GNU 1.4, 2.0 Series Macro Processor Manual

GNU M4, version 1.4.17
A powerful macro processor
Edition 1.4.17, 22 September 2013
by René Seindal, François Pinard,
Gary V. Vaughan, and Eric Blake
([email protected])
This manual (22 September 2013) is for GNU M4 (version 1.4.17), a package containing an
implementation of the m4 macro language.
Copyright c 1989-1994, 2004-2013 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document under the
terms of the GNU Free Documentation License, Version 1.3 or any later version
published by the Free Software Foundation; with no Invariant Sections, no
Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included
in the section entitled “GNU Free Documentation License.”
i
Table of Contents
1
Introduction and preliminaries . . . . . . . . . . . . . . . . 3
1.1
1.2
1.3
1.4
2
line options for operation modes . . . . . . . . . . . . . . . . . . . . . 7
line options for preprocessor features. . . . . . . . . . . . . . . . . 8
line options for limits control . . . . . . . . . . . . . . . . . . . . . . . 10
line options for frozen state . . . . . . . . . . . . . . . . . . . . . . . . . 11
line options for debugging . . . . . . . . . . . . . . . . . . . . . . . . . . 11
input files on the command line . . . . . . . . . . . . . . . . . . . . 12
Macro names . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Quoting input to m4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Comments in m4 input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Other kinds of input tokens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
How m4 copies input to output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15
15
15
16
16
How to invoke macros . . . . . . . . . . . . . . . . . . . . . . . . . 19
4.1
4.2
4.3
4.4
4.5
5
Command
Command
Command
Command
Command
Specifying
Lexical and syntactic conventions . . . . . . . . . . . . 15
3.1
3.2
3.3
3.4
3.5
4
3
3
4
5
Invoking m4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.1
2.2
2.3
2.4
2.5
2.6
3
Introduction to m4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Historical references . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Problems and bugs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Using this manual . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Macro invocation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Preventing macro invocation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Macro arguments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
On Quoting Arguments to macros . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Macro expansion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
19
19
21
23
23
How to define new macros . . . . . . . . . . . . . . . . . . . . 25
5.1
5.2
5.3
5.4
5.5
5.6
5.7
5.8
Defining a macro . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Arguments to macros . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Special arguments to macros . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Deleting a macro . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Renaming macros . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Temporarily redefining macros . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Indirect call of macros. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Indirect call of builtins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
25
26
27
30
31
33
34
35
ii
GNU M4 1.4.17 macro processor
6
Conditionals, loops, and recursion . . . . . . . . . . . 39
6.1
6.2
6.3
6.4
6.5
6.6
6.7
7
Testing if a macro is defined . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
If-else construct, or multibranch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Recursion in m4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Iteration by counting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Iteration by list contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Working with definition stacks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Building macros with macros . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
39
39
41
46
47
49
51
How to debug macros and input . . . . . . . . . . . . . 55
7.1
7.2
7.3
7.4
8
Displaying macro definitions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Tracing macro calls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Controlling debugging output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Saving debugging output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
55
55
58
60
Input control. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
8.1
8.2
8.3
8.4
8.5
9
Deleting whitespace in input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Changing the quote characters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Changing the comment delimiters . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Changing the lexical structure of words . . . . . . . . . . . . . . . . . . . . . . . .
Saving text until end of input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
61
62
65
67
70
File inclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
9.1
9.2
10
Including named files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
Searching for include files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
Diverting and undiverting output . . . . . . . . . . 75
10.1
10.2
10.3
10.4
11
75
76
78
79
Macros for text handling . . . . . . . . . . . . . . . . . . . . 81
11.1
11.2
11.3
11.4
11.5
11.6
11.7
12
Diverting output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Undiverting output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Diversion numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Discarding diverted text . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Calculating length of strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Searching for substrings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Searching for regular expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Extracting substrings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Translating characters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Substituting text by regular expression . . . . . . . . . . . . . . . . . . . . . . .
Formatting strings (printf-like) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
81
81
81
82
83
84
86
Macros for doing arithmetic . . . . . . . . . . . . . . . . . 89
12.1
12.2
Decrement and increment operators . . . . . . . . . . . . . . . . . . . . . . . . . . 89
Evaluating integer expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
iii
13
Macros for running shell commands . . . . . . . . 95
13.1
13.2
13.3
13.4
13.5
14
Using frozen files. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
Frozen file format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
Compatibility with other versions of m4 . . . 109
16.1
16.2
16.3
17
Printing error messages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
Printing current location . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
Exiting from m4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
Fast loading of frozen state . . . . . . . . . . . . . . . . 105
15.1
15.2
16
95
96
97
97
99
Miscellaneous builtin macros . . . . . . . . . . . . . . 101
14.1
14.2
14.3
15
Determining the platform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Executing simple commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Reading the output of commands . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Exit status. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Making temporary files. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Extensions in GNU M4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
Facilities in System V m4 not in GNU m4 . . . . . . . . . . . . . . . . . . . . 110
Other incompatibilities. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
Correct version of some examples . . . . . . . . . 115
17.1
17.2
17.3
17.4
17.5
17.6
17.7
17.8
Solution
Solution
Solution
Solution
Solution
Solution
Solution
Solution
for
for
for
for
for
for
for
for
exch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
forloop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
foreach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
copy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
m4wrap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
cleardivert . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
capitalize . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
fatal_error . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
115
115
117
124
125
126
127
129
Appendix A How to make copies of the overall
M4 package . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
A.1
License for copying the M4 package . . . . . . . . . . . . . . . . . . . . . . . . . . 131
Appendix B How to make copies of this manual
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
B.1
License for copying this manual . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
Appendix C Indices of concepts and macros
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
C.1
C.2
Index for all m4 macros . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
Index for many concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
1
GNU m4 is an implementation of the traditional UNIX macro processor. It is mostly
SVR4 compatible, although it has some extensions (for example, handling more than 9
positional parameters to macros). m4 also has builtin functions for including files, running
shell commands, doing arithmetic, etc. Autoconf needs GNU m4 for generating configure
scripts, but not for running them.
GNU m4 was originally written by René Seindal, with subsequent changes by François
Pinard and other volunteers on the Internet. All names and email addresses can be found
in the files m4-1.4.17/AUTHORS and m4-1.4.17/THANKS from the GNU M4 distribution.
This is release 1.4.17. It is now considered stable: future releases in the 1.4.x series are
only meant to fix bugs, increase speed, or improve documentation. However. . .
An experimental feature, which would improve m4 usefulness, allows for changing the
syntax for what is a word in m4. You should use:
./configure --enable-changeword
if you want this feature compiled in. The current implementation slows down m4 considerably and is hardly acceptable. In the future, m4 2.0 will come with a different set of new
features that provide similar capabilities, but without the inefficiencies, so changeword will
go away and you should not count on it.
Chapter 1: Introduction and preliminaries
3
1 Introduction and preliminaries
This first chapter explains what GNU m4 is, where m4 comes from, how to read and use this
documentation, how to call the m4 program, and how to report bugs about it. It concludes
by giving tips for reading the remainder of the manual.
The following chapters then detail all the features of the m4 language.
1.1 Introduction to m4
m4 is a macro processor, in the sense that it copies its input to the output, expanding
macros as it goes. Macros are either builtin or user-defined, and can take any number
of arguments. Besides just doing macro expansion, m4 has builtin functions for including
named files, running shell commands, doing integer arithmetic, manipulating text in various
ways, performing recursion, etc.. . . m4 can be used either as a front-end to a compiler, or
as a macro processor in its own right.
The m4 macro processor is widely available on all UNIXes, and has been standardized by
POSIX. Usually, only a small percentage of users are aware of its existence. However, those
who find it often become committed users. The popularity of GNU Autoconf, which requires
GNU m4 for generating configure scripts, is an incentive for many to install it, while these
people will not themselves program in m4. GNU m4 is mostly compatible with the System
V, Release 4 version, except for some minor differences. See Chapter 16 [Compatibility],
page 109, for more details.
Some people find m4 to be fairly addictive. They first use m4 for simple problems, then
take bigger and bigger challenges, learning how to write complex sets of m4 macros along
the way. Once really addicted, users pursue writing of sophisticated m4 applications even to
solve simple problems, devoting more time debugging their m4 scripts than doing real work.
Beware that m4 may be dangerous for the health of compulsive programmers.
1.2 Historical references
Macro languages were invented early in the history of computing. In the 1950s Alan Perlis
suggested that the macro language be independent of the language being processed. Techniques such as conditional and recursive macros, and using macros to define other macros,
were described by Doug McIlroy of Bell Labs in “Macro Instruction Extensions of Compiler
Languages”, Communications of the ACM 3, 4 (1960), 214–20, http://dx.doi.org/10.
1145/367177.367223.
An important precursor of m4 was GPM; see C. Strachey, “A general purpose macrogenerator”, Computer Journal 8, 3 (1965), 225–41, http://dx.doi.org/10.1093/comjnl/8.
3.225. GPM is also succinctly described in David Gries’s book Compiler Construction for
Digital Computers, Wiley (1971). Strachey was a brilliant programmer: GPM fit into 250
machine instructions!
Inspired by GPM while visiting Strachey’s Lab in 1968, McIlroy wrote a model preprocessor in that fit into a page of Snobol 3 code, and McIlroy and Robert Morris developed
a series of further models at Bell Labs. Andrew D. Hall followed up with M6, a general
purpose macro processor used to port the Fortran source code of the Altran computer algebra system; see Hall’s “The M6 Macro Processor”, Computing Science Technical Report
4
GNU M4 1.4.17 macro processor
#2, Bell Labs (1972), http://cm.bell-labs.com/cm/cs/cstr/2.pdf. M6’s source code
consisted of about 600 Fortran statements. Its name was the first of the m4 line.
The Brian Kernighan and P.J. Plauger book Software Tools, Addison-Wesley (1976),
describes and implements a Unix macro-processor language, which inspired Dennis Ritchie
to write m3, a macro processor for the AP-3 minicomputer.
Kernighan and Ritchie then joined forces to develop the original m4, described in “The
M4 Macro Processor”, Bell Laboratories (1977), http://wolfram.schneider.org/bsd/
7thEdManVol2/m4/m4.pdf. It had only 21 builtin macros.
While GPM was more pure, m4 is meant to deal with the true intricacies of real life:
macros can be recognized without being pre-announced, skipping whitespace or end-of-lines
is easier, more constructs are builtin instead of derived, etc.
Originally, the Kernighan and Plauger macro-processor, and then m3, formed the engine
for the Rational FORTRAN preprocessor, that is, the Ratfor equivalent of cpp. Later, m4
was used as a front-end for Ratfor, C and Cobol.
René Seindal released his implementation of m4, GNU m4, in 1990, with the aim of
removing the artificial limitations in many of the traditional m4 implementations, such as
maximum line length, macro size, or number of macros.
The late Professor A. Dain Samples described and implemented a further evolution
in the form of M5: “User’s Guide to the M5 Macro Language: 2nd edition”, Electronic
Announcement on comp.compilers newsgroup (1992).
François Pinard took over maintenance of GNU m4 in 1992, until 1994 when he released
GNU m4 1.4, which was the stable release for 10 years. It was at this time that GNU Autoconf decided to require GNU m4 as its underlying engine, since all other implementations
of m4 had too many limitations.
More recently, in 2004, Paul Eggert released 1.4.1 and 1.4.2 which addressed some long
standing bugs in the venerable 1.4 release. Then in 2005, Gary V. Vaughan collected
together the many patches to GNU m4 1.4 that were floating around the net and released
1.4.3 and 1.4.4. And in 2006, Eric Blake joined the team and prepared patches for the
release of 1.4.5, 1.4.6, 1.4.7, and 1.4.8. More bug fixes were incorporated in 2007, with
releases 1.4.9 and 1.4.10. Eric continued with some portability fixes for 1.4.11 and 1.4.12 in
2008, 1.4.13 in 2009, 1.4.14 and 1.4.15 in 2010, and 1.4.16 in 2011.
Meanwhile, development has continued on new features for m4, such as dynamic module
loading and additional builtins. When complete, GNU m4 2.0 will start a new series of
releases.
1.3 Problems and bugs
If you have problems with GNU M4 or think you’ve found a bug, please report it. Before
reporting a bug, make sure you’ve actually found a real bug. Carefully reread the documentation and see if it really says you can do what you’re trying to do. If it’s not clear whether
you should be able to do something or not, report that too; it’s a bug in the documentation!
Before reporting a bug or trying to fix it yourself, try to isolate it to the smallest possible
input file that reproduces the problem. Then send us the input file and the exact results
m4 gave you. Also say what you expected to occur; this will help us decide whether the
problem was really in the documentation.
Chapter 1: Introduction and preliminaries
5
Once you’ve got a precise problem, send e-mail to [email protected]. Please include the
version number of m4 you are using. You can get this information with the command m4
--version. Also provide details about the platform you are executing on.
Non-bug suggestions are always welcome as well. If you have questions about things
that are unclear in the documentation or are just obscure features, please report them too.
1.4 Using this manual
This manual contains a number of examples of m4 input and output, and a simple notation
is used to distinguish input, output and error messages from m4. Examples are set out from
the normal text, and shown in a fixed width font, like this
This is an example of an example!
To distinguish input from output, all output from m4 is prefixed by the string ‘⇒’,
and all error messages by the string ‘ error ’. When showing how command line options
affect matters, the command line is shown with a prompt ‘$ like this’, otherwise, you
can assume that a simple m4 invocation will work. Thus:
$ command line to invoke m4
Example of input line
⇒Output line from m4
error and an error message
The sequence ‘^D’ in an example indicates the end of the input file. The sequence ‘NL’
refers to the newline character. The majority of these examples are self-contained, and you
can run them with similar results by invoking m4 -d. In fact, the testsuite that is bundled
in the GNU M4 package consists of the examples in this document! Some of the examples
assume that your current directory is located where you unpacked the installation, so if you
plan on following along, you may find it helpful to do this now:
$ cd m4-1.4.17
As each of the predefined macros in m4 is described, a prototype call of the macro will
be shown, giving descriptive names to the arguments, e.g.,
example (string, [count = ‘1’], [argument]. . . )
[Composite]
This is a sample prototype. There is not really a macro named example, but this
documents that if there were, it would be a Composite macro, rather than a Builtin.
