. 9
2.5. Make ................................................................................................................. 14
2.6. Debugging .......................................................................................................... 18
2.7. Using Emacs as a Development Environment .............................................................. 21
2.8. Further Reading ................................................................................................... 28
3. Secure Programming ....................................................................................................... 29
3.1. Synopsis ............................................................................................................. 29
3.2. Secure Design Methodology .................................................................................... 29
3.3. Buer Overflows .................................................................................................. 29
3.4. SetUID issues ....................................................................................................... 31
3.5. Limiting your program's environment ...................................................................... 31
3.6. Trust ................................................................................................................. 32
3.7. Race Conditions ................................................................................................... 32
4. Localization and Internationalization - L10N and I18N ............................................................ 35
4.1. Programming I18N Compliant Applications ................................................................ 35
4.2. Localized Messages with POSIX.1 Native Language Support (NLS) .................................... 35
5. Source Tree Guidelines and Policies .................................................................................... 39
5.1. Style Guidelines ................................................................................................... 39
5.2. MAINTAINER on Makefiles ....................................................................................... 39
5.3. Contributed Software ............................................................................................ 39
5.4. Encumbered Files ................................................................................................. 43
5.5. Shared Libraries ................................................................................................... 43
6. Regression and Performance Testing .................................................................................. 45
6.1. Micro Benchmark Checklist .................................................................................... 45
6.2. The FreeBSD Source Tinderbox ............................................................................... 46
II. Interprocess Communication .................................................................................................... 49
7. Sockets ......................................................................................................................... 53
7.1. Synopsis ............................................................................................................. 53
7.2. Networking and Diversity ...................................................................................... 53
7.3. Protocols ............................................................................................................ 53
7.4. The Sockets Model ............................................................................................... 56
7.5. Essential Socket Functions ...................................................................................... 56
7.6. Helper Functions .................................................................................................. 68
7.7. Concurrent Servers ............................................................................................... 70
8. IPv6 Internals ................................................................................................................ 73
8.1. IPv6/IPsec Implementation .................................................................................... 73
III. Kernel ................................................................................................................................ 89
9. Building and Installing a FreeBSD Kernel ............................................................................. 93
9.1. Building a Kernel the “Traditional” Way .................................................................... 93
9.2. Building a Kernel the “New” Way ............................................................................ 93
10. Kernel Debugging .......................................................................................................... 95
10.1. Obtaining a Kernel Crash Dump ............................................................................. 95
10.2. Debugging a Kernel Crash Dump with kgdb .............................................................. 97
10.3. Debugging a Crash Dump with DDD ....................................................................... 100
10.4. On-Line Kernel Debugging Using DDB .................................................................... 101
10.5. On-Line Kernel Debugging Using Remote GDB .......................................................... 104
Table of Contents
10.6. Debugging a Console Driver .................................................................................
10.7. Debugging Deadlocks .........................................................................................
10.8. Kernel debugging with Dcons ...............................................................................
10.9. Glossary of Kernel Options for Debugging ...............................................................
IV. Architectures ......................................................................................................................
11. x86 Assembly Language Programming ..............................................................................
11.1. Synopsis ..........................................................................................................
11.2. The Tools ........................................................................................................
11.3. System Calls .....................................................................................................
11.4. Return Values ...................................................................................................
11.5. Creating Portable Code .......................................................................................
11.6. Our First Program .............................................................................................
11.7. Writing UNIX® Filters ........................................................................................
11.8. Buered Input and Output ..................................................................................
11.9. Command Line Arguments ..................................................................................
11.10. UNIX® Environment ........................................................................................
11.11. Working with Files ...........................................................................................
11.12. One-Pointed Mind ............................................................................................
11.13. Using the FPU .................................................................................................
11.14. Caveats ..........................................................................................................
11.15. Acknowledgements ..........................................................................................
V. Appendices ..........................................................................................................................
Bibliography ...................................................................................................................
Index .....................................................................................................................................
List of Examples
2.1. A sample .emacs le ........................................................................................................... 23
Part I. Basics
Table of Contents
1. Introduction ........................................................................................................................... 5
1.1. Developing on FreeBSD ................................................................................................... 5
1.2. The BSD Vision ............................................................................................................. 5
1.3. Architectural Guidelines .................................................................................................. 5
1.4. The Layout of /usr/src ............................................................................................... 5
2. Programming Tools .................................................................................................................. 7
2.1. Synopsis ...................................................................................................................... 7
2.2. Introduction ................................................................................................................. 7
2.3. Introduction to Programming ........................................................................................... 7
2.4. Compiling with cc ......................................................................................................... 9
2.5. Make ......................................................................................................................... 14
2.6. Debugging .................................................................................................................. 18
2.7. Using Emacs as a Development Environment ...................................................................... 21
2.8. Further Reading ........................................................................................................... 28
3. Secure Programming ............................................................................................................... 29
3.1. Synopsis ..................................................................................................................... 29
3.2. Secure Design Methodology ............................................................................................ 29
3.3. Buer Overflows .......................................................................................................... 29
3.4. SetUID issues ............................................................................................................... 31
3.5. Limiting your program's environment .............................................................................. 31
3.6. Trust ......................................................................................................................... 32
3.7. Race Conditions ........................................................................................................... 32
4. Localization and Internationalization - L10N and I18N .................................................................... 35
4.1. Programming I18N Compliant Applications ........................................................................ 35
4.2. Localized Messages with POSIX.1 Native Language Support (NLS) ............................................ 35
5. Source Tree Guidelines and Policies ............................................................................................ 39
5.1. Style Guidelines ........................................................................................................... 39
5.2. MAINTAINER on Makefiles ............................................................................................... 39
5.3. Contributed Software .................................................................................................... 39
5.4. Encumbered Files ......................................................................................................... 43
5.5. Shared Libraries ........................................................................................................... 43
6. Regression and Performance Testing .......................................................................................... 45
6.1. Micro Benchmark Checklist ............................................................................................ 45
6.2. The FreeBSD Source Tinderbox ....................................................................................... 46
Chapter 1. Introduction
Contributed by Murray Stokely and Jeroen Ruigrok van der Werven.
1.1. Developing on FreeBSD
So here we are. System all installed and you are ready to start programming. But where to start? What does FreeBSD
provide? What can it do for me, as a programmer?
These are some questions which this chapter tries to answer. Of course, programming has different levels of proficiency like any other trade. For some it is a hobby, for others it is their profession. The information in this chapter
might be aimed toward the beginning programmer; indeed, it could serve useful for the programmer unfamiliar
with the FreeBSD platform.
1.2. The BSD Vision
To produce the best UNIX® like operating system package possible, with due respect to the original software tools
ideology as well as usability, performance and stability.
1.3. Architectural Guidelines
Our ideology can be described by the following guidelines
• Do not add new functionality unless an implementor cannot complete a real application without it.
• It is as important to decide what a system is not as to decide what it is. Do not serve all the world's needs; rather,
make the system extensible so that additional needs can be met in an upwardly compatible fashion.
• The only thing worse than generalizing from one example is generalizing from no examples at all.
• If a problem is not completely understood, it is probably best to provide no solution at all.
• If you can get 90 percent of the desired effect for 10 percent of the work, use the simpler solution.
• Isolate complexity as much as possible.
• Provide mechanism, rather than policy. In particular, place user interface policy in the client's hands.
From Scheifler & Gettys: "X Window System"
1.4. The Layout of /usr/src
The complete source code to FreeBSD is available from our public repository. The source code is normally installed
in /usr/src which contains the following subdirectories:
Source for les in /bin
Utilities covered by the Common Development and Distribution License
Source for les from contributed software.
Cryptographical sources
The Layout of /usr/src
Source for les in /etc
Source for les in /usr/games
Utilities covered by the GNU Public License
Source for les in /usr/include
Source for Kerberos version 5
Source for les in /usr/lib
Source for les in /usr/libexec
Files required to produce a FreeBSD release
Build system for the /rescue utilities
Source for les in /sbin
FreeSec sources
Source for les in /usr/share
Kernel source les
The FreeBSD test suite
Tools used for maintenance and testing of FreeBSD
Source for les in /usr/bin
Source for les in /usr/sbin
Chapter 2. Programming Tools
Contributed by James Raynard and Murray Stokely.
2.1. Synopsis
This chapter is an introduction to using some of the programming tools supplied with FreeBSD, although much of
it will be applicable to many other versions of UNIX®. It does not attempt to describe coding in any detail. Most of
the chapter assumes little or no previous programming knowledge, although it is hoped that most programmers
will nd something of value in it.
2.2. Introduction
FreeBSD offers an excellent development environment. Compilers for C and C++ and an assembler come with the
basic system, not to mention classic UNIX® tools such as sed and awk . If that is not enough, there are many more
compilers and interpreters in the Ports collection. The following section, Introduction to Programming, lists some
of the available options. FreeBSD is very compatible with standards such as POSIX® and ANSI C, as well with its
own BSD heritage, so it is possible to write applications that will compile and run with little or no modification
on a wide range of platforms.
However, all this power can be rather overwhelming at rst if you have never written programs on a UNIX® platform before. This document aims to help you get up and running, without getting too deeply into more advanced
topics. The intention is that this document should give you enough of the basics to be able to make some sense
of the documentation.
Most of the document requires little or no knowledge of programming, although it does assume a basic competence
with using UNIX® and a willingness to learn!
2.3. Introduction to Programming
A program is a set of instructions that tell the computer to do various things; sometimes the instruction it has to
perform depends on what happened when it performed a previous instruction. This section gives an overview of
the two main ways in which you can give these instructions, or “commands” as they are usually called. One way
uses an interpreter, the other a compiler. As human languages are too difficult for a computer to understand in an
unambiguous way, commands are usually written in one or other languages specially designed for the purpose.
2.3.1. Interpreters
With an interpreter, the language comes as an environment, where you type in commands at a prompt and the
environment executes them for you. For more complicated programs, you can type the commands into a le and
get the interpreter to load the le and execute the commands in it. If anything goes wrong, many interpreters will
drop you into a debugger to help you track down the problem.
The advantage of this is that you can see the results of your commands immediately, and mistakes can be corrected
readily. The biggest disadvantage comes when you want to share your programs with someone. They must have
the same interpreter, or you must have some way of giving it to them, and they need to understand how to use it.
Also users may not appreciate being thrown into a debugger if they press the wrong key! From a performance point
of view, interpreters can use up a lot of memory, and generally do not generate code as efficiently as compilers.
In my opinion, interpreted languages are the best way to start if you have not done any programming before. This
kind of environment is typically found with languages like Lisp, Smalltalk, Perl and Basic. It could also be argued
that the UNIX® shell (sh, csh ) is itself an interpreter, and many people do in fact write shell “scripts” to help with
various “housekeeping” tasks on their machine. Indeed, part of the original UNIX® philosophy was to provide lots
of small utility programs that could be linked together in shell scripts to perform useful tasks.
Interpreters available with FreeBSD
2.3.2. Interpreters available with FreeBSD
Here is a list of interpreters that are available from the FreeBSD Ports Collection, with a brief discussion of some
of the more popular interpreted languages.
Instructions on how to get and install applications from the Ports Collection can be found in the Ports section
of the handbook.
Short for Beginner's All-purpose Symbolic Instruction Code. Developed in the 1950s for teaching University
students to program and provided with every self-respecting personal computer in the 1980s, BASIC has been
the rst programming language for many programmers. It is also the foundation for Visual Basic.
The Bywater Basic Interpreter can be found in the Ports Collection as lang/bwbasic and the Phil Cockroft's
Basic Interpreter (formerly Rabbit Basic) is available as lang/pbasic.
A language that was developed in the late 1950s as an alternative to the “number-crunching” languages that
were popular at the time. Instead of being based on numbers, Lisp is based on lists; in fact, the name is short
for “List Processing”. It is very popular in AI (Artificial Intelligence) circles.
Lisp is an extremely powerful and sophisticated language, but can be rather large and unwieldy.
Various implementations of Lisp that can run on UNIX® systems are available in the Ports Collection for
FreeBSD. GNU Common Lisp can be found as lang/gcl. CLISP by Bruno Haible and Michael Stoll is available as
lang/clisp. For CMUCL, which includes a highly-optimizing compiler too, or simpler Lisp implementations like
SLisp, which implements most of the Common Lisp constructs in a few hundred lines of C code, lang/cmucl
and lang/slisp are available respectively.
Very popular with system administrators for writing scripts; also often used on World Wide Web servers for
writing CGI scripts.
Perl is available in the Ports Collection as lang/perl5.16 for all FreeBSD releases.
A dialect of Lisp that is rather more compact and cleaner than Common Lisp. Popular in Universities as it is
simple enough to teach to undergraduates as a rst language, while it has a high enough level of abstraction
to be used in research work.
Scheme is available from the Ports Collection as lang/elk for the Elk Scheme Interpreter. The MIT Scheme
Interpreter can be found in lang/mit-scheme and the SCM Scheme Interpreter in lang/scm.
Icon is a high-level language with extensive facilities for processing strings and structures. The version of Icon
for FreeBSD can be found in the Ports Collection as lang/icon.
Logo is a language that is easy to learn, and has been used as an introductory programming language in various
courses. It is an excellent tool to work with when teaching programming to smaller age groups, as it makes
creation of elaborate geometric shapes an easy task.
The latest version of Logo for FreeBSD is available from the Ports Collection in lang/logo.
Python is an Object-Oriented, interpreted language. Its advocates argue that it is one of the best languages
to start programming with, since it is relatively easy to start with, but is not limited in comparison to other
popular interpreted languages that are used for the development of large, complex applications (Perl and Tcl
are two other languages that are popular for such tasks).
Chapter 2. Programming Tools
The latest version of Python is available from the Ports Collection in lang/python.
Ruby is an interpreter, pure object-oriented programming language. It has become widely popular because
of its easy to understand syntax, flexibility when writing code, and the ability to easily develop and maintain
large, complex programs.
Ruby is available from the Ports Collection as lang/ruby18.
Tcl and Tk
Tcl is an embeddable, interpreted language, that has become widely used and became popular mostly because
of its portability to many platforms. It can be used both for quickly writing small, prototype applications, or
(when combined with Tk, a GUI toolkit) fully-edged, featureful programs.
Various versions of Tcl are available as ports for FreeBSD. The latest version, Tcl 8.5, can be found in lang/tcl85.
2.3.3. Compilers
Compilers are rather different. First of all, you write your code in a le (or les) using an editor. You then run the
compiler and see if it accepts your program. If it did not compile, grit your teeth and go back to the editor; if it
did compile and gave you a program, you can run it either at a shell command prompt or in a debugger to see if
it works properly. 1
Obviously, this is not quite as direct as using an interpreter. However it allows you to do a lot of things which
are very difficult or even impossible with an interpreter, such as writing code which interacts closely with the
operating system—or even writing your own operating system! It is also useful if you need to write very efficient
code, as the compiler can take its time and optimize the code, which would not be acceptable in an interpreter.
Moreover, distributing a program written for a compiler is usually more straightforward than one written for an
interpreter—you can just give them a copy of the executable, assuming they have the same operating system as you.
As the edit-compile-run-debug cycle is rather tedious when using separate programs, many commercial compiler
makers have produced Integrated Development Environments (IDEs for short). FreeBSD does not include an IDE
in the base system, but devel/kdevelop is available in the Ports Collection and many use Emacs for this purpose.
Using Emacs as an IDE is discussed in Section 2.7, “Using Emacs as a Development Environment”.
2.4. Compiling with cc
This section deals with the gcc and clang compilers for C and C++, since they come with the FreeBSD base system.
Starting with FreeBSD 10.X clang is installed as cc. The details of producing a program with an interpreter vary
considerably between interpreters, and are usually well covered in the documentation and on-line help for the
Once you have written your masterpiece, the next step is to convert it into something that will (hopefully!) run on
FreeBSD. This usually involves several steps, each of which is done by a separate program.
Pre-process your source code to remove comments and do other tricks like expanding macros in C.
Check the syntax of your code to see if you have obeyed the rules of the language. If you have not, it will
Convert the source code into assembly language—this is very close to machine code, but still understandable
by humans. Allegedly.
Convert the assembly language into machine code—yep, we are talking bits and bytes, ones and zeros here.
Check that you have used things like functions and global variables in a consistent way. For example, if you
have called a non-existent function, it will complain.
If you run it in the shell, you may get a core dump.
Compiling with cc
If you are trying to produce an executable from several source code les, work out how to t them all together.
Work out how to produce something that the system's run-time loader will be able to load into memory and
Finally, write the executable on the filesystem.
The word compiling is often used to refer to just steps 1 to 4—the others are referred to as linking. Sometimes step
1 is referred to as pre-processing and steps 3-4 as assembling.
Fortunately, almost all this detail is hidden from you, as cc is a front end that manages calling all these programs
with the right arguments for you; simply typing
% cc foobar.c
will cause foobar.c to be compiled by all the steps above. If you have more than one le to compile, just do something like
% cc foo.c bar.c
Note that the syntax checking is just that—checking the syntax. It will not check for any logical mistakes you may
have made, like putting the program into an infinite loop, or using a bubble sort when you meant to use a binary
sort. 2
There are lots and lots of options for cc, which are all in the manual page. Here are a few of the most important
ones, with examples of how to use them.
-o filename
The output name of the le. If you do not use this option, cc will produce an executable called a.out . 3
% cc foobar.c
% cc -o foobar foobar.c
executable is a.out
executable is foobar
Just compile the le, do not link it. Useful for toy programs where you just want to check the syntax, or if
you are using a Makefile.
% cc -c foobar.c
This will produce an object le (not an executable) called foobar.o. This can be linked together with other
object les into an executable.
Create a debug version of the executable. This makes the compiler put information into the executable about
which line of which source le corresponds to which function call. A debugger can use this information to
show the source code as you step through the program, which is very useful; the disadvantage is that all this
extra information makes the program much bigger. Normally, you compile with -g while you are developing
a program and then compile a “release version” without -g when you are satisfied it works properly.
% cc -g foobar.c
This will produce a debug version of the program. 4
In case you did not know, a binary sort is an efficient way of sorting things into order and a bubble sort is not.
The reasons for this are buried in the mists of history.
Note, we did not use the -o ag to specify the executable name, so we will get an executable called a.out . Producing a debug version called
foobar is left as an exercise for the reader!
Chapter 2. Programming Tools
Create an optimized version of the executable. The compiler performs various clever tricks to try to produce
an executable that runs faster than normal. You can add a number after the -O to specify a higher level of
optimization, but this often exposes bugs in the compiler's optimizer.
% cc -O -o foobar foobar.c
This will produce an optimized version of foobar.
The following three ags will force cc to check that your code complies to the relevant international standard,
often referred to as the ANSI standard, though strictly speaking it is an ISO standard.
Enable all the warnings which the authors of cc believe are worthwhile. Despite the name, it will not enable
all the warnings cc is capable of.
Turn o most, but not all, of the non-ANSI C features provided by cc. Despite the name, it does not guarantee
strictly that your code will comply to the standard.
Turn o all cc's non-ANSI C features.
Without these ags, cc will allow you to use some of its non-standard extensions to the standard. Some of these
are very useful, but will not work with other compilers—in fact, one of the main aims of the standard is to allow
people to write code that will work with any compiler on any system. This is known as portable code.
Generally, you should try to make your code as portable as possible, as otherwise you may have to completely
rewrite the program later to get it to work somewhere else—and who knows what you may be using in a few years
% cc -Wall -ansi -pedantic -o foobar foobar.c
This will produce an executable foobar after checking foobar.c for standard compliance.
Specify a function library to be used at link time.
The most common example of this is when compiling a program that uses some of the mathematical functions
in C. Unlike most other platforms, these are in a separate library from the standard C one and you have to
tell the compiler to add it.
The rule is that if the library is called libsomething.a, you give cc the argument -lsomething. For example,
the math library is libm.a, so you give cc the argument -lm . A common “gotcha” with the math library is that
it has to be the last library on the command line.
% cc -o foobar foobar.c -lm
This will link the math library functions into foobar.
If you are compiling C++ code, use c++ . c++ can also be invoked as clang++ on FreeBSD.
% c++ -o foobar
This will both produce an executable foobar from the C++ source le
2.4.1. Common cc Queries and Problems
I am trying to write a program which uses the sin() function and I get an error like this. What does it mean?
Common cc Queries and Problems
/var/tmp/cc0143941.o: Undefined symbol `_sin' referenced from text segment
When using mathematical functions like sin() , you have to tell cc to link in the math library, like so:
% cc -o foobar foobar.c -lm
All right, I wrote this simple program to practice using -lm . All it does is raise 2.1 to the power of 6.
#include <stdio.h>
int main() {
float f;
f = pow(2.1, 6);
printf("2.1 ^ 6 = %f\n", f);
return 0;
and I compiled it as:
% cc temp.c -lm
like you said I should, but I get this when I run it:
% ./a.out
2.1 ^ 6 = 1023.000000
This is not the right answer! What is going on?
When the compiler sees you call a function, it checks if it has already seen a prototype for it. If it has not, it
assumes the function returns an int, which is definitely not what you want here.
So how do I x this?
The prototypes for the mathematical functions are in math.h . If you include this le, the compiler will be
able to nd the prototype and it will stop doing strange things to your calculation!
#include <math.h>
#include <stdio.h>
int main() {
After recompiling it as you did before, run it:
% ./a.out
2.1 ^ 6 = 85.766121
If you are using any of the mathematical functions, always include math.h and remember to link in the math
I compiled a le called foobar.c and I cannot nd an executable called foobar. Where has it gone?
Remember, cc will call the executable a.out unless you tell it differently. Use the -o filename option:
% cc -o foobar foobar.c
Chapter 2. Programming Tools
OK, I have an executable called foobar, I can see it when I run ls, but when I type in foobar at the command
prompt it tells me there is no such le. Why can it not nd it?
Unlike MS-DOS®, UNIX® does not look in the current directory when it is trying to nd out which executable
you want it to run, unless you tell it to. Type ./foobar , which means “run the le called foobar in the current
I called my executable test , but nothing happens when I run it. What is going on?
Most UNIX® systems have a program called test in /usr/bin and the shell is picking that one up before it
gets to checking the current directory. Either type:
% ./test
or choose a better name for your program!
I compiled my program and it seemed to run all right at rst, then there was an error and it said something
about core dumped. What does that mean?
The name core dump dates back to the very early days of UNIX®, when the machines used core memory for
storing data. Basically, if the program failed under certain conditions, the system would write the contents
of core memory to disk in a le called core , which the programmer could then pore over to nd out what
went wrong.
Fascinating stu, but what I am supposed to do now?
Use gdb to analyze the core (see Section 2.6, “Debugging”).
When my program dumped core, it said something about a segmentation fault. What is that?
This basically means that your program tried to perform some sort of illegal operation on memory; UNIX®
is designed to protect the operating system and other programs from rogue programs.
Common causes for this are:
• Trying to write to a NULL pointer, eg
char *foo = NULL;
strcpy(foo, "bang!");
• Using a pointer that has not been initialized, eg
char *foo;
strcpy(foo, "bang!");
The pointer will have some random value that, with luck, will point into an area of memory that is not
available to your program and the kernel will kill your program before it can do any damage. If you are
unlucky, it will point somewhere inside your own program and corrupt one of your data structures, causing the program to fail mysteriously.
• Trying to access past the end of an array, eg
int bar[20];
bar[27] = 6;
• Trying to store something in read-only memory, eg
char *foo = "My string";
strcpy(foo, "bang!");
UNIX® compilers often put string literals like "My string" into read-only areas of memory.
• Doing naughty things with malloc() and free() , eg
char bar[80];
char *foo = malloc(27);
Making one of these mistakes will not always lead to an error, but they are always bad practice. Some systems
and compilers are more tolerant than others, which is why programs that ran well on one system can crash
when you try them on an another.
Sometimes when I get a core dump it says bus error. It says in my UNIX® book that this means a hardware
problem, but the computer still seems to be working. Is this true?
No, fortunately not (unless of course you really do have a hardware problem…). This is usually another way
of saying that you accessed memory in a way you should not have.
This dumping core business sounds as though it could be quite useful, if I can make it happen when I want
to. Can I do this, or do I have to wait until there is an error?
Yes, just go to another console or xterm, do
% ps
to nd out the process ID of your program, and do
% kill -ABRT pid
where pid is the process ID you looked up.
This is useful if your program has got stuck in an infinite loop, for instance. If your program happens to trap
SIGABRT, there are several other signals which have a similar effect.
Alternatively, you can create a core dump from inside your program, by calling the abort() function. See
the manual page of abort(3) to learn more.
If you want to create a core dump from outside your program, but do not want the process to terminate, you
can use the gcore program. See the manual page of gcore(1) for more information.
2.5. Make
2.5.1. What is make ?
When you are working on a simple program with only one or two source les, typing in
% cc file1.c file2.c
Chapter 2. Programming Tools
is not too bad, but it quickly becomes very tedious when there are several les—and it can take a while to compile,
One way to get around this is to use object les and only recompile the source le if the source code has changed.
So we could have something like:
% cc file1.o file2.o … file37.c …
if we had changed file37.c , but not any of the others, since the last time we compiled. This may speed up the
compilation quite a bit, but does not solve the typing problem.
Or we could write a shell script to solve the typing problem, but it would have to re-compile everything, making
it very inefficient on a large project.
What happens if we have hundreds of source les lying about? What if we are working in a team with other people
who forget to tell us when they have changed one of their source les that we use?
Perhaps we could put the two solutions together and write something like a shell script that would contain some
kind of magic rule saying when a source le needs compiling. Now all we need now is a program that can understand
these rules, as it is a bit too complicated for the shell.
This program is called make . It reads in a le, called a makefile, that tells it how different les depend on each other,
and works out which les need to be re-compiled and which ones do not. For example, a rule could say something
like “if fromboz.o is older than fromboz.c, that means someone must have changed fromboz.c, so it needs to be
re-compiled.” The makefile also has rules telling make how to re-compile the source le, making it a much more
powerful tool.
Makefiles are typically kept in the same directory as the source they apply to, and can be called makefile, Makefile
or MAKEFILE. Most programmers use the name Makefile, as this puts it near the top of a directory listing, where
it can easily be seen. 5
2.5.2. Example of using make
Here is a very simple make le:
foo: foo.c
cc -o foo foo.c
It consists of two lines, a dependency line and a creation line.
The dependency line here consists of the name of the program (known as the target), followed by a colon, then
whitespace, then the name of the source le. When make reads this line, it looks to see if foo exists; if it exists,
it compares the time foo was last modified to the time foo.c was last modified. If foo does not exist, or is older
than foo.c , it then looks at the creation line to nd out what to do. In other words, this is the rule for working
out when foo.c needs to be re-compiled.
The creation line starts with a tab (press the tab key) and then the command you would type to create foo if you
were doing it at a command prompt. If foo is out of date, or does not exist, make then executes this command to
create it. In other words, this is the rule which tells make how to re-compile foo.c .
So, when you type make , it will make sure that foo is up to date with respect to your latest changes to foo.c . This
principle can be extended to Makefiles with hundreds of targets—in fact, on FreeBSD, it is possible to compile the
entire operating system just by typing make world in the appropriate directory!
Another useful property of makefiles is that the targets do not have to be programs. For instance, we could have
a make le that looks like this:
foo: foo.c
cc -o foo foo.c
They do not use the MAKEFILE form as block capitals are often used for documentation les like README .
Make and include-les
cp foo /home/me
We can tell make which target we want to make by typing:
% make target
make will then only look at that target and ignore any others. For example, if we type make foo with the makefile
above, make will ignore the install target.
If we just type make on its own, make will always look at the rst target and then stop without looking at any others.
So if we typed make here, it will just go to the foo target, re-compile foo if necessary, and then stop without going
on to the install target.
Notice that the install target does not actually depend on anything! This means that the command on the following line is always executed when we try to make that target by typing make install . In this case, it will copy foo
into the user's home directory. This is often used by application makefiles, so that the application can be installed
in the correct directory when it has been correctly compiled.
This is a slightly confusing subject to try to explain. If you do not quite understand how make works, the best thing
to do is to write a simple program like “hello world” and a make le like the one above and experiment. Then
progress to using more than one source le, or having the source le include a header le. The touch command is
very useful here—it changes the date on a le without you having to edit it.
2.5.3. Make and include-files
C code often starts with a list of les to include, for example stdio.h. Some of these les are system-include les,
some of them are from the project you are now working on:
#include <stdio.h>
#include "foo.h"
int main(....
To make sure that this le is recompiled the moment foo.h is changed, you have to add it in your Makefile:
foo: foo.c foo.h
The moment your project is getting bigger and you have more and more own include-les to maintain, it will be
a pain to keep track of all include les and the les which are depending on it. If you change an include-le but
forget to recompile all the les which are depending on it, the results will be devastating. clang has an option to
analyze your les and to produce a list of include-les and their dependencies: -MM .
If you add this to your Makefile:
cc -E -MM *.c > .depend
and run make depend , the le .depend will appear with a list of object-les, C-les and the include-les:
foo.o: foo.c foo.h
If you change foo.h , next time you run make all les depending on foo.h will be recompiled.
Do not forget to run make depend each time you add an include-le to one of your les.
2.5.4. FreeBSD Makefiles
Makefiles can be rather complicated to write. Fortunately, BSD-based systems like FreeBSD come with some very
powerful ones as part of the system. One very good example of this is the FreeBSD ports system. Here is the essential
part of a typical ports Makefile:
Chapter 2. Programming Tools
.include <>
Now, if we go to the directory for this port and type make , the following happens:
A check is made to see if the source code for this port is already on the system.
If it is not, an FTP connection to the URL in MASTER_SITES is set up to download the source.
The checksum for the source is calculated and compared it with one for a known, good, copy of the source.
This is to make sure that the source was not corrupted while in transit.
Any changes required to make the source work on FreeBSD are applied—this is known as patching.
Any special configuration needed for the source is done. (Many UNIX® program distributions try to work out
which version of UNIX® they are being compiled on and which optional UNIX® features are present—this is
where they are given the information in the FreeBSD ports scenario).
The source code for the program is compiled. In effect, we change to the directory where the source was
unpacked and do make —the program's own make le has the necessary information to build the program.
We now have a compiled version of the program. If we wish, we can test it now; when we feel confident about
the program, we can type make install . This will cause the program and any supporting les it needs to be
copied into the correct location; an entry is also made into a package database, so that the port can easily be
uninstalled later if we change our mind about it.
Now I think you will agree that is rather impressive for a four line script!
The secret lies in the last line, which tells make to look in the system makefile called It is easy to
overlook this line, but this is where all the clever stu comes from—someone has written a makefile that tells make
to do all the things above (plus a couple of other things I did not mention, including handling any errors that may
occur) and anyone can get access to that just by putting a single line in their own make le!
If you want to have a look at these system makefiles, they are in /usr/share/mk , but it is probably best to wait
until you have had a bit of practice with makefiles, as they are very complicated (and if you do look at them, make
sure you have a ask of strong coffee handy!)
2.5.5. More advanced uses of make
Make is a very powerful tool, and can do much more than the simple example above shows. Unfortunately, there
are several different versions of make , and they all differ considerably. The best way to learn what they can do is
probably to read the documentation—hopefully this introduction will have given you a base from which you can
do this.
The version of make that comes with FreeBSD is the Berkeley make; there is a tutorial for it in /usr/share/doc/
psd/12.make. To view it, do
% zmore paper.ascii.gz
in that directory.
Many applications in the ports use GNU make, which has a very good set of “info” pages. If you have installed any
of these ports, GNU make will automatically have been installed as gmake . It is also available as a port and package
in its own right.
To view the info pages for GNU make, you will have to edit the dir le in the /usr/local/info directory to add
an entry for it. This involves adding a line like
* Make: (make).
The GNU Make utility.
to the le. Once you have done this, you can type info and then select make from the menu (or in Emacs, do C-h i ).
2.6. Debugging
2.6.1. The Debugger
The debugger that comes with FreeBSD is called gdb (GNU debugger). You start it up by typing
% gdb progname
although many people prefer to run it inside Emacs. You can do this by:
M-x gdb RET progname RET
Using a debugger allows you to run the program under more controlled circumstances. Typically, you can step
through the program a line at a time, inspect the value of variables, change them, tell the debugger to run up to
a certain point and then stop, and so on. You can even attach to a program that is already running, or load a core
le to investigate why the program crashed. It is even possible to debug the kernel, though that is a little trickier
than the user applications we will be discussing in this section.
gdb has quite good on-line help, as well as a set of info pages, so this section will concentrate on a few of the basic
Finally, if you nd its text-based command-prompt style o-putting, there is a graphical front-end for it (devel/xxgdb) in the Ports Collection.
This section is intended to be an introduction to using gdb and does not cover specialized topics such as debugging
the kernel.
2.6.2. Running a program in the debugger
You will need to have compiled the program with the -g option to get the most out of using gdb . It will work
without, but you will only see the name of the function you are in, instead of the source code. If you see a line like:
… (no debugging symbols found) …
when gdb starts up, you will know that the program was not compiled with the -g option.
At the gdb prompt, type break main . This will tell the debugger that you are not interested in watching the preliminary set-up code in the program being run, and that it should stop execution at the beginning of your code.
Now type run to start the program—it will start at the beginning of the set-up code and then get stopped by the
debugger when it calls main() . (If you have ever wondered where main() gets called from, now you know!).
You can now step through the program, a line at a time, by pressing n. If you get to a function call, you can step
into it by pressing s. Once you are in a function call, you can return from stepping into a function call by pressing
f. You can also use up and down to take a quick look at the caller.
Here is a simple example of how to spot a mistake in a program with gdb . This is our program (with a deliberate
#include <stdio.h>
int bazz(int anint);
main() {
int i;
printf("This is my program\n");
Chapter 2. Programming Tools
return 0;
int bazz(int anint) {
printf("You gave me %d\n", anint);
return anint;
This program sets i to be 5 and passes it to a function bazz() which prints out the number we gave it.
When we compile and run the program we get
% cc -g -o temp temp.c
% ./temp
This is my program
anint = 4231
That was not what we expected! Time to see what is going on!
% gdb temp
GDB is free software and you are welcome to distribute copies of it
under certain conditions; type "show copying" to see the conditions.
There is absolutely no warranty for GDB; type "show warranty" for details.
GDB 4.13 (i386-unknown-freebsd), Copyright 1994 Free Software Foundation, Inc.
(gdb) break main
Skip the set-up code
Breakpoint 1 at 0x160f: file temp.c, line 9. gdb puts breakpoint at main()
(gdb) run
Run as far as main()
Starting program: /home/james/tmp/temp Program starts running
Breakpoint 1, main () at temp.c:9 gdb stops at main()
(gdb) n
Go to next line
This is my program
Program prints out
(gdb) s
step into bazz()
bazz (anint=4231) at temp.c:17
gdb displays stack frame
Hang on a minute! How did anint get to be 4231 ? Did we not we set it to be 5 in main() ? Let's move up to main()
and have a look.
(gdb) up
Move up call stack
#1 0x1625 in main () at temp.c:11 gdb displays stack frame
(gdb) p i
Show us the value of i
$1 = 4231
gdb displays 4231
Oh dear! Looking at the code, we forgot to initialize i. We meant to put
main() {
int i;
i = 5;
printf("This is my program\n");
but we left the i=5; line out. As we did not initialize i, it had whatever number happened to be in that area of
memory when the program ran, which in this case happened to be 4231 .
gdb displays the stack frame every time we go into or out of a function, even if we are using
up and down to move around the call stack. This shows the name of the function and the
values of its arguments, which helps us keep track of where we are and what is going on. (The
Examining a core le
stack is a storage area where the program stores information about the arguments passed to
functions and where to go when it returns from a function call).
2.6.3. Examining a core file
A core le is basically a le which contains the complete state of the process when it crashed. In “the good old
days”, programmers had to print out hex listings of core les and sweat over machine code manuals, but now life
is a bit easier. Incidentally, under FreeBSD and other 4.4BSD systems, a core le is called progname.core instead
of just core , to make it clearer which program a core le belongs to.
To examine a core le, start up gdb in the usual way. Instead of typing break or run , type
(gdb) core progname .core
If you are not in the same directory as the core le, you will have to do dir /path/to/core/file rst.
You should see something like this:
% gdb a.out
GDB is free software and you are welcome to distribute copies of it
under certain conditions; type "show copying" to see the conditions.
There is absolutely no warranty for GDB; type "show warranty" for details.
GDB 4.13 (i386-unknown-freebsd), Copyright 1994 Free Software Foundation, Inc.
(gdb) core a.out.core
Core was generated by `a.out'.
Program terminated with signal 11, Segmentation fault.
Cannot access memory at address 0x7020796d.
#0 0x164a in bazz (anint=0x5) at temp.c:17
In this case, the program was called a.out , so the core le is called a.out.core . We can see that the program
crashed due to trying to access an area in memory that was not available to it in a function called bazz .
Sometimes it is useful to be able to see how a function was called, as the problem could have occurred a long way
up the call stack in a complex program. The bt command causes gdb to print out a back-trace of the call stack:
(gdb) bt
#0 0x164a in bazz (anint=0x5) at temp.c:17
#1 0xefbfd888 in end ()
#2 0x162c in main () at temp.c:11
The end() function is called when a program crashes; in this case, the bazz() function was called from main() .
2.6.4. Attaching to a running program
One of the neatest features about gdb is that it can attach to a program that is already running. Of course, that
assumes you have sufficient permissions to do so. A common problem is when you are stepping through a program
that forks, and you want to trace the child, but the debugger will only let you trace the parent.
What you do is start up another gdb , use ps to nd the process ID for the child, and do
(gdb) attach pid
in gdb , and then debug as usual.
“That is all very well,” you are probably thinking, “but by the time I have done that, the child process will be over
the hill and far away”. Fear not, gentle reader, here is how to do it (courtesy of the gdb info pages):
if ((pid = fork()) < 0)
/* _Always_ check this */
Chapter 2. Programming Tools
else if (pid == 0) {
int PauseMode = 1;
/* child */
while (PauseMode)
sleep(10); /* Wait until someone attaches to us */
} else {
/* parent */
Now all you have to do is attach to the child, set PauseMode to 0, and wait for the sleep() call to return!
2.7. Using Emacs as a Development Environment
2.7.1. Emacs
Emacs is a highly customizable editor—indeed, it has been customized to the point where it is more like an operating system than an editor! Many developers and sysadmins do in fact spend practically all their time working
inside Emacs, leaving it only to log out.