It requires at least one argument, string. Remember that in m4, there must not be a
space between the macro name and the opening parenthesis, unless it was intended
to call the macro without any arguments. The brackets around count and argument
show that these arguments are optional. If count is omitted, the macro behaves as
if count were ‘1’, whereas if argument is omitted, the macro behaves as if it were
the empty string. A blank argument is not the same as an omitted argument. For
example, ‘example(‘a’)’, ‘example(‘a’,‘1’)’, and ‘example(‘a’,‘1’,)’ would behave identically with count set to ‘1’; while ‘example(‘a’,)’ and ‘example(‘a’,‘’)’
would explicitly pass the empty string for count. The ellipses (‘...’) show that the
macro processes additional arguments after argument, rather than ignoring them.
All macro arguments in m4 are strings, but some are given special interpretation, e.g., as
numbers, file names, regular expressions, etc. The documentation for each macro will state
6
GNU M4 1.4.17 macro processor
how the parameters are interpreted, and what happens if the argument cannot be parsed
according to the desired interpretation. Unless specified otherwise, a parameter specified
to be a number is parsed as a decimal, even if the argument has leading zeros; and parsing
the empty string as a number results in 0 rather than an error, although a warning will be
issued.
This document consistently writes and uses builtin, without a hyphen, as if it were an
English word. This is how the builtin primitive is spelled within m4.
Chapter 2: Invoking m4
7
2 Invoking m4
The format of the m4 command is:
m4 [option...] [file...]
All options begin with ‘-’, or if long option names are used, with ‘--’. A long option
name need not be written completely, any unambiguous prefix is sufficient. POSIX requires
m4 to recognize arguments intermixed with files, even when POSIXLY_CORRECT is set in the
environment. Most options take effect at startup regardless of their position, but some are
documented below as taking effect after any files that occurred earlier in the command line.
The argument -- is a marker to denote the end of options.
With short options, options that do not take arguments may be combined into a single
command line argument with subsequent options, options with mandatory arguments may
be provided either as a single command line argument or as two arguments, and options
with optional arguments must be provided as a single argument. In other words, m4 QPDfoo -d a -df is equivalent to m4 -Q -P -D foo -d -df -- ./a, although the latter form
is considered canonical.
With long options, options with mandatory arguments may be provided with an equal
sign (‘=’) in a single argument, or as two arguments, and options with optional arguments
must be provided as a single argument. In other words, m4 --def foo --debug a is equivalent to m4 --define=foo --debug= -- ./a, although the latter form is considered canonical
(not to mention more robust, in case a future version of m4 introduces an option named
--default).
m4 understands the following options, grouped by functionality.
2.1 Command line options for operation modes
Several options control the overall operation of m4:
--help
Print a help summary on standard output, then immediately exit m4 without
reading any input files or performing any other actions.
--version
Print the version number of the program on standard output, then immediately
exit m4 without reading any input files or performing any other actions.
-E
--fatal-warnings
Controls the effect of warnings. If unspecified, then execution continues and
exit status is unaffected when a warning is printed. If specified exactly once,
warnings become fatal; when one is issued, execution continues, but the exit
status will be non-zero. If specified multiple times, then execution halts with
non-zero status the first time a warning is issued. The introduction of behavior
levels is new to M4 1.4.9; for behavior consistent with earlier versions, you
should specify -E twice.
-i
--interactive
-e
Makes this invocation of m4 interactive. This means that all output will be
unbuffered, and interrupts will be ignored. The spelling -e exists for compati-
8
GNU M4 1.4.17 macro processor
bility with other m4 implementations, and issues a warning because it may be
withdrawn in a future version of GNU M4.
-P
--prefix-builtins
Internally modify all builtin macro names so they all start with the prefix
‘m4_’. For example, using this option, one should write ‘m4_define’ instead of
‘define’, and ‘m4___file__’ instead of ‘__file__’. This option has no effect
if -R is also specified.
-Q
--quiet
--silent
Suppress warnings, such as missing or superfluous arguments in macro calls, or
treating the empty string as zero.
--warn-macro-sequence[=regexp]
Issue a warning if the regular expression regexp has a non-empty match in
any macro definition (either by define or pushdef). Empty matches are ignored; therefore, supplying the empty string as regexp disables any warning.
If the optional regexp is not supplied, then the default regular expression is
‘\$\({[^}]*}\|[0-9][0-9]+\)’ (a literal ‘$’ followed by multiple digits or by
an open brace), since these sequences will change semantics in the default operation of GNU M4 2.0 (due to a change in how more than 9 arguments in a
macro definition will be handled, see Section 5.2 [Arguments], page 26). Providing an alternate regular expression can provide a useful reverse lookup feature
of finding where a macro is defined to have a given definition.
-W regexp
--word-regexp=regexp
Use regexp as an alternative syntax for macro names. This experimental option
will not be present in all GNU m4 implementations (see Section 8.4 [Changeword], page 67).
2.2 Command line options for preprocessor features
Several options allow m4 to behave more like a preprocessor. Macro definitions and deletions
can be made on the command line, the search path can be altered, and the output file can
track where the input came from. These features occur with the following options:
-D name[=value]
--define=name[=value]
This enters name into the symbol table. If ‘=value’ is missing, the value is
taken to be the empty string. The value can be any string, and the macro can
be defined to take arguments, just as if it was defined from within the input.
This option may be given more than once; order with respect to file names is
significant, and redefining the same name loses the previous value.
Chapter 2: Invoking m4
9
-I directory
--include=directory
Make m4 search directory for included files that are not found in the current
working directory. See Section 9.2 [Search Path], page 74, for more details.
This option may be given more than once.
-s
--synclines
Generate synchronization lines, for use by the C preprocessor or other similar
tools. Order is significant with respect to file names. This option is useful, for
example, when m4 is used as a front end to a compiler. Source file name and
line number information is conveyed by directives of the form ‘#line linenum
"file"’, which are inserted as needed into the middle of the output. Such
directives mean that the following line originated or was expanded from the
contents of input file file at line linenum. The ‘"file"’ part is often omitted
when the file name did not change from the previous directive.
Synchronization directives are always given on complete lines by themselves.
When a synchronization discrepancy occurs in the middle of an output line, the
associated synchronization directive is delayed until the next newline that does
not occur in the middle of a quoted string or comment.
define(‘twoline’, ‘1
2’)
⇒#line 2 "stdin"
⇒
changecom(‘/*’, ‘*/’)
⇒
define(‘comment’, ‘/*1
2*/’)
⇒#line 5
⇒
dnl no line
hello
⇒#line 7
⇒hello
twoline
⇒1
⇒#line 8
⇒2
comment
⇒/*1
⇒2*/
one comment ‘two
three’
⇒#line 10
⇒one /*1
⇒2*/ two
⇒three
10
GNU M4 1.4.17 macro processor
goodbye
⇒#line 12
⇒goodbye
-U name
--undefine=name
This deletes any predefined meaning name might have. Obviously, only predefined macros can be deleted in this way. This option may be given more than
once; undefining a name that does not have a definition is silently ignored.
Order is significant with respect to file names.
2.3 Command line options for limits control
There are some limits within m4 that can be tuned. For compatibility, m4 also accepts
some options that control limits in other implementations, but which are automatically
unbounded (limited only by your hardware and operating system constraints) in GNU m4.
-g
--gnu
Enable all the extensions in this implementation. In this release of M4, this option is always on by default; it is currently only useful when overriding a prior
use of --traditional. However, having GNU behavior as default makes it
impossible to write a strictly POSIX-compliant client that avoids all incompatible GNU M4 extensions, since such a client would have to use the non-POSIX
command-line option to force full POSIX behavior. Thus, a future version of
M4 will be changed to implicitly use the option --traditional if the environment variable POSIXLY_CORRECT is set. Projects that intentionally use GNU
extensions should consider using --gnu to state their intentions, so that the
project will not mysteriously break if the user upgrades to a newer M4 and has
POSIXLY_CORRECT set in their environment.
-G
--traditional
Suppress all the extensions made in this implementation, compared to the System V version. See Chapter 16 [Compatibility], page 109, for a list of these.
-H num
--hashsize=num
Make the internal hash table for symbol lookup be num entries big. For better
performance, the number should be prime, but this is not checked. The default
is 509 entries. It should not be necessary to increase this value, unless you
define an excessive number of macros.
-L num
--nesting-limit=num
Artificially limit the nesting of macro calls to num levels, stopping program
execution if this limit is ever exceeded. When not specified, nesting defaults
to unlimited on platforms that can detect stack overflow, and to 1024 levels
otherwise. A value of zero means unlimited; but then heavily nested code could
potentially cause a stack overflow.
Chapter 2: Invoking m4
11
The precise effect of this option is more correctly associated with textual nesting
than dynamic recursion. It has been useful when some complex m4 input was
generated by mechanical means, and also in diagnosing recursive algorithms
that do not scale well. Most users never need to change this option from its
default.
This option does not have the ability to break endless rescanning loops, since
these do not necessarily consume much memory or stack space. Through clever
usage of rescanning loops, one can request complex, time-consuming computations from m4 with useful results. Putting limitations in this area would break
m4 power. There are many pathological cases: ‘define(‘a’, ‘a’)a’ is only the
simplest example (but see Chapter 16 [Compatibility], page 109). Expecting
GNU m4 to detect these would be a little like expecting a compiler system to
detect and diagnose endless loops: it is a quite hard problem in general, if not
undecidable!
-B num
-S num
-T num
These options are present for compatibility with System V m4, but do nothing
in this implementation. They may disappear in future releases, and issue a
warning to that effect.
-N num
--diversions=num
These options are present only for compatibility with previous versions of GNU
m4, and were controlling the number of possible diversions which could be used
at the same time. They do nothing, because there is no fixed limit anymore.
They may disappear in future releases, and issue a warning to that effect.
2.4 Command line options for frozen state
GNU m4 comes with a feature of freezing internal state (see Chapter 15 [Frozen files],
page 105). This can be used to speed up m4 execution when reusing a common initialization
script.
-F file
--freeze-state=file
Once execution is finished, write out the frozen state on the specified file. It is
conventional, but not required, for file to end in ‘.m4f’.
-R file
--reload-state=file
Before execution starts, recover the internal state from the specified frozen file.
The options -D, -U, and -t take effect after state is reloaded, but before the
input files are read.
2.5 Command line options for debugging
Finally, there are several options for aiding in debugging m4 scripts.
12
GNU M4 1.4.17 macro processor
-d[flags]
--debug[=flags]
Set the debug-level according to the flags flags. The debug-level controls the
format and amount of information presented by the debugging functions. See
Section 7.3 [Debug Levels], page 58, for more details on the format and meaning
of flags. If omitted, flags defaults to ‘aeq’.
--debugfile[=file]
-o file
--error-output=file
Redirect dumpdef output, debug messages, and trace output to the named file.
Warnings, error messages, and errprint output are still printed to standard
error. If these options are not used, or if file is unspecified (only possible for
--debugfile), debug output goes to standard error; if file is the empty string,
debug output is discarded. See Section 7.4 [Debug Output], page 60, for more
details. The option --debugfile may be given more than once, and order is
significant with respect to file names. The spellings -o and --error-output
are misleading and inconsistent with other GNU tools; for now they are silently
accepted as synonyms of --debugfile and only recognized once, but in a future
version of M4, using them will cause a warning to be issued.
-l num
--arglength=num
Restrict the size of the output generated by macro tracing to num characters
per trace line. If unspecified or zero, output is unlimited. See Section 7.3
[Debug Levels], page 58, for more details.
-t name
--trace=name
This enables tracing for the macro name, at any point where it is defined. name
need not be defined when this option is given. This option may be given more
than once, and order is significant with respect to file names. See Section 7.2
[Trace], page 55, for more details.
2.6 Specifying input files on the command line
The remaining arguments on the command line are taken to be input file names. If no
names are present, standard input is read. A file name of - is taken to mean standard
input. It is conventional, but not required, for input files to end in ‘.m4’.
The input files are read in the sequence given. Standard input can be read more than
once, so the file name - may appear multiple times on the command line; this makes a
difference when input is from a terminal or other special file type. It is an error if an input
file ends in the middle of argument collection, a comment, or a quoted string.
The options --define (-D), --undefine (-U), --synclines (-s), and --trace (-t) only
take effect after processing input from any file names that occur earlier on the command
line. For example, assume the file foo contains:
$ cat foo
bar
Chapter 2: Invoking m4
13
The text ‘bar’ can then be redefined over multiple uses of foo:
$ m4 -Dbar=hello foo -Dbar=world foo
⇒hello
⇒world
If none of the input files invoked m4exit (see Section 14.3 [M4exit], page 103), the exit
status of m4 will be 0 for success, 1 for general failure (such as problems with reading an
input file), and 63 for version mismatch (see Section 15.1 [Using frozen files], page 105).
If you need to read a file whose name starts with a -, you can specify it as ‘./-file’,
or use -- to mark the end of options.
Chapter 3: Lexical and syntactic conventions
15
3 Lexical and syntactic conventions
As m4 reads its input, it separates it into tokens. A token is either a name, a quoted
string, or any single character, that is not a part of either a name or a string. Input to
m4 can also contain comments. GNU m4 does not yet understand multibyte locales; all
operations are byte-oriented rather than character-oriented (although if your locale uses a
single byte encoding, such as ISO-8859-1, you will not notice a difference). However, m4
is eight-bit clean, so you can use non-ascii characters in quoted strings (see Section 8.2
[Changequote], page 62), comments (see Section 8.3 [Changecom], page 65), and macro
names (see Section 5.7 [Indir], page 34), with the exception of the nul character (the zero
byte ‘’\0’’).
3.1 Macro names
A name is any sequence of letters, digits, and the character ‘_’ (underscore), where the first
character is not a digit. m4 will use the longest such sequence found in the input. If a
name has a macro definition, it will be subject to macro expansion (see Chapter 4 [Macros],
page 19). Names are case-sensitive.
Examples of legal names are: ‘foo’, ‘_tmp’, and ‘name01’.
3.2 Quoting input to m4
A quoted string is a sequence of characters surrounded by quote strings, defaulting to ‘‘’
and ‘’’, where the nested begin and end quotes within the string are balanced. The value
of a string token is the text, with one level of quotes stripped off. Thus
‘’
⇒
is the empty string, and double-quoting turns into single-quoting.
‘‘quoted’’
⇒‘quoted’
The quote characters can be changed at any time, using the builtin macro changequote.
See Section 8.2 [Changequote], page 62, for more information.
3.3 Comments in m4 input
Comments in m4 are normally delimited by the characters ‘#’ and newline. All characters between the comment delimiters are ignored, but the entire comment (including the
delimiters) is passed through to the output—comments are not discarded by m4.
Comments cannot be nested, so the first newline after a ‘#’ ends the comment. The
commenting effect of the begin-comment string can be inhibited by quoting it.
$ m4
‘quoted text’ # ‘commented text’
⇒quoted text # ‘commented text’
‘quoting inhibits’ ‘#’ ‘comments’
⇒quoting inhibits # comments
The comment delimiters can be changed to any string at any time, using the builtin
macro changecom. See Section 8.3 [Changecom], page 65, for more information.
16
GNU M4 1.4.17 macro processor
3.4 Other kinds of input tokens
Any character, that is neither a part of a name, nor of a quoted string, nor a comment, is
a token by itself. When not in the context of macro expansion, all of these tokens are just
copied to output. However, during macro expansion, whitespace characters (space, tab,
newline, formfeed, carriage return, vertical tab), parentheses (‘(’ and ‘)’), comma (‘,’), and
dollar (‘$’) have additional roles, explained later.
3.5 How m4 copies input to output
As m4 reads the input token by token, it will copy each token directly to the output immediately.
The exception is when it finds a word with a macro definition. In that case m4 will
calculate the macro’s expansion, possibly reading more input to get the arguments. It then
inserts the expansion in front of the remaining input. In other words, the resulting text
from a macro call will be read and parsed into tokens again.
m4 expands a macro as soon as possible. If it finds a macro call when collecting the
arguments to another, it will expand the second call first. This process continues until
there are no more macro calls to expand and all the input has been consumed.
For a running example, examine how m4 handles this input:
format(‘Result is %d’, eval(‘2**15’))
First, m4 sees that the token ‘format’ is a macro name, so it collects the tokens ‘(’, ‘‘Result
is %d’’, ‘,’, and ‘ ’, before encountering another potential macro. Sure enough, ‘eval’ is a
macro name, so the nested argument collection picks up ‘(’, ‘‘2**15’’, and ‘)’, invoking the
eval macro with the lone argument of ‘2**15’. The expansion of ‘eval(2**15)’ is ‘32768’,
which is then rescanned as the five tokens ‘3’, ‘2’, ‘7’, ‘6’, and ‘8’; and combined with the
next ‘)’, the format macro now has all its arguments, as if the user had typed:
format(‘Result is %d’, 32768)
The format macro expands to ‘Result is 32768’, and we have another round of scanning
for the tokens ‘Result’, ‘ ’, ‘is’, ‘ ’, ‘3’, ‘2’, ‘7’, ‘6’, and ‘8’. None of these are macros, so
the final output is
⇒Result is 32768
As a more complicated example, we will contrast an actual code example from the Gnulib
project1 , showing both a buggy approach and the desired results. The user desires to output
a shell assignment statement that takes its argument and turns it into a shell variable by
converting it to uppercase and prepending a prefix. The original attempt looks like this:
changequote([,])dnl
define([gl_STRING_MODULE_INDICATOR],
[
dnl comment
GNULIB_]translit([$1],[a-z],[A-Z])[=1
])dnl
gl_STRING_MODULE_INDICATOR([strcase])
1
Derived from a patch in http://lists.gnu.org/archive/html/bug-gnulib/2007-01/msg00389.html,
and a followup patch in http://lists.gnu.org/archive/html/bug-gnulib/2007-02/msg00000.html
Chapter 3: Lexical and syntactic conventions
⇒
⇒
⇒
17
GNULIB_strcase=1
Oops – the argument did not get capitalized. And although the manual is not able
to easily show it, both lines that appear empty actually contain two trailing spaces. By
stepping through the parse, it is easy to see what happened. First, m4 sees the token
‘changequote’, which it recognizes as a macro, followed by ‘(’, ‘[’, ‘,’, ‘]’, and ‘)’ to
form the argument list. The macro expands to the empty string, but changes the quoting
characters to something more useful for generating shell code (unbalanced ‘‘’ and ‘’’ appear
all the time in shell scripts, but unbalanced ‘[]’ tend to be rare). Also in the first line, m4
sees the token ‘dnl’, which it recognizes as a builtin macro that consumes the rest of the
line, resulting in no output for that line.
The second line starts a macro definition. m4 sees the token ‘define’, which it recognizes
as a macro, followed by a ‘(’, ‘[gl_STRING_MODULE_INDICATOR]’, and ‘,’. Because an
unquoted comma was encountered, the first argument is known to be the expansion of
the single-quoted string token, or ‘gl_STRING_MODULE_INDICATOR’. Next, m4 sees ‘NL’, ‘
’, and ‘ ’, but this whitespace is discarded as part of argument collection. Then comes a
rather lengthy single-quoted string token, ‘[NL
dnl commentNL
GNULIB_]’. This is
followed by the token ‘translit’, which m4 recognizes as a macro name, so a nested macro
expansion has started.
The arguments to the translit are found by the tokens ‘(’, ‘[$1]’, ‘,’, ‘[a-z]’, ‘,’,
‘[A-Z]’, and finally ‘)’. All three string arguments are expanded (or in other words, the
quotes are stripped), and since neither ‘$’ nor ‘1’ need capitalization, the result of the macro
is ‘$1’. This expansion is rescanned, resulting in the two literal characters ‘$’ and ‘1’.
Scanning of the outer macro resumes, and picks up with ‘[=1NL ]’, and finally ‘)’.
The collected pieces of expanded text are concatenated, with the end result that the macro
‘gl_STRING_MODULE_INDICATOR’ is now defined to be the sequence ‘NL
dnl commentNL
GNULIB_$1=1NL ’. Once again, ‘dnl’ is recognized and avoids a newline in the output.
The final line is then parsed, beginning with ‘ ’ and ‘ ’ that are output literally. Then
‘gl_STRING_MODULE_INDICATOR’ is recognized as a macro name, with an argument list of
‘(’, ‘[strcase]’, and ‘)’. Since the definition of the macro contains the sequence ‘$1’,
that sequence is replaced with the argument ‘strcase’ prior to starting the rescan. The
rescan sees ‘NL’ and four spaces, which are output literally, then ‘dnl’, which discards
the text ‘ commentNL’. Next comes four more spaces, also output literally, and the token
‘GNULIB_strcase’, which resulted from the earlier parameter substitution. Since that is
not a macro name, it is output literally, followed by the literal tokens ‘=’, ‘1’, ‘NL’, and
two more spaces. Finally, the original ‘NL’ seen after the macro invocation is scanned and
output literally.
Now for a corrected approach. This rearranges the use of newlines and whitespace so
that less whitespace is output (which, although harmless to shell scripts, can be visually
unappealing), and fixes the quoting issues so that the capitalization occurs when the macro
‘gl_STRING_MODULE_INDICATOR’ is invoked, rather then when it is defined. It also adds
another layer of quoting to the first argument of translit, to ensure that the output will
be rescanned as a string rather than a potential uppercase macro name needing further
expansion.
18
GNU M4 1.4.17 macro processor
changequote([,])dnl
define([gl_STRING_MODULE_INDICATOR],
[dnl comment
GNULIB_[]translit([[$1]], [a-z], [A-Z])=1dnl
])dnl
gl_STRING_MODULE_INDICATOR([strcase])
⇒
GNULIB_STRCASE=1
The parsing of the first line is unchanged. The second line sees the name of the macro
to define, then sees the discarded ‘NL’ and two spaces, as before. But this time, the
next token is ‘[dnl commentNL GNULIB_[]translit([[$1]], [a-z], [A-Z])=1dnlNL]’,
which includes nested quotes, followed by ‘)’ to end the macro definition and ‘dnl’ to skip
the newline. No early expansion of translit occurs, so the entire string becomes the
definition of the macro.
The final line is then parsed, beginning with two spaces that are output literally, and
an invocation of gl_STRING_MODULE_INDICATOR with the argument ‘strcase’. Again, the
‘$1’ in the macro definition is substituted prior to rescanning. Rescanning first encounters
‘dnl’, and discards ‘ commentNL’. Then two spaces are output literally. Next comes the
token ‘GNULIB_’, but that is not a macro, so it is output literally. The token ‘[]’ is an
empty string, so it does not affect output. Then the token ‘translit’ is encountered.
This time, the arguments to translit are parsed as ‘(’, ‘[[strcase]]’, ‘,’, ‘ ’, ‘[a-z]’,
‘,’, ‘ ’, ‘[A-Z]’, and ‘)’. The two spaces are discarded, and the translit results in the desired
result ‘[STRCASE]’. This is rescanned, but since it is a string, the quotes are stripped and
the only output is a literal ‘STRCASE’. Then the scanner sees ‘=’ and ‘1’, which are output
literally, followed by ‘dnl’ which discards the rest of the definition of gl_STRING_MODULE_
INDICATOR. The newline at the end of output is the literal ‘NL’ that appeared after the
invocation of the macro.
The order in which m4 expands the macros can be further explored using the trace
facilities of GNU m4 (see Section 7.2 [Trace], page 55).
Chapter 4: How to invoke macros
19
4 How to invoke macros
This chapter covers macro invocation, macro arguments and how macro expansion is treated.
4.1 Macro invocation
Macro invocations has one of the forms
name
which is a macro invocation without any arguments, or
name(arg1, arg2, ..., argn)
which is a macro invocation with n arguments. Macros can have any number of arguments.
All arguments are strings, but different macros might interpret the arguments in different
ways.
The opening parenthesis must follow the name directly, with no spaces in between. If it
does not, the macro is called with no arguments at all.
For a macro call to have no arguments, the parentheses must be left out. The macro
call
name()
is a macro call with one argument, which is the empty string, not a call with no arguments.
4.2 Preventing macro invocation
An innovation of the m4 language, compared to some of its predecessors (like Strachey’s GPM,
for example), is the ability to recognize macro calls without resorting to any special, prefixed
invocation character. While generally useful, this feature might sometimes be the source of
spurious, unwanted macro calls. So, GNU m4 offers several mechanisms or techniques for
inhibiting the recognition of names as macro calls.
First of all, many builtin macros cannot meaningfully be called without arguments.
As a GNU extension, for any of these macros, whenever an opening parenthesis does not
immediately follow their name, the builtin macro call is not triggered. This solves the most
usual cases, like for ‘include’ or ‘eval’. Later in this document, the sentence “This macro
is recognized only with parameters” refers to this specific provision of GNU M4, also known
as a blind builtin macro. For the builtins defined by POSIX that bear this disclaimer,
POSIX specifically states that invoking those builtins without arguments is unspecified,
because many other implementations simply invoke the builtin as though it were given one
empty argument instead.
$ m4
eval
⇒eval
eval(‘1’)
⇒1
There is also a command line option (--prefix-builtins, or -P, see Section 2.1 [Invoking m4], page 7) that renames all builtin macros with a prefix of ‘m4_’ at startup. The
option has no effect whatsoever on user defined macros. For example, with this option, one
has to write m4_dnl and even m4_m4exit. It also has no effect on whether a macro requires
parameters.
20
GNU M4 1.4.17 macro processor
$ m4 -P
eval
⇒eval
eval(‘1’)
⇒eval(1)
m4_eval
⇒m4_eval
m4_eval(‘1’)
⇒1
Another alternative is to redefine problematic macros to a name less likely to cause
conflicts, using Chapter 5 [Definitions], page 25.
If your version of GNU m4 has the changeword feature compiled in, it offers far more flexibility in specifying the syntax of macro names, both builtin or user-defined. See Section 8.4
[Changeword], page 67, for more information on this experimental feature.
Of course, the simplest way to prevent a name from being interpreted as a call to an
existing macro is to quote it. The remainder of this section studies a little more deeply how
quoting affects macro invocation, and how quoting can be used to inhibit macro invocation.
Even if quoting is usually done over the whole macro name, it can also be done over
only a few characters of this name (provided, of course, that the unquoted portions are not
also a macro). It is also possible to quote the empty string, but this works only inside the
name. For example:
‘divert’
⇒divert
‘d’ivert
⇒divert
di‘ver’t
⇒divert
div‘’ert
⇒divert
all yield the string ‘divert’. While in both:
‘’divert
⇒
divert‘’
⇒
the divert builtin macro will be called, which expands to the empty string.
The output of macro evaluations is always rescanned. In the following example, the input
‘x‘’y’ yields the string ‘bCD’, exactly as if m4 has been given ‘substr(ab‘’cde, ‘1’, ‘3’)’
as input:
define(‘cde’, ‘CDE’)
⇒
define(‘x’, ‘substr(ab’)
⇒
define(‘y’, ‘cde, ‘1’, ‘3’)’)
⇒
x‘’y
Chapter 4: How to invoke macros
21
⇒bCD
Unquoted strings on either side of a quoted string are subject to being recognized as
macro names. In the following example, quoting the empty string allows for the second
macro to be recognized as such:
define(‘macro’, ‘m’)
⇒
macro(‘m’)macro
⇒mmacro
macro(‘m’)‘’macro
⇒mm
Quoting may prevent recognizing as a macro name the concatenation of a macro expansion with the surrounding characters. In this example:
define(‘macro’, ‘di$1’)
⇒
macro(‘v’)‘ert’
⇒divert
macro(‘v’)ert
⇒
the input will produce the string ‘divert’. When the quotes were removed, the divert
builtin was called instead.
4.3 Macro arguments
When a name is seen, and it has a macro definition, it will be expanded as a macro.
If the name is followed by an opening parenthesis, the arguments will be collected before
the macro is called. If too few arguments are supplied, the missing arguments are taken
to be the empty string. However, some builtins are documented to behave differently for
a missing optional argument than for an explicit empty string. If there are too many
arguments, the excess arguments are ignored. Unquoted leading whitespace is stripped off
all arguments, but whitespace generated by a macro expansion or occurring after a macro
that expanded to an empty string remains intact. Whitespace includes space, tab, newline,
carriage return, vertical tab, and formfeed.
define(‘macro’, ‘$1’)
⇒
macro( unquoted leading space lost)
⇒unquoted leading space lost
macro(‘ quoted leading space kept’)
⇒ quoted leading space kept
macro(
divert ‘unquoted space kept after expansion’)
⇒ unquoted space kept after expansion
macro(macro(‘
’)‘whitespace from expansion kept’)
⇒
⇒whitespace from expansion kept
macro(‘unquoted trailing whitespace kept’
22
GNU M4 1.4.17 macro processor
)
⇒unquoted trailing whitespace kept
⇒
Normally m4 will issue warnings if a builtin macro is called with an inappropriate number
of arguments, but it can be suppressed with the --quiet command line option (or --silent,
or -Q, see Section 2.1 [Invoking m4], page 7). For user defined macros, there is no check of
the number of arguments given.