It is impossible even to summarize everything Emacs can do here, but here are some of the features of interest
to developers:
• Very powerful editor, allowing search-and-replace on both strings and regular expressions (patterns), jumping
to start/end of block expression, etc, etc.
• Pull-down menus and online help.
• Language-dependent syntax highlighting and indentation.
• Completely customizable.
• You can compile and debug programs within Emacs.
• On a compilation error, you can jump to the offending line of source code.
• Friendly-ish front-end to the info program used for reading GNU hypertext documentation, including the documentation on Emacs itself.
• Friendly front-end to gdb , allowing you to look at the source code as you step through your program.
And doubtless many more that have been overlooked.
Emacs can be installed on FreeBSD using the editors/emacs port.
Once it is installed, start it up and do C-h t to read an Emacs tutorial—that means hold down the control key,
press h, let go of the control key, and then press t. (Alternatively, you can use the mouse to select Emacs Tutorial
from the Help menu.)
Although Emacs does have menus, it is well worth learning the key bindings, as it is much quicker when you are
editing something to press a couple of keys than to try to nd the mouse and then click on the right place. And,
when you are talking to seasoned Emacs users, you will nd they often casually throw around expressions like “Mx replace-s RET foo RET bar RET ” so it is useful to know what they mean. And in any case, Emacs has far too
many useful functions for them to all t on the menu bars.
Fortunately, it is quite easy to pick up the key-bindings, as they are displayed next to the menu item. My advice is
to use the menu item for, say, opening a le until you understand how it works and feel confident with it, then try
doing C-x C-f. When you are happy with that, move on to another menu command.
If you can not remember what a particular combination of keys does, select Describe Key from the Help menu and
type it in—Emacs will tell you what it does. You can also use the Command Apropos menu item to nd out all the
commands which contain a particular word in them, with the key binding next to it.
Configuring Emacs
By the way, the expression above means hold down the Meta key, press x, release the Meta key, type replace-s
(short for replace-string —another feature of Emacs is that you can abbreviate commands), press the return key,
type foo (the string you want replaced), press the return key, type bar (the string you want to replace foo with)
and press return again. Emacs will then do the search-and-replace operation you have just requested.
If you are wondering what on earth the Meta key is, it is a special key that many UNIX® workstations have. Unfortunately, PC's do not have one, so it is usually the alt key (or if you are unlucky, the escape key).
Oh, and to get out of Emacs, do C-x C-c (that means hold down the control key, press x, press c and release the
control key). If you have any unsaved les open, Emacs will ask you if you want to save them. (Ignore the bit in
the documentation where it says C-z is the usual way to leave Emacs—that leaves Emacs hanging around in the
background, and is only really useful if you are on a system which does not have virtual terminals).
2.7.2. Configuring Emacs
Emacs does many wonderful things; some of them are built in, some of them need to be configured.
Instead of using a proprietary macro language for configuration, Emacs uses a version of Lisp specially adapted
for editors, known as Emacs Lisp. Working with Emacs Lisp can be quite helpful if you want to go on and learn
something like Common Lisp. Emacs Lisp has many features of Common Lisp, although it is considerably smaller
(and thus easier to master).
The best way to learn Emacs Lisp is to download the Emacs Tutorial
However, there is no need to actually know any Lisp to get started with configuring Emacs, as I have included a
sample .emacs le, which should be enough to get you started. Just copy it into your home directory and restart
Emacs if it is already running; it will read the commands from the le and (hopefully) give you a useful basic setup.
2.7.3. A sample .emacs file
Unfortunately, there is far too much here to explain it in detail; however there are one or two points worth mentioning.
• Everything beginning with a ; is a comment and is ignored by Emacs.
• In the rst line, the -*- Emacs-Lisp -*- is so that we can edit the .emacs le itself within Emacs and get all
the fancy features for editing Emacs Lisp. Emacs usually tries to guess this based on the filename, and may not
get it right for .emacs .
• The tab key is bound to an indentation function in some modes, so when you press the tab key, it will indent
the current line of code. If you want to put a tab character in whatever you are writing, hold the control key
down while you are pressing the tab key.
• This le supports syntax highlighting for C, C++, Perl, Lisp and Scheme, by guessing the language from the filename.
• Emacs already has a pre-defined function called next-error . In a compilation output window, this allows you to
move from one compilation error to the next by doing M-n ; we define a complementary function, previous-error, that allows you to go to a previous error by doing M-p . The nicest feature of all is that C-c C-c will open up
the source le in which the error occurred and jump to the appropriate line.
• We enable Emacs's ability to act as a server, so that if you are doing something outside Emacs and you want to
edit a le, you can just type in
% emacsclient filename
and then you can edit the le in your Emacs! 6
Many Emacs users set their EDITOR environment to emacsclient so this happens every time they need to edit a le.
Chapter 2. Programming Tools
Example 2.1. A sample .emacs le
;; -*-Emacs-Lisp-*;; This file is designed to be re-evaled; use the variable first-time
;; to avoid any problems with this.
(defvar first-time t
"Flag signifying this is the first time that .emacs has been evaled")
;; Meta
"\M- " 'set-mark-command)
"\M-\C-h" 'backward-kill-word)
"\M-\C-r" 'query-replace)
"\M-r" 'replace-string)
"\M-g" 'goto-line)
"\M-h" 'help-command)
;; Function keys
(global-set-key [f1] 'manual-entry)
(global-set-key [f2] 'info)
(global-set-key [f3] 'repeat-complex-command)
(global-set-key [f4] 'advertised-undo)
(global-set-key [f5] 'eval-current-buffer)
(global-set-key [f6] 'buffer-menu)
(global-set-key [f7] 'other-window)
(global-set-key [f8] 'find-file)
(global-set-key [f9] 'save-buffer)
(global-set-key [f10] 'next-error)
(global-set-key [f11] 'compile)
(global-set-key [f12] 'grep)
(global-set-key [C-f1] 'compile)
(global-set-key [C-f2] 'grep)
(global-set-key [C-f3] 'next-error)
(global-set-key [C-f4] 'previous-error)
(global-set-key [C-f5] 'display-faces)
(global-set-key [C-f8] 'dired)
(global-set-key [C-f10] 'kill-compilation)
;; Keypad bindings
(global-set-key [up] "\C-p")
(global-set-key [down] "\C-n")
(global-set-key [left] "\C-b")
(global-set-key [right] "\C-f")
(global-set-key [home] "\C-a")
(global-set-key [end] "\C-e")
(global-set-key [prior] "\M-v")
(global-set-key [next] "\C-v")
(global-set-key [C-up] "\M-\C-b")
(global-set-key [C-down] "\M-\C-f")
(global-set-key [C-left] "\M-b")
(global-set-key [C-right] "\M-f")
(global-set-key [C-home] "\M-<")
(global-set-key [C-end] "\M->")
(global-set-key [C-prior] "\M-<")
(global-set-key [C-next] "\M->")
;; Mouse
(global-set-key [mouse-3] 'imenu)
;; Misc
(global-set-key [C-tab] "\C-q\t") ; Control tab quotes a tab.
(setq backup-by-copying-when-mismatch t)
A sample .emacs le
;; Treat 'y' or <CR> as yes, 'n' as no.
(fset 'yes-or-no-p 'y-or-n-p)
(define-key query-replace-map [return] 'act)
(define-key query-replace-map [?\C-m] 'act)
;; Load packages
(require 'desktop)
(require 'tar-mode)
;; Pretty diff mode
(autoload 'ediff-buffers "ediff" "Intelligent Emacs interface to diff" t)
(autoload 'ediff-files "ediff" "Intelligent Emacs interface to diff" t)
(autoload 'ediff-files-remote "ediff"
"Intelligent Emacs interface to diff")
(if first-time
(setq auto-mode-alist
(append '(("\\.cpp$" . c++-mode)
("\\.hpp$" . c++-mode)
("\\.lsp$" . lisp-mode)
("\\.scm$" . scheme-mode)
("\\.pl$" . perl-mode)
) auto-mode-alist)))
;; Auto font lock mode
(defvar font-lock-auto-mode-list
(list 'c-mode 'c++-mode 'c++-c-mode 'emacs-lisp-mode 'lisp-mode 'perl-mode
"List of modes to always start in font-lock-mode")
(defvar font-lock-mode-keyword-alist
'((c++-c-mode . c-font-lock-keywords)
(perl-mode . perl-font-lock-keywords))
"Associations between modes and keywords")
(defun font-lock-auto-mode-select ()
"Automatically select font-lock-mode if the current major mode is in font-lockauto-mode-list"
(if (memq major-mode font-lock-auto-mode-list)
(font-lock-mode t))
(global-set-key [M-f1] 'font-lock-fontify-buffer)
;; New dabbrev stuff
;(require 'new-dabbrev)
(setq dabbrev-always-check-other-buffers t)
(setq dabbrev-abbrev-char-regexp "\\sw\\|\\s_")
(add-hook 'emacs-lisp-mode-hook
'(lambda ()
(set (make-local-variable 'dabbrev-case-fold-search) nil)
(set (make-local-variable 'dabbrev-case-replace) nil)))
(add-hook 'c-mode-hook
'(lambda ()
(set (make-local-variable 'dabbrev-case-fold-search) nil)
(set (make-local-variable 'dabbrev-case-replace) nil)))
(add-hook 'text-mode-hook
'(lambda ()
(set (make-local-variable 'dabbrev-case-fold-search) t)
(set (make-local-variable 'dabbrev-case-replace) t)))
;; C++ and C mode...
(defun my-c++-mode-hook ()
(setq tab-width 4)
Chapter 2. Programming Tools
(define-key c++-mode-map "\C-m" 'reindent-then-newline-and-indent)
(define-key c++-mode-map "\C-ce" 'c-comment-edit)
(setq c++-auto-hungry-initial-state 'none)
(setq c++-delete-function 'backward-delete-char)
(setq c++-tab-always-indent t)
(setq c-indent-level 4)
(setq c-continued-statement-offset 4)
(setq c++-empty-arglist-indent 4))
(defun my-c-mode-hook ()
(setq tab-width 4)
(define-key c-mode-map "\C-m" 'reindent-then-newline-and-indent)
(define-key c-mode-map "\C-ce" 'c-comment-edit)
(setq c-auto-hungry-initial-state 'none)
(setq c-delete-function 'backward-delete-char)
(setq c-tab-always-indent t)
;; BSD-ish indentation style
(setq c-indent-level 4)
(setq c-continued-statement-offset 4)
(setq c-brace-offset -4)
(setq c-argdecl-indent 0)
(setq c-label-offset -4))
;; Perl mode
(defun my-perl-mode-hook ()
(setq tab-width 4)
(define-key c++-mode-map "\C-m" 'reindent-then-newline-and-indent)
(setq perl-indent-level 4)
(setq perl-continued-statement-offset 4))
;; Scheme mode...
(defun my-scheme-mode-hook ()
(define-key scheme-mode-map "\C-m" 'reindent-then-newline-and-indent))
;; Emacs-Lisp mode...
(defun my-lisp-mode-hook ()
(define-key lisp-mode-map "\C-m" 'reindent-then-newline-and-indent)
(define-key lisp-mode-map "\C-i" 'lisp-indent-line)
(define-key lisp-mode-map "\C-j" 'eval-print-last-sexp))
;; Add all of the hooks...
(add-hook 'c++-mode-hook 'my-c++-mode-hook)
(add-hook 'c-mode-hook 'my-c-mode-hook)
(add-hook 'scheme-mode-hook 'my-scheme-mode-hook)
(add-hook 'emacs-lisp-mode-hook 'my-lisp-mode-hook)
(add-hook 'lisp-mode-hook 'my-lisp-mode-hook)
(add-hook 'perl-mode-hook 'my-perl-mode-hook)
;; Complement to next-error
(defun previous-error (n)
"Visit previous compilation error message and corresponding source code."
(interactive "p")
(next-error (- n)))
;; Misc...
(transient-mark-mode 1)
(setq mark-even-if-inactive t)
(setq visible-bell nil)
(setq next-line-add-newlines nil)
(setq compile-command "make")
(setq suggest-key-bindings nil)
(put 'eval-expression 'disabled nil)
(put 'narrow-to-region 'disabled nil)
(put 'set-goal-column 'disabled nil)
(if (>= emacs-major-version 21)
(setq show-trailing-whitespace t))
A sample .emacs le
;; Elisp archive searching
(autoload 'format-lisp-code-directory "lispdir" nil t)
(autoload 'lisp-dir-apropos "lispdir" nil t)
(autoload 'lisp-dir-retrieve "lispdir" nil t)
(autoload 'lisp-dir-verify "lispdir" nil t)
;; Font lock mode
(defun my-make-face (face color &optional bold)
"Create a face from a color and optionally make it bold"
(make-face face)
(copy-face 'default face)
(set-face-foreground face color)
(if bold (make-face-bold face))
(if (eq window-system 'x)
(my-make-face 'blue "blue")
(my-make-face 'red "red")
(my-make-face 'green "dark green")
(setq font-lock-comment-face 'blue)
(setq font-lock-string-face 'bold)
(setq font-lock-type-face 'bold)
(setq font-lock-keyword-face 'bold)
(setq font-lock-function-name-face 'red)
(setq font-lock-doc-string-face 'green)
(add-hook 'find-file-hooks 'font-lock-auto-mode-select)
(setq baud-rate 1000000)
(global-set-key "\C-cmm" 'menu-bar-mode)
(global-set-key "\C-cms" 'scroll-bar-mode)
(global-set-key [backspace] 'backward-delete-char)
(global-set-key [delete] 'delete-char)
(standard-display-european t)
(load-library "iso-transl")))
;; X11 or PC using direct screen writes
(if window-system
(global-set-key [M-f1] 'hilit-repaint-command)
(global-set-key [M-f2] [?\C-u M-f1])
(setq hilit-mode-enable-list
'(not text-mode c-mode c++-mode emacs-lisp-mode lisp-mode
hilit-auto-highlight nil
hilit-auto-rehighlight 'visible
hilit-inhibit-hooks nil
hilit-inhibit-rebinding t)
(require 'hilit19)
(require 'paren))
(setq baud-rate 2400)
; For slow serial connections
;; TTY type terminal
(if (and (not window-system)
(not (equal system-type 'ms-dos)))
(if first-time
(keyboard-translate ?\C-h ?\C-?)
(keyboard-translate ?\C-? ?\C-h)))))
;; Under UNIX
(if (not (equal system-type 'ms-dos))
Chapter 2. Programming Tools
(if first-time
;; Add any face changes here
(add-hook 'term-setup-hook 'my-term-setup-hook)
(defun my-term-setup-hook ()
(if (eq window-system 'pc)
;; (set-face-background 'default "red")
;; Restore the "desktop" - do this as late as possible
(if first-time
;; Indicate that this file has been read at least once
(setq first-time nil)
;; No need to debug anything now
(setq debug-on-error nil)
;; All done
(message "All done, %s%s" (user-login-name) ".")
2.7.4. Extending the Range of Languages Emacs Understands
Now, this is all very well if you only want to program in the languages already catered for in the .emacs le (C, C++,
Perl, Lisp and Scheme), but what happens if a new language called “whizbang” comes out, full of exciting features?
The rst thing to do is nd out if whizbang comes with any les that tell Emacs about the language. These usually
end in .el , short for “Emacs Lisp”. For example, if whizbang is a FreeBSD port, we can locate these les by doing
% find /usr/ports/lang/whizbang -name "*.el" -print
and install them by copying them into the Emacs site Lisp directory. On FreeBSD, this is /usr/local/share/
emacs/site-lisp .
So for example, if the output from the nd command was
we would do
# cp /usr/ports/lang/whizbang/work/misc/whizbang.el /usr/local/share/emacs/site-lisp
Next, we need to decide what extension whizbang source les have. Let's say for the sake of argument that they
all end in .wiz . We need to add an entry to our .emacs le to make sure Emacs will be able to use the information
in whizbang.el .
Find the auto-mode-alist entry in .emacs and add a line for whizbang, such as:
("\\.lsp$" . lisp-mode)
("\\.wiz$" . whizbang-mode)
("\\.scm$" . scheme-mode)
This means that Emacs will automatically go into whizbang-mode when you edit a le ending in .wiz .
Further Reading
Just below this, you will nd the font-lock-auto-mode-list entry. Add whizbang-mode to it like so:
;; Auto font lock mode
(defvar font-lock-auto-mode-list
(list 'c-mode 'c++-mode 'c++-c-mode 'emacs-lisp-mode 'whizbang-mode 'lisp-mode 'perlmode 'scheme-mode)
"List of modes to always start in font-lock-mode")
This means that Emacs will always enable font-lock-mode (ie syntax highlighting) when editing a .wiz le.
And that is all that is needed. If there is anything else you want done automatically when you open up a .wiz le,
you can add a whizbang-mode hook (see my-scheme-mode-hook for a simple example that adds auto-indent ).
2.8. Further Reading
For information about setting up a development environment for contributing fixes to FreeBSD itself, please see
• Brian Harvey and Matthew Wright Simply Scheme MIT 1994. ISBN 0-262-08226-8
• Randall Schwartz Learning Perl O'Reilly 1993 ISBN 1-56592-042-2
• Patrick Henry Winston and Berthold Klaus Paul Horn Lisp (3rd Edition) Addison-Wesley 1989 ISBN 0-201-08319-1
• Brian W. Kernighan and Rob Pike The Unix Programming Environment Prentice-Hall 1984 ISBN 0-13-937681-X
• Brian W. Kernighan and Dennis M. Ritchie The C Programming Language (2nd Edition) Prentice-Hall 1988 ISBN
• Bjarne Stroustrup The C++ Programming Language Addison-Wesley 1991 ISBN 0-201-53992-6
• W. Richard Stevens Advanced Programming in the Unix Environment Addison-Wesley 1992 ISBN 0-201-56317-7
• W. Richard Stevens Unix Network Programming Prentice-Hall 1990 ISBN 0-13-949876-1
Chapter 3. Secure Programming
Contributed by Murray Stokely.
3.1. Synopsis
This chapter describes some of the security issues that have plagued UNIX® programmers for decades and some
of the new tools available to help programmers avoid writing exploitable code.
3.2. Secure Design Methodology
Writing secure applications takes a very scrutinous and pessimistic outlook on life. Applications should be run with
the principle of “least privilege” so that no process is ever running with more than the bare minimum access that
it needs to accomplish its function. Previously tested code should be reused whenever possible to avoid common
mistakes that others may have already xed.
One of the pitfalls of the UNIX® environment is how easy it is to make assumptions about the sanity of the environment. Applications should never trust user input (in all its forms), system resources, inter-process communication,
or the timing of events. UNIX® processes do not execute synchronously so logical operations are rarely atomic.
3.3. Buffer Overflows
Buer Overflows have been around since the very beginnings of the von Neumann 1 architecture. They rst
gained widespread notoriety in 1988 with the Morris Internet worm. Unfortunately, the same basic attack remains
effective today. By far the most common type of buer overflow attack is based on corrupting the stack.
Most modern computer systems use a stack to pass arguments to procedures and to store local variables. A stack
is a last in rst out (LIFO) buer in the high memory area of a process image. When a program invokes a function
a new "stack frame" is created. This stack frame consists of the arguments passed to the function as well as a
dynamic amount of local variable space. The "stack pointer" is a register that holds the current location of the
top of the stack. Since this value is constantly changing as new values are pushed onto the top of the stack, many
implementations also provide a "frame pointer" that is located near the beginning of a stack frame so that local
variables can more easily be addressed relative to this value. 1 The return address for function calls is also stored
on the stack, and this is the cause of stack-overflow exploits since overflowing a local variable in a function can
overwrite the return address of that function, potentially allowing a malicious user to execute any code he or she
Although stack-based attacks are by far the most common, it would also be possible to overrun the stack with a
heap-based (malloc/free) attack.
The C programming language does not perform automatic bounds checking on arrays or pointers as many other
languages do. In addition, the standard C library is lled with a handful of very dangerous functions.
strcpy(char *dest, const char *src)
May overflow the dest buer
strcat(char *dest, const char *src)
May overflow the dest buer
getwd (char *buf)
May overflow the buf buer
gets(char *s)
May overflow the s buer
[vf]scanf(const char *format, ...)
May overflow its arguments.
realpath(char *path, char resolved_path[])
May overflow the path buer
[v]sprintf(char *str, const char *format, ...)
May overflow the str buer.
Example Buer Overflow
3.3.1. Example Buffer Overflow
The following example code contains a buer overflow designed to overwrite the return address and skip the
instruction immediately following the function call. (Inspired by 4)
#include <stdio.h>
void manipulate(char *buffer) {
char newbuffer[80];
int main() {
char ch,buffer[4096];
int i=0;
while ((buffer[i++] = getchar()) != '\n') {};
printf("The value of i is : %d\n",i);
return 0;
Let us examine what the memory image of this process would look like if we were to input 160 spaces into our little
program before hitting return.
[XXX figure here!]
Obviously more malicious input can be devised to execute actual compiled instructions (such as exec(/bin/sh)).
3.3.2. Avoiding Buffer Overflows
The most straightforward solution to the problem of stack-overflows is to always use length restricted memory
and string copy functions. strncpy and strncat are part of the standard C library. These functions accept a length
value as a parameter which should be no larger than the size of the destination buer. These functions will then
copy up to `length' bytes from the source to the destination. However there are a number of problems with these
functions. Neither function guarantees NUL termination if the size of the input buer is as large as the destination.
The length parameter is also used inconsistently between strncpy and strncat so it is easy for programmers to get
confused as to their proper usage. There is also a significant performance loss compared to strcpy when copying
a short string into a large buer since strncpy NUL lls up the size specified.
Another memory copy implementation exists to get around these problems. The strlcpy and strlcat functions
guarantee that they will always null terminate the destination string when given a non-zero length argument. Compiler based run-time bounds checking
Unfortunately there is still a very large assortment of code in public use which blindly copies memory around
without using any of the bounded copy routines we just discussed. Fortunately, there is a way to help prevent such
attacks — run-time bounds checking, which is implemented by several C/C++ compilers.
ProPolice is one such compiler feature, and is integrated into gcc(1) versions 4.1 and later. It replaces and extends
the earlier StackGuard gcc(1) extension.
ProPolice helps to protect against stack-based buer overflows and other attacks by laying pseudo-random numbers in key areas of the stack before calling any function. When a function returns, these “canaries” are checked
and if they are found to have been changed the executable is immediately aborted. Thus any attempt to modify
the return address or other variable stored on the stack in an attempt to get malicious code to run is unlikely to
succeed, as the attacker would have to also manage to leave the pseudo-random canaries untouched.
Chapter 3. Secure Programming
Recompiling your application with ProPolice is an effective means of stopping most buer-overflow attacks, but
it can still be compromised. Library based run-time bounds checking
Compiler-based mechanisms are completely useless for binary-only software for which you cannot recompile. For
these situations there are a number of libraries which re-implement the unsafe functions of the C-library (strcpy,
fscanf, getwd , etc..) and ensure that these functions can never write past the stack pointer.
• libsafe
• libverify
• libparanoia
Unfortunately these library-based defenses have a number of shortcomings. These libraries only protect against a
very small set of security related issues and they neglect to x the actual problem. These defenses may fail if the
application was compiled with -fomit-frame-pointer. Also, the LD_PRELOAD and LD_LIBRARY_PATH environment
variables can be overwritten/unset by the user.
3.4. SetUID issues
There are at least 6 different IDs associated with any given process. Because of this you have to be very careful
with the access that your process has at any given time. In particular, all seteuid applications should give up their
privileges as soon as it is no longer required.
The real user ID can only be changed by a superuser process. The login program sets this when a user initially logs
in and it is seldom changed.
The effective user ID is set by the exec() functions if a program has its seteuid bit set. An application can call
seteuid() at any time to set the effective user ID to either the real user ID or the saved set-user-ID. When the
effective user ID is set by exec() functions, the previous value is saved in the saved set-user-ID.
3.5. Limiting your program's environment
The traditional method of restricting a process is with the chroot() system call. This system call changes the root
directory from which all other paths are referenced for a process and any child processes. For this call to succeed
the process must have execute (search) permission on the directory being referenced. The new environment does
not actually take effect until you chdir() into your new environment. It should also be noted that a process can
easily break out of a chroot environment if it has root privilege. This could be accomplished by creating device
nodes to read kernel memory, attaching a debugger to a process outside of the chroot(8) environment, or in many
other creative ways.
The behavior of the chroot() system call can be controlled somewhat with the kern.chroot_allow_open_directories
sysctl variable. When this value is set to 0, chroot() will fail with EPERM if there are any directories open. If set
to the default value of 1, then chroot() will fail with EPERM if there are any directories open and the process is
already subject to a chroot() call. For any other value, the check for open directories will be bypassed completely.
3.5.1. FreeBSD's jail functionality
The concept of a Jail extends upon the chroot() by limiting the powers of the superuser to create a true `virtual
server'. Once a prison is set up all network communication must take place through the specified IP address, and
the power of "root privilege" in this jail is severely constrained.
POSIX®.1e Process Capabilities
While in a prison, any tests of superuser power within the kernel using the suser() call will fail. However, some
calls to suser() have been changed to a new interface suser_xxx() . This function is responsible for recognizing
or denying access to superuser power for imprisoned processes.
A superuser process within a jailed environment has the power to:
• Manipulate credential with setuid, seteuid, setgid, setegid, setgroups, setreuid, setregid, setlogin
• Set resource limits with setrlimit
• Modify some sysctl nodes (kern.hostname)
• chroot()
• Set ags on a vnode: chflags , fchflags
• Set attributes of a vnode such as le permission, owner, group, size, access time, and modification time.
• Bind to privileged ports in the Internet domain (ports < 1024)
Jail is a very useful tool for running applications in a secure environment but it does have some shortcomings.
Currently, the IPC mechanisms have not been converted to the suser_xxx so applications such as MySQL cannot
be run within a jail. Superuser access may have a very limited meaning within a jail, but there is no way to specify
exactly what "very limited" means.
3.5.2. POSIX®.1e Process Capabilities
POSIX® has released a working draft that adds event auditing, access control lists, ne grained privileges, information labeling, and mandatory access control.
This is a work in progress and is the focus of the TrustedBSD project. Some of the initial work has been committed
to FreeBSD-CURRENT (cap_set_proc(3)).
3.6. Trust
An application should never assume that anything about the users environment is sane. This includes (but is certainly not limited to): user input, signals, environment variables, resources, IPC, mmaps, the filesystem working
directory, le descriptors, the # of open les, etc.
You should never assume that you can catch all forms of invalid input that a user might supply. Instead, your
application should use positive filtering to only allow a specific subset of inputs that you deem safe. Improper data
validation has been the cause of many exploits, especially with CGI scripts on the world wide web. For filenames
you need to be extra careful about paths ("../", "/"), symbolic links, and shell escape characters.
Perl has a really cool feature called "Taint" mode which can be used to prevent scripts from using data derived
outside the program in an unsafe way. This mode will check command line arguments, environment variables,
locale information, the results of certain syscalls (readdir(), readlink(), getpwxxx()), and all le input.
3.7. Race Conditions
A race condition is anomalous behavior caused by the unexpected dependence on the relative timing of events. In
other words, a programmer incorrectly assumed that a particular event would always happen before another.
Some of the common causes of race conditions are signals, access checks, and le opens. Signals are asynchronous
events by nature so special care must be taken in dealing with them. Checking access with access(2) then open(2)
is clearly non-atomic. Users can move les in between the two calls. Instead, privileged applications should se32
Chapter 3. Secure Programming
teuid() and then call open() directly. Along the same lines, an application should always set a proper umask before open() to obviate the need for spurious chmod() calls.
Chapter 4. Localization and
Internationalization - L10N and I18N
4.1. Programming I18N Compliant Applications
To make your application more useful for speakers of other languages, we hope that you will program I18N compliant. The GNU gcc compiler and GUI libraries like QT and GTK support I18N through special handling of strings.
Making a program I18N compliant is very easy. It allows contributors to port your application to other languages
quickly. Refer to the library specific I18N documentation for more details.
In contrast with common perception, I18N compliant code is easy to write. Usually, it only involves wrapping your
strings with library specific functions. In addition, please be sure to allow for wide or multibyte character support.
4.1.1. A Call to Unify the I18N Effort
It has come to our attention that the individual I18N/L10N efforts for each country has been repeating each others'
efforts. Many of us have been reinventing the wheel repeatedly and inefficiently. We hope that the various major
groups in I18N could congregate into a group effort similar to the Core Team's responsibility.
Currently, we hope that, when you write or port I18N programs, you would send it out to each country's related
FreeBSD mailing list for testing. In the future, we hope to create applications that work in all the languages outof-the-box without dirty hacks.
The FreeBSD internationalization mailing list has been established. If you are an I18N/L10N developer, please send
your comments, ideas, questions, and anything you deem related to it.
4.1.2. Perl and Python
Perl and Python have I18N and wide character handling libraries. Please use them for I18N compliance.
4.2. Localized Messages with POSIX.1 Native Language Support
Contributed by Gábor Kövesdán.
Beyond the basic I18N functions, like supporting various input encodings or supporting national conventions,
such as the different decimal separators, at a higher level of I18N, it is possible to localize the messages written to
the output by the various programs. A common way of doing this is using the POSIX.1 NLS functions, which are
provided as a part of the FreeBSD base system.
4.2.1. Organizing Localized Messages into Catalog Files
POSIX.1 NLS is based on catalog les, which contain the localized messages in the desired encoding. The messages
are organized into sets and each message is identified by an integer number in the containing set. The catalog les
are conventionally named after the locale they contain localized messages for, followed by the .msg extension. For
instance, the Hungarian messages for ISO8859-2 encoding should be stored in a le called hu_HU.ISO8859-2 .
These catalog les are common text les that contain the numbered messages. It is possible to write comments
by starting the line with a $ sign. Set boundaries are also separated by special comments, where the keyword set
must directly follow the $ sign. The set keyword is then followed by the set number. For example:
$set 1
Using the Catalog Files from the Source Code
The actual message entries start with the message number and followed by the localized message. The well-known
modifiers from printf(3) are accepted:
15 "File not found: %s\n"
The language catalog les have to be compiled into a binary form before they can be opened from the program.
This conversion is done with the gencat(1) utility. Its rst argument is the filename of the compiled catalog and
its further arguments are the input catalogs. The localized messages can also be organized into more catalog les
and then all of them can be processed with gencat(1).
4.2.2. Using the Catalog Files from the Source Code
Using the catalog les is simple. To use the related functions, nl_types.h must be included. Before using a catalog,
it has to be opened with catopen(3). The function takes two arguments. The rst parameter is the name of the
installed and compiled catalog. Usually, the name of the program is used, such as grep. This name will be used when
looking for the compiled catalog le. The catopen(3) call looks for this le in /usr/share/nls/ locale/catname
and in /usr/local/share/nls/ locale/catname, where locale is the locale set and catname is the catalog name
being discussed. The second parameter is a constant, which can have two values:
• NL_CAT_LOCALE, which means that the used catalog le will be based on LC_MESSAGES .
• 0, which means that LANG has to be used to open the proper catalog.
The catopen(3) call returns a catalog identifier of type nl_catd . Please refer to the manual page for a list of possible
returned error codes.
After opening a catalog catgets(3) can be used to retrieve a message. The rst parameter is the catalog identifier
returned by catopen(3), the second one is the number of the set, the third one is the number of the messages, and
the fourth one is a fallback message, which will be returned if the requested message cannot be retrieved from
the catalog le.
After using the catalog le, it must be closed by calling catclose(3), which has one argument, the catalog id.
4.2.3. A Practical Example
The following example will demonstrate an easy solution on how to use NLS catalogs in a flexible way.
The below lines need to be put into a common header le of the program, which is included into all source les
where localized messages are necessary:
#define getstr(n) nlsstr[n]
#include <nl_types.h>
extern nl_catd catalog;
#define getstr(n) catgets(catalog, 1, n, nlsstr[n])
extern char
Next, put these lines into the global declaration part of the main source le:
#include <nl_types.h>
nl_catd catalog;
* Default messages to use when NLS is disabled or no catalog
Chapter 4. Localization and Internationalization - L10N
and I18N
* is found.
*nlsstr[] = {
/* 1*/ "some random message",
/* 2*/ "some other message"
Next come the real code snippets, which open, read, and close the catalog:
catalog = catopen("myapp", NL_CAT_LOCALE);
#endif Reducing Strings to Localize
There is a good way of reducing the strings that need to be localized by using libc error messages. This is also useful
to just avoid duplication and provide consistent error messages for the common errors that can be encountered
by a great many of programs.
First, here is an example that does not use libc error messages:
#include <err.h>
if (!S_ISDIR(st.st_mode))
errx(1, "argument is not a directory");
This can be transformed to print an error message by reading errno and printing an error message accordingly:
#include <err.h>
#include <errno.h>
if (!S_ISDIR(st.st_mode)) {
errno = ENOTDIR;
err(1, NULL);
In this example, the custom string is eliminated, thus translators will have less work when localizing the program
and users will see the usual “Not a directory” error message when they encounter this error. This message will
probably seem more familiar to them. Please note that it was necessary to include errno.h in order to directly
access errno.
It is worth to note that there are cases when errno is set automatically by a preceding call, so it is not necessary
to set it explicitly:
#include <err.h>
if ((p = malloc(size)) == NULL)
err(1, NULL);
Making use of
4.2.4. Making use of
Using the catalog les requires few repeatable steps, such as compiling the catalogs and installing them to the
proper location. In order to simplify this process even more, introduces some macros. It is not necessary to include explicitly, it is pulled in from the common Makefiles, such as or
Usually it is enough to define NLSNAME, which should have the catalog name mentioned as the rst argument of
catopen(3) and list the catalog les in NLS without their .msg extension. Here is an example, which makes it possible
to to disable NLS when used with the code examples before. The WITHOUT_NLS make(1) variable has to be defined
in order to build the program without NLS support.
.if !defined(WITHOUT_NLS)
NLS= es_ES.ISO8859-1
NLS+= hu_HU.ISO8859-2
NLS+= pt_BR.ISO8859-1
Conventionally, the catalog les are placed under the nls subdirectory and this is the default behavior of It is possible, though to override the location of the catalogs with the NLSSRCDIR make(1) variable. The default
name of the precompiled catalog les also follow the naming convention mentioned before. It can be overridden
by setting the NLSNAME variable. There are other options to ne tune the processing of the catalog les but usually
it is not needed, thus they are not described here. For further information on, please refer to the le
itself, it is short and easy to understand.
Chapter 5. Source Tree Guidelines
and Policies
Contributed by Poul-Henning Kamp and Giorgos Keramidas.
This chapter documents various guidelines and policies in force for the FreeBSD source tree.
5.1. Style Guidelines
Consistent coding style is extremely important, particularly with large projects like FreeBSD. Code should follow
the FreeBSD coding styles described in style(9) and style.Makefile(5).
5.2. MAINTAINER on Makefiles
If a particular portion of the FreeBSD src/ distribution is being maintained by a person or group of persons, this
is communicated through an entry in the src/MAINTAINERS le. Maintainers of ports within the Ports Collection
express their maintainership to the world by adding a MAINTAINER line to the Makefile of the port in question:
MAINTAINER = email-addresses
For other parts of the repository, or for sections not listed as having a maintainer, or when
you are unsure who the active maintainer is, try looking at the recent commit history of the
relevant parts of the source tree. It is quite often the case that a maintainer is not explicitly
named, but the people who are actively working in a part of the source tree for, say, the last
couple of years are interested in reviewing changes. Even if this is not specifically mentioned
in the documentation or the source itself, asking for a review as a form of courtesy is a very
reasonable thing to do.
The role of the maintainer is as follows:
• The maintainer owns and is responsible for that code. This means that he or she is responsible for fixing bugs
and answering problem reports pertaining to that piece of the code, and in the case of contributed software, for
tracking new versions, as appropriate.
• Changes to directories which have a maintainer defined shall be sent to the maintainer for review before being
committed. Only if the maintainer does not respond for an unacceptable period of time, to several emails, will
it be acceptable to commit changes without review by the maintainer. However, it is suggested that you try to
have the changes reviewed by someone else if at all possible.
• It is of course not acceptable to add a person or group as maintainer unless they agree to assume this duty. On
the other hand it does not have to be a committer and it can easily be a group of people.
5.3. Contributed Software
Contributed by Poul-Henning Kamp, David O'Brien and Gavin Atkinson.
Vendor Imports with SVN
Some parts of the FreeBSD distribution consist of software that is actively being maintained outside the FreeBSD
project. For historical reasons, we call this contributed software. Some examples are sendmail, gcc and patch.
Over the last couple of years, various methods have been used in dealing with this type of software and all have
some number of advantages and drawbacks. No clear winner has emerged.
Since this is the case, after some debate one of these methods has been selected as the “official” method and will be
required for future imports of software of this kind. Furthermore, it is strongly suggested that existing contributed
software converge on this model over time, as it has significant advantages over the old method, including the
ability to easily obtain dis relative to the “official” versions of the source by everyone (even without direct repository access). This will make it significantly easier to return changes to the primary developers of the contributed
Ultimately, however, it comes down to the people actually doing the work. If using this model is particularly unsuited to the package being dealt with, exceptions to these rules may be granted only with the approval of the core
team and with the general consensus of the other developers. The ability to maintain the package in the future
will be a key issue in the decisions.
Because it makes it harder to import future versions minor, trivial and/or cosmetic changes
are strongly discouraged on les that are still tracking the vendor branch.
5.3.1. Vendor Imports with SVN
Contributed by Dag-Erling Smørgrav.
This section describes the vendor import procedure with Subversion in details.
Preparing the Tree
If this is your rst import after the switch to SVN, you will have to flatten and clean up the vendor tree, and
bootstrap merge history in the main tree. If not, you can safely omit this step.
During the conversion from CVS to SVN, vendor branches were imported with the same layout as the main
tree. For example, the foo vendor sources ended up in vendor/foo/dist/contrib/ foo , but it is pointless and
rather inconvenient. What we really want is to have the vendor source directly in vendor/foo/dist , like this:
cd vendor/ foo/dist/contrib/ foo
svn move $(svn list) ../..
cd ../..
svn remove contrib
svn propdel -R svn:mergeinfo
svn commit
Note that, the propdel bit is necessary because starting with 1.5, Subversion will automatically add
svn:mergeinfo to any directory you copy or move. In this case, you will not need this information, since you
are not going to merge anything from the tree you deleted.
You may want to flatten the tags as well. The procedure is exactly the same. If you do
this, put o the commit until the end.
Chapter 5. Source Tree Guidelines and Policies
Check the dist tree and perform any cleanup that is deemed to be necessary. You may want to disable keyword
expansion, as it makes no sense on unmodified vendor code. In some cases, it can be even be harmful.
% svn propdel svn:keywords -R .
% svn commit
Bootstrapping of svn:mergeinfo on the target directory (in the main tree) to the revision that corresponds
to the last change was made to the vendor tree prior to importing new sources is also needed:
% cd head/contrib/ foo
% svn merge --record-only
% svn commit
svn_base /vendor/ foo/[email protected] 12345678 .
where svn_base is the base directory of your SVN repository, e.g. svn+ssh:// .
Importing New Sources
Prepare a full, clean tree of the vendor sources. With SVN, we can keep a full distribution in the vendor tree
without bloating the main tree. Import everything but merge only what is needed.
Note that you will need to add any les that were added since the last vendor import, and remove any that
were removed. To facilitate this, you should prepare sorted lists of the contents of the vendor tree and of the
sources you are about to import:
cd vendor/ foo/dist
svn list -R | grep -v '/$' | sort > ../ old
cd ../foo-9.9
find . -type f | cut -c 3- | sort > ../ new
With these two les, the following command will list list removed les (les only in old ):
% comm -23 ../ old ../new
While the command below will list added les (les only in new ):
% comm -13 ../ old ../new
Let's put this together:
cd vendor/ foo/foo-9.9
tar cf - . | tar xf - -C ../dist
cd ../dist
comm -23 ../ old ../new | xargs svn remove
comm -13 ../ old ../new | xargs svn add
If there are new directories in the new distribution, the last command will fail. You
will have to add the directories, and run it again. Conversely, if any directories were
removed, you will have to remove them manually.
Check properties on any new les:
• All text les should have svn:eol-style set to native.
• All binary les should have svn:mime-type set to application/octet-stream , unless there is a more appropriate media type.
• Executable les should have svn:executable set to *.
Vendor Imports with SVN
• There should be no other properties on any le in the tree.
You are ready to commit, but you should rst check the output of svn stat and svn
diff to make sure everything is in order.
Once you have committed the new vendor release, you should tag it for future reference. The best and quickest
way is to do it directly in the repository:
% svn copy svn_base /vendor/ foo/dist svn_base /vendor/ foo/9.9
To get the new tag, you can update your working copy of vendor/foo .
If you choose to do the copy in the checkout instead, do not forget to remove the generated svn:mergeinfo as described above.
Merging to -HEAD
After you have prepared your import, it is time to merge. Option --accept=postpone tells SVN not to handle
merge conflicts yet, because they will be taken care of manually:
% cd head/contrib/ foo
% svn update
% svn merge --accept=postpone
svn_base /vendor/ foo/dist
Resolve any conflicts, and make sure that any les that were added or removed in the vendor tree have been
properly added or removed in the main tree. It is always a good idea to check differences against the vendor
% svn diff --no-diff-deleted --old= svn_base /vendor/ foo/dist --new=.
The --no-diff-deleted option tells SVN not to check les that are in the vendor tree but not in the main tree.
With SVN, there is no concept of on or o the vendor branch. If a le that previously
had local modifications no longer does, just remove any left-over cruft, such as FreeBSD
version tags, so it no longer shows up in dis against the vendor tree.
If any changes are required for the world to build with the new sources, make them now — and test until you
are satisfied that everything build and runs correctly.
Now, you are ready to commit. Make sure you get everything in one go. Ideally, you would have done all steps
in a clean tree, in which case you can just commit from the top of that tree. That is the best way to avoid
Chapter 5. Source Tree Guidelines and Policies
surprises. If you do it properly, the tree will move atomically from a consistent state with the old code to a
consistent state with the new code.
5.4. Encumbered Files
It might occasionally be necessary to include an encumbered le in the FreeBSD source tree. For example, if a
device requires a small piece of binary code to be loaded to it before the device will operate, and we do not have
the source to that code, then the binary le is said to be encumbered. The following policies apply to including
encumbered les in the FreeBSD source tree.
1. Any le which is interpreted or executed by the system CPU(s) and not in source format is encumbered.
2. Any le with a license more restrictive than BSD or GNU is encumbered.
3. A le which contains downloadable binary data for use by the hardware is not encumbered, unless (1) or (2)
apply to it. It must be stored in an architecture neutral ASCII format (le2c or uuencoding is recommended).
4. Any encumbered le requires specific approval from the Core Team before it is added to the repository.
5. Encumbered les go in src/contrib or src/sys/contrib .
6. The entire module should be kept together. There is no point in splitting it, unless there is code-sharing with
non-encumbered code.
7. Object les are named arch/filename.o.uu>.
8. Kernel les:
a. Should always be referenced in conf/files.* (for build simplicity).
b. Should always be in LINT , but the Core Team decides per case if it should be commented out or not. The Core
Team can, of course, change their minds later on.
c. The Release Engineer decides whether or not it goes into the release.
9. User-land les:
a. The Core team decides if the code should be part of make world .
b. The Release Engineering decides if it goes into the release.
5.5. Shared Libraries
Contributed by Satoshi Asami, Peter Wemm and David O'Brien.
If you are adding shared library support to a port or other piece of software that does not have one, the version
numbers should follow these rules. Generally, the resulting numbers will have nothing to do with the release version of the software.
The three principles of shared library building are:
• Start from 1.0
• If there is a change that is backwards compatible, bump minor number (note that ELF systems ignore the minor
• If there is an incompatible change, bump major number
Shared Libraries
For instance, added functions and bugfixes result in the minor version number being bumped, while deleted functions, changed function call syntax, etc. will force the major version number to change.
Stick to version numbers of the form major.minor (x.y). Our a.out dynamic linker does not handle version numbers
of the form x.y.z well. Any version number after the y (i.e. the third digit) is totally ignored when comparing shared
lib version numbers to decide which library to link with. Given two shared libraries that differ only in the “micro”
revision, will link with the higher one. That is, if you link with, the linker only records 3.3
in the headers, and will link with anything starting with >= 3) .(highest available).
Note will always use the highest “minor” revision. For instance, it will use in
preference to , even if the program was initially linked with .
In addition, our ELF dynamic linker does not handle minor version numbers at all. However, one should still specify
a major and minor version number as our Makefiles “do the right thing” based on the type of system.
For non-port libraries, it is also our policy to change the shared library version number only once between releases.
In addition, it is our policy to change the major shared library version number only once between major OS releases
(i.e. from 6.0 to 7.0). When you make a change to a system library that requires the version number to be bumped,
check the Makefile's commit logs. It is the responsibility of the committer to ensure that the rst such change since
the release will result in the shared library version number in the Makefile to be updated, and any subsequent
changes will not.
Chapter 6. Regression and
Performance Testing
Regression tests are used to exercise a particular bit of the system to check that it works as expected, and to make
sure that old bugs are not reintroduced.
The FreeBSD regression testing tools can be found in the FreeBSD source tree in the directory src/tools/regression.
6.1. Micro Benchmark Checklist
This section contains hints for doing proper micro-benchmarking on FreeBSD or of FreeBSD itself.
It is not possible to use all of the suggestions below every single time, but the more used, the better the benchmark's
ability to test small differences will be.
• Disable APM and any other kind of clock fiddling (ACPI ?).
• Run in single user mode. E.g., cron(8), and other daemons only add noise. The sshd(8) daemon can also cause
problems. If ssh access is required during testing either disable the SSHv1 key regeneration, or kill the parent
sshd daemon during the tests.
• Do not run ntpd(8).
• If syslog(3) events are generated, run syslogd(8) with an empty /etc/syslogd.conf , otherwise, do not run it.
• Minimize disk-I/O, avoid it entirely if possible.
• Do not mount le systems that are not needed.
• Mount /, /usr , and any other le system as read-only if possible. This removes atime updates to disk (etc.) from
the I/O picture.
• Reinitialize the read/write test le system with newfs(8) and populate it from a tar(1) or dump(8) le before
every run. Unmount and mount it before starting the test. This results in a consistent le system layout. For
a worldstone test this would apply to /usr/obj (just reinitialize with newfs and mount). To get 100% reproducibility, populate the le system from a dd(1) le (i.e.: dd if=myimage of=/dev/ad0s1h bs=1m )
• Use malloc backed or preloaded md(4) partitions.
• Reboot between individual iterations of the test, this gives a more consistent state.
• Remove all non-essential device drivers from the kernel. For instance if USB is not needed for the test, do not
put USB in the kernel. Drivers which attach often have timeouts ticking away.
• Unconfigure hardware that are not in use. Detach disks with atacontrol(8) and camcontrol(8) if the disks are
not used for the test.
• Do not configure the network unless it is being tested, or wait until after the test has been performed to ship
the results o to another computer.
If the system must be connected to a public network, watch out for spikes of broadcast traffic. Even though it is
hardly noticeable, it will take up CPU cycles. Multicast has similar caveats.
• Put each le system on its own disk. This minimizes jitter from head-seek optimizations.
The FreeBSD Source Tinderbox
• Minimize output to serial or VGA consoles. Running output into les gives less jitter. (Serial consoles easily
become a bottleneck.) Do not touch keyboard while the test is running, even space or back-space shows up in
the numbers.
• Make sure the test is long enough, but not too long. If the test is too short, timestamping is a problem. If it is
too long temperature changes and drift will affect the frequency of the quartz crystals in the computer. Rule of
thumb: more than a minute, less than an hour.
• Try to keep the temperature as stable as possible around the machine. This affects both quartz crystals and disk
drive algorithms. To get real stable clock, consider stabilized clock injection. E.g., get a OCXO + PLL, inject output into clock circuits instead of motherboard xtal. Contact Poul-Henning Kamp <[email protected]> for more
information about this.
• Run the test at least 3 times but it is better to run more than 20 times both for “before” and “after” code. Try
to interleave if possible (i.e.: do not run 20 times before then 20 times after), this makes it possible to spot environmental effects. Do not interleave 1:1, but 3:3, this makes it possible to spot interaction effects.
A good pattern is: bababa{bbbaaa}*. This gives hint after the rst 1+1 runs (so it is possible to stop the test if it
goes entirely the wrong way), a standard deviation after the rst 3+3 (gives a good indication if it is going to be
worth a long run) and trending and interaction numbers later on.
• Use ministat(1) to see if the numbers are significant. Consider buying “Cartoon guide to statistics” ISBN:
0062731025, highly recommended, if you have forgotten or never learned about standard deviation and Student's T.
• Do not use background fsck(8) unless the test is a benchmark of background fsck . Also, disable background_fsck
in /etc/rc.conf unless the benchmark is not started at least 60+“fsck runtime” seconds after the boot, as rc(8)
wakes up and checks if fsck needs to run on any le systems when background fsck is enabled. Likewise, make
sure there are no snapshots lying around unless the benchmark is a test with snapshots.
• If the benchmark show unexpected bad performance, check for things like high interrupt volume from an unexpected source. Some versions of ACPI have been reported to “misbehave” and generate excess interrupts. To
help diagnose odd test results, take a few snapshots of vmstat -i and look for anything unusual.
• Make sure to be careful about optimization parameters for kernel and userspace, likewise debugging. It is easy
to let something slip through and realize later the test was not comparing the same thing.
• Do not ever benchmark with the WITNESS and INVARIANTS kernel options enabled unless the test is interested
to benchmarking those features. WITNESS can cause 400%+ drops in performance. Likewise, userspace malloc(3)
parameters default differently in -CURRENT from the way they ship in production releases.
6.2. The FreeBSD Source Tinderbox
The source Tinderbox consists of:
• A build script, tinderbox, that automates checking out a specific version of the FreeBSD source tree and building
• A supervisor script, tbmaster, that monitors individual Tinderbox instances, logs their output, and emails failure
• A CGI script named index.cgi that reads a set of tbmaster logs and presents an easy-to-read HTML summary
of them.
• A set of build servers that continually test the tip of the most important FreeBSD code branches.
• A webserver that keeps a complete set of Tinderbox logs and displays an up-to-date summary.
Chapter 6. Regression and Performance Testing
The scripts are maintained and were developed by Dag-Erling C. Smørgrav <[email protected]>, and are now written in Perl, a move on from their original incarnation as shell scripts. All scripts and configuration les are kept
in /projects/tinderbox/.
For more information about the tinderbox and tbmaster scripts at this stage, see their respective man pages: tinderbox(1) and tbmaster(1).
6.2.1. The index.cgi Script
The index.cgi script generates the HTML summary of tinderbox and tbmaster logs. Although originally intended
to be used as a CGI script, as indicated by its name, this script can also be run from the command line or from
a cron(8) job, in which case it will look for logs in the directory where the script is located. It will automatically
detect context, generating HTTP headers when it is run as a CGI script. It conforms to XHTML standards and is
styled using CSS.
The script starts in the main() block by attempting to verify that it is running on the official Tinderbox website.
If it is not, a page indicating it is not an official website is produced, and a URL to the official site is provided.
Next, it scans the log directory to get an inventory of configurations, branches and architectures for which log
les exist, to avoid hard-coding a list into the script and potentially ending up with blank rows or columns. This
information is derived from the names of the log les matching the following pattern:
The configurations used on the official Tinderbox build servers are named for the branches they build. For example,
the releng_8 configuration is used to build RELENG_8 as well as all still-supported release branches.
Once all of this startup procedure has been successfully completed, do_config() is called for each configuration.
The do_config() function generates HTML for a single Tinderbox configuration.
It works by rst generating a header row, then iterating over each branch build with the specified configuration,
producing a single row of results for each in the following manner:
• For each item:
• For each machine within that architecture:
• If a brief log le exists, then:
• Call success() to determine the outcome of the build.
• Output the modification size.
• Output the size of the brief log le with a link to the log le itself.
• If a full log le also exists, then:
• Output the size of the full log le with a link to the log le itself.
• Otherwise:
• No output.
The success() function mentioned above scans a brief log le for the string “tinderbox run completed” in order
to determine whether the build was successful.
Configurations and branches are sorted according to their branch rank. This is computed as follows:
• HEAD and CURRENT have rank 9999.
Official Build Servers
• RELENG_x has rank xx99.
• RELENG_x_y has rank xxyy .
This means that HEAD always ranks highest, and RELENG branches are ranked in numerical order, with each STABLE
branch ranking higher than the release branches forked o of it. For instance, for FreeBSD 8, the order from highest
to lowest would be:
• RELENG_8 (branch rank 899).
• RELENG_8_3 (branch rank 803).
• RELENG_8_2 (branch rank 802).
• RELENG_8_1 (branch rank 801).
• RELENG_8_0 (branch rank 800).
The colors that Tinderbox uses for each cell in the table are defined by CSS. Successful builds are displayed with
green text; unsuccessful builds are displayed with red text. The color fades as time passes since the corresponding
build, with every half an hour bringing the color closer to grey.
6.2.2. Official Build Servers
The official Tinderbox build servers are hosted by Sentex Data Communications, who also host the FreeBSD Netperf
Three build servers currently exist: builds:
• HEAD for amd64, arm, i386, i386/pc98, ia64, mips, powerpc, powerpc64, and sparc64.
• RELENG_9 and supported 9.X branches for amd64, arm, i386, i386/pc98, ia64, mips, powerpc, powerpc64, and
sparc64. builds:
• RELENG_8 and supported 8.X branches for amd64, i386, i386/pc98, ia64, mips, powerpc and sparc64. builds:
• RELENG_7 and supported 7.X branches for amd64, i386, i386/pc98, ia64, powerpc, and sparc64.
6.2.3. Official Summary Site
Summaries and logs from the official build servers are available online at, hosted by
Dag-Erling C. Smørgrav <[email protected]> and set up as follows:
• A cron(8) job checks the build servers at regular intervals and downloads any new log les using rsync(1).
• Apache is set up to use index.cgi as DirectoryIndex.
Part II. Interprocess
Table of Contents
7. Sockets .................................................................................................................................
7.1. Synopsis .....................................................................................................................
7.2. Networking and Diversity ..............................................................................................
7.3. Protocols ....................................................................................................................
7.4. The Sockets Model .......................................................................................................
7.5. Essential Socket Functions .............................................................................................
7.6. Helper Functions ..........................................................................................................
7.7. Concurrent Servers .......................................................................................................
8. IPv6 Internals ........................................................................................................................
8.1. IPv6/IPsec Implementation ............................................................................................
Chapter 7. Sockets
Contributed by G. Adam Stanislav.
7.1. Synopsis
BSD sockets take interprocess communications to a new level. It is no longer necessary for the communicating
processes to run on the same machine. They still can, but they do not have to.
Not only do these processes not have to run on the same machine, they do not have to run under the same operating system. Thanks to BSD sockets, your FreeBSD software can smoothly cooperate with a program running on
a Macintosh®, another one running on a Sun™ workstation, yet another one running under Windows® 2000, all
connected with an Ethernet-based local area network.
But your software can equally well cooperate with processes running in another building, or on another continent,
inside a submarine, or a space shuttle.
It can also cooperate with processes that are not part of a computer (at least not in the strict sense of the word),
but of such devices as printers, digital cameras, medical equipment. Just about anything capable of digital communications.
7.2. Networking and Diversity
We have already hinted on the diversity of networking. Many different systems have to talk to each other. And they
have to speak the same language. They also have to understand the same language the same way.
People often think that body language is universal. But it is not. Back in my early teens, my father took me to Bulgaria.
We were sitting at a table in a park in Soa, when a vendor approached us trying to sell us some roasted almonds.
I had not learned much Bulgarian by then, so, instead of saying no, I shook my head from side to side, the “universal”
body language for no. The vendor quickly started serving us some almonds.
I then remembered I had been told that in Bulgaria shaking your head sideways meant yes. Quickly, I started nodding my head up and down. The vendor noticed, took his almonds, and walked away. To an uninformed observer, I
did not change the body language: I continued using the language of shaking and nodding my head. What changed
was the meaning of the body language. At rst, the vendor and I interpreted the same language as having completely different meaning. I had to adjust my own interpretation of that language so the vendor would understand.
It is the same with computers: The same symbols may have different, even outright opposite meaning. Therefore,
for two computers to understand each other, they must not only agree on the same language, but on the same
interpretation of the language.
7.3. Protocols
While various programming languages tend to have complex syntax and use a number of multi-letter reserved
words (which makes them easy for the human programmer to understand), the languages of data communications
tend to be very terse. Instead of multi-byte words, they often use individual bits. There is a very convincing reason
for it: While data travels inside your computer at speeds approaching the speed of light, it often travels considerably
slower between two computers.
Because the languages used in data communications are so terse, we usually refer to them as protocols rather than
As data travels from one computer to another, it always uses more than one protocol. These protocols are layered.
The data can be compared to the inside of an onion: You have to peel o several layers of “skin” to get to the data.
This is best illustrated with a picture:
In this example, we are trying to get an image from a web page we are connected to via an Ethernet.
The image consists of raw data, which is simply a sequence of RGB values that our software can process, i.e., convert
into an image and display on our monitor.
Alas, our software has no way of knowing how the raw data is organized: Is it a sequence of RGB values, or a sequence
of grayscale intensities, or perhaps of CMYK encoded colors? Is the data represented by 8-bit quanta, or are they
16 bits in size, or perhaps 4 bits? How many rows and columns does the image consist of? Should certain pixels
be transparent?
I think you get the picture...
To inform our software how to handle the raw data, it is encoded as a PNG le. It could be a GIF, or a JPEG, but
it is a PNG.
And PNG is a protocol.
At this point, I can hear some of you yelling, “No, it is not! It is a le format!”
Well, of course it is a le format. But from the perspective of data communications, a le format is a protocol: The
le structure is a language, a terse one at that, communicating to our process how the data is organized. Ergo, it
is a protocol.
Alas, if all we received was the PNG le, our software would be facing a serious problem: How is it supposed to
know the data is representing an image, as opposed to some text, or perhaps a sound, or what not? Secondly, how
is it supposed to know the image is in the PNG format as opposed to GIF, or JPEG, or some other image format?
To obtain that information, we are using another protocol: HTTP. This protocol can tell us exactly that the data
represents an image, and that it uses the PNG protocol. It can also tell us some other things, but let us stay focused
on protocol layers here.
Chapter 7. Sockets
So, now we have some data wrapped in the PNG protocol, wrapped in the HTTP protocol. How did we get it from
the server?
By using TCP/IP over Ethernet, that is how. Indeed, that is three more protocols. Instead of continuing inside out,
I am now going to talk about Ethernet, simply because it is easier to explain the rest that way.
Ethernet is an interesting system of connecting computers in a local area network (LAN). Each computer has a network
interface card (NIC), which has a unique 48-bit ID called its address. No two Ethernet NICs in the world have the
same address.
These NICs are all connected with each other. Whenever one computer wants to communicate with another in the
same Ethernet LAN, it sends a message over the network. Every NIC sees the message. But as part of the Ethernet
protocol, the data contains the address of the destination NIC (among other things). So, only one of all the network
interface cards will pay attention to it, the rest will ignore it.
But not all computers are connected to the same network. Just because we have received the data over our Ethernet
does not mean it originated in our own local area network. It could have come to us from some other network
(which may not even be Ethernet based) connected with our own network via the Internet.
All data is transferred over the Internet using IP, which stands for Internet Protocol. Its basic role is to let us know
where in the world the data has arrived from, and where it is supposed to go to. It does not guarantee we will receive
the data, only that we will know where it came from if we do receive it.
Even if we do receive the data, IP does not guarantee we will receive various chunks of data in the same order the
other computer has sent it to us. So, we can receive the center of our image before we receive the upper left corner
and after the lower right, for example.
It is TCP (Transmission Control Protocol) that asks the sender to resend any lost data and that places it all into the
proper order.
All in all, it took ve different protocols for one computer to communicate to another what an image looks like. We
received the data wrapped into the PNG protocol, which was wrapped into the HTTP protocol, which was wrapped
into the TCP protocol, which was wrapped into the IP protocol, which was wrapped into the Ethernet protocol.
Oh, and by the way, there probably were several other protocols involved somewhere on the way. For example, if
our LAN was connected to the Internet through a dial-up call, it used the PPP protocol over the modem which used
one (or several) of the various modem protocols, et cetera, et cetera, et cetera...
As a developer you should be asking by now, “How am I supposed to handle it all?”
Luckily for you, you are not supposed to handle it all. You are supposed to handle some of it, but not all of it.
Specifically, you need not worry about the physical connection (in our case Ethernet and possibly PPP, etc). Nor
do you need to handle the Internet Protocol, or the Transmission Control Protocol.
In other words, you do not have to do anything to receive the data from the other computer. Well, you do have to
ask for it, but that is almost as simple as opening a le.
Once you have received the data, it is up to you to figure out what to do with it. In our case, you would need to
understand the HTTP protocol and the PNG le structure.
To use an analogy, all the internetworking protocols become a gray area: Not so much because we do not understand how it works, but because we are no longer concerned about it. The sockets interface takes care of this gray
area for us:
The Sockets Model
We only need to understand any protocols that tell us how to interpret the data, not how to receive it from another
process, nor how to send it to another process.
7.4. The Sockets Model
BSD sockets are built on the basic UNIX® model: Everything is a le. In our example, then, sockets would let us
receive an HTTP le, so to speak. It would then be up to us to extract the PNG le from it.
Because of the complexity of internetworking, we cannot just use the open system call, or the open() C function.
Instead, we need to take several steps to “opening” a socket.
Once we do, however, we can start treating the socket the same way we treat any le descriptor: We can read from
it, write to it, pipe it, and, eventually, close it.
7.5. Essential Socket Functions
While FreeBSD offers different functions to work with sockets, we only need four to “open” a socket. And in some
cases we only need two.
7.5.1. The Client-Server Difference
Typically, one of the ends of a socket-based data communication is a server, the other is a client. The Common Elements socket
The one function used by both, clients and servers, is socket(2). It is declared this way:
int socket(int domain, int type, int protocol);
Chapter 7. Sockets
The return value is of the same type as that of open , an integer. FreeBSD allocates its value from the same pool as
that of le handles. That is what allows sockets to be treated the same way as les.
The domain argument tells the system what protocol family you want it to use. Many of them exist, some are vendor
specific, others are very common. They are declared in sys/socket.h .
Use PF_INET for UDP, TCP and other Internet protocols (IPv4).
Five values are defined for the type argument, again, in sys/socket.h . All of them start with “SOCK_ ”. The most
common one is SOCK_STREAM , which tells the system you are asking for a reliable stream delivery service (which is
TCP when used with PF_INET ).
If you asked for SOCK_DGRAM, you would be requesting a connectionless datagram delivery service (in our case, UDP).
If you wanted to be in charge of the low-level protocols (such as IP), or even network interfaces (e.g., the Ethernet),
you would need to specify SOCK_RAW .
Finally, the protocol argument depends on the previous two arguments, and is not always meaningful. In that
case, use 0 for its value.
The Unconnected Socket
Nowhere, in the socket function have we specified to what other system we should be connected. Our newly created socket remains unconnected.
This is on purpose: To use a telephone analogy, we have just attached a modem to the phone
line. We have neither told the modem to make a call, nor to answer if the phone rings. sockaddr
Various functions of the sockets family expect the address of (or pointer to, to use C terminology) a small area
of the memory. The various C declarations in the sys/socket.h refer to it as struct sockaddr . This structure is
declared in the same le:
* Structure used by kernel to store most
* addresses.
struct sockaddr {
unsigned char sa_len; /* total length */
sa_family_t sa_family; /* address family */
char sa_data[14]; /* actually longer; address value */
#define SOCK_MAXADDRLEN 255 /* longest possible addresses */
Please note the vagueness with which the sa_data eld is declared, just as an array of 14 bytes, with the comment
hinting there can be more than 14 of them.
This vagueness is quite deliberate. Sockets is a very powerful interface. While most people perhaps think of it as
nothing more than the Internet interface—and most applications probably use it for that nowadays—sockets can
be used for just about any kind of interprocess communications, of which the Internet (or, more precisely, IP) is
only one.
The sys/socket.h refers to the various types of protocols sockets will handle as address families, and lists them
right before the definition of sockaddr:
The Client-Server Difference
* Address families.
#define AF_UNSPEC 0 /* unspecified */
#define AF_LOCAL 1 /* local to host (pipes, portals) */
#define AF_UNIX AF_LOCAL /* backward compatibility */
#define AF_INET 2 /* internetwork: UDP, TCP, etc. */
#define AF_IMPLINK 3 /* arpanet imp addresses */
#define AF_PUP 4 /* pup protocols: e.g. BSP */
#define AF_CHAOS 5 /* mit CHAOS protocols */
#define AF_NS 6 /* XEROX NS protocols */
#define AF_ISO 7 /* ISO protocols */
#define AF_OSI AF_ISO
#define AF_ECMA 8 /* European computer manufacturers */
#define AF_DATAKIT 9 /* datakit protocols */
#define AF_CCITT 10 /* CCITT protocols, X.25 etc */
#define AF_SNA 11 /* IBM SNA */
#define AF_DECnet 12 /* DECnet */
#define AF_DLI 13 /* DEC Direct data link interface */
#define AF_LAT 14 /* LAT */
#define AF_HYLINK 15 /* NSC Hyperchannel */
#define AF_APPLETALK 16 /* Apple Talk */
#define AF_ROUTE 17 /* Internal Routing Protocol */
#define AF_LINK 18 /* Link layer interface */
#define pseudo_AF_XTP 19 /* eXpress Transfer Protocol (no AF) */
#define AF_COIP 20 /* connection-oriented IP, aka ST II */
#define AF_CNT 21 /* Computer Network Technology */
#define pseudo_AF_RTIP 22 /* Help Identify RTIP packets */
#define AF_IPX 23 /* Novell Internet Protocol */
#define AF_SIP 24 /* Simple Internet Protocol */
#define pseudo_AF_PIP 25 /* Help Identify PIP packets */
#define AF_ISDN 26 /* Integrated Services Digital Network*/
#define AF_E164 AF_ISDN /* CCITT E.164 recommendation */
#define pseudo_AF_KEY 27 /* Internal key-management function */
#define AF_INET6 28 /* IPv6 */
#define AF_NATM 29 /* native ATM access */
#define AF_ATM 30 /* ATM */
#define pseudo_AF_HDRCMPLT 31 /* Used by BPF to not rewrite headers
* in interface output routine
#define AF_NETGRAPH 32 /* Netgraph sockets */
#define AF_SLOW 33 /* 802.3ad slow protocol */
#define AF_SCLUSTER 34 /* Sitara cluster protocol */
#define AF_ARP 35
#define AF_BLUETOOTH 36 /* Bluetooth sockets */
#define AF_MAX 37
The one used for IP is AF_INET. It is a symbol for the constant 2.
It is the address family listed in the sa_family eld of sockaddr that decides how exactly the vaguely named bytes
of sa_data will be used.
Specifically, whenever the address family is AF_INET, we can use struct sockaddr_in found in netinet/in.h ,
wherever sockaddr is expected:
* Socket address, internet style.
struct sockaddr_in {
uint8_t sin_len;
sa_family_t sin_family;
in_port_t sin_port;
struct in_addr sin_addr;
char sin_zero[8];
Chapter 7. Sockets
We can visualize its organization this way:
The three important elds are sin_family, which is byte 1 of the structure, sin_port , a 16-bit value found in bytes
2 and 3, and sin_addr, a 32-bit integer representation of the IP address, stored in bytes 4-7.
Now, let us try to ll it out. Let us assume we are trying to write a client for the daytime protocol, which simply
states that its server will write a text string representing the current date and time to port 13. We want to use TCP/
IP, so we need to specify AF_INET in the address family eld. AF_INET is defined as 2. Let us use the IP address of , which is the time server of US federal government ( ).
By the way the sin_addr eld is declared as being of the struct in_addr type, which is defined in netinet/in.h :
* Internet address (a structure for historical reasons)
struct in_addr {
in_addr_t s_addr;
In addition, in_addr_t is a 32-bit integer.
The is just a convenient notation of expressing a 32-bit integer by listing all of its 8-bit bytes,
starting with the most significant one.
So far, we have viewed sockaddr as an abstraction. Our computer does not store short integers as a single 16-bit
entity, but as a sequence of 2 bytes. Similarly, it stores 32-bit integers as a sequence of 4 bytes.
Suppose we coded something like this:
= 13;
sa.sin_addr.s_addr = (((((192 << 8) | 43) << 8) | 244) << 8) | 18;
What would the result look like?
Well, that depends, of course. On a Pentium®, or other x86, based computer, it would look like this:
The Client-Server Difference
On a different system, it might look like this:
And on a PDP it might look different yet. But the above two are the most common ways in use today.
Ordinarily, wanting to write portable code, programmers pretend that these differences do not exist. And they get
away with it (except when they code in assembly language). Alas, you cannot get away with it that easily when
coding for sockets.
Because when communicating with another computer, you usually do not know whether it stores data most significant byte (MSB) or least significant byte (LSB) rst.
You might be wondering, “So, will sockets not handle it for me?”
It will not.
While that answer may surprise you at rst, remember that the general sockets interface only understands the
sa_len and sa_family elds of the sockaddr structure. You do not have to worry about the byte order there (of
course, on FreeBSD sa_family is only 1 byte anyway, but many other UNIX® systems do not have sa_len and use
2 bytes for sa_family, and expect the data in whatever order is native to the computer).
But the rest of the data is just sa_data[14] as far as sockets goes. Depending on the address family, sockets just
forwards that data to its destination.
Indeed, when we enter a port number, it is because we want the other computer to know what service we are asking
for. And, when we are the server, we read the port number so we know what service the other computer is expecting
from us. Either way, sockets only has to forward the port number as data. It does not interpret it in any way.
Similarly, we enter the IP address to tell everyone on the way where to send our data to. Sockets, again, only
forwards it as data.
That is why, we (the programmers, not the sockets) have to distinguish between the byte order used by our computer
and a conventional byte order to send the data in to the other computer.
We will call the byte order our computer uses the host byte order, or just the host order.
There is a convention of sending the multi-byte data over IP MSB rst. This, we will refer to as the network byte
order, or simply the network order.
Now, if we compiled the above code for an Intel based computer, our host byte order would produce:
But the network byte order requires that we store the data MSB rst:
Chapter 7. Sockets
Unfortunately, our host order is the exact opposite of the network order.
We have several ways of dealing with it. One would be to reverse the values in our code:
= 13 << 8;
sa.sin_addr.s_addr = (((((18 << 8) | 244) << 8) | 43) << 8) | 192;
This will trick our compiler into storing the data in the network byte order. In some cases, this is exactly the way to
do it (e.g., when programming in assembly language). In most cases, however, it can cause a problem.
Suppose, you wrote a sockets-based program in C. You know it is going to run on a Pentium®, so you enter all your
constants in reverse and force them to the network byte order. It works well.
Then, some day, your trusted old Pentium® becomes a rusty old Pentium®. You replace it with a system whose
host order is the same as the network order. You need to recompile all your software. All of your software continues
to perform well, except the one program you wrote.
You have since forgotten that you had forced all of your constants to the opposite of the host order. You spend
some quality time tearing out your hair, calling the names of all gods you ever heard of (and some you made up),
hitting your monitor with a nerf bat, and performing all the other traditional ceremonies of trying to figure out
why something that has worked so well is suddenly not working at all.
Eventually, you figure it out, say a couple of swear words, and start rewriting your code.
Luckily, you are not the rst one to face the problem. Someone else has created the htons(3) and htonl(3) C functions to convert a short and long respectively from the host byte order to the network byte order, and the ntohs(3)
and ntohl(3) C functions to go the other way.
On MSB-rst systems these functions do nothing. On LSB-rst systems they convert values to the proper order.
So, regardless of what system your software is compiled on, your data will end up in the correct order if you use
these functions. Client Functions
Typically, the client initiates the connection to the server. The client knows which server it is about to call: It knows
its IP address, and it knows the port the server resides at. It is akin to you picking up the phone and dialing the
number (the address), then, after someone answers, asking for the person in charge of wingdings (the port). connect
Once a client has created a socket, it needs to connect it to a specific port on a remote system. It uses connect(2):
int connect(int s, const struct sockaddr *name, socklen_t namelen);
The s argument is the socket, i.e., the value returned by the socket function. The name is a pointer to sockaddr,
the structure we have talked about extensively. Finally, namelen informs the system how many bytes are in our
sockaddr structure.
If connect is successful, it returns 0. Otherwise it returns -1 and stores the error code in errno.
The Client-Server Difference
There are many reasons why connect may fail. For example, with an attempt to an Internet connection, the IP
address may not exist, or it may be down, or just too busy, or it may not have a server listening at the specified
port. Or it may outright refuse any request for specific code. Our First Client
We now know enough to write a very simple client, one that will get current time from and print
it to stdout.
* daytime.c
* Programmed by G. Adam Stanislav
#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
int main() {
register int s;
register int bytes;
struct sockaddr_in sa;
char buffer[BUFSIZ+1];
if ((s = socket(PF_INET, SOCK_STREAM, 0)) < 0) {
return 1;
bzero(&sa, sizeof sa);
sa.sin_family = AF_INET;
sa.sin_port = htons(13);
sa.sin_addr.s_addr = htonl((((((192 << 8) | 43) << 8) | 244) << 8) | 18);
if (connect(s, (struct sockaddr *)&sa, sizeof sa) < 0) {
return 2;
while ((bytes = read(s, buffer, BUFSIZ)) > 0)
write(1, buffer, bytes);
return 0;
Go ahead, enter it in your editor, save it as daytime.c, then compile and run it:
% cc -O3 -o daytime daytime.c
% ./daytime
52079 01-06-19 02:29:25 50 0 1 543.9 UTC(NIST) *
In this case, the date was June 19, 2001, the time was 02:29:25 UTC. Naturally, your results will vary. Server Functions
The typical server does not initiate the connection. Instead, it waits for a client to call it and request services. It does
not know when the client will call, nor how many clients will call. It may be just sitting there, waiting patiently,
Chapter 7. Sockets
one moment, The next moment, it can nd itself swamped with requests from a number of clients, all calling in
at the same time.
The sockets interface offers three basic functions to handle this. bind
Ports are like extensions to a phone line: After you dial a number, you dial the extension to get to a specific person
or department.
There are 65535 IP ports, but a server usually processes requests that come in on only one of them. It is like telling
the phone room operator that we are now at work and available to answer the phone at a specific extension. We
use bind(2) to tell sockets which port we want to serve.
int bind(int s, const struct sockaddr *addr, socklen_t addrlen);
Beside specifying the port in addr, the server may include its IP address. However, it can just use the symbolic
constant INADDR_ANY to indicate it will serve all requests to the specified port regardless of what its IP address
is. This symbol, along with several similar ones, is declared in netinet/in.h
#define INADDR_ANY
Suppose we were writing a server for the daytime protocol over TCP/IP. Recall that it uses port 13. Our sockaddr_in
structure would look like this: listen
To continue our office phone analogy, after you have told the phone central operator what extension you will be
at, you now walk into your office, and make sure your own phone is plugged in and the ringer is turned on. Plus,
you make sure your call waiting is activated, so you can hear the phone ring even while you are talking to someone.
The server ensures all of that with the listen(2) function.
int listen(int s, int backlog);
In here, the backlog variable tells sockets how many incoming requests to accept while you are busy processing
the last request. In other words, it determines the maximum size of the queue of pending connections. accept
After you hear the phone ringing, you accept the call by answering the call. You have now established a connection
with your client. This connection remains active until either you or your client hang up.
The server accepts the connection by using the accept(2) function.
int accept(int s, struct sockaddr *addr, socklen_t *addrlen);
Note that this time addrlen is a pointer. This is necessary because in this case it is the socket that lls out addr,
the sockaddr_in structure.
The Client-Server Difference
The return value is an integer. Indeed, the accept returns a new socket. You will use this new socket to communicate
with the client.
What happens to the old socket? It continues to listen for more requests (remember the backlog variable we passed
to listen?) until we close it.
Now, the new socket is meant only for communications. It is fully connected. We cannot pass it to listen again,
trying to accept additional connections. Our First Server
Our rst server will be somewhat more complex than our rst client was: Not only do we have more sockets functions to use, but we need to write it as a daemon.
This is best achieved by creating a child process after binding the port. The main process then exits and returns
control to the shell (or whatever program invoked it).
The child calls listen, then starts an endless loop, which accepts a connection, serves it, and eventually closes
its socket.