$ m4
index(‘abc’)
error m4:stdin:1: Warning: too few arguments to builtin ‘index’
⇒0
index(‘abc’,)
⇒0
index(‘abc’, ‘b’, ‘ignored’)
error m4:stdin:3: Warning: excess arguments to builtin ‘index’ ignored
⇒1
$ m4 -Q
index(‘abc’)
⇒0
index(‘abc’,)
⇒0
index(‘abc’, ‘b’, ‘ignored’)
⇒1
Macros are expanded normally during argument collection, and whatever commas,
quotes and parentheses that might show up in the resulting expanded text will serve to
define the arguments as well. Thus, if foo expands to ‘, b, c’, the macro call
bar(a foo, d)
is a macro call with four arguments, which are ‘a ’, ‘b’, ‘c’ and ‘d’. To understand why the
first argument contains whitespace, remember that unquoted leading whitespace is never
part of an argument, but trailing whitespace always is.
It is possible for a macro’s definition to change during argument collection, in which case
the expansion uses the definition that was in effect at the time the opening ‘(’ was seen.
define(‘f’, ‘1’)
⇒
f(define(‘f’, ‘2’))
⇒1
f
⇒2
It is an error if the end of file occurs while collecting arguments.
hello world
⇒hello world
define(
^D
error m4:stdin:2: ERROR: end of file in argument list
Chapter 4: How to invoke macros
23
4.4 On Quoting Arguments to macros
Each argument has unquoted leading whitespace removed. Within each argument, all unquoted parentheses must match. For example, if foo is a macro,
foo(() (‘(’) ‘(’)
is a macro call, with one argument, whose value is ‘() (() (’. Commas separate arguments,
except when they occur inside quotes, comments, or unquoted parentheses. See Section 5.3
[Pseudo Arguments], page 27, for examples.
It is common practice to quote all arguments to macros, unless you are sure you want
the arguments expanded. Thus, in the above example with the parentheses, the ‘right’ way
to do it is like this:
foo(‘() (() (’)
It is, however, in certain cases necessary (because nested expansion must occur to create
the arguments for the outer macro) or convenient (because it uses fewer characters) to leave
out quotes for some arguments, and there is nothing wrong in doing it. It just makes life a
bit harder, if you are not careful to follow a consistent quoting style. For consistency, this
manual follows the rule of thumb that each layer of parentheses introduces another layer
of single quoting, except when showing the consequences of quoting rules. This is done
even when the quoted string cannot be a macro, such as with integers when you have not
changed the syntax via changeword (see Section 8.4 [Changeword], page 67).
The quoting rule of thumb of one level of quoting per parentheses has a nice property:
when a macro name appears inside parentheses, you can determine when it will be expanded.
If it is not quoted, it will be expanded prior to the outer macro, so that its expansion becomes
the argument. If it is single-quoted, it will be expanded after the outer macro. And if it is
double-quoted, it will be used as literal text instead of a macro name.
define(‘active’, ‘ACT, IVE’)
⇒
define(‘show’, ‘$1 $1’)
⇒
show(active)
⇒ACT ACT
show(‘active’)
⇒ACT, IVE ACT, IVE
show(‘‘active’’)
⇒active active
4.5 Macro expansion
When the arguments, if any, to a macro call have been collected, the macro is expanded, and
the expansion text is pushed back onto the input (unquoted), and reread. The expansion
text from one macro call might therefore result in more macros being called, if the calls are
included, completely or partially, in the first macro calls’ expansion.
Taking a very simple example, if foo expands to ‘bar’, and bar expands to ‘Hello’, the
input
$ m4 -Dbar=Hello -Dfoo=bar
foo
24
GNU M4 1.4.17 macro processor
⇒Hello
will expand first to ‘bar’, and when this is reread and expanded, into ‘Hello’.
Chapter 5: How to define new macros
25
5 How to define new macros
Macros can be defined, redefined and deleted in several different ways. Also, it is possible
to redefine a macro without losing a previous value, and bring back the original value at a
later time.
5.1 Defining a macro
The normal way to define or redefine macros is to use the builtin define:
define (name, [expansion])
[Builtin]
Defines name to expand to expansion. If expansion is not given, it is taken to be
empty.
The expansion of define is void. The macro define is recognized only with parameters.
The following example defines the macro foo to expand to the text ‘Hello World.’.
define(‘foo’, ‘Hello world.’)
⇒
foo
⇒Hello world.
The empty line in the output is there because the newline is not a part of the macro
definition, and it is consequently copied to the output. This can be avoided by use of the
macro dnl. See Section 8.1 [Dnl], page 61, for details.
The first argument to define should be quoted; otherwise, if the macro is already
defined, you will be defining a different macro. This example shows the problems with
underquoting, since we did not want to redefine one:
define(foo, one)
⇒
define(foo, two)
⇒
one
⇒two
GNU m4 normally replaces only the topmost definition of a macro if it has several definitions from pushdef (see Section 5.6 [Pushdef], page 33). Some other implementations
of m4 replace all definitions of a macro with define. See Section 16.2 [Incompatibilities],
page 110, for more details.
As a GNU extension, the first argument to define does not have to be a simple word. It
can be any text string, even the empty string. A macro with a non-standard name cannot
be invoked in the normal way, as the name is not recognized. It can only be referenced
by the builtins indir (see Section 5.7 [Indir], page 34) and defn (see Section 5.5 [Defn],
page 31).
Arrays and associative arrays can be simulated by using non-standard macro names.
array (index)
array_set (index, [value])
[Composite]
[Composite]
Provide access to entries within an array. array reads the entry at location index,
and array_set assigns value to location index.
26
GNU M4 1.4.17 macro processor
define(‘array’, ‘defn(format(‘‘array[%d]’’, ‘$1’))’)
⇒
define(‘array_set’, ‘define(format(‘‘array[%d]’’, ‘$1’), ‘$2’)’)
⇒
array_set(‘4’, ‘array element no. 4’)
⇒
array_set(‘17’, ‘array element no. 17’)
⇒
array(‘4’)
⇒array element no. 4
array(eval(‘10 + 7’))
⇒array element no. 17
Change the ‘%d’ to ‘%s’ and it is an associative array.
5.2 Arguments to macros
Macros can have arguments. The nth argument is denoted by $n in the expansion text,
and is replaced by the nth actual argument, when the macro is expanded. Replacement
of arguments happens before rescanning, regardless of how many nesting levels of quoting
appear in the expansion. Here is an example of a macro with two arguments.
exch (arg1, arg2)
[Composite]
Expands to arg2 followed by arg1, effectively exchanging their order.
define(‘exch’, ‘$2, $1’)
⇒
exch(‘arg1’, ‘arg2’)
⇒arg2, arg1
This can be used, for example, if you like the arguments to define to be reversed.
define(‘exch’, ‘$2, $1’)
⇒
define(exch(‘‘expansion text’’, ‘‘macro’’))
⇒
macro
⇒expansion text
See Section 4.4 [Quoting Arguments], page 23, for an explanation of the double quotes.
(You should try and improve this example so that clients of exch do not have to double
quote; or see Section 17.1 [Answers], page 115).
As a special case, the zeroth argument, $0, is always the name of the macro being
expanded.
define(‘test’, ‘‘Macro name: $0’’)
⇒
test
⇒Macro name: test
If you want quoted text to appear as part of the expansion text, remember that quotes
can be nested in quoted strings. Thus, in
Chapter 5: How to define new macros
27
define(‘foo’, ‘This is macro ‘foo’.’)
⇒
foo
⇒This is macro foo.
The ‘foo’ in the expansion text is not expanded, since it is a quoted string, and not a name.
GNU m4 allows the number following the ‘$’ to consist of one or more digits, allowing
macros to have any number of arguments. The extension of accepting multiple digits is
incompatible with POSIX, and is different than traditional implementations of m4, which
only recognize one digit. Therefore, future versions of GNU M4 will phase out this feature.
To portably access beyond the ninth argument, you can use the argn macro documented
later (see Section 6.3 [Shift], page 41).
POSIX also states that ‘$’ followed immediately by ‘{’ in a macro definition is
implementation-defined. This version of M4 passes the literal characters ‘${’ through
unchanged, but M4 2.0 will implement an optional feature similar to sh, where ‘${11}’
expands to the eleventh argument, to replace the current recognition of ‘$11’. Meanwhile,
if you want to guarantee that you will get a literal ‘${’ in output when expanding a macro,
even when you upgrade to M4 2.0, you can use nested quoting to your advantage:
define(‘foo’, ‘single quoted $‘’{1} output’)
⇒
define(‘bar’, ‘‘double quoted $’‘{2} output’’)
⇒
foo(‘a’, ‘b’)
⇒single quoted ${1} output
bar(‘a’, ‘b’)
⇒double quoted ${2} output
To help you detect places in your M4 input files that might change in behavior due to
the changed behavior of M4 2.0, you can use the --warn-macro-sequence command-line
option (see Section 2.1 [Invoking m4], page 7) with the default regular expression. This
will add a warning any time a macro definition includes ‘$’ followed by multiple digits, or
by ‘{’. The warning is not enabled by default, because it triggers a number of warnings in
Autoconf 2.61 (and Autoconf uses -E to treat warnings as errors), and because it will still
be possible to restore older behavior in M4 2.0.
$ m4 --warn-macro-sequence
define(‘foo’, ‘$001 ${1} $1’)
error m4:stdin:1: Warning: definition of ‘foo’ contains sequence ‘$001’
error m4:stdin:1: Warning: definition of ‘foo’ contains sequence ‘${1}’
⇒
foo(‘bar’)
⇒bar ${1} bar
5.3 Special arguments to macros
There is a special notation for the number of actual arguments supplied, and for all the
actual arguments.
The number of actual arguments in a macro call is denoted by $# in the expansion text.
28
GNU M4 1.4.17 macro processor
nargs (. . . )
[Composite]
Expands to a count of the number of arguments supplied.
define(‘nargs’, ‘$#’)
⇒
nargs
⇒0
nargs()
⇒1
nargs(‘arg1’, ‘arg2’, ‘arg3’)
⇒3
nargs(‘commas can be quoted, like this’)
⇒1
nargs(arg1#inside comments, commas do not separate arguments
still arg1)
⇒1
nargs((unquoted parentheses, like this, group arguments))
⇒1
Remember that ‘#’ defaults to the comment character; if you forget quotes to inhibit the
comment behavior, your macro definition may not end where you expected.
dnl Attempt to define a macro to just ‘$#’
define(underquoted, $#)
oops)
⇒
underquoted
⇒0)
⇒oops
The notation $* can be used in the expansion text to denote all the actual arguments,
unquoted, with commas in between. For example
define(‘echo’, ‘$*’)
⇒
echo(arg1,
arg2, arg3 , arg4)
⇒arg1,arg2,arg3 ,arg4
Often each argument should be quoted, and the notation $@ handles that. It is just like
$*, except that it quotes each argument. A simple example of that is:
define(‘echo’, ‘$@’)
⇒
echo(arg1,
arg2, arg3 , arg4)
⇒arg1,arg2,arg3 ,arg4
Where did the quotes go? Of course, they were eaten, when the expanded text were
reread by m4. To show the difference, try
define(‘echo1’, ‘$*’)
⇒
define(‘echo2’, ‘$@’)
⇒
define(‘foo’, ‘This is macro ‘foo’.’)
Chapter 5: How to define new macros
29
⇒
echo1(foo)
⇒This is macro This is macro foo..
echo1(‘foo’)
⇒This is macro foo.
echo2(foo)
⇒This is macro foo.
echo2(‘foo’)
⇒foo
See Section 7.2 [Trace], page 55, if you do not understand this. As another example of the
difference, remember that comments encountered in arguments are passed untouched to the
macro, and that quoting disables comments.
define(‘echo1’, ‘$*’)
⇒
define(‘echo2’, ‘$@’)
⇒
define(‘foo’, ‘bar’)
⇒
echo1(#foo’foo
foo)
⇒#foo’foo
⇒bar
echo2(#foo’foo
foo)
⇒#foobar
⇒bar’
A ‘$’ sign in the expansion text, that is not followed by anything m4 understands, is
simply copied to the macro expansion, as any other text is.
define(‘foo’, ‘$$$ hello $$$’)
⇒
foo
⇒$$$ hello $$$
If you want a macro to expand to something like ‘$12’, the judicious use of nested quoting
can put a safe character between the $ and the next character, relying on the rescanning to
remove the nested quote. This will prevent m4 from interpreting the $ sign as a reference
to an argument.
define(‘foo’, ‘no nested quote: $1’)
⇒
foo(‘arg’)
⇒no nested quote: arg
define(‘foo’, ‘nested quote around $: ‘$’1’)
⇒
foo(‘arg’)
⇒nested quote around $: $1
define(‘foo’, ‘nested empty quote after $: $‘’1’)
30
GNU M4 1.4.17 macro processor
⇒
foo(‘arg’)
⇒nested empty quote after $: $1
define(‘foo’, ‘nested quote around next character: $‘1’’)
⇒
foo(‘arg’)
⇒nested quote around next character: $1
define(‘foo’, ‘nested quote around both: ‘$1’’)
⇒
foo(‘arg’)
⇒nested quote around both: arg
5.4 Deleting a macro
A macro definition can be removed with undefine:
undefine (name . . . )
[Builtin]
For each argument, remove the macro name. The macro names must necessarily be
quoted, since they will be expanded otherwise.
The expansion of undefine is void. The macro undefine is recognized only with
parameters.
foo bar blah
⇒foo bar blah
define(‘foo’, ‘some’)define(‘bar’, ‘other’)define(‘blah’, ‘text’)
⇒
foo bar blah
⇒some other text
undefine(‘foo’)
⇒
foo bar blah
⇒foo other text
undefine(‘bar’, ‘blah’)
⇒
foo bar blah
⇒foo bar blah
Undefining a macro inside that macro’s expansion is safe; the macro still expands to the
definition that was in effect at the ‘(’.
define(‘f’, ‘‘$0’:$1’)
⇒
f(f(f(undefine(‘f’)‘hello world’)))
⇒f:f:f:hello world
f(‘bye’)
⇒f(bye)
It is not an error for name to have no macro definition. In that case, undefine does
nothing.
Chapter 5: How to define new macros
31
5.5 Renaming macros
It is possible to rename an already defined macro. To do this, you need the builtin defn:
defn (name . . . )
[Builtin]
Expands to the quoted definition of each name. If an argument is not a defined macro,
the expansion for that argument is empty.
If name is a user-defined macro, the quoted definition is simply the quoted expansion
text. If, instead, there is only one name and it is a builtin, the expansion is a special
token, which points to the builtin’s internal definition. This token is only meaningful
as the second argument to define (and pushdef), and is silently converted to an
empty string in most other contexts. Combining a builtin with anything else is not
supported; a warning is issued and the builtin is omitted from the final expansion.
The macro defn is recognized only with parameters.
Its normal use is best understood through an example, which shows how to rename
undefine to zap:
define(‘zap’, defn(‘undefine’))
⇒
zap(‘undefine’)
⇒
undefine(‘zap’)
⇒undefine(zap)
In this way, defn can be used to copy macro definitions, and also definitions of builtin
macros. Even if the original macro is removed, the other name can still be used to access
the definition.
The fact that macro definitions can be transferred also explains why you should use $0,
rather than retyping a macro’s name in its definition:
define(‘foo’, ‘This is ‘$0’’)
⇒
define(‘bar’, defn(‘foo’))
⇒
bar
⇒This is bar
Macros used as string variables should be referred through defn, to avoid unwanted
expansion of the text:
define(‘string’, ‘The macro dnl is very useful
’)
⇒
string
⇒The macro
defn(‘string’)
⇒The macro dnl is very useful
⇒
However, it is important to remember that m4 rescanning is purely textual. If an unbalanced end-quote string occurs in a macro definition, the rescan will see that embedded
32
GNU M4 1.4.17 macro processor
quote as the termination of the quoted string, and the remainder of the macro’s definition
will be rescanned unquoted. Thus it is a good idea to avoid unbalanced end-quotes in macro
definitions or arguments to macros.
define(‘foo’, a’a)
⇒
define(‘a’, ‘A’)
⇒
define(‘echo’, ‘$@’)
⇒
foo
⇒A’A
defn(‘foo’)
⇒aA’
echo(foo)
⇒AA’
On the other hand, it is possible to exploit the fact that defn can concatenate multiple
macros prior to the rescanning phase, in order to join the definitions of macros that, in
isolation, have unbalanced quotes. This is particularly useful when one has used several
macros to accumulate text that M4 should rescan as a whole. In the example below, note
how the use of defn on l in isolation opens a string, which is not closed until the next line;
but used on l and r together results in nested quoting.
define(‘l’, ‘<[>’)define(‘r’, ‘<]>’)
⇒
changequote(‘[’, ‘]’)
⇒
defn([l])defn([r])
])
⇒<[>]defn([r])
⇒)
defn([l], [r])
⇒<[>][<]>
Using defn to generate special tokens for builtin macros outside of expected contexts
can sometimes trigger warnings. But most of the time, such tokens are silently converted
to the empty string.
$ m4 -d
defn(‘defn’)
⇒
define(defn(‘divnum’), ‘cannot redefine a builtin token’)
error m4:stdin:2: Warning: define: invalid macro name ignored
⇒
divnum
⇒0
len(defn(‘divnum’))
⇒0
Also note that defn with multiple arguments can only join text macros, not builtins,
although a future version of GNU M4 may lift this restriction.
Chapter 5: How to define new macros
33
$ m4 -d
define(‘a’, ‘A’)define(‘AA’, ‘b’)
⇒
traceon(‘defn’, ‘define’)
⇒
defn(‘a’, ‘divnum’, ‘a’)
error m4:stdin:3: Warning: cannot concatenate builtin ‘divnum’
error m4trace: -1- defn(‘a’, ‘divnum’, ‘a’) -> ‘‘A’‘A’’
⇒AA
define(‘mydivnum’, defn(‘divnum’, ‘divnum’))mydivnum
error m4:stdin:4: Warning: cannot concatenate builtin ‘divnum’
error m4:stdin:4: Warning: cannot concatenate builtin ‘divnum’
error m4trace: -2- defn(‘divnum’, ‘divnum’)
error m4trace: -1- define(‘mydivnum’, ‘’)
⇒
traceoff(‘defn’, ‘define’)
⇒
5.6 Temporarily redefining macros
It is possible to redefine a macro temporarily, reverting to the previous definition at a later
time. This is done with the builtins pushdef and popdef:
pushdef (name, [expansion])
popdef (name . . . )
[Builtin]
[Builtin]
Analogous to define and undefine.
These macros work in a stack-like fashion. A macro is temporarily redefined with
pushdef, which replaces an existing definition of name, while saving the previous
definition, before the new one is installed. If there is no previous definition, pushdef
behaves exactly like define.
If a macro has several definitions (of which only one is accessible), the topmost definition can be removed with popdef. If there is no previous definition, popdef behaves
like undefine.
The expansion of both pushdef and popdef is void. The macros pushdef and popdef
are recognized only with parameters.
define(‘foo’, ‘Expansion one.’)
⇒
foo
⇒Expansion one.
pushdef(‘foo’, ‘Expansion two.’)
⇒
foo
⇒Expansion two.
pushdef(‘foo’, ‘Expansion three.’)
⇒
pushdef(‘foo’, ‘Expansion four.’)
⇒
34
GNU M4 1.4.17 macro processor
popdef(‘foo’)
⇒
foo
⇒Expansion three.
popdef(‘foo’, ‘foo’)
⇒
foo
⇒Expansion one.
popdef(‘foo’)
⇒
foo
⇒foo
If a macro with several definitions is redefined with define, the topmost definition is
replaced with the new definition. If it is removed with undefine, all the definitions are
removed, and not only the topmost one. However, POSIX allows other implementations
that treat define as replacing an entire stack of definitions with a single new definition,
so to be portable to other implementations, it may be worth explicitly using popdef and
pushdef rather than relying on the GNU behavior of define.
define(‘foo’, ‘Expansion one.’)
⇒
foo
⇒Expansion one.
pushdef(‘foo’, ‘Expansion two.’)
⇒
foo
⇒Expansion two.
define(‘foo’, ‘Second expansion two.’)
⇒
foo
⇒Second expansion two.
undefine(‘foo’)
⇒
foo
⇒foo
Local variables within macros are made with pushdef and popdef. At the start of the
macro a new definition is pushed, within the macro it is manipulated and at the end it is
popped, revealing the former definition.
It is possible to temporarily redefine a builtin with pushdef and defn.
5.7 Indirect call of macros
Any macro can be called indirectly with indir:
indir (name, [args...])
[Builtin]
Results in a call to the macro name, which is passed the rest of the arguments args.
If name is not defined, an error message is printed, and the expansion is void.
The macro indir is recognized only with parameters.
Chapter 5: How to define new macros
35
This can be used to call macros with computed or “invalid” names (define allows such
names to be defined):
define(‘$$internal$macro’, ‘Internal macro (name ‘$0’)’)
⇒
$$internal$macro
⇒$$internal$macro
indir(‘$$internal$macro’)
⇒Internal macro (name $$internal$macro)
The point is, here, that larger macro packages can have private macros defined, that will
not be called by accident. They can only be called through the builtin indir.
One other point to observe is that argument collection occurs before indir invokes
name, so if argument collection changes the value of name, that will be reflected in the final
expansion. This is different than the behavior when invoking macros directly, where the
definition that was in effect before argument collection is used.
$ m4 -d
define(‘f’, ‘1’)
⇒
f(define(‘f’, ‘2’))
⇒1
indir(‘f’, define(‘f’, ‘3’))
⇒3
indir(‘f’, undefine(‘f’))
error m4:stdin:4: undefined macro ‘f’
⇒
When handed the result of defn (see Section 5.5 [Defn], page 31) as one of its arguments,
indir defers to the invoked name for whether a token representing a builtin is recognized
or flattened to the empty string.
$ m4 -d
indir(defn(‘defn’), ‘divnum’)
error m4:stdin:1: Warning: indir: invalid macro name ignored
⇒
indir(‘define’, defn(‘defn’), ‘divnum’)
error m4:stdin:2: Warning: define: invalid macro name ignored
⇒
indir(‘define’, ‘foo’, defn(‘divnum’))
⇒
foo
⇒0
indir(‘divert’, defn(‘foo’))
error m4:stdin:5: empty string treated as 0 in builtin ‘divert’
⇒
5.8 Indirect call of builtins
Builtin macros can be called indirectly with builtin:
36
GNU M4 1.4.17 macro processor
builtin (name, [args...])
[Builtin]
Results in a call to the builtin name, which is passed the rest of the arguments args.
If name does not name a builtin, an error message is printed, and the expansion is
void.
The macro builtin is recognized only with parameters.
This can be used even if name has been given another definition that has covered the
original, or been undefined so that no macro maps to the builtin.
pushdef(‘define’, ‘hidden’)
⇒
undefine(‘undefine’)
⇒
define(‘foo’, ‘bar’)
⇒hidden
foo
⇒foo
builtin(‘define’, ‘foo’, defn(‘divnum’))
⇒
foo
⇒0
builtin(‘define’, ‘foo’, ‘BAR’)
⇒
foo
⇒BAR
undefine(‘foo’)
⇒undefine(foo)
foo
⇒BAR
builtin(‘undefine’, ‘foo’)
⇒
foo
⇒foo
The name argument only matches the original name of the builtin, even when the -prefix-builtins option (or -P, see Section 2.1 [Invoking m4], page 7) is in effect. This is
different from indir, which only tracks current macro names.
$ m4 -P
m4_builtin(‘divnum’)
⇒0
m4_builtin(‘m4_divnum’)
error m4:stdin:2: undefined builtin ‘m4_divnum’
⇒
m4_indir(‘divnum’)
error m4:stdin:3: undefined macro ‘divnum’
⇒
m4_indir(‘m4_divnum’)
⇒0
Chapter 5: How to define new macros
37
Note that indir and builtin can be used to invoke builtins without arguments, even
when they normally require parameters to be recognized; but it will provoke a warning, and
result in a void expansion.
builtin
⇒builtin
builtin()
error m4:stdin:2: undefined builtin ‘’
⇒
builtin(‘builtin’)
error m4:stdin:3: Warning: too few arguments to builtin ‘builtin’
⇒
builtin(‘builtin’,)
error m4:stdin:4: undefined builtin ‘’
⇒
builtin(‘builtin’, ‘‘’
’)
error m4:stdin:5: undefined builtin ‘‘’
error ’
⇒
indir(‘index’)
error m4:stdin:7: Warning: too few arguments to builtin ‘index’
⇒
Chapter 6: Conditionals, loops, and recursion
39
6 Conditionals, loops, and recursion
Macros, expanding to plain text, perhaps with arguments, are not quite enough. We would
like to have macros expand to different things, based on decisions taken at run-time. For
that, we need some kind of conditionals. Also, we would like to have some kind of loop
construct, so we could do something a number of times, or while some condition is true.
6.1 Testing if a macro is defined
There are two different builtin conditionals in m4. The first is ifdef:
ifdef (name, string-1, [string-2])
[Builtin]
If name is defined as a macro, ifdef expands to string-1, otherwise to string-2. If
string-2 is omitted, it is taken to be the empty string (according to the normal rules).
The macro ifdef is recognized only with parameters.
ifdef(‘foo’, ‘‘foo’ is defined’, ‘‘foo’ is not defined’)
⇒foo is not defined
define(‘foo’, ‘’)
⇒
ifdef(‘foo’, ‘‘foo’ is defined’, ‘‘foo’ is not defined’)
⇒foo is defined
ifdef(‘no_such_macro’, ‘yes’, ‘no’, ‘extra argument’)
error m4:stdin:4: Warning: excess arguments to builtin ‘ifdef’ ignored
⇒no
6.2 If-else construct, or multibranch
The other conditional, ifelse, is much more powerful. It can be used as a way to introduce
a long comment, as an if-else construct, or as a multibranch, depending on the number of
arguments supplied:
ifelse (comment)
ifelse (string-1, string-2, equal, [not-equal])
ifelse (string-1, string-2, equal-1, string-3, string-4, equal-2,
. . . , [not-equal])
[Builtin]
[Builtin]
[Builtin]
Used with only one argument, the ifelse simply discards it and produces no output.
If called with three or four arguments, ifelse expands into equal, if string-1 and
string-2 are equal (character for character), otherwise it expands to not-equal. A
final fifth argument is ignored, after triggering a warning.
If called with six or more arguments, and string-1 and string-2 are equal, ifelse
expands into equal-1, otherwise the first three arguments are discarded and the processing starts again.
The macro ifelse is recognized only with parameters.
Using only one argument is a common m4 idiom for introducing a block comment, as an
alternative to repeatedly using dnl. This special usage is recognized by GNU m4, so that
in this case, the warning about missing arguments is never triggered.
40
GNU M4 1.4.17 macro processor
ifelse(‘some comments’)
⇒
ifelse(‘foo’, ‘bar’)
error m4:stdin:2: Warning: too few arguments to builtin ‘ifelse’
⇒
Using three or four arguments provides decision points.
ifelse(‘foo’, ‘bar’, ‘true’)
⇒
ifelse(‘foo’, ‘foo’, ‘true’)
⇒true
define(‘foo’, ‘bar’)
⇒
ifelse(foo, ‘bar’, ‘true’, ‘false’)
⇒true
ifelse(foo, ‘foo’, ‘true’, ‘false’)
⇒false
Notice how the first argument was used unquoted; it is common to compare the expansion
of a macro with a string. With this macro, you can now reproduce the behavior of blind
builtins, where the macro is recognized only with arguments.
define(‘foo’, ‘ifelse(‘$#’, ‘0’, ‘‘$0’’, ‘arguments:$#’)’)
⇒
foo
⇒foo
foo()
⇒arguments:1
foo(‘a’, ‘b’, ‘c’)
⇒arguments:3
For an example of a way to make defining blind macros easier, see Section 6.7 [Composition], page 51.
The macro ifelse can take more than four arguments. If given more than four arguments, ifelse works like a case or switch statement in traditional programming languages.
If string-1 and string-2 are equal, ifelse expands into equal-1, otherwise the procedure is
repeated with the first three arguments discarded. This calls for an example:
ifelse(‘foo’, ‘bar’, ‘third’, ‘gnu’, ‘gnats’)
error m4:stdin:1: Warning: excess arguments to builtin ‘ifelse’ ignored
⇒gnu
ifelse(‘foo’, ‘bar’, ‘third’, ‘gnu’, ‘gnats’, ‘sixth’)
⇒
ifelse(‘foo’, ‘bar’, ‘third’, ‘gnu’, ‘gnats’, ‘sixth’, ‘seventh’)
⇒seventh
ifelse(‘foo’, ‘bar’, ‘3’, ‘gnu’, ‘gnats’, ‘6’, ‘7’, ‘8’)
error m4:stdin:4: Warning: excess arguments to builtin ‘ifelse’ ignored
⇒7
Naturally, the normal case will be slightly more advanced than these examples. A
common use of ifelse is in macros implementing loops of various kinds.