* daytimed - a port 13 server
* Programmed by G. Adam Stanislav
* June 19, 2001
#include <stdio.h>
#include <string.h>
#include <time.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#define BACKLOG 4
int main() {
register int s, c;
int b;
struct sockaddr_in sa;
time_t t;
struct tm *tm;
FILE *client;
if ((s = socket(PF_INET, SOCK_STREAM, 0)) < 0) {
return 1;
bzero(&sa, sizeof sa);
sa.sin_family = AF_INET;
sa.sin_port = htons(13);
sa.sin_addr.s_addr = htonl(INADDR_ANY);
if (bind(s, (struct sockaddr *)&sa, sizeof sa) < 0) {
return 2;
switch (fork()) {
case -1:
Chapter 7. Sockets
return 3;
return 0;
case 0:
listen(s, BACKLOG);
for (;;) {
b = sizeof sa;
if ((c = accept(s, (struct sockaddr *)&sa, &b)) < 0) {
perror("daytimed accept");
return 4;
if ((client = fdopen(c, "w")) == NULL) {
perror("daytimed fdopen");
return 5;
if ((t = time(NULL)) < 0) {
perror("daytimed time");
return 6;
tm = gmtime(&t);
fprintf(client, "%.4i-%.2i-%.2iT%.2i:%.2i:%.2iZ\n",
tm->tm_year + 1900,
tm->tm_mon + 1,
We start by creating a socket. Then we ll out the sockaddr_in structure in sa. Note the conditional use of INADDR_ANY:
sa.sin_addr.s_addr = htonl(INADDR_ANY);
Its value is 0. Since we have just used bzero on the entire structure, it would be redundant to set it to 0 again.
But if we port our code to some other system where INADDR_ANY is perhaps not a zero, we need to assign it to
sa.sin_addr.s_addr. Most modern C compilers are clever enough to notice that INADDR_ANY is a constant. As
long as it is a zero, they will optimize the entire conditional statement out of the code.
After we have called bind successfully, we are ready to become a daemon: We use fork to create a child process. In
both, the parent and the child, the s variable is our socket. The parent process will not need it, so it calls close ,
then it returns 0 to inform its own parent it had terminated successfully.
Meanwhile, the child process continues working in the background. It calls listen and sets its backlog to 4. It does
not need a large value here because daytime is not a protocol many clients request all the time, and because it can
process each request instantly anyway.
The Client-Server Difference
Finally, the daemon starts an endless loop, which performs the following steps:
Call accept. It waits here until a client contacts it. At that point, it receives a new socket, c, which it can use
to communicate with this particular client.
It uses the C function fdopen to turn the socket from a low-level le descriptor to a C-style FILE pointer. This
will allow the use of fprintf later on.
It checks the time, and prints it in the ISO 8601 format to the client “le”. It then uses fclose to close the
le. That will automatically close the socket as well.
We can generalize this, and use it as a model for many other servers:
This flowchart is good for sequential servers, i.e., servers that can serve one client at a time, just as we were able
to with our daytime server. This is only possible whenever there is no real “conversation” going on between the
Chapter 7. Sockets
client and the server: As soon as the server detects a connection to the client, it sends out some data and closes
the connection. The entire operation may take nanoseconds, and it is finished.
The advantage of this flowchart is that, except for the brief moment after the parent fork s and before it exits,
there is always only one process active: Our server does not take up much memory and other system resources.
Note that we have added initialize daemon in our flowchart. We did not need to initialize our own daemon, but this
is a good place in the ow of the program to set up any signal handlers, open any les we may need, etc.
Just about everything in the ow chart can be used literally on many different servers. The serve entry is the exception. We think of it as a “black box”, i.e., something you design specifically for your own server, and just “plug
it into the rest.”
Not all protocols are that simple. Many receive a request from the client, reply to it, then receive another request
from the same client. Because of that, they do not know in advance how long they will be serving the client. Such
servers usually start a new process for each client. While the new process is serving its client, the daemon can
continue listening for more connections.
Now, go ahead, save the above source code as daytimed.c (it is customary to end the names of daemons with the
letter d). After you have compiled it, try running it:
% ./daytimed
bind: Permission denied
What happened here? As you will recall, the daytime protocol uses port 13. But all ports below 1024 are reserved to
the superuser (otherwise, anyone could start a daemon pretending to serve a commonly used port, while causing
a security breach).
Try again, this time as the superuser:
# ./daytimed
What... Nothing? Let us try again:
# ./daytimed
bind: Address already in use
Every port can only be bound by one program at a time. Our rst attempt was indeed successful: It started the
child daemon and returned quietly. It is still running and will continue to run until you either kill it, or any of its
system calls fail, or you reboot the system.
Fine, we know it is running in the background. But is it working? How do we know it is a proper daytime server?
% telnet localhost 13
Trying ::1...
telnet: connect to address ::1: Connection refused
Connected to localhost.
Escape character is '^]'.
Connection closed by foreign host.
telnet tried the new IPv6, and failed. It retried with IPv4 and succeeded. The daemon works.
Helper Functions
If you have access to another UNIX® system via telnet, you can use it to test accessing the server remotely. My
computer does not have a static IP address, so this is what I did:
% who
ttyp0 Jun 19 16:59
ttyp1 Jun 19 16:06
% telnet 13
Connected to
Escape character is '^]'.
Connection closed by foreign host.
Again, it worked. Will it work using the domain name?
% telnet 13
Connected to
Escape character is '^]'.
Connection closed by foreign host.
By the way, telnet prints the Connection closed by foreign host message after our daemon has closed the socket. This
shows us that, indeed, using fclose(client); in our code works as advertised.
7.6. Helper Functions
FreeBSD C library contains many helper functions for sockets programming. For example, in our sample client we
hard coded the IP address. But we do not always know the IP address. Even if we do, our software
is more flexible if it allows the user to enter the IP address, or even the domain name.
7.6.1. gethostbyname
While there is no way to pass the domain name directly to any of the sockets functions, the FreeBSD C library
comes with the gethostbyname(3) and gethostbyname2(3) functions, declared in netdb.h.
struct hostent * gethostbyname(const char *name);
struct hostent * gethostbyname2(const char *name, int af);
Both return a pointer to the hostent structure, with much information about the domain. For our purposes, the
h_addr_list[0] eld of the structure points at h_length bytes of the correct address, already stored in the network
byte order.
This allows us to create a much more flexible—and much more useful—version of our daytime program:
* daytime.c
* Programmed by G. Adam Stanislav
* 19 June 2001
#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
Chapter 7. Sockets
#include <netdb.h>
int main(int argc, char *argv[]) {
register int s;
register int bytes;
struct sockaddr_in sa;
struct hostent *he;
char buf[BUFSIZ+1];
char *host;
if ((s = socket(PF_INET, SOCK_STREAM, 0)) < 0) {
return 1;
bzero(&sa, sizeof sa);
sa.sin_family = AF_INET;
sa.sin_port = htons(13);
host = (argc > 1) ? (char *)argv[1] : "";
if ((he = gethostbyname(host)) == NULL) {
return 2;
bcopy(he->h_addr_list[0],&sa.sin_addr, he->h_length);
if (connect(s, (struct sockaddr *)&sa, sizeof sa) < 0) {
return 3;
while ((bytes = read(s, buf, BUFSIZ)) > 0)
write(1, buf, bytes);
return 0;
We now can type a domain name (or an IP address, it works both ways) on the command line, and the program
will try to connect to its daytime server. Otherwise, it will still default to . However, even in this
case we will use gethostbyname rather than hard coding . That way, even if its IP address changes
in the future, we will still nd it.
Since it takes virtually no time to get the time from your local server, you could run daytime twice in a row: First
to get the time from , the second time from your own system. You can then compare the results
and see how exact your system clock is:
% daytime ­; daytime localhost
52080 01-06-20 04:02:33 50 0 0 390.2 UTC(NIST) *
As you can see, my system was two seconds ahead of the NIST time.
7.6.2. getservbyname
Sometimes you may not be sure what port a certain service uses. The getservbyname(3) function, also declared in
netdb.h comes in very handy in those cases:
Concurrent Servers
struct servent * getservbyname(const char *name, const char *proto);
The servent structure contains the s_port , which contains the proper port, already in network byte order.
Had we not known the correct port for the daytime service, we could have found it this way:
struct servent *se;
if ((se = getservbyname("daytime", "tcp")) == NULL {
fprintf(stderr, "Cannot determine which port to use.\n");
return 7;
sa.sin_port = se->s_port;
You usually do know the port. But if you are developing a new protocol, you may be testing it on an unofficial
port. Some day, you will register the protocol and its port (if nowhere else, at least in your /etc/services , which
is where getservbyname looks). Instead of returning an error in the above code, you just use the temporary port
number. Once you have listed the protocol in /etc/services , your software will nd its port without you having
to rewrite the code.
7.7. Concurrent Servers
Unlike a sequential server, a concurrent server has to be able to serve more than one client at a time. For example,
a chat server may be serving a specific client for hours—it cannot wait till it stops serving a client before it serves
the next one.
This requires a significant change in our flowchart:
Chapter 7. Sockets
We moved the serve from the daemon process to its own server process. However, because each child process inherits
all open les (and a socket is treated just like a le), the new process inherits not only the “accepted handle,” i.e.,
the socket returned by the accept call, but also the top socket, i.e., the one opened by the top process right at the
However, the server process does not need this socket and should close it immediately. Similarly, the daemon process
no longer needs the accepted socket, and not only should, but must close it—otherwise, it will run out of available
le descriptors sooner or later.
After the server process is done serving, it should close the accepted socket. Instead of returning to accept, it now
Under UNIX®, a process does not really exit. Instead, it returns to its parent. Typically, a parent process wait s
for its child process, and obtains a return value. However, our daemon process cannot simply stop and wait. That
would defeat the whole purpose of creating additional processes. But if it never does wait , its children will become
zombies—no longer functional but still roaming around.
For that reason, the daemon process needs to set signal handlers in its initialize daemon phase. At least a SIGCHLD signal
has to be processed, so the daemon can remove the zombie return values from the system and release the system
resources they are taking up.
That is why our flowchart now contains a process signals box, which is not connected to any other box. By the way,
many servers also process SIGHUP, and typically interpret as the signal from the superuser that they should reread
their configuration les. This allows us to change settings without having to kill and restart these servers.
Chapter 8. IPv6 Internals
8.1. IPv6/IPsec Implementation
Contributed by Yoshinobu Inoue.
This section should explain IPv6 and IPsec related implementation internals. These functionalities are derived
from KAME project
8.1.1. IPv6 Conformance
The IPv6 related functions conforms, or tries to conform to the latest set of IPv6 specifications. For future reference
we list some of the relevant documents below (NOTE: this is not a complete list - this is too hard to maintain...).
For details please refer to specific chapter in the document, RFCs, manual pages, or comments in the source code.
Conformance tests have been performed on the KAME STABLE kit at TAHI project. Results can be viewed at http:// . We also attended Univ. of New Hampshire IOL tests ( )
in the past, with our past snapshots.
• RFC1639: FTP Operation Over Big Address Records (FOOBAR)
• RFC2428 is preferred over RFC1639. FTP clients will rst try RFC2428, then RFC1639 if failed.
• RFC1886: DNS Extensions to support IPv6
• RFC1933: Transition Mechanisms for IPv6 Hosts and Routers
• IPv4 compatible address is not supported.
• automatic tunneling (described in 4.3 of this RFC) is not supported.
• gif(4) interface implements IPv[46]-over-IPv[46] tunnel in a generic way, and it covers "configured tunnel"
described in the spec. See in this document for details.
• RFC1981: Path MTU Discovery for IPv6
• RFC2080: RIPng for IPv6
• usr.sbin/route6d support this.
• RFC2292: Advanced Sockets API for IPv6
• For supported library functions/kernel APIs, see sys/netinet6/ADVAPI .
• RFC2362: Protocol Independent Multicast-Sparse Mode (PIM-SM)
• RFC2362 defines packet formats for PIM-SM. draft-ietf-pim-ipv6-01.txt
• RFC2373: IPv6 Addressing Architecture
• supports node required addresses, and conforms to the scope requirement.
• RFC2374: An IPv6 Aggregatable Global Unicast Address Format
• supports 64-bit length of Interface ID.
is written based on this.
• RFC2375: IPv6 Multicast Address Assignments
• Userland applications use the well-known addresses assigned in the RFC.
• RFC2428: FTP Extensions for IPv6 and NATs
• RFC2428 is preferred over RFC1639. FTP clients will rst try RFC2428, then RFC1639 if failed.
• RFC2460: IPv6 specification
• RFC2461: Neighbor discovery for IPv6
• See in this document for details.
• RFC2462: IPv6 Stateless Address Autoconfiguration
• See in this document for details.
• RFC2463: ICMPv6 for IPv6 specification
• See in this document for details.
• RFC2464: Transmission of IPv6 Packets over Ethernet Networks
• RFC2465: MIB for IPv6: Textual Conventions and General Group
• Necessary statistics are gathered by the kernel. Actual IPv6 MIB support is provided as a patchkit for ucdsnmp.
• RFC2466: MIB for IPv6: ICMPv6 group
• Necessary statistics are gathered by the kernel. Actual IPv6 MIB support is provided as patchkit for ucd-snmp.
• RFC2467: Transmission of IPv6 Packets over FDDI Networks
• RFC2497: Transmission of IPv6 packet over ARCnet Networks
• RFC2553: Basic Socket Interface Extensions for IPv6
• IPv4 mapped address (3.7) and special behavior of IPv6 wildcard bind socket (3.8) are supported. See
in this document for details.
• RFC2675: IPv6 Jumbograms
• See in this document for details.
• RFC2710: Multicast Listener Discovery for IPv6
• RFC2711: IPv6 router alert option
• draft-ietf-ipngwg-router-renum-08 : Router renumbering for IPv6
• draft-ietf-ipngwg-icmp-namelookups-02 : IPv6 Name Lookups Through ICMP
• draft-ietf-ipngwg-icmp-name-lookups-03 : IPv6 Name Lookups Through ICMP
• draft-ietf-pim-ipv6-01.txt : PIM for IPv6
• pim6dd(8) implements dense mode. pim6sd(8) implements sparse mode.
• draft-itojun-ipv6-tcp-to-anycast-00 : Disconnecting TCP connection toward IPv6 anycast address
• draft-yamamoto-wideipv6-comm-model-00
Chapter 8. IPv6 Internals
• See in this document for details.
• draft-ietf-ipngwg-scopedaddr-format-00.txt : An Extension of Format for IPv6 Scoped Addresses Neighbor Discovery
Neighbor Discovery is fairly stable. Currently Address Resolution, Duplicated Address Detection, and Neighbor Unreachability Detection are supported. In the near future we will be adding Proxy Neighbor Advertisement support
in the kernel and Unsolicited Neighbor Advertisement transmission command as admin tool.
If DAD fails, the address will be marked "duplicated" and message will be generated to syslog (and usually to console). The "duplicated" mark can be checked with ifconfig(8). It is administrators' responsibility to check for and
recover from DAD failures. The behavior should be improved in the near future.
Some of the network driver loops multicast packets back to itself, even if instructed not to do so (especially in
promiscuous mode). In such cases DAD may fail, because DAD engine sees inbound NS packet (actually from the
node itself) and considers it as a sign of duplicate. You may want to look at #if condition marked "heuristics" in
sys/netinet6/nd6_nbr.c:nd6_dad_timer() as workaround (note that the code fragment in "heuristics" section is not
spec conformant).
Neighbor Discovery specification (RFC2461) does not talk about neighbor cache handling in the following cases:
1. when there was no neighbor cache entry, node received unsolicited RS/NS/NA/redirect packet without linklayer address
2. neighbor cache handling on medium without link-layer address (we need a neighbor cache entry for IsRouter
For rst case, we implemented workaround based on discussions on IETF ipngwg mailing list. For more details, see
the comments in the source code and email thread started from (IPng 7155), dated Feb 6 1999.
IPv6 on-link determination rule (RFC2461) is quite different from assumptions in BSD network code. At this moment, no on-link determination rule is supported where default router list is empty (RFC2461, section 5.2, last sentence in 2nd paragraph - note that the spec misuse the word "host" and "node" in several places in the section).
To avoid possible DoS attacks and infinite loops, only 10 options on ND packet is accepted now. Therefore, if you
have 20 prefix options attached to RA, only the rst 10 prefixes will be recognized. If this troubles you, please
ask it on FREEBSD-CURRENT mailing list and/or modify nd6_maxndopt in sys/netinet6/nd6.c . If there are high
demands we may provide sysctl knob for the variable. Scope Index
IPv6 uses scoped addresses. Therefore, it is very important to specify scope index (interface index for link-local
address, or site index for site-local address) with an IPv6 address. Without scope index, scoped IPv6 address is
ambiguous to the kernel, and kernel will not be able to determine the outbound interface for a packet.
Ordinary userland applications should use advanced API (RFC2292) to specify scope index, or interface index. For
similar purpose, sin6_scope_id member in sockaddr_in6 structure is defined in RFC2553. However, the semantics
for sin6_scope_id is rather vague. If you care about portability of your application, we suggest you to use advanced
API rather than sin6_scope_id.
In the kernel, an interface index for link-local scoped address is embedded into 2nd 16bit-word (3rd and 4th byte)
in IPv6 address. For example, you may see something like:
in the routing table and interface address structure (struct in6_ifaddr). The address above is a link-local unicast
address which belongs to a network interface whose interface identifier is 1. The embedded index enables us to
identify IPv6 link local addresses over multiple interfaces effectively and with only a little code change.
Routing daemons and configuration programs, like route6d(8) and ifconfig(8), will need to manipulate the "embedded" scope index. These programs use routing sockets and ioctls (like SIOCGIFADDR_IN6) and the kernel API
will return IPv6 addresses with 2nd 16bit-word lled in. The APIs are for manipulating kernel internal structure.
Programs that use these APIs have to be prepared about differences in kernels anyway.
When you specify scoped address to the command line, NEVER write the embedded form (such as 02:1::1 or
fe80:2::fedc). This is not supposed to work. Always use standard form, like 02::1 or fe80::fedc, with command line
option for specifying interface (like ping6 -I ne0 ff02::1 ). In general, if a command does not have command
line option to specify outgoing interface, that command is not ready to accept scoped address. This may seem to
be opposite from IPv6's premise to support "dentist office" situation. We believe that specifications need some
improvements for this.
Some of the userland tools support extended numeric IPv6 syntax, as documented in draft-ietf-ipngwg-scopedaddr-format-00.txt . You can specify outgoing link, by using name of the outgoing interface like "fe80::1%ne0".
This way you will be able to specify link-local scoped address without much trouble.
To use this extension in your program, you will need to use getaddrinfo(3), and getnameinfo(3) with NI_WITHSCOPEID. The implementation currently assumes 1-to-1 relationship between a link and an interface, which is
stronger than what specs say. Plug and Play
Most of the IPv6 stateless address autoconfiguration is implemented in the kernel. Neighbor Discovery functions
are implemented in the kernel as a whole. Router Advertisement (RA) input for hosts is implemented in the kernel.
Router Solicitation (RS) output for endhosts, RS input for routers, and RA output for routers are implemented in
the userland. Assignment of link-local, and special addresses
IPv6 link-local address is generated from IEEE802 address (Ethernet MAC address). Each of interface is assigned an
IPv6 link-local address automatically, when the interface becomes up (IFF_UP). Also, direct route for the link-local
address is added to routing table.
Here is an output of netstat command:
Netif Expire
Interfaces that has no IEEE802 address (pseudo interfaces like tunnel interfaces, or ppp interfaces) will borrow
IEEE802 address from other interfaces, such as Ethernet interfaces, whenever possible. If there is no IEEE802 hardware attached, a last resort pseudo-random value, MD5(hostname), will be used as source of link-local address. If
it is not suitable for your usage, you will need to configure the link-local address manually.
If an interface is not capable of handling IPv6 (such as lack of multicast support), link-local address will not be
assigned to that interface. See section 2 for details.
Each interface joins the solicited multicast address and the link-local all-nodes multicast addresses (e.g. fe80::1:ff01:6317 and 02::1, respectively, on the link the interface is attached). In addition to a link-local address, the loopback address (::1) will be assigned to the loopback interface. Also, ::1/128 and 01::/32 are automatically added to
routing table, and loopback interface joins node-local multicast group 01::1. Stateless address autoconfiguration on hosts
In IPv6 specification, nodes are separated into two categories: routers and hosts. Routers forward packets addressed
to others, hosts does not forward the packets. net.inet6.ip6.forwarding defines whether this node is router or host
(router if it is 1, host if it is 0).
Chapter 8. IPv6 Internals
When a host hears Router Advertisement from the router, a host may autoconfigure itself by stateless address
autoconfiguration. This behavior can be controlled by net.inet6.ip6.accept_rtadv (host autoconfigures itself if it is
set to 1). By autoconfiguration, network address prefix for the receiving interface (usually global address prefix)
is added. Default route is also configured. Routers periodically generate Router Advertisement packets. To request
an adjacent router to generate RA packet, a host can transmit Router Solicitation. To generate a RS packet at any
time, use the rtsol command. rtsold(8) daemon is also available. rtsold(8) generates Router Solicitation whenever
necessary, and it works great for nomadic usage (notebooks/laptops). If one wishes to ignore Router Advertisements, use sysctl to set net.inet6.ip6.accept_rtadv to 0.
To generate Router Advertisement from a router, use the rtadvd(8) daemon.
Note that, IPv6 specification assumes the following items, and nonconforming cases are left unspecified:
• Only hosts will listen to router advertisements
• Hosts have single network interface (except loopback)
Therefore, this is unwise to enable net.inet6.ip6.accept_rtadv on routers, or multi-interface host. A misconfigured
node can behave strange (nonconforming configuration allowed for those who would like to do some experiments).
To summarize the sysctl knob:
accept_rtadv forwarding role of the node
--- --- --0 0 host (to be manually configured)
0 1 router
1 0 autoconfigured host
(spec assumes that host has single
interface only, autoconfigured host
with multiple interface is
1 1 invalid, or experimental
(out-of-scope of spec)
RFC2462 has validation rule against incoming RA prefix information option, in 5.5.3 (e). This is to protect hosts
from malicious (or misconfigured) routers that advertise very short prefix lifetime. There was an update from Jim
Bound to ipngwg mailing list (look for "(ipng 6712)" in the archive) and it is implemented Jim's update.
See in the document for relationship between DAD and autoconfiguration. Generic tunnel interface
GIF (Generic InterFace) is a pseudo interface for configured tunnel. Details are described in gif(4). Currently
• v6 in v6
• v6 in v4
• v4 in v6
• v4 in v4
are available. Use gifconfig(8) to assign physical (outer) source and destination address to gif interfaces. Configuration that uses same address family for inner and outer IP header (v4 in v4, or v6 in v6) is dangerous. It is very
easy to configure interfaces and routing tables to perform infinite level of tunneling. Please be warned.
gif can be configured to be ECN-friendly. See for ECN-friendliness of tunnels, and gif(4) for how to configure.
If you would like to configure an IPv4-in-IPv6 tunnel with gif interface, read gif(4) carefully. You will need to remove
IPv6 link-local address automatically assigned to the gif interface.
IPv6 Source Address Selection
Current source selection rule is scope oriented (there are some exceptions - see below). For a given destination, a
source IPv6 address is selected by the following rule:
1. If the source address is explicitly specified by the user (e.g. via the advanced API), the specified address is used.
2. If there is an address assigned to the outgoing interface (which is usually determined by looking up the routing
table) that has the same scope as the destination address, the address is used.
This is the most typical case.
3. If there is no address that satisfies the above condition, choose a global address assigned to one of the interfaces
on the sending node.
4. If there is no address that satisfies the above condition, and destination address is site local scope, choose a site
local address assigned to one of the interfaces on the sending node.
5. If there is no address that satisfies the above condition, choose the address associated with the routing table
entry for the destination. This is the last resort, which may cause scope violation.
For instance, ::1 is selected for 01::1, fe80:1::200:f8ff:fe01:6317 for fe80:1::2a0:24ff:feab:839b (note that embedded
interface index - described in - helps us choose the right source address. Those embedded indices will not
be on the wire). If the outgoing interface has multiple address for the scope, a source is selected longest match
basis (rule 3). Suppose 2001:0DB8:808:1:200:f8ff:fe01:6317 and 2001:0DB8:9:124:200:f8ff:fe01:6317 are given to the
outgoing interface. 2001:0DB8:808:1:200:f8ff:fe01:6317 is chosen as the source for the destination 2001:0DB8:800::1.
Note that the above rule is not documented in the IPv6 spec. It is considered "up to implementation" item. There
are some cases where we do not use the above rule. One example is connected TCP session, and we use the address
kept in tcb as the source. Another example is source address for Neighbor Advertisement. Under the spec (RFC2461
7.2.2) NA's source should be the target address of the corresponding NS's target. In this case we follow the spec
rather than the above longest-match rule.
For new connections (when rule 1 does not apply), deprecated addresses (addresses with preferred lifetime = 0)
will not be chosen as source address if other choices are available. If no other choices are available, deprecated
address will be used as a last resort. If there are multiple choice of deprecated addresses, the above scope rule will
be used to choose from those deprecated addresses. If you would like to prohibit the use of deprecated address for
some reason, configure net.inet6.ip6.use_deprecated to 0. The issue related to deprecated address is described in
RFC2462 5.5.4 (NOTE: there is some debate underway in IETF ipngwg on how to use "deprecated" address). Jumbo Payload
The Jumbo Payload hop-by-hop option is implemented and can be used to send IPv6 packets with payloads longer
than 65,535 octets. But currently no physical interface whose MTU is more than 65,535 is supported, so such payloads can be seen only on the loopback interface (i.e. lo0).
If you want to try jumbo payloads, you rst have to reconfigure the kernel so that the MTU of the loopback interface
is more than 65,535 bytes; add the following to the kernel configuration le:
options "LARGE_LOMTU" #To test jumbo payload
and recompile the new kernel.
Then you can test jumbo payloads by the ping6(8) command with -b and -s options. The -b option must be specified
to enlarge the size of the socket buer and the -s option specifies the length of the packet, which should be more
than 65,535. For example, type as follows:
% ping6 -b 70000 -s 68000 ::1
Chapter 8. IPv6 Internals
The IPv6 specification requires that the Jumbo Payload option must not be used in a packet that carries a fragment
header. If this condition is broken, an ICMPv6 Parameter Problem message must be sent to the sender. specification
is followed, but you cannot usually see an ICMPv6 error caused by this requirement.
When an IPv6 packet is received, the frame length is checked and compared to the length specified in the payload
length eld of the IPv6 header or in the value of the Jumbo Payload option, if any. If the former is shorter than
the latter, the packet is discarded and statistics are incremented. You can see the statistics as output of netstat(8)
command with `-s -p ip6' option:
% netstat -s -p ip6
1 with data size < data length
So, kernel does not send an ICMPv6 error unless the erroneous packet is an actual Jumbo Payload, that is, its packet
size is more than 65,535 bytes. As described above, currently no physical interface with such a huge MTU is supported, so it rarely returns an ICMPv6 error.
TCP/UDP over jumbogram is not supported at this moment. This is because we have no medium (other than loopback) to test this. Contact us if you need this.
IPsec does not work on jumbograms. This is due to some specification twists in supporting AH with jumbograms
(AH header size influences payload length, and this makes it real hard to authenticate inbound packet with jumbo
payload option as well as AH).
There are fundamental issues in *BSD support for jumbograms. We would like to address those, but we need more
time to finalize these. To name a few:
• mbuf pkthdr.len eld is typed as "int" in 4.4BSD, so it will not hold jumbogram with len > 2G on 32bit architecture
CPUs. If we would like to support jumbogram properly, the eld must be expanded to hold 4G + IPv6 header +
link-layer header. Therefore, it must be expanded to at least int64_t (u_int32_t is NOT enough).
• We mistakingly use "int" to hold packet length in many places. We need to convert them into larger integral
type. It needs a great care, as we may experience overflow during packet length computation.
• We mistakingly check for ip6_plen eld of IPv6 header for packet payload length in various places. We should
be checking mbuf pkthdr.len instead. ip6_input() will perform sanity check on jumbo payload option on input,
and we can safely use mbuf pkthdr.len afterwards.
• TCP code needs a careful update in bunch of places, of course. Loop prevention in header processing
IPv6 specification allows arbitrary number of extension headers to be placed onto packets. If we implement IPv6
packet processing code in the way BSD IPv4 code is implemented, kernel stack may overflow due to long function
call chain. sys/netinet6 code is carefully designed to avoid kernel stack overflow. Because of this, sys/netinet6 code
defines its own protocol switch structure, as "struct ip6protosw" (see netinet6/ip6protosw.h ). There is no such
update to IPv4 part (sys/netinet) for compatibility, but small change is added to its pr_input() prototype. So "struct
ipprotosw" is also defined. Because of this, if you receive IPsec-over-IPv4 packet with massive number of IPsec
headers, kernel stack may blow up. IPsec-over-IPv6 is okay. (O-course, for those all IPsec headers to be processed,
each such IPsec header must pass each IPsec check. So an anonymous attacker will not be able to do such an attack.) ICMPv6
After RFC2463 was published, IETF ipngwg has decided to disallow ICMPv6 error packet against ICMPv6 redirect,
to prevent ICMPv6 storm on a network medium. This is already implemented into the kernel. Applications
For userland programming, we support IPv6 socket API as specified in RFC2553, RFC2292 and upcoming Internet
TCP/UDP over IPv6 is available and quite stable. You can enjoy telnet(1), ftp(1), rlogin(1), rsh(1), ssh(1), etc. These
applications are protocol independent. That is, they automatically chooses IPv4 or IPv6 according to DNS. Kernel Internals
While ip_forward() calls ip_output(), ip6_forward() directly calls if_output() since routers must not divide IPv6
packets into fragments.
ICMPv6 should contain the original packet as long as possible up to 1280. UDP6/IP6 port unreach, for instance,
should contain all extension headers and the *unchanged* UDP6 and IP6 headers. So, all IP6 functions except TCP
never convert network byte order into host byte order, to save the original packet.
tcp_input(), udp6_input() and icmp6_input() can not assume that IP6 header is preceding the transport headers
due to extension headers. So, in6_cksum() was implemented to handle packets whose IP6 header and transport
header is not continuous. TCP/IP6 nor UDP6/IP6 header structures do not exist for checksum calculation.
To process IP6 header, extension headers and transport headers easily, network drivers are now required to store
packets in one internal mbuf or one or more external mbufs. A typical old driver prepares two internal mbufs for
96 - 204 bytes data, however, now such packet data is stored in one external mbuf.
netstat -s -p ip6 tells you whether or not your driver conforms such requirement. In the following example,
"cce0" violates the requirement. (For more information, refer to Section 2.)
Mbuf statistics:
317 one mbuf
two or more mbuf::
lo0 = 8
cce0 = 10
3282 one ext mbuf
0 two or more ext mbuf
Each input function calls IP6_EXTHDR_CHECK in the beginning to check if the region between IP6 and its header is
continuous. IP6_EXTHDR_CHECK calls m_pullup() only if the mbuf has M_LOOP ag, that is, the packet comes from
the loopback interface. m_pullup() is never called for packets coming from physical network interfaces.
Both IP and IP6 reassemble functions never call m_pullup(). IPv4 mapped address and IPv6 wildcard socket
RFC2553 describes IPv4 mapped address (3.7) and special behavior of IPv6 wildcard bind socket (3.8). The spec
allows you to:
• Accept IPv4 connections by AF_INET6 wildcard bind socket.
• Transmit IPv4 packet over AF_INET6 socket by using special form of the address like :::
but the spec itself is very complicated and does not specify how the socket layer should behave. Here we call the
former one "listening side" and the latter one "initiating side", for reference purposes.
You can perform wildcard bind on both of the address families, on the same port.
The following table show the behavior of FreeBSD 4.x.
listening side
FreeBSD 4.x
initiating side
(AF_INET6 wildcard
(connection to ::ffff:
socket gets IPv4 conn.)
---- configurable
default: enabled
Chapter 8. IPv6 Internals
The following sections will give you more details, and how you can configure the behavior.
Comments on listening side:
It looks that RFC2553 talks too little on wildcard bind issue, especially on the port space issue, failure mode and relationship between AF_INET/INET6 wildcard bind. There can be several separate interpretation for this RFC which
conform to it but behaves differently. So, to implement portable application you should assume nothing about the
behavior in the kernel. Using getaddrinfo(3) is the safest way. Port number space and wildcard bind issues were
discussed in detail on ipv6imp mailing list, in mid March 1999 and it looks that there is no concrete consensus
(means, up to implementers). You may want to check the mailing list archives.
If a server application would like to accept IPv4 and IPv6 connections, there will be two alternatives.
One is using AF_INET and AF_INET6 socket (you will need two sockets). Use getaddrinfo(3) with AI_PASSIVE into
ai_ags, and socket(2) and bind(2) to all the addresses returned. By opening multiple sockets, you can accept connections onto the socket with proper address family. IPv4 connections will be accepted by AF_INET socket, and
IPv6 connections will be accepted by AF_INET6 socket.
Another way is using one AF_INET6 wildcard bind socket. Use getaddrinfo(3) with AI_PASSIVE into ai_ags and
with AF_INET6 into ai_family, and set the 1st argument hostname to NULL. And socket(2) and bind(2) to the address
returned. (should be IPv6 unspecified addr). You can accept either of IPv4 and IPv6 packet via this one socket.
To support only IPv6 traffic on AF_INET6 wildcard binded socket portably, always check the peer address when
a connection is made toward AF_INET6 listening socket. If the address is IPv4 mapped address, you may want to
reject the connection. You can check the condition by using IN6_IS_ADDR_V4MAPPED() macro.
To resolve this issue more easily, there is system dependent setsockopt(2) option, IPV6_BINDV6ONLY, used like
int on;
setsockopt(s, IPPROTO_IPV6, IPV6_BINDV6ONLY,
(char *)&on, sizeof (on)) < 0));
When this call succeed, then this socket only receive IPv6 packets.
Comments on initiating side:
Advise to application implementers: to implement a portable IPv6 application (which works on multiple IPv6 kernels), we believe that the following is the key to the success:
• NEVER hardcode AF_INET nor AF_INET6.
• Use getaddrinfo(3) and getnameinfo(3) throughout the system. Never use gethostby*(), getaddrby*(), inet_*()
or getipnodeby*(). (To update existing applications to be IPv6 aware easily, sometime getipnodeby*() will be
useful. But if possible, try to rewrite the code to use getaddrinfo(3) and getnameinfo(3).)
• If you would like to connect to destination, use getaddrinfo(3) and try all the destination returned, like telnet(1)
• Some of the IPv6 stack is shipped with buggy getaddrinfo(3). Ship a minimal working version with your application and use that as last resort.
If you would like to use AF_INET6 socket for both IPv4 and IPv6 outgoing connection, you will need to use getipnodebyname(3). When you would like to update your existing application to be IPv6 aware with minimal effort, this
approach might be chosen. But please note that it is a temporal solution, because getipnodebyname(3) itself is not
recommended as it does not handle scoped IPv6 addresses at all. For IPv6 name resolution, getaddrinfo(3) is the
preferred API. So you should rewrite your application to use getaddrinfo(3), when you get the time to do it.
When writing applications that make outgoing connections, story goes much simpler if you treat AF_INET and
AF_INET6 as totally separate address family. {set,get}sockopt issue goes simpler, DNS issue will be made simpler.
We do not recommend you to rely upon IPv4 mapped address. unified tcp and inpcb code
FreeBSD 4.x uses shared tcp code between IPv4 and IPv6 (from sys/netinet/tcp*) and separate udp4/6 code. It uses
unified inpcb structure.
The platform can be configured to support IPv4 mapped address. Kernel configuration is summarized as follows:
• By default, AF_INET6 socket will grab IPv4 connections in certain condition, and can initiate connection to IPv4
destination embedded in IPv4 mapped IPv6 address.
• You can disable it on entire system with sysctl like below.
sysctl net.inet6.ip6.mapped_addr=0 listening side
Each socket can be configured to support special AF_INET6 wildcard bind (enabled by default). You can disable it
on each socket basis with setsockopt(2) like below.
int on;
setsockopt(s, IPPROTO_IPV6, IPV6_BINDV6ONLY,
(char *)&on, sizeof (on)) < 0));
Wildcard AF_INET6 socket grabs IPv4 connection if and only if the following conditions are satisfied:
• there is no AF_INET socket that matches the IPv4 connection
• the AF_INET6 socket is configured to accept IPv4 traffic, i.e. getsockopt(IPV6_BINDV6ONLY) returns 0.
There is no problem with open/close ordering. initiating side
FreeBSD 4.x supports outgoing connection to IPv4 mapped address (:::, if the node is configured to
support IPv4 mapped address. sockaddr_storage
When RFC2553 was about to be finalized, there was discussion on how struct sockaddr_storage members are named.
One proposal is to prepend "__" to the members (like "__ss_len") as they should not be touched. The other proposal
was not to prepend it (like "ss_len") as we need to touch those members directly. There was no clear consensus on it.
As a result, RFC2553 defines struct sockaddr_storage as follows:
struct sockaddr_storage {
u_char __ss_len; /* address length */
u_char __ss_family; /* address family */
/* and bunch of padding */
On the contrary, XNET draft defines as follows:
struct sockaddr_storage {
u_char ss_len; /* address length */
u_char ss_family; /* address family */
Chapter 8. IPv6 Internals
/* and bunch of padding */
In December 1999, it was agreed that RFC2553bis should pick the latter (XNET) definition.
Current implementation conforms to XNET definition, based on RFC2553bis discussion.
If you look at multiple IPv6 implementations, you will be able to see both definitions. As an userland programmer,
the most portable way of dealing with it is to:
1. ensure ss_family and/or ss_len are available on the platform, by using GNU autoconf,
2. have -Dss_family=__ss_family to unify all occurrences (including header le) into __ss_family, or
3. never touch __ss_family. cast to sockaddr * and use sa_family like:
struct sockaddr_storage ss;
family = ((struct sockaddr *)&ss)->sa_family
8.1.2. Network Drivers
Now following two items are required to be supported by standard drivers:
1. mbuf clustering requirement. In this stable release, we changed MINCLSIZE into MHLEN+1 for all the operating
systems in order to make all the drivers behave as we expect.
2. multicast. If ifmcstat(8) yields no multicast group for a interface, that interface has to be patched.
If any of the drivers do not support the requirements, then the drivers can not be used for IPv6 and/or IPsec
communication. If you nd any problem with your card using IPv6/IPsec, then, please report it to the FreeBSD
problem reports mailing list.