Chapter 6: Conditionals, loops, and recursion
41
6.3 Recursion in m4
There is no direct support for loops in m4, but macros can be recursive. There is no limit on
the number of recursion levels, other than those enforced by your hardware and operating
system.
Loops can be programmed using recursion and the conditionals described previously.
There is a builtin macro, shift, which can, among other things, be used for iterating
through the actual arguments to a macro:
shift (arg1, . . . )
[Builtin]
Takes any number of arguments, and expands to all its arguments except arg1, separated by commas, with each argument quoted.
The macro shift is recognized only with parameters.
shift
⇒shift
shift(‘bar’)
⇒
shift(‘foo’, ‘bar’, ‘baz’)
⇒bar,baz
An example of the use of shift is this macro:
reverse (. . . )
[Composite]
Takes any number of arguments, and reverses their order.
It is implemented as:
define(‘reverse’, ‘ifelse(‘$#’, ‘0’, , ‘$#’, ‘1’, ‘‘$1’’,
‘reverse(shift($@)), ‘$1’’)’)
⇒
reverse
⇒
reverse(‘foo’)
⇒foo
reverse(‘foo’, ‘bar’, ‘gnats’, ‘and gnus’)
⇒and gnus, gnats, bar, foo
While not a very interesting macro, it does show how simple loops can be made with
shift, ifelse and recursion. It also shows that shift is usually used with ‘$@’. Another
example of this is an implementation of a short-circuiting conditional operator.
cond (test-1, string-1, equal-1, [test-2], [string-2], [equal-2],
. . . , [not-equal])
[Composite]
Similar to ifelse, where an equal comparison between the first two strings results in
the third, otherwise the first three arguments are discarded and the process repeats.
The difference is that each test-<n> is expanded only when it is encountered. This
means that every third argument to cond is normally given one more level of quoting
than the corresponding argument to ifelse.
42
GNU M4 1.4.17 macro processor
Here is the implementation of cond, along with a demonstration of how it can shortcircuit the side effects in side. Notice how all the unquoted side effects happen regardless
of how many comparisons are made with ifelse, compared with only the relevant effects
with cond.
define(‘cond’,
‘ifelse(‘$#’, ‘1’, ‘$1’,
‘ifelse($1, ‘$2’, ‘$3’,
‘$0(shift(shift(shift($@))))’)’)’)dnl
define(‘side’, ‘define(‘counter’, incr(counter))$1’)dnl
define(‘example1’,
‘define(‘counter’, ‘0’)dnl
ifelse(side(‘$1’), ‘yes’, ‘one comparison: ’,
side(‘$1’), ‘no’, ‘two comparisons: ’,
side(‘$1’), ‘maybe’, ‘three comparisons: ’,
‘side(‘default answer: ’)’)counter’)dnl
define(‘example2’,
‘define(‘counter’, ‘0’)dnl
cond(‘side(‘$1’)’, ‘yes’, ‘one comparison: ’,
‘side(‘$1’)’, ‘no’, ‘two comparisons: ’,
‘side(‘$1’)’, ‘maybe’, ‘three comparisons: ’,
‘side(‘default answer: ’)’)counter’)dnl
example1(‘yes’)
⇒one comparison: 3
example1(‘no’)
⇒two comparisons: 3
example1(‘maybe’)
⇒three comparisons: 3
example1(‘feeling rather indecisive today’)
⇒default answer: 4
example2(‘yes’)
⇒one comparison: 1
example2(‘no’)
⇒two comparisons: 2
example2(‘maybe’)
⇒three comparisons: 3
example2(‘feeling rather indecisive today’)
⇒default answer: 4
Another common task that requires iteration is joining a list of arguments into a single
string.
join ([separator], [args...])
joinall ([separator], [args...])
[Composite]
[Composite]
Generate a single-quoted string, consisting of each arg separated by separator. While
joinall always outputs a separator between arguments, join avoids the separator
for an empty arg.
Chapter 6: Conditionals, loops, and recursion
43
Here are some examples of its usage, based on the implementation m4-1.4.17/examples/
join.m4 distributed in this package:
$ m4 -I examples
include(‘join.m4’)
⇒
join,join(‘-’),join(‘-’, ‘’),join(‘-’, ‘’, ‘’)
⇒,,,
joinall,joinall(‘-’),joinall(‘-’, ‘’),joinall(‘-’, ‘’, ‘’)
⇒,,,join(‘-’, ‘1’)
⇒1
join(‘-’, ‘1’, ‘2’, ‘3’)
⇒1-2-3
join(‘’, ‘1’, ‘2’, ‘3’)
⇒123
join(‘-’, ‘’, ‘1’, ‘’, ‘’, ‘2’, ‘’)
⇒1-2
joinall(‘-’, ‘’, ‘1’, ‘’, ‘’, ‘2’, ‘’)
⇒-1---2join(‘,’, ‘1’, ‘2’, ‘3’)
⇒1,2,3
define(‘nargs’, ‘$#’)dnl
nargs(join(‘,’, ‘1’, ‘2’, ‘3’))
⇒1
Examining the implementation shows some interesting points about several m4 programming idioms.
$ m4 -I examples
undivert(‘join.m4’)dnl
⇒divert(‘-1’)
⇒# join(sep, args) - join each non-empty ARG into a single
⇒# string, with each element separated by SEP
⇒define(‘join’,
⇒‘ifelse(‘$#’, ‘2’, ‘‘$2’’,
⇒ ‘ifelse(‘$2’, ‘’, ‘’, ‘‘$2’_’)$0(‘$1’, shift(shift($@)))’)’)
⇒define(‘_join’,
⇒‘ifelse(‘$#$2’, ‘2’, ‘’,
⇒ ‘ifelse(‘$2’, ‘’, ‘’, ‘‘$1$2’’)$0(‘$1’, shift(shift($@)))’)’)
⇒# joinall(sep, args) - join each ARG, including empty ones,
⇒# into a single string, with each element separated by SEP
⇒define(‘joinall’, ‘‘$2’_$0(‘$1’, shift($@))’)
⇒define(‘_joinall’,
⇒‘ifelse(‘$#’, ‘2’, ‘’, ‘‘$1$3’$0(‘$1’, shift(shift($@)))’)’)
⇒divert‘’dnl
First, notice that this implementation creates helper macros _join and _joinall. This
division of labor makes it easier to output the correct number of separator instances: join
and joinall are responsible for the first argument, without a separator, while _join and
44
GNU M4 1.4.17 macro processor
_joinall are responsible for all remaining arguments, always outputting a separator when
outputting an argument.
Next, observe how join decides to iterate to itself, because the first arg was empty, or
to output the argument and swap over to _join. If the argument is non-empty, then the
nested ifelse results in an unquoted ‘_’, which is concatenated with the ‘$0’ to form the
next macro name to invoke. The joinall implementation is simpler since it does not have
to suppress empty arg; it always executes once then defers to _joinall.
Another important idiom is the idea that separator is reused for each iteration.
Each iteration has one less argument, but rather than discarding ‘$1’ by iterating with
$0(shift($@)), the macro discards ‘$2’ by using $0(‘$1’, shift(shift($@))).
Next, notice that it is possible to compare more than one condition in a single ifelse
test. The test of ‘$#$2’ against ‘2’ allows _join to iterate for two separate reasons—either
there are still more than two arguments, or there are exactly two arguments but the last
argument is not empty.
Finally, notice that these macros require exactly two arguments to terminate recursion,
but that they still correctly result in empty output when given no args (i.e., zero or one
macro argument). On the first pass when there are too few arguments, the shift results
in no output, but leaves an empty string to serve as the required second argument for the
second pass. Put another way, ‘‘$1’, shift($@)’ is not the same as ‘$@’, since only the
former guarantees at least two arguments.
Sometimes, a recursive algorithm requires adding quotes to each element, or treating
multiple arguments as a single element:
quote (. . . )
dquote (. . . )
dquote_elt (. . . )
[Composite]
[Composite]
[Composite]
Takes any number of arguments, and adds quoting. With quote, only one level of
quoting is added, effectively removing whitespace after commas and turning multiple
arguments into a single string. With dquote, two levels of quoting are added, one
around each element, and one around the list. And with dquote_elt, two levels of
quoting are added around each element.
An actual implementation of these three macros is distributed as m4-1.4.17/examples/
quote.m4 in this package. First, let’s examine their usage:
$ m4 -I examples
include(‘quote.m4’)
⇒
-quote-dquote-dquote_elt⇒----quote()-dquote()-dquote_elt()⇒--‘’-‘’-quote(‘1’)-dquote(‘1’)-dquote_elt(‘1’)⇒-1-‘1’-‘1’-quote(‘1’, ‘2’)-dquote(‘1’, ‘2’)-dquote_elt(‘1’, ‘2’)⇒-1,2-‘1’,‘2’-‘1’,‘2’define(‘n’, ‘$#’)dnl
Chapter 6: Conditionals, loops, and recursion
45
-n(quote(‘1’, ‘2’))-n(dquote(‘1’, ‘2’))-n(dquote_elt(‘1’, ‘2’))⇒-1-1-2dquote(dquote_elt(‘1’, ‘2’))
⇒‘‘1’’,‘‘2’’
dquote_elt(dquote(‘1’, ‘2’))
⇒‘‘1’,‘2’’
The last two lines show that when given two arguments, dquote results in one string,
while dquote_elt results in two. Now, examine the implementation. Note that quote
and dquote_elt make decisions based on their number of arguments, so that when called
without arguments, they result in nothing instead of a quoted empty string; this is so that
it is possible to distinguish between no arguments and an empty first argument. dquote, on
the other hand, results in a string no matter what, since it is still possible to tell whether
it was invoked without arguments based on the resulting string.
$ m4 -I examples
undivert(‘quote.m4’)dnl
⇒divert(‘-1’)
⇒# quote(args) - convert args to single-quoted string
⇒define(‘quote’, ‘ifelse(‘$#’, ‘0’, ‘’, ‘‘$*’’)’)
⇒# dquote(args) - convert args to quoted list of quoted strings
⇒define(‘dquote’, ‘‘$@’’)
⇒# dquote_elt(args) - convert args to list of double-quoted strings
⇒define(‘dquote_elt’, ‘ifelse(‘$#’, ‘0’, ‘’, ‘$#’, ‘1’, ‘‘‘$1’’’,
⇒
‘‘‘$1’’,$0(shift($@))’)’)
⇒divert‘’dnl
It is worth pointing out that ‘quote(args)’ is more efficient than ‘joinall(‘,’, args)’
for producing the same output.
One more useful macro based on shift allows portably selecting an arbitrary argument
(usually greater than the ninth argument), without relying on the GNU extension of multidigit arguments (see Section 5.2 [Arguments], page 26).
argn (n, . . . )
[Composite]
Expands to argument n out of the remaining arguments. n must be a positive number.
Usually invoked as ‘argn(‘n’,$@)’.
It is implemented as:
define(‘argn’, ‘ifelse(‘$1’, 1, ‘‘$2’’,
‘argn(decr(‘$1’), shift(shift($@)))’)’)
⇒
argn(‘1’, ‘a’)
⇒a
define(‘foo’, ‘argn(‘11’, $@)’)
⇒
foo(‘a’, ‘b’, ‘c’, ‘d’, ‘e’, ‘f’, ‘g’, ‘h’, ‘i’, ‘j’, ‘k’, ‘l’)
⇒k
46
GNU M4 1.4.17 macro processor
6.4 Iteration by counting
Here is an example of a loop macro that implements a simple for loop.
forloop (iterator, start, end, text)
[Composite]
Takes the name in iterator, which must be a valid macro name, and successively
assign it each integer value from start to end, inclusive. For each assignment to
iterator, append text to the expansion of the forloop. text may refer to iterator.
Any definition of iterator prior to this invocation is restored.
It can, for example, be used for simple counting:
$ m4 -I examples
include(‘forloop.m4’)
⇒
forloop(‘i’, ‘1’, ‘8’, ‘i ’)
⇒1 2 3 4 5 6 7 8
For-loops can be nested, like:
$ m4 -I examples
include(‘forloop.m4’)
⇒
forloop(‘i’, ‘1’, ‘4’, ‘forloop(‘j’, ‘1’,
’)
⇒ (1, 1) (1, 2) (1, 3) (1, 4) (1, 5) (1,
⇒ (2, 1) (2, 2) (2, 3) (2, 4) (2, 5) (2,
⇒ (3, 1) (3, 2) (3, 3) (3, 4) (3, 5) (3,
⇒ (4, 1) (4, 2) (4, 3) (4, 4) (4, 5) (4,
⇒
‘8’, ‘ (i, j)’)
6)
6)
6)
6)
(1,
(2,
(3,
(4,
7)
7)
7)
7)
(1,
(2,
(3,
(4,
8)
8)
8)
8)
The implementation of the forloop macro is fairly straightforward. The forloop macro
itself is simply a wrapper, which saves the previous definition of the first argument, calls
the internal macro _forloop, and re-establishes the saved definition of the first argument.
The macro _forloop expands the fourth argument once, and tests to see if the iterator
has reached the final value. If it has not finished, it increments the iterator (using the
predefined macro incr, see Section 12.1 [Incr], page 89), and recurses.
Here is an actual implementation of forloop, distributed as m4-1.4.17/examples/
forloop.m4 in this package:
$ m4 -I examples
undivert(‘forloop.m4’)dnl
⇒divert(‘-1’)
⇒# forloop(var, from, to, stmt) - simple version
⇒define(‘forloop’, ‘pushdef(‘$1’, ‘$2’)_forloop($@)popdef(‘$1’)’)
⇒define(‘_forloop’,
⇒
‘$4‘’ifelse($1, ‘$3’, ‘’, ‘define(‘$1’, incr($1))$0($@)’)’)
⇒divert‘’dnl
Notice the careful use of quotes. Certain macro arguments are left unquoted, each for its
own reason. Try to find out why these arguments are left unquoted, and see what happens
if they are quoted. (As presented, these two macros are useful but not very robust for
Chapter 6: Conditionals, loops, and recursion
47
general use. They lack even basic error handling for cases like start less than end, end not
numeric, or iterator not being a macro name. See if you can improve these macros; or see
Section 17.2 [Answers], page 115).