(NOTE: In the past we required all PCMCIA drivers to have a call to in6_ifattach(). We have no such requirement
any more)
8.1.3. Translator
We categorize IPv4/IPv6 translator into 4 types:
• Translator A --- It is used in the early stage of transition to make it possible to establish a connection from an
IPv6 host in an IPv6 island to an IPv4 host in the IPv4 ocean.
• Translator B --- It is used in the early stage of transition to make it possible to establish a connection from an IPv4
host in the IPv4 ocean to an IPv6 host in an IPv6 island.
• Translator C --- It is used in the late stage of transition to make it possible to establish a connection from an IPv4
host in an IPv4 island to an IPv6 host in the IPv6 ocean.
• Translator D --- It is used in the late stage of transition to make it possible to establish a connection from an IPv6
host in the IPv6 ocean to an IPv4 host in an IPv4 island.
TCP relay translator for category A is supported. This is called "FAITH". We also provide IP header translator for
category A. (The latter is not yet put into FreeBSD 4.x yet.) FAITH TCP relay translator
FAITH system uses TCP relay daemon called faithd(8) helped by the kernel. FAITH will reserve an IPv6 address
prefix, and relay TCP connection toward that prefix to IPv4 destination.
For example, if the reserved IPv6 prefix is 2001:0DB8:0200:ffff::, and the IPv6 destination for TCP connection is
2001:0DB8:0200:ffff::, the connection will be relayed toward IPv4 destination
destination IPv4 node (
| IPv4 tcp toward
FAITH-relay dual stack node
| IPv6 TCP toward 2001:0DB8:0200:ffff::
source IPv6 node
faithd(8) must be invoked on FAITH-relay dual stack node.
For more details, consult src/usr.sbin/faithd/README
8.1.4. IPsec
IPsec is mainly organized by three components.
1. Policy Management
2. Key Management
3. AH and ESP handling Policy Management
The kernel implements experimental policy management code. There are two way to manage security policy. One is
to configure per-socket policy using setsockopt(2). In this cases, policy configuration is described in ipsec_set_policy(3). The other is to configure kernel packet filter-based policy using PF_KEY interface, via setkey(8).
The policy entry is not re-ordered with its indexes, so the order of entry when you add is very significant. Key Management
The key management code implemented in this kit (sys/netkey) is a home-brew PFKEY v2 implementation. This
conforms to RFC2367.
The home-brew IKE daemon, "racoon" is included in the kit (kame/kame/racoon). Basically you will need to run
racoon as daemon, then set up a policy to require keys (like ping -P 'out ipsec esp/transport//use' ). The
kernel will contact racoon daemon as necessary to exchange keys. AH and ESP handling
IPsec module is implemented as "hooks" to the standard IPv4/IPv6 processing. When sending a packet, ip{,6}_output() checks if ESP/AH processing is required by checking if a matching SPD (Security Policy Database) is found.
If ESP/AH is needed, {esp,ah}{4,6}_output() will be called and mbuf will be updated accordingly. When a packet is
received, {esp,ah}4_input() will be called based on protocol number, i.e. (*inetsw[proto])(). {esp,ah}4_input() will
decrypt/check authenticity of the packet, and strips o daisy-chained header and padding for ESP/AH. It is safe to
strip o the ESP/AH header on packet reception, since we will never use the received packet in "as is" form.
By using ESP/AH, TCP4/6 effective data segment size will be affected by extra daisy-chained headers inserted by
ESP/AH. Our code takes care of the case.
Basic crypto functions can be found in directory "sys/crypto". ESP/AH transform are listed in {esp,ah}_core.c with
wrapper functions. If you wish to add some algorithm, add wrapper function in {esp,ah}_core.c, and add your crypto
algorithm code into sys/crypto.
Tunnel mode is partially supported in this release, with the following restrictions:
Chapter 8. IPv6 Internals
• IPsec tunnel is not combined with GIF generic tunneling interface. It needs a great care because we may create an
infinite loop between ip_output() and tunnelifp->if_output(). Opinion varies if it is better to unify them, or not.
• MTU and Don't Fragment bit (IPv4) considerations need more checking, but basically works ne.
• Authentication model for AH tunnel must be revisited. We will need to improve the policy management engine,
eventually. Conformance to RFCs and IDs
The IPsec code in the kernel conforms (or, tries to conform) to the following standards:
"old IPsec" specification documented in rfc182[5-9].txt
"new IPsec" specification documented in rfc240[1-6].txt , rfc241[01].txt , rfc2451.txt and draft-mcdonald-simple-ipsec-api-01.txt (draft expired, but you can take from
(NOTE: IKE specifications, rfc241[7-9].txt are implemented in userland, as "racoon" IKE daemon)
Currently supported algorithms are:
• old IPsec AH
• null crypto checksum (no document, just for debugging)
• keyed MD5 with 128bit crypto checksum (rfc1828.txt )
• keyed SHA1 with 128bit crypto checksum (no document)
• HMAC MD5 with 128bit crypto checksum (rfc2085.txt )
• HMAC SHA1 with 128bit crypto checksum (no document)
• old IPsec ESP
• null encryption (no document, similar to rfc2410.txt )
• DES-CBC mode (rfc1829.txt )
• new IPsec AH
• null crypto checksum (no document, just for debugging)
• keyed MD5 with 96bit crypto checksum (no document)
• keyed SHA1 with 96bit crypto checksum (no document)
• HMAC MD5 with 96bit crypto checksum (rfc2403.txt )
• HMAC SHA1 with 96bit crypto checksum (rfc2404.txt )
• new IPsec ESP
• null encryption (rfc2410.txt )
• DES-CBC with derived IV (draft-ietf-ipsec-ciph-des-derived-01.txt , draft expired)
• DES-CBC with explicit IV (rfc2405.txt )
• 3DES-CBC with explicit IV (rfc2451.txt )
• BLOWFISH CBC (rfc2451.txt )
• CAST128 CBC (rfc2451.txt )
• RC5 CBC (rfc2451.txt )
• each of the above can be combined with:
• ESP authentication with HMAC-MD5(96bit)
• ESP authentication with HMAC-SHA1(96bit)
The following algorithms are NOT supported:
• old IPsec AH
• HMAC MD5 with 128bit crypto checksum + 64bit replay prevention (rfc2085.txt )
• keyed SHA1 with 160bit crypto checksum + 32bit padding (rfc1852.txt )
IPsec (in kernel) and IKE (in userland as "racoon") has been tested at several interoperability test events, and it is
known to interoperate with many other implementations well. Also, current IPsec implementation as quite wide
coverage for IPsec crypto algorithms documented in RFC (we cover algorithms without intellectual property issues
only). ECN consideration on IPsec tunnels
ECN-friendly IPsec tunnel is supported as described in draft-ipsec-ecn-00.txt .
Normal IPsec tunnel is described in RFC2401. On encapsulation, IPv4 TOS eld (or, IPv6 traffic class eld) will be
copied from inner IP header to outer IP header. On decapsulation outer IP header will be simply dropped. The
decapsulation rule is not compatible with ECN, since ECN bit on the outer IP TOS/traffic class eld will be lost.
To make IPsec tunnel ECN-friendly, we should modify encapsulation and decapsulation procedure. This is described
inoyd/papers/draft-ipsec-ecn-00.txt, chapter 3.
IPsec tunnel implementation can give you three behaviors, by setting net.inet.ipsec.ecn (or net.inet6.ipsec6.ecn)
to some value:
• RFC2401: no consideration for ECN (sysctl value -1)
• ECN forbidden (sysctl value 0)
• ECN allowed (sysctl value 1)
Note that the behavior is configurable in per-node manner, not per-SA manner (draft-ipsec-ecn-00 wants per-SA
configuration, but it looks too much for me).
The behavior is summarized as follows (see source code for more detail):
-- copy all TOS bits
from inner to outer.
-- drop TOS bits on outer
(use inner TOS bits as is)
ECN forbidden
copy TOS bits except for ECN
(masked with 0xfc) from inner
to outer. set ECN bits to 0.
drop TOS bits on outer
(use inner TOS bits as is)
ECN allowed
copy TOS bits except for ECN
CE (masked with 0xfe) from
inner to outer.
use inner TOS bits with some
change. if outer ECN CE bit
is 1, enable ECN CE bit on
Chapter 8. IPv6 Internals
set ECN CE bit to 0.
the inner.
General strategy for configuration is as follows:
• if both IPsec tunnel endpoint are capable of ECN-friendly behavior, you should better configure both end to
“ECN allowed” (sysctl value 1).
• if the other end is very strict about TOS bit, use "RFC2401" (sysctl value -1).
• in other cases, use "ECN forbidden" (sysctl value 0).
The default behavior is "ECN forbidden" (sysctl value 0).
For more information, please refer to:oyd/papers/draft-ipsec-ecn-00.txt, RFC2481 (Explicit Congestion Notification), src/sys/
(Thanks goes to Kenjiro Cho <[email protected]> for detailed analysis) Interoperability
Here are (some of) platforms that KAME code have tested IPsec/IKE interoperability in the past. Note that both
ends may have modified their implementation, so use the following list just for reference purposes.
Altiga, Ashley-laurent (, Data Fellows (F-Secure), Ericsson ACC, FreeS/WAN, HITACHI, IBM AIX®, IIJ,
Intel, Microsoft® Windows NT®, NIST (linux IPsec + plutoplus), Netscreen, OpenBSD, RedCreek, Routerware, SSH,
Secure Computing, Soliton, Toshiba, VPNet, Yamaha RT100i
Part III. Kernel
Table of Contents
9. Building and Installing a FreeBSD Kernel ..................................................................................... 93
9.1. Building a Kernel the “Traditional” Way ............................................................................ 93
9.2. Building a Kernel the “New” Way .................................................................................... 93
10. Kernel Debugging ................................................................................................................. 95
10.1. Obtaining a Kernel Crash Dump ..................................................................................... 95
10.2. Debugging a Kernel Crash Dump with kgdb ...................................................................... 97
10.3. Debugging a Crash Dump with DDD ............................................................................... 100
10.4. On-Line Kernel Debugging Using DDB ............................................................................ 101
10.5. On-Line Kernel Debugging Using Remote GDB ................................................................. 104
10.6. Debugging a Console Driver ......................................................................................... 104
10.7. Debugging Deadlocks ................................................................................................. 105
10.8. Kernel debugging with Dcons ....................................................................................... 105
10.9. Glossary of Kernel Options for Debugging ....................................................................... 107
Chapter 9. Building and Installing a
FreeBSD Kernel
Being a kernel developer requires understanding of the kernel build process. To debug the FreeBSD kernel it is
required to be able to build one. There are two known ways to do so:
• The “Traditional” Way
• The “New” Way
It is supposed that the reader of this chapter is familiar with the information described in the
Building and Installing a Custom Kernel chapter of the FreeBSD Handbook. If this is not the
case, please read through the above mentioned chapter to understand how the build process
9.1. Building a Kernel the “Traditional” Way
Up to version 4.X of FreeBSD this was the recommended way to build a new kernel. It can still be used on newer
versions (instead of the “buildkernel” target of the toplevel /usr/src/ makefiles). Building the kernel this way
may be useful when working on the kernel code and it may actually be faster than the “New” procedure when only
a single option or two were tweaked in the kernel configuration le. On the other hand, it might lead to unexpected
kernel build breakage when used by beginners on newer versions of FreeBSD.
Run config(8) to generate the kernel source code:
# /usr/sbin/config
Change into the build directory. config(8) will print the name of this directory after being run as above.
# cd ../compile/ MYKERNEL
Compile the kernel:
# make depend
# make
Install the new kernel:
# make install
9.2. Building a Kernel the “New” Way
This procedure is well supported and recommended under the latest FreeBSD releases and is documented in the
Building and Installing a Custom Kernel chapter of the FreeBSD Handbook.
Chapter 10. Kernel Debugging
Contributed by Paul Richards, Jörg Wunsch and Robert Watson.
10.1. Obtaining a Kernel Crash Dump
When running a development kernel (e.g., FreeBSD-CURRENT), such as a kernel under extreme conditions (e.g.,
very high load averages, tens of thousands of connections, exceedingly high number of concurrent users, hundreds
of jail(8)s, etc.), or using a new feature or device driver on FreeBSD-STABLE (e.g., PAE), sometimes a kernel will
panic. In the event that it does, this chapter will demonstrate how to extract useful information out of a crash.
A system reboot is inevitable once a kernel panics. Once a system is rebooted, the contents of a system's physical
memory (RAM) is lost, as well as any bits that are on the swap device before the panic. To preserve the bits in
physical memory, the kernel makes use of the swap device as a temporary place to store the bits that are in RAM
across a reboot after a crash. In doing this, when FreeBSD boots after a crash, a kernel image can now be extracted
and debugging can take place.
A swap device that has been configured as a dump device still acts as a swap device. Dumps
to non-swap devices (such as tapes or CDRWs, for example) are not supported at this time. A
“swap device” is synonymous with a “swap partition.”
Several types of kernel crash dumps are available:
Full memory dumps
Hold the complete contents of physical memory.
Hold only memory pages in use by the kernel (FreeBSD 6.2 and higher).
Hold captured, scripted, or interactive debugger output (FreeBSD 7.1 and higher).
Minidumps are the default dump type as of FreeBSD 7.0, and in most cases will capture all necessary information
present in a full memory dump, as most problems can be isolated only using kernel state.
10.1.1. Configuring the Dump Device
Before the kernel will dump the contents of its physical memory to a dump device, a dump device must be configured. A dump device is specified by using the dumpon(8) command to tell the kernel where to save kernel crash
dumps. The dumpon(8) program must be called after the swap partition has been configured with swapon(8). This
is normally handled by setting the dumpdev variable in rc.conf(5) to the path of the swap device (the recommended
way to extract a kernel dump) or AUTO to use the rst configured swap device. The default for dumpdev is AUTO in
HEAD, and changed to NO on RELENG_* branches (except for RELENG_7, which was left set to AUTO). On FreeBSD 9.0RELEASE and later versions, bsdinstall will ask whether crash dumps should be enabled on the target system during the install process.
Extracting a Kernel Dump
Check /etc/fstab or swapinfo(8) for a list of swap devices.
Make sure the dumpdir specified in rc.conf(5) exists before a kernel crash!
# mkdir /var/crash
# chmod 700 /var/crash
Also, remember that the contents of /var/crash is sensitive and very likely contains confidential information such as passwords.
10.1.2. Extracting a Kernel Dump
Once a dump has been written to a dump device, the dump must be extracted before the swap device is mounted. To
extract a dump from a dump device, use the savecore(8) program. If dumpdev has been set in rc.conf(5), savecore(8)
will be called automatically on the rst multi-user boot after the crash and before the swap device is mounted.
The location of the extracted core is placed in the rc.conf(5) value dumpdir , by default /var/crash and will be
named vmcore.0.
In the event that there is already a le called vmcore.0 in /var/crash (or whatever dumpdir is set to), the kernel
will increment the trailing number for every crash to avoid overwriting an existing vmcore (e.g., vmcore.1). While
debugging, it is highly likely that you will want to use the highest version vmcore in /var/crash when searching
for the right vmcore.
If you are testing a new kernel but need to boot a different one in order to get your system up
and running again, boot it only into single user mode using the -s ag at the boot prompt,
and then perform the following steps:
fsck -p
mount -a -t ufs
# make sure /var/crash is writable
savecore /var/crash /dev/ad0s1b
# exit to multi-user
This instructs savecore(8) to extract a kernel dump from /dev/ad0s1b and place the contents in /var/crash . Do not forget to make sure the destination directory /var/crash has
enough space for the dump. Also, do not forget to specify the correct path to your swap device as it is likely different than /dev/ad0s1b !
Chapter 10. Kernel Debugging
10.2. Debugging a Kernel Crash Dump with kgdb
This section covers kgdb(1) as found in FreeBSD 5.3 and later. In previous versions, one must
use gdb -k to read a core dump le.
Once a dump has been obtained, getting useful information out of the dump is relatively easy for simple problems.
Before launching into the internals of kgdb(1) to debug the crash dump, locate the debug version of your kernel
(normally called kernel.debug) and the path to the source les used to build your kernel (normally /usr/obj/
usr/src/sys/ KERNCONF, where KERNCONF is the ident specified in a kernel config(5)). With those two pieces of
info, let the debugging commence!
To enter into the debugger and begin getting information from the dump, the following steps are required at a
# cd /usr/obj/usr/src/sys/ KERNCONF
# kgdb kernel.debug /var/crash/vmcore.0
You can debug the crash dump using the kernel sources just like you can for any other program.
This rst dump is from a 5.2-BETA kernel and the crash comes from deep within the kernel. The output below has
been modified to include line numbers on the left. This rst trace inspects the instruction pointer and obtains a
back trace. The address that is used on line 41 for the list command is the instruction pointer and can be found
on line 17. Most developers will request having at least this information sent to them if you are unable to debug the
problem yourself. If, however, you do solve the problem, make sure that your patch winds its way into the source
tree via a problem report, mailing lists, or by being able to commit it!
1:# cd /usr/obj/usr/src/sys/ KERNCONF
2:# kgdb kernel.debug /var/crash/vmcore.0
3:GNU gdb 5.2.1 (FreeBSD)
4:Copyright 2002 Free Software Foundation, Inc.
5:GDB is free software, covered by the GNU General Public License, and you are
6:welcome to change it and/or distribute copies of it under certain conditions.
7:Type "show copying" to see the conditions.
8:There is absolutely no warranty for GDB. Type "show warranty" for details.
9:This GDB was configured as "i386-undermydesk-freebsd"...
10:panic: page fault
11:panic messages:
12:--13:Fatal trap 12: page fault while in kernel mode
14:cpuid = 0; apic id = 00
15:fault virtual address = 0x300
16:fault code:
= supervisor read, page not present
17:instruction pointer
= 0x8:0xc0713860
18:stack pointer
= 0x10:0xdc1d0b70
19:frame pointer
= 0x10:0xdc1d0b7c
20:code segment
= base 0x0, limit 0xfffff, type 0x1b
= DPL 0, pres 1, def32 1, gran 1
22:processor eflags
= resume, IOPL = 0
23:current process
= 14394 (uname)
24:trap number
= 12
25:panic: page fault
cpuid = 0;
27:Stack backtrace:
29:syncing disks, buffers remaining... 2199 2199 panic: mi_switch: switch in a critical ↺
30:cpuid = 0;
31:Uptime: 2h43m19s
Debugging a Kernel Crash Dump with kgdb
32:Dumping 255 MB
33: 16 32 48 64 80 96 112 128 144 160 176 192 208 224 240
34:--35:Reading symbols from /boot/kernel/snd_maestro3.ko...done.
36:Loaded symbols for /boot/kernel/snd_maestro3.ko
37:Reading symbols from /boot/kernel/snd_pcm.ko...done.
38:Loaded symbols for /boot/kernel/snd_pcm.ko
39:#0 doadump () at /usr/src/sys/kern/kern_shutdown.c:240
41:(kgdb) list *0xc0713860
42:0xc0713860 is in lapic_ipi_wait (/usr/src/sys/i386/i386/local_apic.c:663).
incr = 0;
delay = 1;
} else
incr = 1;
for (x = 0; x < delay; x += incr) {
if ((lapic->icr_lo & APIC_DELSTAT_MASK) == APIC_DELSTAT_IDLE)
return (1);
return (0);
53:(kgdb) backtrace
54:#0 doadump () at /usr/src/sys/kern/kern_shutdown.c:240
55:#1 0xc055fd9b in boot (howto=260) at /usr/src/sys/kern/kern_shutdown.c:372
56:#2 0xc056019d in panic () at /usr/src/sys/kern/kern_shutdown.c:550
57:#3 0xc0567ef5 in mi_switch () at /usr/src/sys/kern/kern_synch.c:470
58:#4 0xc055fa87 in boot (howto=256) at /usr/src/sys/kern/kern_shutdown.c:312
59:#5 0xc056019d in panic () at /usr/src/sys/kern/kern_shutdown.c:550
60:#6 0xc0720c66 in trap_fatal (frame=0xdc1d0b30, eva=0)
at /usr/src/sys/i386/i386/trap.c:821
62:#7 0xc07202b3 in trap (frame=
{tf_fs = -1065484264, tf_es = -1065484272, tf_ds = -1065484272, tf_edi = 1, ↺
tf_esi = 0, tf_ebp = -602076292, tf_isp = -602076324, tf_ebx = 0, tf_edx = 0, tf_ecx ↺
= 1000000, tf_eax = 243, tf_trapno = 12, tf_err = 0, tf_eip = -1066321824, tf_cs = 8, ↺
tf_eflags = 65671, tf_esp = 243, tf_ss = 0})
at /usr/src/sys/i386/i386/trap.c:250
65:#8 0xc070c9f8 in calltrap () at {standard input}:94
66:#9 0xc07139f3 in lapic_ipi_vectored (vector=0, dest=0)
at /usr/src/sys/i386/i386/local_apic.c:733
68:#10 0xc0718b23 in ipi_selected (cpus=1, ipi=1)
at /usr/src/sys/i386/i386/mp_machdep.c:1115
70:#11 0xc057473e in kseq_notify (ke=0xcc05e360, cpu=0)
at /usr/src/sys/kern/sched_ule.c:520
72:#12 0xc0575cad in sched_add (td=0xcbcf5c80)
at /usr/src/sys/kern/sched_ule.c:1366
74:#13 0xc05666c6 in setrunqueue (td=0xcc05e360)
at /usr/src/sys/kern/kern_switch.c:422
76:#14 0xc05752f4 in sched_wakeup (td=0xcbcf5c80)
at /usr/src/sys/kern/sched_ule.c:999
78:#15 0xc056816c in setrunnable (td=0xcbcf5c80)
at /usr/src/sys/kern/kern_synch.c:570
80:#16 0xc0567d53 in wakeup (ident=0xcbcf5c80)
at /usr/src/sys/kern/kern_synch.c:411
82:#17 0xc05490a8 in exit1 (td=0xcbcf5b40, rv=0)
at /usr/src/sys/kern/kern_exit.c:509
84:#18 0xc0548011 in sys_exit () at /usr/src/sys/kern/kern_exit.c:102
85:#19 0xc0720fd0 in syscall (frame=
{tf_fs = 47, tf_es = 47, tf_ds = 47, tf_edi = 0, tf_esi = -1, tf_ebp =
-1077940712, tf_isp = -602075788, tf_ebx = 672411944, tf_edx = 10, tf_ecx = 672411600, ↺
tf_eax = 1, tf_trapno = 12, tf_err = 2, tf_eip = 671899563, tf_cs = 31, tf_eflags = ↺
642, tf_esp = -1077940740, tf_ss = 47})
at /usr/src/sys/i386/i386/trap.c:1010
88:#20 0xc070ca4d in Xint0x80_syscall () at {standard input}:136
89:---Can't read userspace from dump, or kernel process--90:(kgdb) quit
Chapter 10. Kernel Debugging
This next trace is an older dump from the FreeBSD 2 time frame, but is more involved and demonstrates more of
the features of gdb . Long lines have been folded to improve readability, and the lines are numbered for reference.
Despite this, it is a real-world error trace taken during the development of the pcvt console driver.
1:Script started on Fri Dec 30 23:15:22 1994
2:# cd /sys/compile/URIAH
3:# gdb -k kernel /var/crash/vmcore.1
4:Reading symbol data from /usr/src/sys/compile/URIAH/kernel
5:IdlePTD 1f3000
6:panic: because you said to!
7:current pcb at 1e3f70
8:Reading in symbols for ../../i386/i386/machdep.c...done.
9:(kgdb) backtrace
10:#0 boot (arghowto=256) (../../i386/i386/machdep.c line 767)
11:#1 0xf0115159 in panic ()
12:#2 0xf01955bd in diediedie () (../../i386/i386/machdep.c line 698)
13:#3 0xf010185e in db_fncall ()
14:#4 0xf0101586 in db_command (-266509132, -266509516, -267381073)
15:#5 0xf0101711 in db_command_loop ()
16:#6 0xf01040a0 in db_trap ()
17:#7 0xf0192976 in kdb_trap (12, 0, -272630436, -266743723)
18:#8 0xf019d2eb in trap_fatal (...)
19:#9 0xf019ce60 in trap_pfault (...)
20:#10 0xf019cb2f in trap (...)
21:#11 0xf01932a1 in exception:calltrap ()
22:#12 0xf0191503 in cnopen (...)
23:#13 0xf0132c34 in spec_open ()
24:#14 0xf012d014 in vn_open ()
25:#15 0xf012a183 in open ()
26:#16 0xf019d4eb in syscall (...)
27:(kgdb) up 10
28:Reading in symbols for ../../i386/i386/trap.c...done.
29:#10 0xf019cb2f in trap (frame={tf_es = -260440048, tf_ds = 16, tf_\
30:edi = 3072, tf_esi = -266445372, tf_ebp = -272630356, tf_isp = -27\
31:2630396, tf_ebx = -266427884, tf_edx = 12, tf_ecx = -266427884, tf\
32:_eax = 64772224, tf_trapno = 12, tf_err = -272695296, tf_eip = -26\
33:6672343, tf_cs = -266469368, tf_eflags = 66066, tf_esp = 3072, tf_\
34:ss = -266427884}) (../../i386/i386/trap.c line 283)
(void) trap_pfault(&frame, FALSE);
36:(kgdb) frame frame->tf_ebp frame->tf_eip
37:Reading in symbols for ../../i386/isa/pcvt/pcvt_drv.c...done.
38:#0 0xf01ae729 in pcopen (dev=3072, flag=3, mode=8192, p=(struct p\
39:roc *) 0xf07c0c00) (../../i386/isa/pcvt/pcvt_drv.c line 403)
return ((*linesw[tp->t_line].l_open)(dev, tp));
41:(kgdb) list
tp->t_state |= TS_CARR_ON;
tp->t_cflag |= CLOCAL; /* cannot be a modem (:-) */
return ((*linesw[tp->t_line].l_open)(dev, tp));
return ((*linesw[tp->t_line].l_open)(dev, tp, flag));
#endif /* PCVT_NETBSD || (PCVT_FREEBSD >= 200) */
52:(kgdb) print tp
53:Reading in symbols for ../../i386/i386/cons.c...done.
54:$1 = (struct tty *) 0x1bae
55:(kgdb) print tp->t_line
56:$2 = 1767990816
57:(kgdb) up
58:#1 0xf0191503 in cnopen (dev=0x00000000, flag=3, mode=8192, p=(st\
59:ruct proc *) 0xf07c0c00) (../../i386/i386/cons.c line 126)
return ((*cdevsw[major(dev)].d_open)(dev, flag, mode, p));
61:(kgdb) up
Debugging a Crash Dump with DDD
62:#2 0xf0132c34 in spec_open ()
63:(kgdb) up
64:#3 0xf012d014 in vn_open ()
65:(kgdb) up
66:#4 0xf012a183 in open ()
67:(kgdb) up
68:#5 0xf019d4eb in syscall (frame={tf_es = 39, tf_ds = 39, tf_edi =\
69: 2158592, tf_esi = 0, tf_ebp = -272638436, tf_isp = -272629788, tf\
70:_ebx = 7086, tf_edx = 1, tf_ecx = 0, tf_eax = 5, tf_trapno = 582, \
71:tf_err = 582, tf_eip = 75749, tf_cs = 31, tf_eflags = 582, tf_esp \
72:= -272638456, tf_ss = 39}) (../../i386/i386/trap.c line 673)
error = (*callp->sy_call)(p, args, rval);
74:(kgdb) up
75:Initial frame selected; you cannot go up.
76:(kgdb) quit
Comments to the above script:
line 6:
This is a dump taken from within DDB (see below), hence the panic comment “because you said to!”, and a
rather long stack trace; the initial reason for going into DDB has been a page fault trap though.
line 20:
This is the location of function trap() in the stack trace.
line 36:
Force usage of a new stack frame; this is no longer necessary. The stack frames are supposed to point to the
right locations now, even in case of a trap. From looking at the code in source line 403, there is a high probability
that either the pointer access for “tp” was messed up, or the array access was out of bounds.
line 52:
The pointer looks suspicious, but happens to be a valid address.
line 56:
However, it obviously points to garbage, so we have found our error! (For those unfamiliar with that particular
piece of code: tp->t_line refers to the line discipline of the console device here, which must be a rather small
integer number.)
If your system is crashing regularly and you are running out of disk space, deleting old vmcore
les in /var/crash could save a considerable amount of disk space!
10.3. Debugging a Crash Dump with DDD
Examining a kernel crash dump with a graphical debugger like ddd is also possible (you will need to install the
devel/ddd port in order to use the ddd debugger). Add the -k option to the ddd command line you would use
normally. For example;
# ddd --debugger kgdb kernel.debug /var/crash/vmcore.0
You should then be able to go about looking at the crash dump using ddd 's graphical interface.
Chapter 10. Kernel Debugging
10.4. On-Line Kernel Debugging Using DDB
While kgdb as an o-line debugger provides a very high level of user interface, there are some things it cannot do.
The most important ones being breakpointing and single-stepping kernel code.
If you need to do low-level debugging on your kernel, there is an on-line debugger available called DDB. It allows
setting of breakpoints, single-stepping kernel functions, examining and changing kernel variables, etc. However,
it cannot access kernel source les, and only has access to the global and static symbols, not to the full debug
information like gdb does.
To configure your kernel to include DDB, add the options
options KDB
options DDB
to your config le, and rebuild. (See The FreeBSD Handbook for details on configuring the FreeBSD kernel).
If you have an older version of the boot blocks, your debugger symbols might not be loaded
at all. Update the boot blocks; the recent ones load the DDB symbols automatically.
Once your DDB kernel is running, there are several ways to enter DDB. The rst, and earliest way is to type the
boot ag -d right at the boot prompt. The kernel will start up in debug mode and enter DDB prior to any device
probing. Hence you can even debug the device probe/attach functions. Users of FreeBSD-CURRENT will need to
use the boot menu option, six, to escape to a command prompt.
The second scenario is to drop to the debugger once the system has booted. There are two simple ways to accomplish this. If you would like to break to the debugger from the command prompt, simply type the command:
# sysctl debug.kdb.enter=1
To force a panic on the y, issue the following command:
# sysctl debug.kdb.panic=1
Alternatively, if you are at the system console, you may use a hot-key on the keyboard. The default break-todebugger sequence is Ctrl+Alt+ESC. For syscons, this sequence can be remapped and some of the distributed maps
out there do this, so check to make sure you know the right sequence to use. There is an option available for serial
consoles that allows the use of a serial line BREAK on the console line to enter DDB (options BREAK_TO_DEBUGGER in
the kernel config le). It is not the default since there are a lot of serial adapters around that gratuitously generate
a BREAK condition, for example when pulling the cable.
The third way is that any panic condition will branch to DDB if the kernel is configured to use it. For this reason,
it is not wise to configure a kernel with DDB for a machine running unattended.
To obtain the unattended functionality, add:
to the kernel configuration le and rebuild/reinstall.
On-Line Kernel Debugging Using DDB
The DDB commands roughly resemble some gdb commands. The rst thing you probably need to do is to set a
break function-name address
Numbers are taken hexadecimal by default, but to make them distinct from symbol names; hexadecimal numbers
starting with the letters a-f need to be preceded with 0x (this is optional for other numbers). Simple expressions
are allowed, for example: function-name + 0x103 .
To exit the debugger and continue execution, type:
To get a stack trace, use:
Note that when entering DDB via a hot-key, the kernel is currently servicing an interrupt, so
the stack trace might be not of much use to you.
If you want to remove a breakpoint, use
del address-expression
The rst form will be accepted immediately after a breakpoint hit, and deletes the current breakpoint. The second
form can remove any breakpoint, but you need to specify the exact address; this can be obtained from:
show b
show break
To single-step the kernel, try:
This will step into functions, but you can make DDB trace them until the matching return statement is reached by:
This is different from gdb 's next statement; it is like gdb 's finish. Pressing n more than once
will cause a continue.
To examine data from memory, use (for example):
x/wx 0xf0133fe0,40
x/hd db_symtab_space
x/bc termbuf,10
x/s stringbuf
Chapter 10. Kernel Debugging
for word/halfword/byte access, and hexadecimal/decimal/character/ string display. The number after the comma
is the object count. To display the next 0x10 items, simply use:
x ,10
Similarly, use
x/ia foofunc,10
to disassemble the rst 0x10 instructions of foofunc, and display them along with their offset from the beginning
of foofunc.
To modify memory, use the write command:
w/b termbuf 0xa 0xb 0
w/w 0xf0010030 0 0
The command modifier (b/h/w) specifies the size of the data to be written, the rst following expression is the
address to write to and the remainder is interpreted as data to write to successive memory locations.
If you need to know the current registers, use:
show reg
Alternatively, you can display a single register value by e.g.
p $eax
and modify it by:
set $eax new-value
Should you need to call some kernel functions from DDB, simply say:
call func(arg1, arg2, ...)
The return value will be printed.
For a ps(1) style summary of all running processes, use:
Now you have examined why your kernel failed, and you wish to reboot. Remember that, depending on the severity
of previous malfunctioning, not all parts of the kernel might still be working as expected. Perform one of the
following actions to shut down and reboot your system:
This will cause your kernel to dump core and reboot, so you can later analyze the core on a higher level with gdb .
This command usually must be followed by another continue statement.
call boot(0)
Might be a good way to cleanly shut down the running system, sync() all disks, and finally, in some cases, reboot.
As long as the disk and filesystem interfaces of the kernel are not damaged, this could be a good way for an almost
clean shutdown.
call cpu_reset()
This is the final way out of disaster and almost the same as hitting the Big Red Button.
If you need a short command summary, simply type:
On-Line Kernel Debugging Using Remote GDB
It is highly recommended to have a printed copy of the ddb(4) manual page ready for a debugging session. Remember that it is hard to read the on-line manual while single-stepping the kernel.
10.5. On-Line Kernel Debugging Using Remote GDB
This feature has been supported since FreeBSD 2.2, and it is actually a very neat one.
GDB has already supported remote debugging for a long time. This is done using a very simple protocol along a serial
line. Unlike the other methods described above, you will need two machines for doing this. One is the host providing
the debugging environment, including all the sources, and a copy of the kernel binary with all the symbols in it,
and the other one is the target machine that simply runs a similar copy of the very same kernel (but stripped of
the debugging information).
You should configure the kernel in question with config -g if building the “traditional” way. If building the “new”
way, make sure that makeoptions DEBUG=-g is in the configuration. In both cases, include DDB in the configuration,
and compile it as usual. This gives a large binary, due to the debugging information. Copy this kernel to the target
machine, strip the debugging symbols o with strip -x , and boot it using the -d boot option. Connect the serial
line of the target machine that has "ags 080" set on its uart device to any serial line of the debugging host. See
uart(4) for information on how to set the ags on an uart device. Now, on the debugging machine, go to the compile
directory of the target kernel, and start gdb :
% kgdb kernel
GDB is free software and you are welcome to distribute copies of it
under certain conditions; type "show copying" to see the conditions.
There is absolutely no warranty for GDB; type "show warranty" for details.
GDB 4.16 (i386-unknown-freebsd),
Copyright 1996 Free Software Foundation, Inc...
Initialize the remote debugging session (assuming the rst serial port is being used) by:
(kgdb) target remote /dev/cuau0
Now, on the target host (the one that entered DDB right before even starting the device probe), type:
Debugger("Boot flags requested debugger")
Stopped at Debugger+0x35: movb $0, edata+0x51bc
db> gdb
DDB will respond with:
Next trap will enter GDB remote protocol mode
Every time you type gdb , the mode will be toggled between remote GDB and local DDB. In order to force a next trap
immediately, simply type s (step). Your hosting GDB will now gain control over the target kernel:
Remote debugging using /dev/cuau0
Debugger (msg=0xf01b0383 "Boot flags requested debugger")
at ../../i386/i386/db_interface.c:257
You can use this session almost as any other GDB session, including full access to the source, running it in gudmode inside an Emacs window (which gives you an automatic source code display in another Emacs window), etc.
10.6. Debugging a Console Driver
Since you need a console driver to run DDB on, things are more complicated if the console driver itself is failing.
You might remember the use of a serial console (either with modified boot blocks, or by specifying -h at the Boot:
Chapter 10. Kernel Debugging
prompt), and hook up a standard terminal onto your rst serial port. DDB works on any configured console driver,
including a serial console.
10.7. Debugging Deadlocks
You may experience so called deadlocks, a situation where a system stops doing useful work. To provide a helpful
bug report in this situation, use ddb(4) as described in the previous section. Include the output of ps and trace
for suspected processes in the report.
If possible, consider doing further investigation. The recipe below is especially useful if you suspect that a deadlock
occurs in the VFS layer. Add these options to the kernel configuration le.
makeoptions DEBUG=-g
options WITNESS
When a deadlock occurs, in addition to the output of the ps command, provide information from the show pcpu ,
show allpcpu , show locks , show alllocks , show lockedvnods and alltrace.
To obtain meaningful backtraces for threaded processes, use thread thread-id to switch to the thread stack, and
do a backtrace with where .
10.8. Kernel debugging with Dcons
dcons(4) is a very simple console driver that is not directly connected with any physical devices. It just reads and
writes characters from and to a buer in a kernel or loader. Due to its simple nature, it is very useful for kernel
debugging, especially with a FireWire® device. Currently, FreeBSD provides two ways to interact with the buer
from outside of the kernel using dconschat(8).
10.8.1. Dcons over FireWire®
Most FireWire® (IEEE1394) host controllers are based on the OHCI specification that supports physical access to
the host memory. This means that once the host controller is initialized, we can access the host memory without
the help of software (kernel). We can exploit this facility for interaction with dcons(4). dcons(4) provides similar
functionality as a serial console. It emulates two serial ports, one for the console and DDB, the other for GDB.
Because remote memory access is fully handled by the hardware, the dcons(4) buer is accessible even when the
system crashes.
FireWire® devices are not limited to those integrated into motherboards. PCI cards exist for desktops, and a cardbus interface can be purchased for laptops. Enabling FireWire® and Dcons support on the target machine
To enable FireWire® and Dcons support in the kernel of the target machine:
• Make sure your kernel supports dcons , dcons_crom and firewire . Dcons should be statically linked with the
kernel. For dcons_crom and firewire , modules should be OK.
• Make sure physical DMA is enabled. You may need to add hw.firewire.phydma_enable=1 to /boot/
loader.conf .
• Add options for debugging.
Dcons over FireWire®
• Add dcons_gdb=1 in /boot/loader.conf if you use GDB over FireWire®.
• Enable dcons in /etc/ttys .
• Optionally, to force dcons to be the high-level console, add hw.firewire.dcons_crom.force_console=1 to
loader.conf .
To enable FireWire® and Dcons support in loader(8) on i386 or amd64:
Add LOADER_FIREWIRE_SUPPORT=YES in /etc/make.conf and rebuild loader(8):
# cd /sys/boot/i386 && make clean && make && make install
To enable dcons(4) as an active low-level console, add boot_multicons="YES" to /boot/loader.conf .
Here are a few configuration examples. A sample kernel configuration le would contain:
device dcons
device dcons_crom
options KDB
options DDB
options GDB
And a sample /boot/loader.conf would contain:
hw.firewire.dcons_crom.force_console=1 Enabling FireWire® and Dcons support on the host machine
To enable FireWire® support in the kernel on the host machine:
# kldload firewire
Find out the EUI64 (the unique 64 bit identifier) of the FireWire® host controller, and use fwcontrol(8) or dmesg
to nd the EUI64 of the target machine.
Run dconschat(8), with:
# dconschat -e \# -br -G 12345 -t
The following key combinations can be used once dconschat(8) is running:
~ Ctrl+B
~ Ctrl+R
RESET target
~ Ctrl+Z
Suspend dconschat
Attach remote GDB by starting kgdb(1) with a remote debugging session:
kgdb -r :12345 kernel Some general tips
Here are some general tips:
To take full advantage of the speed of FireWire®, disable other slow console drivers:
Chapter 10. Kernel Debugging
# conscontrol delete ttyd0
# serial console
# conscontrol delete consolectl # video/keyboard
There exists a GDB mode for emacs(1); this is what you will need to add to your .emacs :
(setq gud-gdba-command-name "kgdb -a -a -a -r :12345")
(setq gdb-many-windows t)
(xterm-mouse-mode 1)
M-x gdba
And for DDD (devel/ddd ):
# remote serial protocol
LANG=C ddd --debugger kgdb -r :12345 kernel
# live core debug
LANG=C ddd --debugger kgdb kernel /dev/fwmem0.2
10.8.2. Dcons with KVM
We can directly read the dcons(4) buer via /dev/mem for live systems, and in the core dump for crashed systems.
These give you similar output to dmesg -a , but the dcons(4) buer includes more information. Using Dcons with KVM
To use dcons(4) with KVM:
Dump a dcons(4) buer of a live system:
# dconschat -1
Dump a dcons(4) buer of a crash dump:
# dconschat -1 -M vmcore.XX
Live core debugging can be done via:
# fwcontrol -m target_eui64
# kgdb kernel /dev/fwmem0.2
10.9. Glossary of Kernel Options for Debugging
This section provides a brief glossary of compile-time kernel options used for debugging:
• options KDB : compiles in the kernel debugger framework. Required for options DDB and options GDB . Little
or no performance overhead. By default, the debugger will be entered on panic instead of an automatic reboot.
• options KDB_UNATTENDED: change the default value of the debug.debugger_on_panic sysctl to 0, which controls
whether the debugger is entered on panic. When options KDB is not compiled into the kernel, the behavior is
to automatically reboot on panic; when it is compiled into the kernel, the default behavior is to drop into the
debugger unless options KDB_UNATTENDED is compiled in. If you want to leave the kernel debugger compiled
into the kernel but want the system to come back up unless you're on-hand to use the debugger for diagnostics,
use this option.
• options KDB_TRACE: change the default value of the debug.trace_on_panic sysctl to 1, which controls whether
the debugger automatically prints a stack trace on panic. Especially if running with options KDB_UNATTENDED,
this can be helpful to gather basic debugging information on the serial or rewire console while still rebooting
to recover.
• options DDB : compile in support for the console debugger, DDB. This interactive debugger runs on whatever the
active low-level console of the system is, which includes the video console, serial console, or rewire console.
Glossary of Kernel Options for Debugging
It provides basic integrated debugging facilities, such as stack tracing, process and thread listing, dumping of
lock state, VM state, le system state, and kernel memory management. DDB does not require software running
on a second machine or being able to generate a core dump or full debugging kernel symbols, and provides
detailed diagnostics of the kernel at run-time. Many bugs can be fully diagnosed using only DDB output. This
option depends on options KDB .
• options GDB : compile in support for the remote debugger, GDB, which can operate over serial cable or rewire.
When the debugger is entered, GDB may be attached to inspect structure contents, generate stack traces, etc.
Some kernel state is more awkward to access than in DDB, which is able to generate useful summaries of kernel
state automatically, such as automatically walking lock debugging or kernel memory management structures,
and a second machine running the debugger is required. On the other hand, GDB combines information from
the kernel source and full debugging symbols, and is aware of full data structure definitions, local variables, and
is scriptable. This option is not required to run GDB on a kernel core dump. This option depends on options KDB .
• options BREAK_TO_DEBUGGER, options ALT_BREAK_TO_DEBUGGER: allow a break signal or alternative signal on the
console to enter the debugger. If the system hangs without a panic, this is a useful way to reach the debugger.
Due to the current kernel locking, a break signal generated on a serial console is significantly more reliable at
getting into the debugger, and is generally recommended. This option has little or no performance impact.
• options INVARIANTS: compile into the kernel a large number of run-time assertion checks and tests, which
constantly test the integrity of kernel data structures and the invariants of kernel algorithms. These tests can
be expensive, so are not compiled in by default, but help provide useful "fail stop" behavior, in which certain
classes of undesired behavior enter the debugger before kernel data corruption occurs, making them easier to
debug. Tests include memory scrubbing and use-after-free testing, which is one of the more significant sources
of overhead. This option depends on options INVARIANT_SUPPORT.
• options INVARIANT_SUPPORT: many of the tests present in options INVARIANTS require modified data structures
or additional kernel symbols to be defined.
• options WITNESS: this option enables run-time lock order tracking and verification, and is an invaluable tool
for deadlock diagnosis. WITNESS maintains a graph of acquired lock orders by lock type, and checks the graph
at each acquire for cycles (implicit or explicit). If a cycle is detected, a warning and stack trace are generated
to the console, indicating that a potential deadlock might have occurred. WITNESS is required in order to use
the show locks , show witness and show alllocks DDB commands. This debug option has significant performance overhead, which may be somewhat mitigated through the use of options WITNESS_SKIPSPIN. Detailed
documentation may be found in witness(4).
• options WITNESS_SKIPSPIN: disable run-time checking of spinlock lock order with WITNESS. As spin locks are
acquired most frequently in the scheduler, and scheduler events occur often, this option can significantly speed
up systems running with WITNESS. This option depends on options WITNESS.
• options WITNESS_KDB: change the default value of the debug.witness.kdb sysctl to 1, which causes WITNESS to
enter the debugger when a lock order violation is detected, rather than simply printing a warning. This option
depends on options WITNESS.
• options SOCKBUF_DEBUG: perform extensive run-time consistency checking on socket buers, which can be
useful for debugging both socket bugs and race conditions in protocols and device drivers that interact with
sockets. This option significantly impacts network performance, and may change the timing in device driver
• options DEBUG_VFS_LOCKS: track lock acquisition points for lockmgr/vnode locks, expanding the amount of
information displayed by show lockedvnods in DDB. This option has a measurable performance impact.
• options DEBUG_MEMGUARD: a replacement for the malloc(9) kernel memory allocator that uses the VM system to
detect reads or writes from allocated memory after free. Details may be found in memguard(9). This option has
a significant performance impact, but can be very helpful in debugging kernel memory corruption bugs.
• options DIAGNOSTIC: enable additional, more expensive diagnostic tests along the lines of options INVARIANTS.
Part IV. Architectures
Table of Contents
11. x86 Assembly Language Programming ......................................................................................
11.1. Synopsis ..................................................................................................................
11.2. The Tools ................................................................................................................
11.3. System Calls .............................................................................................................
11.4. Return Values ...........................................................................................................
11.5. Creating Portable Code ...............................................................................................
11.6. Our First Program .....................................................................................................
11.7. Writing UNIX® Filters ................................................................................................
11.8. Buered Input and Output ..........................................................................................
11.9. Command Line Arguments ..........................................................................................
11.10. UNIX® Environment ................................................................................................
11.11. Working with Files ...................................................................................................
11.12. One-Pointed Mind ....................................................................................................
11.13. Using the FPU .........................................................................................................
11.14. Caveats ..................................................................................................................
11.15. Acknowledgements ..................................................................................................
Chapter 11. x86 Assembly Language
This chapter was written by G. Adam Stanislav <[email protected]>.
11.1. Synopsis
Assembly language programming under UNIX® is highly undocumented. It is generally assumed that no one would
ever want to use it because various UNIX® systems run on different microprocessors, so everything should be
written in C for portability.
In reality, C portability is quite a myth. Even C programs need to be modified when ported from one UNIX® to
another, regardless of what processor each runs on. Typically, such a program is full of conditional statements
depending on the system it is compiled for.
Even if we believe that all of UNIX® software should be written in C, or some other high-level language, we still
need assembly language programmers: Who else would write the section of C library that accesses the kernel?
In this chapter I will attempt to show you how you can use assembly language writing UNIX® programs, specifically
under FreeBSD.
This chapter does not explain the basics of assembly language. There are enough resources about that (for a complete online course in assembly language, see Randall Hyde's Art of Assembly Language; or if you prefer a printed book, take a look at Je Duntemann's Assembly Language Step-by-Step (ISBN: 0471375233). However, once the
chapter is finished, any assembly language programmer will be able to write programs for FreeBSD quickly and
Copyright © 2000-2001 G. Adam Stanislav. All rights reserved.
11.2. The Tools
11.2.1. The Assembler
The most important tool for assembly language programming is the assembler, the software that converts assembly language code into machine language.
Two very different assemblers are available for FreeBSD. One is as(1), which uses the traditional UNIX® assembly
language syntax. It comes with the system.
The other is /usr/ports/devel/nasm. It uses the Intel syntax. Its main advantage is that it can assemble code for
many operating systems. It needs to be installed separately, but is completely free.
This chapter uses nasm syntax because most assembly language programmers coming to FreeBSD from other operating systems will nd it easier to understand. And, because, quite frankly, that is what I am used to.
11.2.2. The Linker
The output of the assembler, like that of any compiler, needs to be linked to form an executable le.
The standard ld(1) linker comes with FreeBSD. It works with the code assembled with either assembler.
System Calls
11.3. System Calls
11.3.1. Default Calling Convention
By default, the FreeBSD kernel uses the C calling convention. Further, although the kernel is accessed using int
80h , it is assumed the program will call a function that issues int 80h , rather than issuing int 80h directly.
This convention is very convenient, and quite superior to the Microsoft® convention used by MS-DOS®. Why?
Because the UNIX® convention allows any program written in any language to access the kernel.
An assembly language program can do that as well. For example, we could open a le:
int 80h ; Call kernel
push dword mode
push dword flags
push dword path
mov eax, 5
call kernel
add esp, byte 12
This is a very clean and portable way of coding. If you need to port the code to a UNIX® system which uses a
different interrupt, or a different way of passing parameters, all you need to change is the kernel procedure.
But assembly language programmers like to shave o cycles. The above example requires a call/ret combination.
We can eliminate it by pushing an extra dword:
push dword mode
push dword flags
push dword path
mov eax, 5
push eax ; Or any other dword
int 80h
add esp, byte 16
The 5 that we have placed in EAX identifies the kernel function, in this case open .
11.3.2. Alternate Calling Convention
FreeBSD is an extremely flexible system. It offers other ways of calling the kernel. For it to work, however, the
system must have Linux emulation installed.
Linux is a UNIX® like system. However, its kernel uses the same system-call convention of passing parameters in
registers MS-DOS® does. As with the UNIX® convention, the function number is placed in EAX . The parameters,
however, are not passed on the stack but in EBX, ECX, EDX, ESI, EDI, EBP :
mov eax, 5
mov ebx, path
mov ecx, flags
mov edx, mode
int 80h
This convention has a great disadvantage over the UNIX® way, at least as far as assembly language programming
is concerned: Every time you make a kernel call you must push the registers, then pop them later. This makes your
code bulkier and slower. Nevertheless, FreeBSD gives you a choice.
Chapter 11. x86 Assembly Language Programming
If you do choose the Linux convention, you must let the system know about it. After your program is assembled
and linked, you need to brand the executable:
% brandelf -t Linux filename
11.3.3. Which Convention Should You Use?
If you are coding specifically for FreeBSD, you should always use the UNIX® convention: It is faster, you can store
global variables in registers, you do not have to brand the executable, and you do not impose the installation of
the Linux emulation package on the target system.
If you want to create portable code that can also run on Linux, you will probably still want to give the FreeBSD
users as efficient a code as possible. I will show you how you can accomplish that after I have explained the basics.
11.3.4. Call Numbers
To tell the kernel which system service you are calling, place its number in EAX . Of course, you need to know what
the number is. The syscalls File
The numbers are listed in syscalls . locate syscalls nds this le in several different formats, all produced
automatically from syscalls.master.
You can nd the master le for the default UNIX® calling convention in /usr/src/sys/kern/syscalls.master .
If you need to use the other convention implemented in the Linux emulation mode, read /usr/src/sys/i386/
linux/syscalls.master .
Not only do FreeBSD and Linux use different calling conventions, they sometimes use different numbers for the same functions.
syscalls.master describes how the call is to be made:
0 STD NOHIDE { int nosys(void); } syscall nosys_args int
1 STD NOHIDE { void exit(int rval); } exit rexit_args void
2 STD POSIX { int fork(void); }
3 STD POSIX { ssize_t read(int fd, void *buf, size_t nbyte); }
4 STD POSIX { ssize_t write(int fd, const void *buf, size_t nbyte); }
5 STD POSIX { int open(char *path, int flags, int mode); }
6 STD POSIX { int close(int fd); }
It is the leftmost column that tells us the number to place in EAX .
The rightmost column tells us what parameters to push . They are push ed from right to left.
For example, to open a le, we need to push the mode rst, then flags , then the address at which the path is stored.
11.4. Return Values
A system call would not be useful most of the time if it did not return some kind of a value: The le descriptor of
an open le, the number of bytes read to a buer, the system time, etc.
Man Pages
Additionally, the system needs to inform us if an error occurs: A le does not exist, system resources are exhausted,
we passed an invalid parameter, etc.
11.4.1. Man Pages
The traditional place to look for information about various system calls under UNIX® systems are the manual
pages. FreeBSD describes its system calls in section 2, sometimes in section 3.
For example, open(2) says:
If successful, open() returns a non-negative integer, termed a le descriptor. It returns -1 on
failure, and sets errno to indicate the error.
The assembly language programmer new to UNIX® and FreeBSD will immediately ask the puzzling question:
Where is errno and how do I get to it?
The information presented in the manual pages applies to C programs. The assembly language programmer needs additional information.
11.4.2. Where Are the Return Values?
Unfortunately, it depends... For most system calls it is in EAX , but not for all. A good rule of thumb, when working
with a system call for the rst time, is to look for the return value in EAX . If it is not there, you need further research.
I am aware of one system call that returns the value in EDX : SYS_fork . All others I have
worked with use EAX . But I have not worked with them all yet.
If you cannot nd the answer here or anywhere else, study libc source code and see how it
interfaces with the kernel.
11.4.3. Where Is errno?
Actually, nowhere...
errno is part of the C language, not the UNIX® kernel. When accessing kernel services directly, the error code is
returned in EAX , the same register the proper return value generally ends up in.
This makes perfect sense. If there is no error, there is no error code. If there is an error, there is no return value.
One register can contain either.
11.4.4. Determining an Error Occurred
When using the standard FreeBSD calling convention, the carry flag is cleared upon success, set upon failure.
When using the Linux emulation mode, the signed value in EAX is non-negative upon success, and contains the
return value. In case of an error, the value is negative, i.e., -errno .
Chapter 11. x86 Assembly Language Programming
11.5. Creating Portable Code
Portability is generally not one of the strengths of assembly language. Yet, writing assembly language programs
for different platforms is possible, especially with nasm. I have written assembly language libraries that can be
assembled for such different operating systems as Windows® and FreeBSD.
It is all the more possible when you want your code to run on two platforms which, while different, are based on
similar architectures.
For example, FreeBSD is UNIX®, Linux is UNIX® like. I only mentioned three differences between them (from
an assembly language programmer's perspective): The calling convention, the function numbers, and the way of
returning values.
11.5.1. Dealing with Function Numbers
In many cases the function numbers are the same. However, even when they are not, the problem is easy to deal
with: Instead of using numbers in your code, use constants which you have declared differently depending on the
target architecture:
%ifdef LINUX
%define SYS_execve 11
%define SYS_execve 59
11.5.2. Dealing with Conventions
Both, the calling convention, and the return value (the errno problem) can be resolved with macros:
%ifdef LINUX
%macro system 0
call kernel
align 4
push ebx
push ecx
push edx
push esi
push edi
push ebp
ebx, [esp+32]
ecx, [esp+36]
edx, [esp+40]
esi, [esp+44]
ebp, [esp+48]
or eax, eax
js .errno
Dealing with Other Portability Issues
neg eax
%macro system 0
int 80h
11.5.3. Dealing with Other Portability Issues
The above solutions can handle most cases of writing code portable between FreeBSD and Linux. Nevertheless,
with some kernel services the differences are deeper.
In that case, you need to write two different handlers for those particular system calls, and use conditional assembly. Luckily, most of your code does something other than calling the kernel, so usually you will only need a few
such conditional sections in your code.
11.5.4. Using a Library
You can avoid portability issues in your main code altogether by writing a library of system calls. Create a separate
library for FreeBSD, a different one for Linux, and yet other libraries for more operating systems.
In your library, write a separate function (or procedure, if you prefer the traditional assembly language terminology) for each system call. Use the C calling convention of passing parameters. But still use EAX to pass the call
number in. In that case, your FreeBSD library can be very simple, as many seemingly different functions can be
just labels to the same code:
int 80h
Your Linux library will require more different functions. But even here you can group system calls using the same
number of parameters:
[etc... one-parameter functions]
push ebx
mov ebx, [esp+12]
int 80h
pop ebx
jmp sys.return
or eax, eax
js sys.err
neg eax
Chapter 11. x86 Assembly Language Programming
The library approach may seem inconvenient at rst because it requires you to produce a separate le your code
depends on. But it has many advantages: For one, you only need to write it once and can use it for all your programs.
You can even let other assembly language programmers use it, or perhaps use one written by someone else. But
perhaps the greatest advantage of the library is that your code can be ported to other systems, even by other
programmers, by simply writing a new library without any changes to your code.
If you do not like the idea of having a library, you can at least place all your system calls in a separate assembly
language le and link it with your main program. Here, again, all porters have to do is create a new object le to
link with your main program.
11.5.5. Using an Include File
If you are releasing your software as (or with) source code, you can use macros and place them in a separate le,
which you include in your code.
Porters of your software will simply write a new include le. No library or external object le is necessary, yet your
code is portable without any need to edit the code.
This is the approach we will use throughout this chapter. We will name our include le, and add to it whenever we deal with a new system call.
We can start our by declaring the standard le descriptors:
%define stdin 0
%define stdout 1
%define stderr 2
Next, we create a symbolic name for each system call:
%define SYS_nosys 0
%define SYS_exit 1
%define SYS_fork 2
%define SYS_read 3
%define SYS_write 4
; [etc...­]
We add a short, non-global procedure with a long name, so we do not accidentally reuse the name in our code:
section .text
align 4
int 80h
We create a macro which takes one argument, the syscall number:
%macro system 1
mov eax, %1
call access.the.bsd.kernel
Finally, we create macros for each syscall. These macros take no arguments.
Our First Program
%macro sys.exit 0
system SYS_exit
%macro sys.fork 0
system SYS_fork
%macro 0
system SYS_read
%macro sys.write 0
system SYS_write
; [etc...­]
Go ahead, enter it into your editor and save it as We will add more to it as we discuss more syscalls.
11.6. Our First Program
We are now ready for our rst program, the mandatory Hello, World!
%include ''
section .data
hello db 'Hello, World!', 0Ah
hbytes equ $-hello
section .text
global _start
push dword hbytes
push dword hello
push dword stdout
push dword 0
Here is what it does: Line 1 includes the defines, the macros, and the code from
Lines 3-5 are the data: Line 3 starts the data section/segment. Line 4 contains the string "Hello, World!" followed
by a new line (0Ah ). Line 5 creates a constant that contains the length of the string from line 4 in bytes.
Lines 7-16 contain the code. Note that FreeBSD uses the elf le format for its executables, which requires every
program to start at the point labeled _start (or, more precisely, the linker expects that). This label has to be global.
Lines 10-13 ask the system to write hbytes bytes of the hello string to stdout.
Lines 15-16 ask the system to end the program with the return value of 0. The SYS_exit syscall never returns, so
the code ends there.
If you have come to UNIX® from MS-DOS® assembly language background, you may be used
to writing directly to the video hardware. You will never have to worry about this in FreeBSD,
or any other flavor of UNIX®. As far as you are concerned, you are writing to a le known as
Chapter 11. x86 Assembly Language Programming
stdout. This can be the video screen, or a telnet terminal, or an actual le, or even the input
of another program. Which one it is, is for the system to figure out.
11.6.1. Assembling the Code
Type the code (except the line numbers) in an editor, and save it in a le named hello.asm. You need nasm to
assemble it. Installing nasm
If you do not have nasm, type:
% su
Password:your root password
# cd /usr/ports/devel/nasm
# make install
# exit
You may type make install clean instead of just make install if you do not want to keep nasm source code.
Either way, FreeBSD will automatically download nasm from the Internet, compile it, and install it on your system.
If your system is not FreeBSD, you need to get nasm from its home page. You can still use it
to assemble FreeBSD code.
Now you can assemble, link, and run the code:
% nasm -f elf hello.asm
% ld -s -o hello hello.o
% ./hello
Hello, World!
11.7. Writing UNIX® Filters
A common type of UNIX® application is a filter—a program that reads data from the stdin , processes it somehow,
then writes the result to stdout.
In this chapter, we shall develop a simple filter, and learn how to read from stdin and write to stdout. This filter
will convert each byte of its input into a hexadecimal number followed by a blank space.
%include ''
section .data
hex db '0123456789ABCDEF'
buffer db 0, 0, ' '
section .text
global _start
; read a byte from stdin
push dword 1
push dword buffer
push dword stdin
Writing UNIX® Filters
add esp, byte 12
or eax, eax
je .done
; convert it to hex
movzx eax, byte [buffer]
mov edx, eax
shr dl, 4
mov dl, [hex+edx]
mov [buffer], dl
and al, 0Fh
mov al, [hex+eax]
mov [buffer+1], al
; print it
push dword 3
push dword buffer
push dword stdout
add esp, byte 12
jmp short _start
push dword 0
In the data section we create an array called hex . It contains the 16 hexadecimal digits in ascending order. The
array is followed by a buer which we will use for both input and output. The rst two bytes of the buer are
initially set to 0. This is where we will write the two hexadecimal digits (the rst byte also is where we will read
the input). The third byte is a space.
The code section consists of four parts: Reading the byte, converting it to a hexadecimal number, writing the result,
and eventually exiting the program.
To read the byte, we ask the system to read one byte from stdin , and store it in the rst byte of the buffer . The
system returns the number of bytes read in EAX . This will be 1 while data is coming, or 0, when no more input data
is available. Therefore, we check the value of EAX . If it is 0, we jump to .done , otherwise we continue.
For simplicity sake, we are ignoring the possibility of an error condition at this time.
The hexadecimal conversion reads the byte from the buffer into EAX , or actually just AL, while clearing the remaining bits of EAX to zeros. We also copy the byte to EDX because we need to convert the upper four bits (nibble)
separately from the lower four bits. We store the result in the rst two bytes of the buer.
Next, we ask the system to write the three bytes of the buer, i.e., the two hexadecimal digits and the blank space,
to stdout. We then jump back to the beginning of the program and process the next byte.
Once there is no more input left, we ask the system to exit our program, returning a zero, which is the traditional
value meaning the program was successful.
Go ahead, and save the code in a le named hex.asm , then type the following (the ^D means press the control key
and type D while holding the control key down):
% nasm -f elf hex.asm
% ld -s -o hex hex.o
% ./hex
Chapter 11. x86 Assembly Language Programming
Hello, World!
48 65 6C 6C 6F 2C 20 57 6F 72 6C 64 21 0A Here I come!
48 65 72 65 20 49 20 63 6F 6D 65 21 0A ^D %
If you are migrating to UNIX® from MS-DOS®, you may be wondering why each line ends
with 0A instead of 0D 0A . This is because UNIX® does not use the cr/lf convention, but a
"new line" convention, which is 0A in hexadecimal.
Can we improve this? Well, for one, it is a bit confusing because once we have converted a line of text, our input
no longer starts at the beginning of the line. We can modify it to print a new line instead of a space after each 0A:
%include ''
section .data
hex db '0123456789ABCDEF'
buffer db 0, 0, ' '
section .text
global _start
mov cl, ' '
; read a byte from stdin
push dword 1
push dword buffer
push dword stdin
add esp, byte 12
or eax, eax
je .done
; convert it to hex
movzx eax, byte [buffer]
mov [buffer+2], cl
cmp al, 0Ah
jne .hex
mov [buffer+2], al
mov edx, eax
shr dl, 4
mov dl, [hex+edx]
mov [buffer], dl
and al, 0Fh
mov al, [hex+eax]
mov [buffer+1], al
; print it
push dword 3
push dword buffer
push dword stdout
add esp, byte 12
jmp short .loop
push dword 0
Buered Input and Output
We have stored the space in the CL register. We can do this safely because, unlike Microsoft® Windows®, UNIX®
system calls do not modify the value of any register they do not use to return a value in.
That means we only need to set CL once. We have, therefore, added a new label .loop and jump to it for the next
byte instead of jumping at _start . We have also added the .hex label so we can either have a blank space or a new
line as the third byte of the buffer .
Once you have changed hex.asm to reflect these changes, type:
% nasm -f elf hex.asm
% ld -s -o hex hex.o
% ./hex
Hello, World!
48 65 6C 6C 6F 2C 20 57 6F 72 6C 64 21 0A
Here I come!
48 65 72 65 20 49 20 63 6F 6D 65 21 0A
^D %
That looks better. But this code is quite inefficient! We are making a system call for every single byte twice (once
to read it, another time to write the output).
11.8. Buffered Input and Output
We can improve the efficiency of our code by buffering our input and output. We create an input buer and read
a whole sequence of bytes at one time. Then we fetch them one by one from the buer.
We also create an output buer. We store our output in it until it is full. At that time we ask the kernel to write
the contents of the buer to stdout.
The program ends when there is no more input. But we still need to ask the kernel to write the contents of our
output buer to stdout one last time, otherwise some of our output would make it to the output buer, but never
be sent out. Do not forget that, or you will be wondering why some of your output is missing.
%include ''
%define BUFSIZE 2048
section .data
hex db '0123456789ABCDEF'
section .bss
ibuffer resb BUFSIZE
obuffer resb BUFSIZE
section .text
global _start
sub eax, eax
sub ebx, ebx
sub ecx, ecx
mov edi, obuffer
; read a byte from stdin
call getchar
; convert it to hex
mov dl, al
shr al, 4
mov al, [hex+eax]
call putchar
Chapter 11. x86 Assembly Language Programming
mov al, dl
and al, 0Fh
mov al, [hex+eax]
call putchar
al, ' '
dl, 0Ah
al, dl
call putchar
jmp short .loop
align 4
or ebx, ebx
jne .fetch
call read
dec ebx
push dword BUFSIZE
mov esi, ibuffer
push esi
push dword stdin
add esp, byte 12
mov ebx, eax
or eax, eax
je .done
sub eax, eax
align 4
call write ; flush output buffer
push dword 0
align 4
inc ecx
cmp ecx, BUFSIZE
je write
align 4
sub edi, ecx ; start of buffer
push ecx
push edi
push dword stdout
add esp, byte 12
sub eax, eax
sub ecx, ecx ; buffer is empty now
Buered Input and Output
We now have a third section in the source code, named .bss . This section is not included in our executable le, and,
therefore, cannot be initialized. We use resb instead of db. It simply reserves the requested size of uninitialized
memory for our use.
We take advantage of the fact that the system does not modify the registers: We use registers for what, otherwise,
would have to be global variables stored in the .data section. This is also why the UNIX® convention of passing
parameters to system calls on the stack is superior to the Microsoft convention of passing them in the registers:
We can keep the registers for our own use.
We use EDI and ESI as pointers to the next byte to be read from or written to. We use EBX and ECX to keep count
of the number of bytes in the two buers, so we know when to dump the output to, or read more input from, the
Let us see how it works now:
% nasm -f elf hex.asm
% ld -s -o hex hex.o
% ./hex
Hello, World!
Here I come!
48 65 6C 6C 6F 2C 20 57 6F 72 6C 64 21 0A
48 65 72 65 20 49 20 63 6F 6D 65 21 0A
^D %
Not what you expected? The program did not print the output until we pressed ^D. That is easy to x by inserting
three lines of code to write the output every time we have converted a new line to 0A. I have marked the three
lines with > (do not copy the > in your hex.asm).
%include ''
%define BUFSIZE 2048
section .data
hex db '0123456789ABCDEF'
section .bss
ibuffer resb BUFSIZE
obuffer resb BUFSIZE
section .text
global _start
sub eax, eax
sub ebx, ebx
sub ecx, ecx
mov edi, obuffer
; read a byte from stdin
call getchar
; convert it to hex
mov dl, al
shr al, 4
mov al, [hex+eax]
call putchar
mov al, dl
and al, 0Fh
mov al, [hex+eax]
call putchar
mov al, ' '
Chapter 11. x86 Assembly Language Programming
cmp dl, 0Ah
jne .put
mov al, dl
call putchar
> cmp al, 0Ah
> jne .loop
> call write
jmp short .loop
align 4
or ebx, ebx
jne .fetch
call read
dec ebx
push dword BUFSIZE
mov esi, ibuffer
push esi
push dword stdin
add esp, byte 12
mov ebx, eax
or eax, eax
je .done
sub eax, eax
align 4
call write ; flush output buffer
push dword 0
align 4
inc ecx
cmp ecx, BUFSIZE
je write
align 4
sub edi, ecx ; start of buffer
push ecx
push edi
push dword stdout
add esp, byte 12
sub eax, eax
sub ecx, ecx ; buffer is empty now
Now, let us see how it works:
% nasm -f elf hex.asm
% ld -s -o hex hex.o
% ./hex
How to Unread a Character
Hello, World!
48 65 6C 6C 6F 2C 20 57 6F 72 6C 64 21 0A
Here I come!
48 65 72 65 20 49 20 63 6F 6D 65 21 0A
^D %
Not bad for a 644-byte executable, is it!
This approach to buered input/output still contains a hidden danger. I will discuss—and x
—it later, when I talk about the dark side of buffering.
11.8.1. How to Unread a Character
This may be a somewhat advanced topic, mostly of interest to programmers familiar with the
theory of compilers. If you wish, you may skip to the next section, and perhaps read this later.
While our sample program does not require it, more sophisticated filters often need to look ahead. In other words,
they may need to see what the next character is (or even several characters). If the next character is of a certain
value, it is part of the token currently being processed. Otherwise, it is not.
For example, you may be parsing the input stream for a textual string (e.g., when implementing a language compiler): If a character is followed by another character, or perhaps a digit, it is part of the token you are processing.
If it is followed by white space, or some other value, then it is not part of the current token.
This presents an interesting problem: How to return the next character back to the input stream, so it can be read
again later?
One possible solution is to store it in a character variable, then set a ag. We can modify getchar to check the
ag, and if it is set, fetch the byte from that variable instead of the input buer, and reset the ag. But, of course,
that slows us down.
The C language has an ungetc() function, just for that purpose. Is there a quick way to implement it in our code?
I would like you to scroll back up and take a look at the getchar procedure and see if you can nd a nice and fast
solution before reading the next paragraph. Then come back here and see my own solution.
The key to returning a character back to the stream is in how we are getting the characters to start with:
First we check if the buer is empty by testing the value of EBX . If it is zero, we call the read procedure.
If we do have a character available, we use lodsb , then decrease the value of EBX . The lodsb instruction is effectively identical to:
mov al, [esi]
inc esi
The byte we have fetched remains in the buer until the next time read is called. We do not know when that
happens, but we do know it will not happen until the next call to getchar . Hence, to "return" the last-read byte
back to the stream, all we have to do is decrease the value of ESI and increase the value of EBX :
Chapter 11. x86 Assembly Language Programming
dec esi
inc ebx
But, be careful! We are perfectly safe doing this if our look-ahead is at most one character at a time. If we are
examining more than one upcoming character and call ungetc several times in a row, it will work most of the time,
but not all the time (and will be tough to debug). Why?
Because as long as getchar does not have to call read , all of the pre-read bytes are still in the buer, and our
ungetc works without a glitch. But the moment getchar calls read , the contents of the buer change.
We can always rely on ungetc working properly on the last character we have read with getchar , but not on
anything we have read before that.
If your program reads more than one byte ahead, you have at least two choices:
If possible, modify the program so it only reads one byte ahead. This is the simplest solution.
If that option is not available, rst of all determine the maximum number of characters your program needs to
return to the input stream at one time. Increase that number slightly, just to be sure, preferably to a multiple of
16—so it aligns nicely. Then modify the .bss section of your code, and create a small "spare" buer right before
your input buer, something like this:
section .bss
resb 16 ; or whatever the value you came up with
ibuffer resb BUFSIZE
obuffer resb BUFSIZE
You also need to modify your ungetc to pass the value of the byte to unget in AL:
dec esi
inc ebx
mov [esi], al
With this modification, you can call ungetc up to 17 times in a row safely (the rst call will still be within the buer,
the remaining 16 may be either within the buer or within the "spare").
11.9. Command Line Arguments
Our hex program will be more useful if it can read the names of an input and output le from its command line,
i.e., if it can process the command line arguments. But... Where are they?
Before a UNIX® system starts a program, it pushes some data on the stack, then jumps at the _start label of the
program. Yes, I said jumps, not calls. That means the data can be accessed by reading [esp+offset] , or by simply
popping it.
The value at the top of the stack contains the number of command line arguments. It is traditionally called argc ,
for "argument count."
Command line arguments follow next, all argc of them. These are typically referred to as argv , for "argument
value(s)." That is, we get argv[0] , argv[1] , ... , argv[argc-1] . These are not the actual arguments, but pointers
to arguments, i.e., memory addresses of the actual arguments. The arguments themselves are NUL-terminated
character strings.
The argv list is followed by a NULL pointer, which is simply a 0. There is more, but this is enough for our purposes
right now.
Command Line Arguments
If you have come from the MS-DOS® programming environment, the main difference is that
each argument is in a separate string. The second difference is that there is no practical limit
on how many arguments there can be.
Armed with this knowledge, we are almost ready for the next version of hex.asm . First, however, we need to add
a few lines to
First, we need to add two new entries to our list of system call numbers:
%define SYS_open 5
%define SYS_close 6
Then we add two new macros at the end of the le:
%macro 0
system SYS_open
%macro sys.close 0
system SYS_close
Here, then, is our modified source code:
%include ''
%define BUFSIZE 2048
section .data dd stdin
fd.out dd stdout
hex db '0123456789ABCDEF'
section .bss
ibuffer resb BUFSIZE
obuffer resb BUFSIZE
section .text
align 4
push dword 1
; return failure
align 4
global _start
add esp, byte 8 ; discard argc and argv[0]
pop ecx
jecxz .init
; no more arguments
; ECX contains the path to input file
push dword 0 ; O_RDONLY
push ecx
jc err ; open failed
Chapter 11. x86 Assembly Language Programming
add esp, byte 8
mov [], eax
pop ecx
jecxz .init
; no more arguments
; ECX contains the path to output file
push dword 420 ; file mode (644 octal)
push dword 0200h | 0400h | 01h
push ecx
jc err
add esp, byte 12
mov [fd.out], eax
sub eax, eax
sub ebx, ebx
sub ecx, ecx
mov edi, obuffer
; read a byte from input file or stdin
call getchar
; convert it to hex
mov dl, al
shr al, 4
mov al, [hex+eax]
call putchar
mov al, dl
and al, 0Fh
mov al, [hex+eax]
call putchar
al, ' '
dl, 0Ah
al, dl
call putchar
cmp al, dl
jne .loop
call write
jmp short .loop
align 4
or ebx, ebx
jne .fetch
call read
dec ebx
push dword BUFSIZE
mov esi, ibuffer
Command Line Arguments
push esi
push dword []
add esp, byte 12
mov ebx, eax
or eax, eax
je .done
sub eax, eax
align 4
call write
; flush output buffer
; close files
push dword []
push dword [fd.out]
; return success
push dword 0
align 4
inc ecx
cmp ecx, BUFSIZE
je write
align 4
sub edi, ecx ; start of buffer
push ecx
push edi
push dword [fd.out]
add esp, byte 12
sub eax, eax
sub ecx, ecx ; buffer is empty now
In our .data section we now have two new variables, and fd.out . We store the input and output le descriptors here.
In the .text section we have replaced the references to stdin and stdout with [] and [fd.out] .
The .text section now starts with a simple error handler, which does nothing but exit the program with a return
value of 1. The error handler is before _start so we are within a short distance from where the errors occur.
Naturally, the program execution still begins at _start . First, we remove argc and argv[0] from the stack: They
are of no interest to us (in this program, that is).
We pop argv[1] to ECX . This register is particularly suited for pointers, as we can handle NULL pointers with
jecxz . If argv[1] is not NULL, we try to open the le named in the rst argument. Otherwise, we continue the
program as before: Reading from stdin , writing to stdout. If we fail to open the input le (e.g., it does not exist),
we jump to the error handler and quit.
If all went well, we now check for the second argument. If it is there, we open the output le. Otherwise, we send
the output to stdout. If we fail to open the output le (e.g., it exists and we do not have the write permission), we,
again, jump to the error handler.
Chapter 11. x86 Assembly Language Programming
The rest of the code is the same as before, except we close the input and output les before exiting, and, as mentioned, we use [] and [fd.out] .
Our executable is now a whopping 768 bytes long.
Can we still improve it? Of course! Every program can be improved. Here are a few ideas of what we could do:
• Have our error handler print a message to stderr .
• Add error handlers to the read and write functions.