6.5 Iteration by list contents
Here is an example of a loop macro that implements list iteration.
foreach (iterator, paren-list, text)
foreachq (iterator, quote-list, text)
[Composite]
[Composite]
Takes the name in iterator, which must be a valid macro name, and successively
assign it each value from paren-list or quote-list. In foreach, paren-list is a commaseparated list of elements contained in parentheses. In foreachq, quote-list is a
comma-separated list of elements contained in a quoted string. For each assignment
to iterator, append text to the overall expansion. text may refer to iterator. Any
definition of iterator prior to this invocation is restored.
As an example, this displays each word in a list inside of a sentence, using an implementation of foreach distributed as m4-1.4.17/examples/foreach.m4, and foreachq in
m4-1.4.17/examples/foreachq.m4.
$ m4 -I examples
include(‘foreach.m4’)
⇒
foreach(‘x’, (foo, bar, foobar), ‘Word was: x
’)dnl
⇒Word was: foo
⇒Word was: bar
⇒Word was: foobar
include(‘foreachq.m4’)
⇒
foreachq(‘x’, ‘foo, bar, foobar’, ‘Word was: x
’)dnl
⇒Word was: foo
⇒Word was: bar
⇒Word was: foobar
It is possible to be more complex; each element of the paren-list or quote-list can itself
be a list, to pass as further arguments to a helper macro. This example generates a shell
case statement:
$ m4 -I examples
include(‘foreach.m4’)
⇒
define(‘_case’, ‘ $1)
$2=" $1";;
’)dnl
define(‘_cat’, ‘$1$2’)dnl
case $‘’1 in
⇒case $1 in
48
GNU M4 1.4.17 macro processor
foreach(‘x’, ‘(‘(‘a’, ‘vara’)’, ‘(‘b’, ‘varb’)’, ‘(‘c’, ‘varc’)’)’,
‘_cat(‘_case’, x)’)dnl
⇒ a)
⇒
vara=" a";;
⇒ b)
⇒
varb=" b";;
⇒ c)
⇒
varc=" c";;
esac
⇒esac
The implementation of the foreach macro is a bit more involved; it is a wrapper around
two helper macros. First, _arg1 is needed to grab the first element of a list. Second,
_foreach implements the recursion, successively walking through the original list. Here is
a simple implementation of foreach:
$ m4 -I examples
undivert(‘foreach.m4’)dnl
⇒divert(‘-1’)
⇒# foreach(x, (item_1, item_2, ..., item_n), stmt)
⇒#
parenthesized list, simple version
⇒define(‘foreach’, ‘pushdef(‘$1’)_foreach($@)popdef(‘$1’)’)
⇒define(‘_arg1’, ‘$1’)
⇒define(‘_foreach’, ‘ifelse(‘$2’, ‘()’, ‘’,
⇒ ‘define(‘$1’, _arg1$2)$3‘’$0(‘$1’, (shift$2), ‘$3’)’)’)
⇒divert‘’dnl
Unfortunately, that implementation is not robust to macro names as list elements. Each
iteration of _foreach is stripping another layer of quotes, leading to erratic results if list
elements are not already fully expanded. The first cut at implementing foreachq takes
this into account. Also, when using quoted elements in a paren-list, the overall list must
be quoted. A quote-list has the nice property of requiring fewer characters to create a list
containing the same quoted elements. To see the difference between the two macros, we
attempt to pass double-quoted macro names in a list, expecting the macro name on output
after one layer of quotes is removed during list iteration and the final layer removed during
the final rescan:
$ m4 -I examples
define(‘a’, ‘1’)define(‘b’, ‘2’)define(‘c’, ‘3’)
⇒
include(‘foreach.m4’)
⇒
include(‘foreachq.m4’)
⇒
foreach(‘x’, ‘(‘‘a’’, ‘‘(b’’, ‘‘c)’’)’, ‘x
’)
⇒1
⇒(2)1
⇒
⇒, x
Chapter 6: Conditionals, loops, and recursion
49
⇒)
foreachq(‘x’, ‘‘‘a’’, ‘‘(b’’, ‘‘c)’’’, ‘x
’)dnl
⇒a
⇒(b
⇒c)
Obviously, foreachq did a better job; here is its implementation:
$ m4 -I examples
undivert(‘foreachq.m4’)dnl
⇒include(‘quote.m4’)dnl
⇒divert(‘-1’)
⇒# foreachq(x, ‘item_1, item_2, ..., item_n’, stmt)
⇒#
quoted list, simple version
⇒define(‘foreachq’, ‘pushdef(‘$1’)_foreachq($@)popdef(‘$1’)’)
⇒define(‘_arg1’, ‘$1’)
⇒define(‘_foreachq’, ‘ifelse(quote($2), ‘’, ‘’,
⇒ ‘define(‘$1’, ‘_arg1($2)’)$3‘’$0(‘$1’, ‘shift($2)’, ‘$3’)’)’)
⇒divert‘’dnl
Notice that _foreachq had to use the helper macro quote defined earlier (see Section 6.3
[Shift], page 41), to ensure that the embedded ifelse call does not go haywire if a list
element contains a comma. Unfortunately, this implementation of foreachq has its own
severe flaw. Whereas the foreach implementation was linear, this macro is quadratic in the
number of list elements, and is much more likely to trip up the limit set by the command
line option --nesting-limit (or -L, see Section 2.3 [Invoking m4], page 10). Additionally,
this implementation does not expand ‘defn(‘iterator’)’ very well, when compared with
foreach.
$ m4 -I examples
include(‘foreach.m4’)include(‘foreachq.m4’)
⇒
foreach(‘name’, ‘(‘a’, ‘b’)’, ‘ defn(‘name’)’)
⇒ a b
foreachq(‘name’, ‘‘a’, ‘b’’, ‘ defn(‘name’)’)
⇒ _arg1(‘a’, ‘b’) _arg1(shift(‘a’, ‘b’))
It is possible to have robust iteration with linear behavior and sane iterator contents for
either list style. See if you can learn from the best elements of both of these implementations
to create robust macros (or see Section 17.3 [Answers], page 117).
6.6 Working with definition stacks
Thanks to pushdef, manipulation of a stack is an intrinsic operation in m4. Normally, only
the topmost definition in a stack is important, but sometimes, it is desirable to manipulate
the entire definition stack.
stack_foreach (macro, action)
stack_foreach_lifo (macro, action)
[Composite]
[Composite]
For each of the pushdef definitions associated with macro, invoke the macro action
with a single argument of that definition. stack_foreach visits the oldest definition
50
GNU M4 1.4.17 macro processor
first, while stack_foreach_lifo visits the current definition first. action should not
modify or dereference macro. There are a few special macros, such as defn, which
cannot be used as the macro parameter.
A sample implementation of these macros is distributed in the file m4-1.4.17/examples/
stack.m4.
$ m4 -I examples
include(‘stack.m4’)
⇒
pushdef(‘a’, ‘1’)pushdef(‘a’, ‘2’)pushdef(‘a’, ‘3’)
⇒
define(‘show’, ‘‘$1’
’)
⇒
stack_foreach(‘a’, ‘show’)dnl
⇒1
⇒2
⇒3
stack_foreach_lifo(‘a’, ‘show’)dnl
⇒3
⇒2
⇒1
Now for the implementation. Note the definition of a helper macro, _stack_reverse,
which destructively swaps the contents of one stack of definitions into the reverse order in
the temporary macro ‘tmp-$1’. By calling the helper twice, the original order is restored
back into the macro ‘$1’; since the operation is destructive, this explains why ‘$1’ must
not be modified or dereferenced during the traversal. The caller can then inject additional
code to pass the definition currently being visited to ‘$2’. The choice of helper names is
intentional; since ‘-’ is not valid as part of a macro name, there is no risk of conflict with
a valid macro name, and the code is guaranteed to use defn where necessary. Finally, note
that any macro used in the traversal of a pushdef stack, such as pushdef or defn, cannot
be handled by stack_foreach, since the macro would temporarily be undefined during the
algorithm.
$ m4 -I examples
undivert(‘stack.m4’)dnl
⇒divert(‘-1’)
⇒# stack_foreach(macro, action)
⇒# Invoke ACTION with a single argument of each definition
⇒# from the definition stack of MACRO, starting with the oldest.
⇒define(‘stack_foreach’,
⇒‘_stack_reverse(‘$1’, ‘tmp-$1’)’dnl
⇒‘_stack_reverse(‘tmp-$1’, ‘$1’, ‘$2(defn(‘$1’))’)’)
⇒# stack_foreach_lifo(macro, action)
⇒# Invoke ACTION with a single argument of each definition
⇒# from the definition stack of MACRO, starting with the newest.
⇒define(‘stack_foreach_lifo’,
⇒‘_stack_reverse(‘$1’, ‘tmp-$1’, ‘$2(defn(‘$1’))’)’dnl
Chapter 6: Conditionals, loops, and recursion
51
⇒‘_stack_reverse(‘tmp-$1’, ‘$1’)’)
⇒define(‘_stack_reverse’,
⇒‘ifdef(‘$1’, ‘pushdef(‘$2’, defn(‘$1’))$3‘’popdef(‘$1’)$0($@)’)’)
⇒divert‘’dnl
6.7 Building macros with macros
Since m4 is a macro language, it is possible to write macros that can build other macros.
First on the list is a way to automate the creation of blind macros.
define_blind (name, [value])
[Composite]
Defines name as a blind macro, such that name will expand to value only when given
explicit arguments. value should not be the result of defn (see Section 5.5 [Defn],
page 31). This macro is only recognized with parameters, and results in an empty
string.
Defining a macro to define another macro can be a bit tricky. We want to use a literal
‘$#’ in the argument to the nested define. However, if ‘$’ and ‘#’ are adjacent in the
definition of define_blind, then it would be expanded as the number of arguments to
define_blind rather than the intended number of arguments to name. The solution is to
pass the difficult characters through extra arguments to a helper macro _define_blind.
When composing macros, it is a common idiom to need a helper macro to concatenate
text that forms parameters in the composed macro, rather than interpreting the text as a
parameter of the composing macro.
As for the limitation against using defn, there are two reasons. If a macro was previously
defined with define_blind, then it can safely be renamed to a new blind macro using plain
define; using define_blind to rename it just adds another layer of ifelse, occupying
memory and slowing down execution. And if a macro is a builtin, then it would result in an
attempt to define a macro consisting of both text and a builtin token; this is not supported,
and the builtin token is flattened to an empty string.
With that explanation, here’s the definition, and some sample usage.
define_blind is itself a blind macro.
$ m4 -d
define(‘define_blind’, ‘ifelse(‘$#’, ‘0’, ‘‘$0’’,
‘_$0(‘$1’, ‘$2’, ‘$’‘#’, ‘$’‘0’)’)’)
⇒
define(‘_define_blind’, ‘define(‘$1’,
‘ifelse(‘$3’, ‘0’, ‘‘$4’’, ‘$2’)’)’)
⇒
define_blind
⇒define_blind
define_blind(‘foo’, ‘arguments were $*’)
⇒
foo
⇒foo
foo(‘bar’)
⇒arguments were bar
Notice that
52
GNU M4 1.4.17 macro processor
define(‘blah’, defn(‘foo’))
⇒
blah
⇒blah
blah(‘a’, ‘b’)
⇒arguments were a,b
defn(‘blah’)
⇒ifelse(‘$#’, ‘0’, ‘‘$0’’, ‘arguments were $*’)
Another interesting composition tactic is argument currying, or factoring a macro that
takes multiple arguments for use in a context that provides exactly one argument.
curry (macro, . . . )
[Composite]
Expand to a macro call that takes exactly one argument, then appends that argument
to the original arguments and invokes macro with the resulting list of arguments.
A demonstration of currying makes the intent of this macro a little more obvious. The
macro stack_foreach mentioned earlier is an example of a context that provides exactly
one argument to a macro name. But coupled with currying, we can invoke reverse with
two arguments for each definition of a macro stack. This example uses the file m4-1.4.17/
examples/curry.m4 included in the distribution.
$ m4 -I examples
include(‘curry.m4’)include(‘stack.m4’)
⇒
define(‘reverse’, ‘ifelse(‘$#’, ‘0’, , ‘$#’, ‘1’, ‘‘$1’’,
‘reverse(shift($@)), ‘$1’’)’)
⇒
pushdef(‘a’, ‘1’)pushdef(‘a’, ‘2’)pushdef(‘a’, ‘3’)
⇒
stack_foreach(‘a’, ‘:curry(‘reverse’, ‘4’)’)
⇒:1, 4:2, 4:3, 4
curry(‘curry’, ‘reverse’, ‘1’)(‘2’)(‘3’)
⇒3, 2, 1
Now for the implementation. Notice how curry leaves off with a macro name but no
open parenthesis, while still in the middle of collecting arguments for ‘$1’. The macro
_curry is the helper macro that takes one argument, then adds it to the list and finally
supplies the closing parenthesis. The use of a comma inside the shift call allows currying
to also work for a macro that takes one argument, although it often makes more sense to
invoke that macro directly rather than going through curry.
$ m4 -I examples
undivert(‘curry.m4’)dnl
⇒divert(‘-1’)
⇒# curry(macro, args)
⇒# Expand to a macro call that takes one argument, then invoke
⇒# macro(args, extra).
⇒define(‘curry’, ‘$1(shift($@,)_$0’)
⇒define(‘_curry’, ‘‘$1’)’)
⇒divert‘’dnl
Chapter 6: Conditionals, loops, and recursion
53
Unfortunately, with M4 1.4.x, curry is unable to handle builtin tokens, which are silently
flattened to the empty string when passed through another text macro. This limitation will
be lifted in a future release of M4.
Putting the last few concepts together, it is possible to copy or rename an entire stack
of macro definitions.
copy (source, dest)
rename (source, dest)
[Composite]
[Composite]
Ensure that dest is undefined, then define it to the same stack of definitions currently
in source. copy leaves source unchanged, while rename undefines source. There are
only a few macros, such as copy or defn, which cannot be copied via this macro.
The implementation is relatively straightforward (although since it uses curry, it is
unable to copy builtin macros, such as the second definition of a as a synonym for divnum.
See if you can design a version that works around this limitation, or see Section 17.4
[Answers], page 124).