• Close stdin when we open an input le, stdout when we open an output le.
• Add command line switches, such as -i and -o, so we can list the input and output les in any order, or perhaps
read from stdin and write to a le.
• Print a usage message if command line arguments are incorrect.
I shall leave these enhancements as an exercise to the reader: You already know everything you need to know to
implement them.
11.10. UNIX® Environment
An important UNIX® concept is the environment, which is defined by environment variables. Some are set by the
system, others by you, yet others by the shell, or any program that loads another program.
11.10.1. How to Find Environment Variables
I said earlier that when a program starts executing, the stack contains argc followed by the NULL-terminated
argv array, followed by something else. The "something else" is the environment, or, to be more precise, a NULLterminated array of pointers to environment variables. This is often referred to as env .
The structure of env is the same as that of argv , a list of memory addresses followed by a NULL (0). In this case,
there is no "envc"—we figure out where the array ends by searching for the final NULL.
The variables usually come in the name=value format, but sometimes the =value part may be missing. We need
to account for that possibility.
11.10.2. webvars
I could just show you some code that prints the environment the same way the UNIX® env command does. But I
thought it would be more interesting to write a simple assembly language CGI utility. CGI: A Quick Overview
I have a detailed CGI tutorial on my web site, but here is a very quick overview of CGI:
• The web server communicates with the CGI program by setting environment variables.
• The CGI program sends its output to stdout. The web server reads it from there.
• It must start with an HTTP header followed by two blank lines.
• It then prints the HTML code, or whatever other type of data it is producing.
While certain environment variables use standard names, others vary, depending on the web
server. That makes webvars quite a useful diagnostic tool. The Code
Our webvars program, then, must send out the HTTP header followed by some HTML mark-up. It then must read
the environment variables one by one and send them out as part of the HTML page.
The code follows. I placed comments and explanations right inside the code:
;;;;;;; webvars.asm ­;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; Copyright (c) 2000 G. Adam Stanislav
; All rights reserved.
; Redistribution and use in source and binary forms, with or without
; modification, are permitted provided that the following conditions
; are met:
; 1. Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
; 2. Redistributions in binary form must reproduce the above copyright
notice, this list of conditions and the following disclaimer in the
documentation and/or other materials provided with the distribution.
; Version 1.0
; Started: 8-Dec-2000
; Updated: 8-Dec-2000
%include ''
section .data
http db 'Content-type: text/html', 0Ah, 0Ah
db '<?xml version="1.0" encoding="utf-8"?>', 0Ah
db '<!DOCTYPE html PUBLIC "-//W3C/DTD XHTML Strict//EN" '
db '"DTD/xhtml1-strict.dtd">', 0Ah
db '<html xmlns="" '
db 'xml.lang="en" lang="en">', 0Ah
db '<head>', 0Ah
db '<title>Web Environment</title>', 0Ah
db '<meta name="author" content="G. Adam Stanislav" />', 0Ah
db '</head>', 0Ah, 0Ah
db '<body bgcolor="#ffffff" text="#000000" link="#0000ff" '
db 'vlink="#840084" alink="#0000ff">', 0Ah
Chapter 11. x86 Assembly Language Programming
db '<div class="webvars">', 0Ah
db '<h1>Web Environment</h1>', 0Ah
db '<p>The following <b>environment variables</b> are defined '
db 'on this web server:</p>', 0Ah, 0Ah
db '<table align="center" width="80" border="0" cellpadding="10" '
db 'cellspacing="0" class="webvars">', 0Ah
httplen equ $-http
left db '<tr>', 0Ah
db '<td class="name"><tt>'
leftlen equ $-left
middle db '</tt></td>', 0Ah
db '<td class="value"><tt><b>'
midlen equ $-middle
undef db '<i>(undefined)</i>'
undeflen equ $-undef
right db '</b></tt></td>', 0Ah
db '</tr>', 0Ah
rightlen equ $-right
wrap db '</table>', 0Ah
db '</div>', 0Ah
db '</body>', 0Ah
db '</html>', 0Ah, 0Ah
wraplen equ $-wrap
section .text
global _start
; First, send out all the http and xhtml stuff that is
; needed before we start showing the environment
push dword httplen
push dword http
push dword stdout
; Now find how far on the stack the environment pointers
; are. We have 12 bytes we have pushed before "argc"
mov eax, [esp+12]
; We need to remove the following from the stack:
; The 12 bytes we pushed for sys.write
; The 4 bytes of argc
; The EAX*4 bytes of argv
; The 4 bytes of the NULL after argv
; Total:
; 20 + eax * 4
; Because stack grows down, we need to ADD that many bytes
; to ESP.
lea esp, [esp+20+eax*4]
cld ; This should already be the case, but let's be sure.
; Loop through the environment, printing it out
pop edi
or edi, edi ; Done yet?
je near .wrap
; Print the left part of HTML
push dword leftlen
push dword left
push dword stdout
; It may be tempting to search for the '=' in the env string next.
; But it is possible there is no '=', so we search for the
; terminating NUL first.
mov esi, edi ; Save start of string
sub ecx, ecx
not ecx ; ECX = FFFFFFFF
sub eax, eax
repne scasb
not ecx ; ECX = string length + 1
mov ebx, ecx ; Save it in EBX
; Now is the time to find '='
mov edi, esi ; Start of string
mov al, '='
repne scasb
not ecx
add ecx, ebx ; Length of name
push ecx
push esi
push dword stdout
; Print the middle part of HTML table code
push dword midlen
push dword middle
push dword stdout
; Find the length of the value
not ecx
lea ebx, [ebx+ecx-1]
; Print "undefined" if 0
or ebx, ebx
jne .value
mov ebx, undeflen
mov edi, undef
push ebx
push edi
push dword stdout
; Print the right part of the table row
push dword rightlen
push dword right
push dword stdout
; Get rid of the 60 bytes we have pushed
add esp, byte 60
; Get the next variable
jmp .loop
; Print the rest of HTML
push dword wraplen
push dword wrap
push dword stdout
; Return success
push dword 0
Chapter 11. x86 Assembly Language Programming
This code produces a 1,396-byte executable. Most of it is data, i.e., the HTML mark-up we need to send out.
Assemble and link it as usual:
% nasm -f elf webvars.asm
% ld -s -o webvars webvars.o
To use it, you need to upload webvars to your web server. Depending on how your web server is set up, you may
have to store it in a special cgi-bin directory, or perhaps rename it with a .cgi extension.
Then you need to use your browser to view its output. To see its output on my web server, please go to http:// . If curious about the additional environment variables present in a password protected
web directory, go to , using the name asm and password programmer.
11.11. Working with Files
We have already done some basic le work: We know how to open and close them, how to read and write them
using buers. But UNIX® offers much more functionality when it comes to les. We will examine some of it in this
section, and end up with a nice le conversion utility.
Indeed, let us start at the end, that is, with the le conversion utility. It always makes programming easier when
we know from the start what the end product is supposed to do.
One of the rst programs I wrote for UNIX® was tuc, a text-to-UNIX® le converter. It converts a text le from
other operating systems to a UNIX® text le. In other words, it changes from different kind of line endings to the
newline convention of UNIX®. It saves the output in a different le. Optionally, it converts a UNIX® text le to
a DOS text le.
I have used tuc extensively, but always only to convert from some other OS to UNIX®, never the other way. I have
always wished it would just overwrite the le instead of me having to send the output to a different le. Most of
the time, I end up using it like this:
% tuc myfile tempfile
% mv tempfile myfile
It would be nice to have a ftuc, i.e., fast tuc, and use it like this:
% ftuc myfile
In this chapter, then, we will write ftuc in assembly language (the original tuc is in C), and study various leoriented kernel services in the process.
At rst sight, such a le conversion is very simple: All you have to do is strip the carriage returns, right?
If you answered yes, think again: That approach will work most of the time (at least with MS DOS text les), but
will fail occasionally.
The problem is that not all non UNIX® text les end their line with the carriage return / line feed sequence. Some
use carriage returns without line feeds. Others combine several blank lines into a single carriage return followed
by several line feeds. And so on.
A text le converter, then, must be able to handle any possible line endings:
• carriage return / line feed
• carriage return
• line feed / carriage return
Finite State Machine
• line feed
It should also handle les that use some kind of a combination of the above (e.g., carriage return followed by several
line feeds).
11.11.1. Finite State Machine
The problem is easily solved by the use of a technique called finite state machine, originally developed by the designers of digital electronic circuits. A finite state machine is a digital circuit whose output is dependent not only
on its input but on its previous input, i.e., on its state. The microprocessor is an example of a finite state machine:
Our assembly language code is assembled to machine language in which some assembly language code produces
a single byte of machine language, while others produce several bytes. As the microprocessor fetches the bytes
from the memory one by one, some of them simply change its state rather than produce some output. When all the
bytes of the op code are fetched, the microprocessor produces some output, or changes the value of a register, etc.
Because of that, all software is essentially a sequence of state instructions for the microprocessor. Nevertheless,
the concept of finite state machine is useful in software design as well.
Our text le converter can be designed as a finite state machine with three possible states. We could call them states
0-2, but it will make our life easier if we give them symbolic names:
• ordinary
• cr
• lf
Our program will start in the ordinary state. During this state, the program action depends on its input as follows:
• If the input is anything other than a carriage return or line feed, the input is simply passed on to the output.
The state remains unchanged.
• If the input is a carriage return, the state is changed to cr. The input is then discarded, i.e., no output is made.
• If the input is a line feed, the state is changed to lf. The input is then discarded.
Whenever we are in the cr state, it is because the last input was a carriage return, which was unprocessed. What
our software does in this state again depends on the current input:
• If the input is anything other than a carriage return or line feed, output a line feed, then output the input, then
change the state to ordinary.
• If the input is a carriage return, we have received two (or more) carriage returns in a row. We discard the input,
we output a line feed, and leave the state unchanged.
• If the input is a line feed, we output the line feed and change the state to ordinary. Note that this is not the same
as the rst case above – if we tried to combine them, we would be outputting two line feeds instead of one.
Finally, we are in the lf state after we have received a line feed that was not preceded by a carriage return. This
will happen when our le already is in UNIX® format, or whenever several lines in a row are expressed by a single
carriage return followed by several line feeds, or when line ends with a line feed / carriage return sequence. Here
is how we need to handle our input in this state:
• If the input is anything other than a carriage return or line feed, we output a line feed, then output the input,
then change the state to ordinary. This is exactly the same action as in the cr state upon receiving the same
kind of input.
• If the input is a carriage return, we discard the input, we output a line feed, then change the state to ordinary.
• If the input is a line feed, we output the line feed, and leave the state unchanged.
Chapter 11. x86 Assembly Language Programming The Final State
The above finite state machine works for the entire le, but leaves the possibility that the final line end will be
ignored. That will happen whenever the le ends with a single carriage return or a single line feed. I did not think
of it when I wrote tuc, just to discover that occasionally it strips the last line ending.
This problem is easily xed by checking the state after the entire le was processed. If the state is not ordinary,
we simply need to output one last line feed.
Now that we have expressed our algorithm as a finite state machine, we could easily design a
dedicated digital electronic circuit (a "chip") to do the conversion for us. Of course, doing so
would be considerably more expensive than writing an assembly language program. The Output Counter
Because our le conversion program may be combining two characters into one, we need to use an output counter.
We initialize it to 0, and increase it every time we send a character to the output. At the end of the program, the
counter will tell us what size we need to set the le to.
11.11.2. Implementing FSM in Software
The hardest part of working with a finite state machine is analyzing the problem and expressing it as a finite state
machine. That accomplished, the software almost writes itself.
In a high-level language, such as C, there are several main approaches. One is to use a switch statement which
chooses what function should be run. For example,
switch (state) {
case CR:
case LF:
Another approach is by using an array of function pointers, something like this:
Yet another is to have state be a function pointer, set to point at the appropriate function:
This is the approach we will use in our program because it is very easy to do in assembly language, and very fast,
too. We will simply keep the address of the right procedure in EBX , and then just issue:
call ebx
Memory Mapped Files
This is possibly faster than hardcoding the address in the code because the microprocessor does not have to fetch
the address from the memory—it is already stored in one of its registers. I said possibly because with the caching
modern microprocessors do, either way may be equally fast.
11.11.3. Memory Mapped Files
Because our program works on a single le, we cannot use the approach that worked for us before, i.e., to read
from an input le and to write to an output le.
UNIX® allows us to map a le, or a section of a le, into memory. To do that, we rst need to open the le with the
appropriate read/write ags. Then we use the mmap system call to map it into the memory. One nice thing about
mmap is that it automatically works with virtual memory: We can map more of the le into the memory than we
have physical memory available, yet still access it through regular memory op codes, such as mov , lods , and stos .
Whatever changes we make to the memory image of the le will be written to the le by the system. We do not
even have to keep the le open: As long as it stays mapped, we can read from it and write to it.
The 32-bit Intel microprocessors can access up to four gigabytes of memory – physical or virtual. The FreeBSD
system allows us to use up to a half of it for le mapping.
For simplicity sake, in this tutorial we will only convert les that can be mapped into the memory in their entirety.
There are probably not too many text les that exceed two gigabytes in size. If our program encounters one, it will
simply display a message suggesting we use the original tuc instead.
If you examine your copy of syscalls.master, you will nd two separate syscalls named mmap . This is because of
evolution of UNIX®: There was the traditional BSD mmap , syscall 71. That one was superseded by the POSIX® mmap ,
syscall 197. The FreeBSD system supports both because older programs were written by using the original BSD
version. But new software uses the POSIX® version, which is what we will use.
The syscalls.master le lists the POSIX® version like this:
197 STD BSD { caddr_t mmap(caddr_t addr, size_t len, int prot, \
int flags, int fd, long pad, off_t pos); }
This differs slightly from what mmap(2) says. That is because mmap(2) describes the C version.
The difference is in the long pad argument, which is not present in the C version. However, the FreeBSD syscalls
add a 32-bit pad after pushing a 64-bit argument. In this case, off_t is a 64-bit value.
When we are finished working with a memory-mapped le, we unmap it with the munmap syscall:
For an in-depth treatment of mmap , see W. Richard Stevens' Unix Network Programming, Volume 2, Chapter 12.
11.11.4. Determining File Size
Because we need to tell mmap how many bytes of the le to map into the memory, and because we want to map the
entire le, we need to determine the size of the le.
We can use the fstat syscall to get all the information about an open le that the system can give us. That includes
the le size.
Again, syscalls.master lists two versions of fstat, a traditional one (syscall 62), and a POSIX® one (syscall 189).
Naturally, we will use the POSIX® version:
189 STD POSIX { int fstat(int fd, struct stat *sb); }
Chapter 11. x86 Assembly Language Programming
This is a very straightforward call: We pass to it the address of a stat structure and the descriptor of an open le.
It will ll out the contents of the stat structure.
I do, however, have to say that I tried to declare the stat structure in the .bss section, and fstat did not like it:
It set the carry ag indicating an error. After I changed the code to allocate the structure on the stack, everything
was working ne.
11.11.5. Changing the File Size
Because our program may combine carriage return / line feed sequences into straight line feeds, our output may
be smaller than our input. However, since we are placing our output into the same le we read the input from, we
may have to change the size of the le.
The ftruncate system call allows us to do just that. Despite its somewhat misleading name, the ftruncate system
call can be used to both truncate the le (make it smaller) and to grow it.
And yes, we will nd two versions of ftruncate in syscalls.master, an older one (130), and a newer one (201).
We will use the newer one:
201 STD BSD { int ftruncate(int fd, int pad, off_t length); }
Please note that this one contains a int pad again.
11.11.6. ftuc
We now know everything we need to write ftuc. We start by adding some new lines in First, we define
some constants and structures, somewhere at or near the beginning of the le:
;;;;;;; open flags
%define O_RDONLY 0
%define O_WRONLY 1
%define O_RDWR 2
;;;;;;; mmap flags
%define PROT_NONE 0
%define PROT_READ 1
%define PROT_WRITE 2
%define PROT_EXEC 4
%define MAP_SHARED 0001h
%define MAP_PRIVATE 0002h
;;;;;;; stat structure
struc stat
st_dev resd 1 ; = 0
st_ino resd 1 ; = 4
st_mode resw 1 ; = 8, size is 16 bits
st_nlink resw 1 ; = 10, ditto
st_uid resd 1 ; = 12
st_gid resd 1 ; = 16
st_rdev resd 1 ; = 20
st_atime resd 1 ; = 24
st_atimensec resd 1 ; = 28
st_mtime resd 1 ; = 32
st_mtimensec resd 1 ; = 36
st_ctime resd 1 ; = 40
st_ctimensec resd 1 ; = 44
st_size resd 2 ; = 48, size is 64 bits
st_blocks resd 2 ; = 56, ditto
st_blksize resd 1 ; = 64
st_flags resd 1 ; = 68
st_gen resd 1 ; = 72
st_lspare resd 1 ; = 76
st_qspare resd 4 ; = 80
We define the new syscalls:
SYS_mmap 197
SYS_munmap 73
SYS_fstat 189
SYS_ftruncate 201
We add the macros for their use:
%macro sys.mmap 0
system SYS_mmap
%macro sys.munmap 0
system SYS_munmap
%macro sys.ftruncate 0
system SYS_ftruncate
%macro sys.fstat 0
system SYS_fstat
And here is our code:
;;;;;;; Fast Text-to-Unix Conversion (ftuc.asm) ­;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Started: 21-Dec-2000
;; Updated: 22-Dec-2000
;; Copyright 2000 G. Adam Stanislav.
;; All rights reserved.
;;;;;;; v.1 ­;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
%include ''
section .data
db 'Copyright 2000 G. Adam Stanislav.', 0Ah
db 'All rights reserved.', 0Ah
usg db 'Usage: ftuc filename', 0Ah
usglen equ $-usg
co db "ftuc: Can't open file.", 0Ah
colen equ $-co
fae db 'ftuc: File access error.', 0Ah
faelen equ $-fae
ftl db 'ftuc: File too long, use regular tuc instead.', 0Ah
ftllen equ $-ftl
mae db 'ftuc: Memory allocation error.', 0Ah
maelen equ $-mae
section .text
align 4
push dword maelen
push dword mae
jmp short error
Chapter 11. x86 Assembly Language Programming
align 4
push dword ftllen
push dword ftl
jmp short error
align 4
push dword faelen
push dword fae
jmp short error
align 4
push dword colen
push dword co
jmp short error
align 4
push dword usglen
push dword usg
push dword stderr
push dword 1
align 4
global _start
pop eax ; argc
pop eax ; program name
pop ecx ; file to convert
jecxz usage
pop eax
or eax, eax ; Too many arguments?
jne usage
; Open the file
push dword O_RDWR
push ecx
jc cantopen
mov ebp, eax ; Save fd
sub esp, byte stat_size
mov ebx, esp
; Find file size
push ebx
push ebp ; fd
jc facerr
mov edx, [ebx + st_size + 4]
; File is too long if EDX != 0 ...
or edx, edx
jne near toolong
mov ecx, [ebx + st_size]
; ... or if it is above 2 GB
or ecx, ecx
js near toolong
; Do nothing if the file is 0 bytes in size
jecxz .quit
; Map the entire file in memory
push edx
push edx ; starting at offset 0
push edx ; pad
push ebp ; fd
push dword MAP_SHARED
push ecx ; entire file size
push edx ; let system decide on the address
jc near memerr
mov edi, eax
mov esi, eax
push ecx ; for SYS_munmap
push edi
; Use EBX for state machine
mov ebx, ordinary
mov ah, 0Ah
call ebx
loop .loop
cmp ebx, ordinary
je .filesize
; Output final lf
mov al, ah
inc edx
; truncate file to new size
push dword 0 ; high dword
push edx ; low dword
push eax ; pad
push ebp
; close it (ebp still pushed)
add esp, byte 16
push dword 0
align 4
cmp al, 0Dh
je .cr
cmp al, ah
je .lf
Chapter 11. x86 Assembly Language Programming
inc edx
align 4
mov ebx, cr
align 4
mov ebx, lf
align 4
cmp al, 0Dh
je .cr
cmp al, ah
je .lf
xchg al, ah
inc edx
xchg al, ah
; fall through
inc edx
mov ebx, ordinary
align 4
mov al, ah
inc edx
align 4
cmp al, ah
je .lf
cmp al, 0Dh
je .cr
xchg al, ah
inc edx
xchg al, ah
inc edx
mov ebx, ordinary
align 4
mov ebx, ordinary
mov al, ah
; fall through
One-Pointed Mind
inc edx
Do not use this program on les stored on a disk formatted by MS-DOS® or Windows®. There
seems to be a subtle bug in the FreeBSD code when using mmap on these drives mounted
under FreeBSD: If the le is over a certain size, mmap will just ll the memory with zeros, and
then copy them to the le overwriting its contents.
11.12. One-Pointed Mind
As a student of Zen, I like the idea of a one-pointed mind: Do one thing at a time, and do it well.
This, indeed, is very much how UNIX® works as well. While a typical Windows® application is attempting to do
everything imaginable (and is, therefore, riddled with bugs), a typical UNIX® program does only one thing, and
it does it well.
The typical UNIX® user then essentially assembles his own applications by writing a shell script which combines
the various existing programs by piping the output of one program to the input of another.
When writing your own UNIX® software, it is generally a good idea to see what parts of the problem you need to
solve can be handled by existing programs, and only write your own programs for that part of the problem that
you do not have an existing solution for.
11.12.1. CSV
I will illustrate this principle with a specific real-life example I was faced with recently:
I needed to extract the 11th eld of each record from a database I downloaded from a web site. The database was
a CSV le, i.e., a list of comma-separated values. That is quite a standard format for sharing data among people who
may be using different database software.
The rst line of the le contains the list of various elds separated by commas. The rest of the le contains the
data listed line by line, with values separated by commas.
I tried awk, using the comma as a separator. But because several lines contained a quoted comma, awk was extracting the wrong eld from those lines.
Therefore, I needed to write my own software to extract the 11th eld from the CSV le. However, going with the
UNIX® spirit, I only needed to write a simple filter that would do the following:
• Remove the rst line from the le;
• Change all unquoted commas to a different character;
• Remove all quotation marks.
Strictly speaking, I could use sed to remove the rst line from the le, but doing so in my own program was very
easy, so I decided to do it and reduce the size of the pipeline.
At any rate, writing a program like this took me about 20 minutes. Writing a program that extracts the 11th eld
from the CSV le would take a lot longer, and I could not reuse it to extract some other eld from some other
Chapter 11. x86 Assembly Language Programming
This time I decided to let it do a little more work than a typical tutorial program would:
• It parses its command line for options;
• It displays proper usage if it nds wrong arguments;
• It produces meaningful error messages.
Here is its usage message:
Usage: csv [-t<delim>] [-c<comma>] [-p] [-o <outfile>] [-i <infile>]
All parameters are optional, and can appear in any order.
The -t parameter declares what to replace the commas with. The tab is the default here. For example, -t; will
replace all unquoted commas with semicolons.
I did not need the -c option, but it may come in handy in the future. It lets me declare that I want a character other
than a comma replaced with something else. For example, [email protected] will replace all at signs (useful if you want to split
a list of email addresses to their user names and domains).
The -p option preserves the rst line, i.e., it does not delete it. By default, we delete the rst line because in a CSV
le it contains the eld names rather than data.
The -i and -o options let me specify the input and the output les. Defaults are stdin and stdout, so this is a
regular UNIX® filter.
I made sure that both -i filename and -ifilename are accepted. I also made sure that only one input and one
output les may be specified.
To get the 11th eld of each record, I can now do:
% csv '-t;' data.csv | awk '-F;' '{print $11}'
The code stores the options (except for the le descriptors) in EDX : The comma in DH, the new separator in DL, and
the ag for the -p option in the highest bit of EDX , so a check for its sign will give us a quick decision what to do.
Here is the code:
;;;;;;; csv.asm ­;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; Convert a comma-separated file to a something-else separated file.
; Started: 31-May-2001
; Updated: 1-Jun-2001
; Copyright (c) 2001 G. Adam Stanislav
; All rights reserved.
%include ''
%define BUFSIZE 2048
section .data dd stdin
fd.out dd stdout
usg db 'Usage: csv [-t<delim>] [-c<comma>] [-p] [-o <outfile>] [-i <infile>]', 0Ah
usglen equ $-usg
iemsg db "csv: Can't open input file", 0Ah
iemlen equ $-iemsg
oemsg db "csv: Can't create output file", 0Ah
oemlen equ $-oemsg
section .bss
ibuffer resb BUFSIZE
obuffer resb BUFSIZE
section .text
align 4
push dword iemlen
push dword iemsg
push dword stderr
push dword 1 ; return failure
align 4
push dword oemlen
push dword oemsg
push dword stderr
push dword 2
align 4
push dword usglen
push dword usg
push dword stderr
push dword 3
align 4
global _start
add esp, byte 8 ; discard argc and argv[0]
mov edx, (',' << 8) | 9
pop ecx
or ecx, ecx
je near .init
; no more arguments
; ECX contains the pointer to an argument
cmp byte [ecx], '-'
jne usage
inc ecx
mov ax, [ecx]
cmp al, 'o'
jne .i
; Make sure we are not asked for the output file twice
cmp dword [fd.out], stdout
jne usage
; Find the path to output file - it is either at [ECX+1],
; i.e., -ofile -; or in the next argument,
; i.e., -o file
inc ecx
or ah, ah
jne .openoutput
Chapter 11. x86 Assembly Language Programming
pop ecx
jecxz usage
push dword 420 ; file mode (644 octal)
push dword 0200h | 0400h | 01h
push ecx
jc near oerr
add esp, byte 12
mov [fd.out], eax
jmp short .arg
cmp al, 'i'
jne .p
; Make sure we are not asked twice
cmp dword [], stdin
jne near usage
; Find the path to the input file
inc ecx
or ah, ah
jne .openinput
pop ecx
or ecx, ecx
je near usage
push dword 0
push ecx
jc near ierr
; open failed
add esp, byte 8
mov [], eax
jmp .arg
cmp al, 'p'
jne .t
or ah, ah
jne near usage
or edx, 1 << 31
jmp .arg
cmp al, 't' ; redefine output delimiter
jne .c
or ah, ah
je near usage
mov dl, ah
jmp .arg
cmp al, 'c'
jne near usage
or ah, ah
je near usage
mov dh, ah
jmp .arg
align 4
sub eax, eax
sub ebx, ebx
sub ecx, ecx
mov edi, obuffer
; See if we are to preserve the first line
or edx, edx
js .loop
; get rid of the first line
call getchar
cmp al, 0Ah
jne .firstline
; read a byte from stdin
call getchar
; is it a comma (or whatever the user asked for)?
cmp al, dh
jne .quote
; Replace the comma with a tab (or whatever the user wants)
mov al, dl
call putchar
jmp short .loop
cmp al, '"'
jne .put
; Print everything until you get another quote or EOL. If it
; is a quote, skip it. If it is EOL, print it.
call getchar
cmp al, '"'
je .loop
cmp al, 0Ah
je .put
call putchar
jmp short .qloop
align 4
or ebx, ebx
jne .fetch
call read
dec ebx
jecxz .read
call write
push dword BUFSIZE
Chapter 11. x86 Assembly Language Programming
mov esi, ibuffer
push esi
push dword []
add esp, byte 12
mov ebx, eax
or eax, eax
je .done
sub eax, eax
align 4
call write
; flush output buffer
; close files
push dword []
push dword [fd.out]
; return success
push dword 0
align 4
inc ecx
cmp ecx, BUFSIZE
je write
align 4
jecxz .ret ; nothing to write
sub edi, ecx ; start of buffer
push ecx
push edi
push dword [fd.out]
add esp, byte 12
sub eax, eax
sub ecx, ecx ; buffer is empty now
Much of it is taken from hex.asm above. But there is one important difference: I no longer call write whenever I
am outputting a line feed. Yet, the code can be used interactively.
I have found a better solution for the interactive problem since I rst started writing this chapter. I wanted to
make sure each line is printed out separately only when needed. After all, there is no need to ush out every line
when used non-interactively.
The new solution I use now is to call write every time I nd the input buer empty. That way, when running in
the interactive mode, the program reads one line from the user's keyboard, processes it, and sees its input buer
is empty. It flushes its output and reads the next line. The Dark Side of Buffering
This change prevents a mysterious lockup in a very specific case. I refer to it as the dark side of buffering, mostly
because it presents a danger that is not quite obvious.
Using the FPU
It is unlikely to happen with a program like the csv above, so let us consider yet another filter: In this case we
expect our input to be raw data representing color values, such as the red, green, and blue intensities of a pixel. Our
output will be the negative of our input.
Such a filter would be very simple to write. Most of it would look just like all the other filters we have written so
far, so I am only going to show you its inner loop:
call getchar
not al ; Create a negative
call putchar
jmp short .loop
Because this filter works with raw data, it is unlikely to be used interactively.
But it could be called by image manipulation software. And, unless it calls write before each call to read , chances
are it will lock up.
Here is what might happen:
The image editor will load our filter using the C function popen() .
It will read the rst row of pixels from a bitmap or pixmap.
It will write the rst row of pixels to the pipe leading to the of our filter.
Our filter will read each pixel from its input, turn it to a negative, and write it to its output buer.
Our filter will call getchar to fetch the next pixel.
getchar will nd an empty input buer, so it will call read .
read will call the SYS_read system call.
The kernel will suspend our filter until the image editor sends more data to the pipe.
The image editor will read from the other pipe, connected to the fd.out of our filter so it can set the rst row
of the output image before it sends us the second row of the input.
10. The kernel suspends the image editor until it receives some output from our filter, so it can pass it on to the
image editor.
At this point our filter waits for the image editor to send it more data to process, while the image editor is waiting
for our filter to send it the result of the processing of the rst row. But the result sits in our output buer.
The filter and the image editor will continue waiting for each other forever (or, at least, until they are killed). Our
software has just entered a race condition.
This problem does not exist if our filter flushes its output buer before asking the kernel for more input data.
11.13. Using the FPU
Strangely enough, most of assembly language literature does not even mention the existence of the FPU, or floating
point unit, let alone discuss programming it.
Yet, never does assembly language shine more than when we create highly optimized FPU code by doing things
that can be done only in assembly language.
Chapter 11. x86 Assembly Language Programming
11.13.1. Organization of the FPU
The FPU consists of 8 80–bit floating–point registers. These are organized in a stack fashion—you can push a value
on TOS (top of stack) and you can pop it.
That said, the assembly language op codes are not push and pop because those are already taken.
You can push a value on TOS by using fld , fild , and fbld . Several other op codes let you push many common
constants—such as pi—on the TOS.
Similarly, you can pop a value by using fst , fstp , fist , fistp , and fbstp. Actually, only the op codes that end
with a p will literally pop the value, the rest will store it somewhere else without removing it from the TOS.
We can transfer the data between the TOS and the computer memory either as a 32–bit, 64–bit, or 80–bit real, a 16–
bit, 32–bit, or 64–bit integer, or an 80–bit packed decimal.
The 80–bit packed decimal is a special case of binary coded decimal which is very convenient when converting between
the ASCII representation of data and the internal data of the FPU. It allows us to use 18 significant digits.
No matter how we represent data in the memory, the FPU always stores it in the 80–bit real format in its registers.
Its internal precision is at least 19 decimal digits, so even if we choose to display results as ASCII in the full 18–digit
precision, we are still showing correct results.
We can perform mathematical operations on the TOS: We can calculate its sine, we can scale it (i.e., we can multiply
or divide it by a power of 2), we can calculate its base–2 logarithm, and many other things.
We can also multiply or divide it by, add it to, or subtract it from, any of the FPU registers (including itself).
The official Intel op code for the TOS is st, and for the registers st(0) –st(7) . st and st(0) , then, refer to the
same register.
For whatever reasons, the original author of nasm has decided to use different op codes, namely st0 –st7 . In other
words, there are no parentheses, and the TOS is always st0 , never just st. The Packed Decimal Format
The packed decimal format uses 10 bytes (80 bits) of memory to represent 18 digits. The number represented there
is always an integer.
You can use it to get decimal places by multiplying the TOS by a power of 10 rst.
The highest bit of the highest byte (byte 9) is the sign bit: If it is set, the number is negative, otherwise, it is positive.
The rest of the bits of this byte are unused/ignored.
The remaining 9 bytes store the 18 digits of the number: 2 digits per byte.
The more significant digit is stored in the high nibble (4 bits), the less significant digit in the low nibble.
That said, you might think that -1234567 would be stored in the memory like this (using hexadecimal notation):
80 00 00 00 00 00 01 23 45 67
Alas it is not! As with everything else of Intel make, even the packed decimal is little–endian.
That means our -1234567 is stored like this:
Excursion to Pinhole Photography
67 45 23 01 00 00 00 00 00 80
Remember that, or you will be pulling your hair out in desperation!
The book to read—if you can nd it—is Richard Startz' 8087/80287/80387 for the IBM PC
& Compatibles. Though it does seem to take the fact about the little–endian storage of the
packed decimal for granted. I kid you not about the desperation of trying to figure out what
was wrong with the filter I show below before it occurred to me I should try the little–endian
order even for this type of data.
11.13.2. Excursion to Pinhole Photography
To write meaningful software, we must not only understand our programming tools, but also the eld we are
creating software for.
Our next filter will help us whenever we want to build a pinhole camera, so, we need some background in pinhole
photography before we can continue. The Camera
The easiest way to describe any camera ever built is as some empty space enclosed in some lightproof material,
with a small hole in the enclosure.
The enclosure is usually sturdy (e.g., a box), though sometimes it is flexible (the bellows). It is quite dark inside the
camera. However, the hole lets light rays in through a single point (though in some cases there may be several).
These light rays form an image, a representation of whatever is outside the camera, in front of the hole.
If some light sensitive material (such as lm) is placed inside the camera, it can capture the image.
The hole often contains a lens, or a lens assembly, often called the objective. The Pinhole
But, strictly speaking, the lens is not necessary: The original cameras did not use a lens but a pinhole. Even today,
pinholes are used, both as a tool to study how cameras work, and to achieve a special kind of image.
The image produced by the pinhole is all equally sharp. Or blurred. There is an ideal size for a pinhole: If it is either
larger or smaller, the image loses its sharpness. Focal Length
This ideal pinhole diameter is a function of the square root of focal length, which is the distance of the pinhole from
the lm.
D = PC * sqrt(FL)
In here, D is the ideal diameter of the pinhole, FL is the focal length, and PC is a pinhole constant. According to
Jay Bender, its value is 0.04 , while Kenneth Connors has determined it to be 0.037 . Others have proposed other
values. Plus, this value is for the daylight only: Other types of light will require a different constant, whose value
can only be determined by experimentation. The F–Number
The f–number is a very useful measure of how much light reaches the lm. A light meter can determine that, for
example, to expose a lm of specific sensitivity with f5.6 may require the exposure to last 1/1000 sec.
Chapter 11. x86 Assembly Language Programming
It does not matter whether it is a 35–mm camera, or a 6x9cm camera, etc. As long as we know the f–number, we
can determine the proper exposure.
The f–number is easy to calculate:
F = FL / D
In other words, the f–number equals the focal length divided by the diameter of the pinhole. It also means a higher
f–number either implies a smaller pinhole or a larger focal distance, or both. That, in turn, implies, the higher the
f–number, the longer the exposure has to be.
Furthermore, while pinhole diameter and focal distance are one–dimensional measurements, both, the lm and
the pinhole, are two–dimensional. That means that if you have measured the exposure at f–number A as t, then
the exposure at f–number B is:
t * (B / A)² Normalized F–Number
While many modern cameras can change the diameter of their pinhole, and thus their f–number, quite smoothly
and gradually, such was not always the case.
To allow for different f–numbers, cameras typically contained a metal plate with several holes of different sizes
drilled to them.
Their sizes were chosen according to the above formula in such a way that the resultant f–number was one of
standard f–numbers used on all cameras everywhere. For example, a very old Kodak Duaflex IV camera in my
possession has three such holes for f–numbers 8, 11, and 16.
A more recently made camera may offer f–numbers of 2.8, 4, 5.6, 8, 11, 16, 22, and 32 (as well as others). These
numbers were not chosen arbitrarily: They all are powers of the square root of 2, though they may be rounded
somewhat. The F–Stop
A typical camera is designed in such a way that setting any of the normalized f–numbers changes the feel of the
dial. It will naturally stop in that position. Because of that, these positions of the dial are called f–stops.
Since the f–numbers at each stop are powers of the square root of 2, moving the dial by 1 stop will double the
amount of light required for proper exposure. Moving it by 2 stops will quadruple the required exposure. Moving
the dial by 3 stops will require the increase in exposure 8 times, etc.
11.13.3. Designing the Pinhole Software
We are now ready to decide what exactly we want our pinhole software to do. Processing Program Input
Since its main purpose is to help us design a working pinhole camera, we will use the focal length as the input to
the program. This is something we can determine without software: Proper focal length is determined by the size
of the lm and by the need to shoot "regular" pictures, wide angle pictures, or telephoto pictures.
Most of the programs we have written so far worked with individual characters, or bytes, as their input: The hex
program converted individual bytes into a hexadecimal number, the csv program either let a character through,
or deleted it, or changed it to a different character, etc.
One program, ftuc used the state machine to consider at most two input bytes at a time.
But our pinhole program cannot just work with individual characters, it has to deal with larger syntactic units.
Designing the Pinhole Software
For example, if we want the program to calculate the pinhole diameter (and other values we will discuss later) at
the focal lengths of 100 mm , 150 mm , and 210 mm , we may want to enter something like this:
100, 150, 210
Our program needs to consider more than a single byte of input at a time. When it sees the rst 1, it must understand
it is seeing the rst digit of a decimal number. When it sees the 0 and the other 0, it must know it is seeing more
digits of the same number.
When it encounters the rst comma, it must know it is no longer receiving the digits of the rst number. It must
be able to convert the digits of the rst number into the value of 100 . And the digits of the second number into the
value of 150 . And, of course, the digits of the third number into the numeric value of 210 .
We need to decide what delimiters to accept: Do the input numbers have to be separated by a comma? If so, how
do we treat two numbers separated by something else?
Personally, I like to keep it simple. Something either is a number, so I process it. Or it is not a number, so I discard
it. I do not like the computer complaining about me typing in an extra character when it is obvious that it is an
extra character. Duh!
Plus, it allows me to break up the monotony of computing and type in a query instead of just a number:
What is the best pinhole diameter for the focal length of 150?