$ m4 -I examples
include(‘curry.m4’)include(‘stack.m4’)
⇒
define(‘rename’, ‘copy($@)undefine(‘$1’)’)dnl
define(‘copy’, ‘ifdef(‘$2’, ‘errprint(‘$2 already defined
’)m4exit(‘1’)’,
‘stack_foreach(‘$1’, ‘curry(‘pushdef’, ‘$2’)’)’)’)dnl
pushdef(‘a’, ‘1’)pushdef(‘a’, defn(‘divnum’))pushdef(‘a’, ‘2’)
⇒
copy(‘a’, ‘b’)
⇒
rename(‘b’, ‘c’)
⇒
a b c
⇒2 b 2
popdef(‘a’, ‘c’)c a
⇒ 0
popdef(‘a’, ‘c’)a c
⇒1 1
Chapter 7: How to debug macros and input
55
7 How to debug macros and input
When writing macros for m4, they often do not work as intended on the first try (as is the
case with most programming languages). Fortunately, there is support for macro debugging
in m4.
7.1 Displaying macro definitions
If you want to see what a name expands into, you can use the builtin dumpdef:
dumpdef ([names...])
[Builtin]
Accepts any number of arguments. If called without any arguments, it displays the
definitions of all known names, otherwise it displays the definitions of the names
given. The output is printed to the current debug file (usually standard error), and
is sorted by name. If an unknown name is encountered, a warning is printed.
The expansion of dumpdef is void.
$ m4 -d
define(‘foo’, ‘Hello world.’)
⇒
dumpdef(‘foo’)
error foo: ‘Hello world.’
⇒
dumpdef(‘define’)
error define: <define>
⇒
The last example shows how builtin macros definitions are displayed. The definition that
is dumped corresponds to what would occur if the macro were to be called at that point,
even if other definitions are still live due to redefining a macro during argument collection.
$ m4 -d
pushdef(‘f’, ‘‘$0’1’)pushdef(‘f’, ‘‘$0’2’)
⇒
f(popdef(‘f’)dumpdef(‘f’))
error f: ‘‘$0’1’
⇒f2
f(popdef(‘f’)dumpdef(‘f’))
error m4:stdin:3: undefined macro ‘f’
⇒f1
See Section 7.3 [Debug Levels], page 58, for information on controlling the details of the
display.
7.2 Tracing macro calls
It is possible to trace macro calls and expansions through the builtins traceon and
traceoff:
56
GNU M4 1.4.17 macro processor
traceon ([names...])
traceoff ([names...])
[Builtin]
[Builtin]
When called without any arguments, traceon and traceoff will turn tracing on and
off, respectively, for all currently defined macros.
When called with arguments, only the macros listed in names are affected, whether
or not they are currently defined.
The expansion of traceon and traceoff is void.
Whenever a traced macro is called and the arguments have been collected, the call is
displayed. If the expansion of the macro call is not void, the expansion can be displayed
after the call. The output is printed to the current debug file (defaulting to standard error,
see Section 7.4 [Debug Output], page 60).
$ m4 -d
define(‘foo’, ‘Hello World.’)
⇒
define(‘echo’, ‘$@’)
⇒
traceon(‘foo’, ‘echo’)
⇒
foo
error m4trace: -1- foo -> ‘Hello World.’
⇒Hello World.
echo(‘gnus’, ‘and gnats’)
error m4trace: -1- echo(‘gnus’, ‘and gnats’) -> ‘‘gnus’,‘and gnats’’
⇒gnus,and gnats
The number between dashes is the depth of the expansion. It is one most of the time,
signifying an expansion at the outermost level, but it increases when macro arguments
contain unquoted macro calls. The maximum number that will appear between dashes is
controlled by the option --nesting-limit (or -L, see Section 2.3 [Invoking m4], page 10).
Additionally, the option --trace (or -t) can be used to invoke traceon(name) before
parsing input.
$ m4 -L 3 -t ifelse
ifelse(‘one level’)
error m4trace: -1- ifelse
⇒
ifelse(ifelse(ifelse(‘three levels’)))
error m4trace: -3- ifelse
error m4trace: -2- ifelse
error m4trace: -1- ifelse
⇒
ifelse(ifelse(ifelse(ifelse(‘four levels’))))
error m4:stdin:3: recursion limit of 3 exceeded, use -L<N> to change it
Tracing by name is an attribute that is preserved whether the macro is defined or not.
This allows the selection of macros to trace before those macros are defined.
$ m4 -d
traceoff(‘foo’)
Chapter 7: How to debug macros and input
57
⇒
traceon(‘foo’)
⇒
foo
⇒foo
defn(‘foo’)
⇒
define(‘foo’, ‘bar’)
⇒
foo
error m4trace: -1- foo -> ‘bar’
⇒bar
undefine(‘foo’)
⇒
ifdef(‘foo’, ‘yes’, ‘no’)
⇒no
indir(‘foo’)
error m4:stdin:9: undefined macro ‘foo’
⇒
define(‘foo’, ‘blah’)
⇒
foo
error m4trace: -1- foo -> ‘blah’
⇒blah
traceoff
⇒
foo
⇒blah
Tracing even works on builtins. However, defn (see Section 5.5 [Defn], page 31) does
not transfer tracing status.
$ m4 -d
traceon(‘traceon’)
⇒
traceon(‘traceoff’)
error m4trace: -1- traceon(‘traceoff’)
⇒
traceoff(‘traceoff’)
error m4trace: -1- traceoff(‘traceoff’)
⇒
traceoff(‘traceon’)
⇒
traceon(‘eval’, ‘m4_divnum’)
⇒
define(‘m4_eval’, defn(‘eval’))
⇒
define(‘m4_divnum’, defn(‘divnum’))
58
GNU M4 1.4.17 macro processor
⇒
eval(divnum)
error m4trace: -1- eval(‘0’) -> ‘0’
⇒0
m4_eval(m4_divnum)
error m4trace: -2- m4_divnum -> ‘0’
⇒0
See Section 7.3 [Debug Levels], page 58, for information on controlling the details of
the display. The format of the trace output is not specified by POSIX, and varies between
implementations of m4.
7.3 Controlling debugging output
The -d option to m4 (or --debug, see Section 2.5 [Invoking m4], page 11) controls the amount
of details presented in three categories of output. Trace output is requested by traceon (see
Section 7.2 [Trace], page 55), and each line is prefixed by ‘m4trace:’ in relation to a macro
invocation. Debug output tracks useful events not associated with a macro invocation, and
each line is prefixed by ‘m4debug:’. Finally, dumpdef (see Section 7.1 [Dumpdef], page 55)
output is affected, with no prefix added to the output lines.
The flags following the option can be one or more of the following:
a
In trace output, show the actual arguments that were collected before invoking
the macro. This applies to all macro calls if the ‘t’ flag is used, otherwise
only the macros covered by calls of traceon. Arguments are subject to length
truncation specified by the command line option --arglength (or -l).
c
In trace output, show several trace lines for each macro call. A line is shown
when the macro is seen, but before the arguments are collected; a second line
when the arguments have been collected and a third line after the call has
completed.
e
In trace output, show the expansion of each macro call, if it is not void. This
applies to all macro calls if the ‘t’ flag is used, otherwise only the macros covered
by calls of traceon. The expansion is subject to length truncation specified by
the command line option --arglength (or -l).
f
In debug and trace output, include the name of the current input file in the
output line.
i
In debug output, print a message each time the current input file is changed.
l
In debug and trace output, include the current input line number in the output
line.
p
In debug output, print a message when a named file is found through the path
search mechanism (see Section 9.2 [Search Path], page 74), giving the actual
file name used.
q
In trace and dumpdef output, quote actual arguments and macro expansions
in the display with the current quotes. This is useful in connection with the ‘a’
and ‘e’ flags above.
Chapter 7: How to debug macros and input
59
t
In trace output, trace all macro calls made in this invocation of m4, regardless
of the settings of traceon.
x
In trace output, add a unique ‘macro call id’ to each line of the trace output.
This is useful in connection with the ‘c’ flag above.
V
A shorthand for all of the above flags.
If no flags are specified with the -d option, the default is ‘aeq’. The examples throughout
this manual assume the default flags.
There is a builtin macro debugmode, which allows on-the-fly control of the debugging
output format:
debugmode ([flags])
[Builtin]
The argument flags should be a subset of the letters listed above. As special cases, if
the argument starts with a ‘+’, the flags are added to the current debug flags, and if
it starts with a ‘-’, they are removed. If no argument is present, all debugging flags
are cleared (as if no -d was given), and with an empty argument the flags are reset
to the default of ‘aeq’.
The expansion of debugmode is void.
$ m4
define(‘foo’, ‘FOO’)
⇒
traceon(‘foo’)
⇒
debugmode()
⇒
foo
error m4trace: -1- foo -> ‘FOO’
⇒FOO
debugmode
⇒
foo
error m4trace: -1- foo
⇒FOO
debugmode(‘+l’)
⇒
foo
error m4trace:8: -1- foo
⇒FOO
The following example demonstrates the behavior of length truncation, when specified on
the command line. Note that each argument and the final result are individually truncated.
Also, the special tokens for builtin functions are not truncated.
$ m4 -d -l 6
define(‘echo’, ‘$@’)debugmode(‘+t’)
⇒
echo(‘1’, ‘long string’)
error m4trace: -1- echo(‘1’, ‘long s...’) -> ‘‘1’,‘l...’
60
GNU M4 1.4.17 macro processor
⇒1,long string
indir(‘echo’, defn(‘changequote’))
error m4trace: -2- defn(‘change...’)
error m4trace: -1- indir(‘echo’, <changequote>) -> ‘‘’’
⇒
This example shows the effects of the debug flags that are not related to macro tracing.
$ m4 -dip -I examples
error m4debug: input read from stdin
include(‘foo’)dnl
error m4debug: path search for ‘foo’ found ‘examples/foo’
error m4debug: input read from examples/foo
⇒bar
error m4debug: input reverted to stdin, line 1
^D
error m4debug: input exhausted
7.4 Saving debugging output
Debug and tracing output can be redirected to files using either the --debugfile option
to m4 (see Section 2.5 [Invoking m4], page 11), or with the builtin macro debugfile:
debugfile ([file])
[Builtin]
Sends all further debug and trace output to file, opened in append mode. If file
is the empty string, debug and trace output are discarded. If debugfile is called
without any arguments, debug and trace output are sent to standard error. This does
not affect warnings, error messages, or errprint output, which are always sent to
standard error. If file cannot be opened, the current debug file is unchanged, and an
error is issued.
The expansion of debugfile is void.
$ m4 -d
traceon(‘divnum’)
⇒
divnum(‘extra’)
error m4:stdin:2: Warning: excess arguments to builtin ‘divnum’ ignored
error m4trace: -1- divnum(‘extra’) -> ‘0’
⇒0
debugfile()
⇒
divnum(‘extra’)
error m4:stdin:4: Warning: excess arguments to builtin ‘divnum’ ignored
⇒0
debugfile
⇒
divnum
error m4trace: -1- divnum -> ‘0’
⇒0
Chapter 8: Input control
61
8 Input control
This chapter describes various builtin macros for controlling the input to m4.
8.1 Deleting whitespace in input
The builtin dnl stands for “Discard to Next Line”:
dnl
[Builtin]
All characters, up to and including the next newline, are discarded without performing
any macro expansion. A warning is issued if the end of the file is encountered without
a newline.
The expansion of dnl is void.
It is often used in connection with define, to remove the newline that follows the call
to define. Thus
define(‘foo’, ‘Macro ‘foo’.’)dnl A very simple macro, indeed.
foo
⇒Macro foo.
The input up to and including the next newline is discarded, as opposed to the way
comments are treated (see Section 3.3 [Comments], page 15).
Usually, dnl is immediately followed by an end of line or some other whitespace. GNU
m4 will produce a warning diagnostic if dnl is followed by an open parenthesis. In this
case, dnl will collect and process all arguments, looking for a matching close parenthesis.
All predictable side effects resulting from this collection will take place. dnl will return no
output. The input following the matching close parenthesis up to and including the next
newline, on whatever line containing it, will still be discarded.
dnl(‘args are ignored, but side effects occur’,
define(‘foo’, ‘like this’)) while this text is ignored: undefine(‘foo’)
error m4:stdin:1: Warning: excess arguments to builtin ‘dnl’ ignored
See how ‘foo’ was defined, foo?
⇒See how foo was defined, like this?
If the end of file is encountered without a newline character, a warning is issued and dnl
stops consuming input.
m4wrap(‘m4wrap(‘2 hi
’)0 hi dnl 1 hi’)
⇒
define(‘hi’, ‘HI’)
⇒
^D
error m4:stdin:1: Warning: end of file treated as newline
⇒0 HI 2 HI
62
GNU M4 1.4.17 macro processor
8.2 Changing the quote characters
The default quote delimiters can be changed with the builtin changequote:
changequote ([start = ‘‘’], [end = ‘’’])
[Builtin]
This sets start as the new begin-quote delimiter and end as the new end-quote delimiter. If both arguments are missing, the default quotes (‘ and ’) are used. If start
is void, then quoting is disabled. Otherwise, if end is missing or void, the default
end-quote delimiter (’) is used. The quote delimiters can be of any length.
The expansion of changequote is void.
changequote(‘[’, ‘]’)
⇒
define([foo], [Macro [foo].])
⇒
foo
⇒Macro foo.
The quotation strings can safely contain eight-bit characters. If no single character is
appropriate, start and end can be of any length. Other implementations cap the delimiter
length to five characters, but GNU has no inherent limit.
changequote(‘[[[’, ‘]]]’)
⇒
define([[[foo]]], [[[Macro [[[[[foo]]]]].]]])
⇒
foo
⇒Macro [[foo]].
Calling changequote with start as the empty string will effectively disable the quoting
mechanism, leaving no way to quote text. However, using an empty string is not portable,
as some other implementations of m4 revert to the default quoting, while others preserve the
prior non-empty delimiter. If start is not empty, then an empty end will use the default endquote delimiter of ‘’’, as otherwise, it would be impossible to end a quoted string. Again,
this is not portable, as some other m4 implementations reuse start as the end-quote delimiter,
while others preserve the previous non-empty value. Omitting both arguments restores the
default begin-quote and end-quote delimiters; fortunately this behavior is portable to all
implementations of m4.
define(‘foo’, ‘Macro ‘FOO’.’)
⇒
changequote(‘’, ‘’)
⇒
foo
⇒Macro ‘FOO’.
‘foo’
⇒‘Macro ‘FOO’.’
changequote(‘,)
⇒
foo
⇒Macro FOO.
Chapter 8: Input control
63
There is no way in m4 to quote a string containing an unmatched begin-quote, except
using changequote to change the current quotes.