There is no reason for the computer to spit out a number of complaints:
Syntax error: What
Syntax error: is
Syntax error: the
Syntax error: best
Et cetera, et cetera, et cetera.
Secondly, I like the # character to denote the start of a comment which extends to the end of the line. This does
not take too much effort to code, and lets me treat input les for my software as executable scripts.
In our case, we also need to decide what units the input should come in: We choose millimeters because that is how
most photographers measure the focus length.
Finally, we need to decide whether to allow the use of the decimal point (in which case we must also consider the
fact that much of the world uses a decimal comma).
In our case allowing for the decimal point/comma would offer a false sense of precision: There is little if any
noticeable difference between the focus lengths of 50 and 51, so allowing the user to input something like 50.5 is
not a good idea. This is my opinion, mind you, but I am the one writing this program. You can make other choices
in yours, of course. Offering Options
The most important thing we need to know when building a pinhole camera is the diameter of the pinhole. Since
we want to shoot sharp images, we will use the above formula to calculate the pinhole diameter from focal length.
As experts are offering several different values for the PC constant, we will need to have the choice.
It is traditional in UNIX® programming to have two main ways of choosing program parameters, plus to have a
default for the time the user does not make a choice.
Why have two ways of choosing?
One is to allow a (relatively) permanent choice that applies automatically each time the software is run without us
having to tell it over and over what we want it to do.
Chapter 11. x86 Assembly Language Programming
The permanent choices may be stored in a configuration le, typically found in the user's home directory. The le
usually has the same name as the application but is started with a dot. Often "rc" is added to the le name. So, ours
could be ~/.pinhole or ~/.pinholerc. (The ~/ means current user's home directory.)
The configuration le is used mostly by programs that have many configurable parameters. Those that have only
one (or a few) often use a different method: They expect to nd the parameter in an environment variable. In our
case, we might look at an environment variable named PINHOLE.
Usually, a program uses one or the other of the above methods. Otherwise, if a configuration le said one thing,
but an environment variable another, the program might get confused (or just too complicated).
Because we only need to choose one such parameter, we will go with the second method and search the environment for a variable named PINHOLE.
The other way allows us to make ad hoc decisions: "Though I usually want you to use 0.039, this time I want 0.03872." In
other words, it allows us to override the permanent choice.
This type of choice is usually done with command line parameters.
Finally, a program always needs a default. The user may not make any choices. Perhaps he does not know what to
choose. Perhaps he is "just browsing." Preferably, the default will be the value most users would choose anyway.
That way they do not need to choose. Or, rather, they can choose the default without an additional effort.
Given this system, the program may nd conflicting options, and handle them this way:
If it nds an ad hoc choice (e.g., command line parameter), it should accept that choice. It must ignore any
permanent choice and any default.
Otherwise, if it nds a permanent option (e.g., an environment variable), it should accept it, and ignore the
Otherwise, it should use the default.
We also need to decide what format our PC option should have.
At rst site, it seems obvious to use the PINHOLE=0.04 format for the environment variable, and -p0.04 for the
command line.
Allowing that is actually a security risk. The PC constant is a very small number. Naturally, we will test our software
using various small values of PC. But what will happen if someone runs the program choosing a huge value?
It may crash the program because we have not designed it to handle huge numbers.
Or, we may spend more time on the program so it can handle huge numbers. We might do that if we were writing
commercial software for computer illiterate audience.
Or, we might say, "Tough! The user should know better.""
Or, we just may make it impossible for the user to enter a huge number. This is the approach we will take: We will
use an implied 0. prefix.
In other words, if the user wants 0.04 , we will expect him to type -p04 , or set PINHOLE=04 in his environment. So,
if he says -p9999999 , we will interpret it as 0.9999999 —still ridiculous but at least safer.
Secondly, many users will just want to go with either Bender's constant or Connors' constant. To make it easier on
them, we will interpret -b as identical to -p04 , and -c as identical to -p037 . The Output
We need to decide what we want our software to send to the output, and in what format.
Designing the Pinhole Software
Since our input allows for an unspecified number of focal length entries, it makes sense to use a traditional database–style output of showing the result of the calculation for each focal length on a separate line, while separating
all values on one line by a tab character.
Optionally, we should also allow the user to specify the use of the CSV format we have studied earlier. In this case,
we will print out a line of comma–separated names describing each eld of every line, then show our results as
before, but substituting a comma for the tab .
We need a command line option for the CSV format. We cannot use -c because that already means use Connors'
constant. For some strange reason, many web sites refer to CSV les as "Excel spreadsheet" (though the CSV format
predates Excel). We will, therefore, use the -e switch to inform our software we want the output in the CSV format.
We will start each line of the output with the focal length. This may sound repetitious at rst, especially in the
interactive mode: The user types in the focal length, and we are repeating it.
But the user can type several focal lengths on one line. The input can also come in from a le or from the output
of another program. In that case the user does not see the input at all.
By the same token, the output can go to a le which we will want to examine later, or it could go to the printer,
or become the input of another program.
So, it makes perfect sense to start each line with the focal length as entered by the user.
No, wait! Not as entered by the user. What if the user types in something like this:
Clearly, we need to strip those leading zeros.
So, we might consider reading the user input as is, converting it to binary inside the FPU, and printing it out from
What if the user types something like this:
Ha! The packed decimal FPU format lets us input 18–digit numbers. But the user has entered more than 18 digits.
How do we handle that?
Well, we could modify our code to read the rst 18 digits, enter it to the FPU, then read more, multiply what we
already have on the TOS by 10 raised to the number of additional digits, then add to it.
Yes, we could do that. But in this program it would be ridiculous (in a different one it may be just the thing to
do): Even the circumference of the Earth expressed in millimeters only takes 11 digits. Clearly, we cannot build a
camera that large (not yet, anyway).
So, if the user enters such a huge number, he is either bored, or testing us, or trying to break into the system, or
playing games—doing anything but designing a pinhole camera.
What will we do?
We will slap him in the face, in a manner of speaking:
17459765723452353453534535353530530534563507309676764423 ??? ??? ??? ??? ???
To achieve that, we will simply ignore any leading zeros. Once we nd a non–zero digit, we will initialize a counter
to 0 and start taking three steps:
Send the digit to the output.
Append the digit to a buer we will use later to produce the packed decimal we can send to the FPU.
Chapter 11. x86 Assembly Language Programming
Increase the counter.
Now, while we are taking these three steps, we also need to watch out for one of two conditions:
• If the counter grows above 18, we stop appending to the buer. We continue reading the digits and sending them
to the output.
• If, or rather when, the next input character is not a digit, we are done inputting for now.
Incidentally, we can simply discard the non–digit, unless it is a #, which we must return to the input stream. It
starts a comment, so we must see it after we are done producing output and start looking for more input.
That still leaves one possibility uncovered: If all the user enters is a zero (or several zeros), we will never nd a
non–zero to display.
We can determine this has happened whenever our counter stays at 0. In that case we need to send 0 to the output,
and perform another "slap in the face":
0 ??? ??? ??? ??? ???
Once we have displayed the focal length and determined it is valid (greater than 0 but not exceeding 18 digits),
we can calculate the pinhole diameter.
It is not by coincidence that pinhole contains the word pin. Indeed, many a pinhole literally is a pin hole, a hole
carefully punched with the tip of a pin.
That is because a typical pinhole is very small. Our formula gets the result in millimeters. We will multiply it by
1000 , so we can output the result in microns.
At this point we have yet another trap to face: Too much precision.
Yes, the FPU was designed for high precision mathematics. But we are not dealing with high precision mathematics.
We are dealing with physics (optics, specifically).
Suppose we want to convert a truck into a pinhole camera (we would not be the rst ones to do that!). Suppose its
box is 12 meters long, so we have the focal length of 12000 . Well, using Bender's constant, it gives us square root
of 12000 multiplied by 0.04 , which is 4.381780460 millimeters, or 4381.780460 microns.
Put either way, the result is absurdly precise. Our truck is not exactly 12000 millimeters long. We did not measure
its length with such a precision, so stating we need a pinhole with the diameter of 4.381780460 millimeters is,
well, deceiving. 4.4 millimeters would do just ne.
I "only" used ten digits in the above example. Imagine the absurdity of going for all 18!
We need to limit the number of significant digits of our result. One way of doing it is by using an integer representing microns. So, our truck would need a pinhole with the diameter of 4382 microns. Looking at that number,
we still decide that 4400 microns, or 4.4 millimeters is close enough.
Additionally, we can decide that no matter how big a result we get, we only want to display four significant digits
(or any other number of them, of course). Alas, the FPU does not offer rounding to a specific number of digits (after
all, it does not view the numbers as decimal but as binary).
We, therefore, must devise an algorithm to reduce the number of significant digits.
Here is mine (I think it is awkward—if you know a better one, please, let me know):
Designing the Pinhole Software
Initialize a counter to 0.
While the number is greater than or equal to 10000 , divide it by 10 and increase the counter.
Output the result.
While the counter is greater than 0, output 0 and decrease the counter.
The 10000 is only good if you want four significant digits. For any other number of significant
digits, replace 10000 with 10 raised to the number of significant digits.
We will, then, output the pinhole diameter in microns, rounded o to four significant digits.
At this point, we know the focal length and the pinhole diameter. That means we have enough information to also
calculate the f–number.
We will display the f–number, rounded to four significant digits. Chances are the f–number will tell us very little. To
make it more meaningful, we can nd the nearest normalized f–number, i.e., the nearest power of the square root of 2.
We do that by multiplying the actual f–number by itself, which, of course, will give us its square . We will then
calculate its base–2 logarithm, which is much easier to do than calculating the base–square–root–of–2 logarithm!
We will round the result to the nearest integer. Next, we will raise 2 to the result. Actually, the FPU gives us a good
shortcut to do that: We can use the fscale op code to "scale" 1, which is analogous to shifting an integer left.
Finally, we calculate the square root of it all, and we have the nearest normalized f–number.
If all that sounds overwhelming—or too much work, perhaps—it may become much clearer if you see the code. It
takes 9 op codes altogether:
fmul st0, st0
fld st1
fstp st1
The rst line, fmul st0, st0 , squares the contents of the TOS (top of the stack, same as st, called st0 by nasm).
The fld1 pushes 1 on the TOS.
The next line, fld st1 , pushes the square back to the TOS. At this point the square is both in st and st(2) (it will
become clear why we leave a second copy on the stack in a moment). st(1) contains 1.
Next, fyl2x calculates base–2 logarithm of st multiplied by st(1) . That is why we placed 1 on st(1) before.
At this point, st contains the logarithm we have just calculated, st(1) contains the square of the actual f–number
we saved for later.
frndint rounds the TOS to the nearest integer. fld1 pushes a 1. fscale shifts the 1 we have on the TOS by the
value in st(1) , effectively raising 2 to st(1) .
Finally, fsqrt calculates the square root of the result, i.e., the nearest normalized f–number.
We now have the nearest normalized f–number on the TOS, the base–2 logarithm rounded to the nearest integer
in st(1) , and the square of the actual f–number in st(2) . We are saving the value in st(2) for later.
Chapter 11. x86 Assembly Language Programming
But we do not need the contents of st(1) anymore. The last line, fstp st1 , places the contents of st to st(1) , and
pops. As a result, what was st(1) is now st, what was st(2) is now st(1) , etc. The new st contains the normalized
f–number. The new st(1) contains the square of the actual f–number we have stored there for posterity.
At this point, we are ready to output the normalized f–number. Because it is normalized, we will not round it o
to four significant digits, but will send it out in its full precision.
The normalized f-number is useful as long as it is reasonably small and can be found on our light meter. Otherwise
we need a different method of determining proper exposure.
Earlier we have figured out the formula of calculating proper exposure at an arbitrary f–number from that measured at a different f–number.
Every light meter I have ever seen can determine proper exposure at f5.6. We will, therefore, calculate an "f5.6
multiplier," i.e., by how much we need to multiply the exposure measured at f5.6 to determine the proper exposure
for our pinhole camera.
From the above formula we know this factor can be calculated by dividing our f–number (the actual one, not the
normalized one) by 5.6 , and squaring the result.
Mathematically, dividing the square of our f–number by the square of 5.6 will give us the same result.
Computationally, we do not want to square two numbers when we can only square one. So, the rst solution seems
better at rst.
5.6 is a constant. We do not have to have our FPU waste precious cycles. We can just tell it to divide the square
of the f–number by whatever 5.6² equals to. Or we can divide the f–number by 5.6 , and then square the result.
The two ways now seem equal.
But, they are not!
Having studied the principles of photography above, we remember that the 5.6 is actually square root of 2 raised
to the fth power. An irrational number. The square of this number is exactly 32.
Not only is 32 an integer, it is a power of 2. We do not need to divide the square of the f–number by 32. We only
need to use fscale to shift it right by ve positions. In the FPU lingo it means we will fscale it with st(1) equal
to -5. That is much faster than a division.
So, now it has become clear why we have saved the square of the f–number on the top of the FPU stack. The
calculation of the f5.6 multiplier is the easiest calculation of this entire program! We will output it rounded to four
significant digits.
There is one more useful number we can calculate: The number of stops our f–number is from f5.6. This may help
us if our f–number is just outside the range of our light meter, but we have a shutter which lets us set various
speeds, and this shutter uses stops.
Say, our f–number is 5 stops from f5.6, and the light meter says we should use 1/1000 sec. Then we can set our
shutter speed to 1/1000 rst, then move the dial by 5 stops.
This calculation is quite easy as well. All we have to do is to calculate the base-2 logarithm of the f5.6 multiplier we
had just calculated (though we need its value from before we rounded it o). We then output the result rounded to
the nearest integer. We do not need to worry about having more than four significant digits in this one: The result
is most likely to have only one or two digits anyway.
11.13.4. FPU Optimizations
In assembly language we can optimize the FPU code in ways impossible in high languages, including C.
FPU Optimizations
Whenever a C function needs to calculate a floating–point value, it loads all necessary variables and constants
into FPU registers. It then does whatever calculation is required to get the correct result. Good C compilers can
optimize that part of the code really well.
It "returns" the value by leaving the result on the TOS. However, before it returns, it cleans up. Any variables and
constants it used in its calculation are now gone from the FPU.
It cannot do what we just did above: We calculated the square of the f–number and kept it on the stack for later
use by another function.
We knew we would need that value later on. We also knew we had enough room on the stack (which only has room
for 8 numbers) to store it there.
A C compiler has no way of knowing that a value it has on the stack will be required again in the very near future.
Of course, the C programmer may know it. But the only recourse he has is to store the value in a memory variable.
That means, for one, the value will be changed from the 80-bit precision used internally by the FPU to a C double
(64 bits) or even single (32 bits).
That also means that the value must be moved from the TOS into the memory, and then back again. Alas, of all FPU
operations, the ones that access the computer memory are the slowest.
So, whenever programming the FPU in assembly language, look for the ways of keeping intermediate results on
the FPU stack.
We can take that idea even further! In our program we are using a constant (the one we named PC).
It does not matter how many pinhole diameters we are calculating: 1, 10, 20, 1000, we are always using the same
constant. Therefore, we can optimize our program by keeping the constant on the stack all the time.
Early on in our program, we are calculating the value of the above constant. We need to divide our input by 10
for every digit in the constant.
It is much faster to multiply than to divide. So, at the start of our program, we divide 10 into 1 to obtain 0.1 , which
we then keep on the stack: Instead of dividing the input by 10 for every digit, we multiply it by 0.1 .
By the way, we do not input 0.1 directly, even though we could. We have a reason for that: While 0.1 can be
expressed with just one decimal place, we do not know how many binary places it takes. We, therefore, let the FPU
calculate its binary value to its own high precision.
We are using other constants: We multiply the pinhole diameter by 1000 to convert it from millimeters to microns.
We compare numbers to 10000 when we are rounding them o to four significant digits. So, we keep both, 1000
and 10000 , on the stack. And, of course, we reuse the 0.1 when rounding o numbers to four digits.
Last but not least, we keep -5 on the stack. We need it to scale the square of the f–number, instead of dividing it by
32. It is not by coincidence we load this constant last. That makes it the top of the stack when only the constants
are on it. So, when the square of the f–number is being scaled, the -5 is at st(1) , precisely where fscale expects
it to be.
It is common to create certain constants from scratch instead of loading them from the memory. That is what we
are doing with -5:
; TOS = 1
fadd st0, st0 ; TOS = 2
fadd st0, st0 ; TOS = 4
; TOS = 1
faddp st1, st0 ; TOS = 5
Chapter 11. x86 Assembly Language Programming
; TOS = -5
We can generalize all these optimizations into one rule: Keep repeat values on the stack!
PostScript® is a stack–oriented programming language. There are many more books available
about PostScript® than about the FPU assembly language: Mastering PostScript® will help
you master the FPU.
11.13.5. pinhole—The Code
;;;;;;; pinhole.asm ­;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; Find various parameters of a pinhole camera construction and use
; Started: 9-Jun-2001
; Updated: 10-Jun-2001
; Copyright (c) 2001 G. Adam Stanislav
; All rights reserved.
%include ''
%define BUFSIZE 2048
section .data
align 4
ten dd 10
thousand dd 1000
tthou dd 10000 dd stdin
fd.out dd stdout
envar db 'PINHOLE=' ; Exactly 8 bytes, or 2 dwords long
pinhole db '04,', ; Bender's constant (0.04)
connors db '037', 0Ah ; Connors' constant
usg db 'Usage: pinhole [-b] [-c] [-e] [-p <value>] [-o <outfile>] [-i <infile>]', 0Ah
usglen equ $-usg
iemsg db "pinhole: Can't open input file", 0Ah
iemlen equ $-iemsg
oemsg db "pinhole: Can't create output file", 0Ah
oemlen equ $-oemsg
pinmsg db "pinhole: The PINHOLE constant must not be 0", 0Ah
pinlen equ $-pinmsg
toobig db "pinhole: The PINHOLE constant may not exceed 18 decimal places", 0Ah
biglen equ $-toobig
huhmsg db 9, '???'
separ db 9, '???'
sep2 db 9, '???'
sep3 db 9, '???'
sep4 db 9, '???', 0Ah
huhlen equ $-huhmsg
header db 'focal length in millimeters,pinhole diameter in microns,'
db 'F-number,normalized F-number,F-5.6 multiplier,stops '
db 'from F-5.6', 0Ah
headlen equ $-header
section .bss
ibuffer resb BUFSIZE
pinhole—The Code
obuffer resb BUFSIZE
dbuffer resb 20 ; decimal input buffer
bbuffer resb 10 ; BCD buffer
section .text
align 4
call write
push dword huhlen
push dword huhmsg
push dword [fd.out]
add esp, byte 12
align 4
push dword pinlen
push dword pinmsg
push dword stderr
push dword 4 ; return failure
align 4
push dword biglen
push dword toobig
push dword stderr
push dword 5 ; return failure
align 4
push dword iemlen
push dword iemsg
push dword stderr
push dword 1 ; return failure
align 4
push dword oemlen
push dword oemsg
push dword stderr
push dword 2
align 4
push dword usglen
push dword usg
push dword stderr
push dword 3
align 4
global _start
add esp, byte 8 ; discard argc and argv[0]
sub esi, esi
Chapter 11. x86 Assembly Language Programming
pop ecx
or ecx, ecx
je near .getenv
; no more arguments
; ECX contains the pointer to an argument
cmp byte [ecx], '-'
jne usage
inc ecx
mov ax, [ecx]
inc ecx
cmp al, 'o'
jne .i
; Make sure we are not asked for the output file twice
cmp dword [fd.out], stdout
jne usage
; Find the path to output file - it is either at [ECX+1],
; i.e., -ofile -; or in the next argument,
; i.e., -o file
or ah, ah
jne .openoutput
pop ecx
jecxz usage
push dword 420 ; file mode (644 octal)
push dword 0200h | 0400h | 01h
push ecx
jc near oerr
add esp, byte 12
mov [fd.out], eax
jmp short .arg
cmp al, 'i'
jne .p
; Make sure we are not asked twice
cmp dword [], stdin
jne near usage
; Find the path to the input file
or ah, ah
jne .openinput
pop ecx
or ecx, ecx
je near usage
push dword 0
push ecx
jc near ierr
; open failed
add esp, byte 8
mov [], eax
pinhole—The Code
jmp .arg
cmp al, 'p'
jne .c
or ah, ah
jne .pcheck
pop ecx
or ecx, ecx
je near usage
mov ah, [ecx]
cmp ah, '0'
jl near usage
cmp ah, '9'
ja near usage
mov esi, ecx
jmp .arg
cmp al, 'c'
jne .b
or ah, ah
jne near usage
mov esi, connors
jmp .arg
cmp al, 'b'
jne .e
or ah, ah
jne near usage
mov esi, pinhole
jmp .arg
cmp al, 'e'
jne near usage
or ah, ah
jne near usage
mov al, ','
mov [huhmsg], al
mov [separ], al
mov [sep2], al
mov [sep3], al
mov [sep4], al
jmp .arg
align 4
; If ESI = 0, we did not have a -p argument,
; and need to check the environment for "PINHOLE="
or esi, esi
jne .init
sub ecx, ecx
pop esi
or esi, esi
je .default ; no PINHOLE envar found
; check if this envar starts with 'PINHOLE='
Chapter 11. x86 Assembly Language Programming
mov edi, envar
mov cl, 2 ; 'PINHOLE=' is 2 dwords long
rep cmpsd
jne .nextenv
; Check if it is followed by a digit
mov al, [esi]
cmp al, '0'
jl .default
cmp al, '9'
jbe .init
; fall through
align 4
; We got here because we had no -p argument,
; and did not find the PINHOLE envar.
mov esi, pinhole
; fall through
align 4
sub eax, eax
sub ebx, ebx
sub ecx, ecx
sub edx, edx
mov edi, dbuffer+1
mov byte [dbuffer], '0'
; Convert the pinhole constant to real
cmp al, '9'
ja .setconst
cmp al, '0'
je .processconst
jb .setconst
inc dl
inc cl
cmp cl, 18
ja near consttoobig
jmp short .constloop
align 4
or dl, dl
je near perr
fild dword [tthou]
fild dword [ten]
fdivp st1, st0
fild dword [thousand]
mov edi, obuffer
mov ebp, ecx
call bcdload
pinhole—The Code
fmul st0, st2
loop .constdiv
fadd st0, st0
fadd st0, st0
faddp st1, st0
; If we are creating a CSV file,
; print header
cmp byte [separ], ','
jne .bigloop
push dword headlen
push dword header
push dword [fd.out]
call getchar
jc near done
; Skip to the end of the line if you got '#'
cmp al, '#'
jne .num
call skiptoeol
jmp short .bigloop
; See if you got a number
cmp al, '0'
jl .bigloop
cmp al, '9'
ja .bigloop
; Yes, we have a number
sub ebp, ebp
sub edx, edx
cmp al, '0'
je .number0
mov dl, 1
or dl, dl ; Skip leading 0's
je .nextnumber
push eax
call putchar
pop eax
inc ebp
cmp ebp, 19
jae .nextnumber
mov [dbuffer+ebp], al
call getchar
jc .work
cmp al, '#'
je .ungetc
cmp al, '0'
jl .work
cmp al, '9'
ja .work
Chapter 11. x86 Assembly Language Programming
jmp short .number
dec esi
inc ebx
; Now, do all the work
or dl, dl
je near .work0
cmp ebp, 19
jae near .toobig
call bcdload
; Calculate pinhole diameter
fld st0 ; save it
fmul st0, st3
fld st0
fmul st5
sub ebp, ebp
; Round off to 4 significant digits
fcom st0, st7
fstsw ax
jb .printdiameter
fmul st0, st6
inc ebp
jmp short .diameter
call printnumber ; pinhole diameter
; Calculate F-number
fdivp st1, st0
fld st0
sub ebp, ebp
fcom st0, st6
fstsw ax
jb .printfnumber
fmul st0, st5
inc ebp
jmp short .fnumber
call printnumber ; F number
; Calculate normalized F-number
fmul st0, st0
fld st1
pinhole—The Code
fstp st1
sub ebp, ebp
call printnumber
; Calculate time multiplier from F-5.6
fld st0
; Round off to 4 significant digits
fcom st0, st6
fstsw ax
jb .printfmul
inc ebp
fmul st0, st5
jmp short .fmul
call printnumber ; F multiplier
; Calculate F-stops from 5.6
fxch st1
sub ebp, ebp
call printnumber
mov al, 0Ah
call putchar
jmp .bigloop
mov al, '0'
call putchar
align 4
call huh
jmp .bigloop
align 4
call write
; flush output buffer
; close files
push dword []
push dword [fd.out]
; return success
push dword 0
align 4
; Keep reading until you come to cr, lf, or eof
Chapter 11. x86 Assembly Language Programming
call getchar
jc done
cmp al, 0Ah
jne .cr
cmp al, 0Dh
jne skiptoeol
align 4
or ebx, ebx
jne .fetch
call read
dec ebx
jecxz .read
call write
push dword BUFSIZE
mov esi, ibuffer
push esi
push dword []
add esp, byte 12
mov ebx, eax
or eax, eax
je .empty
sub eax, eax
align 4
add esp, byte 4
align 4
inc ecx
cmp ecx, BUFSIZE
je write
align 4
jecxz .ret ; nothing to write
sub edi, ecx ; start of buffer
push ecx
push edi
push dword [fd.out]
add esp, byte 12
sub eax, eax
sub ecx, ecx ; buffer is empty now
pinhole—The Code
align 4
; EBP contains the number of chars in dbuffer
push ecx
push esi
push edi
lea ecx, [ebp+1]
lea esi, [dbuffer+ebp-1]
shr ecx, 1
edi, bbuffer
eax, eax
[edi], eax
[edi+4], eax
[edi+2], ax
sub ax, 3030h
shl al, 4
or al, ah
mov [edi], al
inc edi
loop .loop
fbld [bbuffer]
eax, eax
align 4
push ebp
mov al, [separ]
call putchar
; Print the integer at the TOS
mov ebp, bbuffer+9
fbstp [bbuffer]
; Check the sign
mov al, [ebp]
dec ebp
or al, al
jns .leading
; We got a negative number (should never happen)
mov al, '-'
call putchar
; Skip leading zeros
mov al, [ebp]
dec ebp
or al, al
jne .first
Chapter 11. x86 Assembly Language Programming
cmp ebp, bbuffer
jae .leading
; We are here because the result was 0.
; Print '0' and return
mov al, '0'
jmp putchar
; We have found the first non-zero.
; But it is still packed
test al, 0F0h
jz .second
push eax
shr al, 4
add al, '0'
call putchar
pop eax
and al, 0Fh
add al, '0'
call putchar
cmp ebp, bbuffer
jb .done
mov al, [ebp]
push eax
shr al, 4
add al, '0'
call putchar
pop eax
and al, 0Fh
add al, '0'
call putchar
dec ebp
jmp short .next
pop ebp
or ebp, ebp
je .ret
mov al, '0'
call putchar
dec ebp
jne .zeros
The code follows the same format as all the other filters we have seen before, with one subtle exception:
We are no longer assuming that the end of input implies the end of things to do, something we
took for granted in the character–oriented filters.
This filter does not process characters. It processes a language (albeit a very simple one, consisting only of numbers).
When we have no more input, it can mean one of two things:
• We are done and can quit. This is the same as before.
Using pinhole
• The last character we have read was a digit. We have stored it at the end of our ASCII–to–
oat conversion buer. We now need to convert the contents of that buer into a number
and write the last line of our output.
For that reason, we have modified our getchar and our read routines to return with the carry
flag clear whenever we are fetching another character from the input, or the carry flag set
whenever there is no more input.
Of course, we are still using assembly language magic to do that! Take a good look at getchar .
It always returns with the carry flag clear.
Yet, our main code relies on the carry flag to tell it when to quit—and it works.
The magic is in read . Whenever it receives more input from the system, it just returns to
getchar , which fetches a character from the input buer, clears the carry flag and returns.
But when read receives no more input from the system, it does not return to getchar at all.
Instead, the add esp, byte 4 op code adds 4 to ESP , sets the carry flag , and returns.
So, where does it return to? Whenever a program uses the call op code, the microprocessor
pushes the return address, i.e., it stores it on the top of the stack (not the FPU stack, the system
stack, which is in the memory). When a program uses the ret op code, the microprocessor pop s
the return value from the stack, and jumps to the address that was stored there.
But since we added 4 to ESP (which is the stack pointer register), we have effectively given the
microprocessor a minor case of amnesia: It no longer remembers it was getchar that call ed
read .
And since getchar never push ed anything before calling read , the top of the stack now contains the return address to whatever or whoever call ed getchar . As far as that caller is concerned, he call ed getchar , which returned with the carry flag set!
Other than that, the bcdload routine is caught up in the middle of a Lilliputian conflict between the Big–Endians
and the Little–Endians.
It is converting the text representation of a number into that number: The text is stored in the big–endian order,
but the packed decimal is little–endian.
To solve the conflict, we use the std op code early on. We cancel it with cld later on: It is quite important we do
not call anything that may depend on the default setting of the direction ag while std is active.
Everything else in this code should be quite clear, providing you have read the entire chapter that precedes it.
It is a classical example of the adage that programming requires a lot of thought and only a little coding. Once we
have thought through every tiny detail, the code almost writes itself.
11.13.6. Using pinhole
Because we have decided to make the program ignore any input except for numbers (and even those inside a comment), we can actually perform textual queries. We do not have to, but we can.
In my humble opinion, forming a textual query, instead of having to follow a very strict syntax, makes software
much more user friendly.
Suppose we want to build a pinhole camera to use the 4x5 inch lm. The standard focal length for that lm is about
150mm. We want to ne–tune our focal length so the pinhole diameter is as round a number as possible. Let us also
suppose we are quite comfortable with cameras but somewhat intimidated by computers. Rather than just have to
type in a bunch of numbers, we want to ask a couple of questions.
Chapter 11. x86 Assembly Language Programming
Our session might look like this:
% pinhole
What size pinhole do I need for the focal length of 150?
150 490 306 362 2930 12
Hmmm... How about 160?
160 506 316 362 3125 12
Let's make it 155, please.
155 498 311 362 3027 12
Ah, let's try 157...
157 501 313 362 3066 12
156 500 312 362 3047 12
That's it! Perfect! Thank you very much!
We have found that while for the focal length of 150, our pinhole diameter should be 490 microns, or 0.49 mm, if
we go with the almost identical focal length of 156 mm, we can get away with a pinhole diameter of exactly one
half of a millimeter.
11.13.7. Scripting
Because we have chosen the # character to denote the start of a comment, we can treat our pinhole software as
a scripting language.
You have probably seen shell scripts that start with:
#! /bin/sh
...because the blank space after the #! is optional.
Whenever UNIX® is asked to run an executable le which starts with the #!, it assumes the le is a script. It adds
the command to the rest of the rst line of the script, and tries to execute that.
Suppose now that we have installed pinhole in /usr/local/bin/, we can now write a script to calculate various
pinhole diameters suitable for various focal lengths commonly used with the 120 lm.
The script might look something like this:
#! /usr/local/bin/pinhole -b -i
# Find the best pinhole diameter
# for the 120 film
### Standard
### Wide angle
30, 40, 50, 60, 70
### Telephoto
100, 120, 140
Because 120 is a medium size lm, we may name this le medium.
We can set its permissions to execute, and run it as if it were a program:
% chmod 755 medium
% ./medium
UNIX® will interpret that last command as:
% /usr/local/bin/pinhole -b -i ./medium
It will run that command and display:
80 358 224 256 1562 11
30 219 137 128 586 9
40 253 158 181 781 10
50 283 177 181 977 10
60 310 194 181 1172 10
70 335 209 181 1367 10
100 400 250 256 1953 11
120 438 274 256 2344 11
140 473 296 256 2734 11
Now, let us enter:
% ./medium -c
UNIX® will treat that as:
% /usr/local/bin/pinhole -b -i ./medium -c
That gives it two conflicting options: -b and -c (Use Bender's constant and use Connors' constant). We have programmed it so later options override early ones—our program will calculate everything using Connors' constant:
80 331 242 256 1826 11
30 203 148 128 685 9
40 234 171 181 913 10
50 262 191 181 1141 10
60 287 209 181 1370 10
70 310 226 256 1598 11
100 370 270 256 2283 11
120 405 296 256 2739 11
140 438 320 362 3196 12
We decide we want to go with Bender's constant after all. We want to save its values as a comma–separated le:
% ./medium -b -e > bender
% cat bender
focal length in millimeters,pinhole diameter in microns,F-number,normalized Fnumber,F-5.6 multiplier,stops from F-5.6
11.14. Caveats
Assembly language programmers who "grew up" under MS-DOS® and Windows® often tend to take shortcuts.
Reading the keyboard scan codes and writing directly to video memory are two classical examples of practices
which, under MS-DOS® are not frowned upon but considered the right thing to do.
Chapter 11. x86 Assembly Language Programming
The reason? Both the PC BIOS and MS-DOS® are notoriously slow when performing these operations.
You may be tempted to continue similar practices in the UNIX® environment. For example, I have seen a web site
which explains how to access the keyboard scan codes on a popular UNIX® clone.
That is generally a very bad idea in UNIX® environment! Let me explain why.
11.14.1. UNIX® Is Protected
For one thing, it may simply not be possible. UNIX® runs in protected mode. Only the kernel and device drivers are
allowed to access hardware directly. Perhaps a particular UNIX® clone will let you read the keyboard scan codes,
but chances are a real UNIX® operating system will not. And even if one version may let you do it, the next one
may not, so your carefully crafted software may become a dinosaur overnight.
11.14.2. UNIX® Is an Abstraction
But there is a much more important reason not to try accessing the hardware directly (unless, of course, you are
writing a device driver), even on the UNIX® like systems that let you do it:
UNIX® is an abstraction!
There is a major difference in the philosophy of design between MS-DOS® and UNIX®. MS-DOS® was designed as a
single-user system. It is run on a computer with a keyboard and a video screen attached directly to that computer.
User input is almost guaranteed to come from that keyboard. Your program's output virtually always ends up on
that screen.
This is NEVER guaranteed under UNIX®. It is quite common for a UNIX® user to pipe and redirect program input
and output:
% program1 | program2 | program3 > file1
If you have written program2, your input does not come from the keyboard but from the output of program1.
Similarly, your output does not go to the screen but becomes the input for program3 whose output, in turn, goes
to file1 .
But there is more! Even if you made sure that your input comes from, and your output goes to, the terminal, there
is no guarantee the terminal is a PC: It may not have its video memory where you expect it, nor may its keyboard
be producing PC-style scan codes. It may be a Macintosh®, or any other computer.
Now you may be shaking your head: My software is in PC assembly language, how can it run on a Macintosh®? But
I did not say your software would be running on a Macintosh®, only that its terminal may be a Macintosh®.
Under UNIX®, the terminal does not have to be directly attached to the computer that runs your software, it can
even be on another continent, or, for that matter, on another planet. It is perfectly possible that a Macintosh®
user in Australia connects to a UNIX® system in North America (or anywhere else) via telnet. The software then
runs on one computer, while the terminal is on a different computer: If you try to read the scan codes, you will
get the wrong input!
Same holds true about any other hardware: A le you are reading may be on a disk you have no direct access to. A
camera you are reading images from may be on a space shuttle, connected to you via satellites.
That is why under UNIX® you must never make any assumptions about where your data is coming from and going
to. Always let the system handle the physical access to the hardware.
These are caveats, not absolute rules. Exceptions are possible. For example, if a text editor
has determined it is running on a local machine, it may want to read the scan codes directly
for improved control. I am not mentioning these caveats to tell you what to do or what not
to do, just to make you aware of certain pitfalls that await you if you have just arrived to
UNIX® form MS-DOS®. Of course, creative people often break rules, and it is OK as long as
they know they are breaking them and why.
11.15. Acknowledgements
This tutorial would never have been possible without the help of many experienced FreeBSD programmers from
the FreeBSD technical discussions mailing list, many of whom have patiently answered my questions, and pointed
me in the right direction in my attempts to explore the inner workings of UNIX® system programming in general
and FreeBSD in particular.
Thomas M. Sommers opened the door for me. His How do I write "Hello, world" in FreeBSD assembler? web page
was my rst encounter with an example of assembly language programming under FreeBSD.
Jake Burkholder has kept the door open by willingly answering all of my questions and supplying me with example
assembly language source code.
Copyright © 2000-2001 G. Adam Stanislav. All rights reserved.
Part V. Appendices
Table of Contents
Bibliography ........................................................................................................................... 183
[1] Dave A Patterson and John L Hennessy. Copyright © 1998 Morgan Kaufmann Publishers, Inc.. 1-55860-428-6.
Morgan Kaufmann Publishers, Inc.. Computer Organization and Design. The Hardware / Software Interface.
[2] W. Richard Stevens. Copyright © 1993 Addison Wesley Longman, Inc.. 0-201-56317-7. Addison Wesley Longman,
Inc.. Advanced Programming in the Unix Environment. 1-2.
[3] Marshall Kirk McKusick and George Neville-Neil. Copyright © 2004 Addison-Wesley. 0-201-70245-2. Addison-Wesley. The Design and Implementation of the FreeBSD Operating System. 1-2.
[4] Aleph One. Phrack 49; "Smashing the Stack for Fun and Profit".
[5] Chrispin Cowan, Calton Pu, and Dave Maier. StackGuard; Automatic Adaptive Detection and Prevention of BuerOverflow Attacks.
[6] Todd Miller and Theo de Raadt. strlcpy and strlcat -- consistent, safe string copy and concatenation..
arguments, 29
bounds checking
compiler-based, 30
library-based, 31
buer overflow, 29, 31
chroot(), 31
contributed software, 39
core team, 43
data validation, 32
frame pointer, 29
gcc, 30
GTK, 35
jail, 31
LIFO, 29
Morris Internet worm, 29
NUL termination, 30
Perl, 35
Perl Taint mode, 32
ports maintainer, 39
positive filtering, 32
POSIX.1e Process Capabilities, 32
process image
frame pointer, 29
stack pointer, 29
ProPolice, 30
Python, 35
Qt, 35
race conditions
access checks, 32
le opens, 32
signals, 32
release engineering, 43
return address, 29
seteuid, 31
stack, 29
stack frame, 29
stack pointer, 29
stack-overflow, 29
StackGuard, 30
string copy functions
strlcat, 30
strlcpy, 30
strncat, 30
strncpy, 30
style, 39
TrustedBSD, 32
user IDs
effective user ID, 31
real user ID, 31
von Neumann, 29
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF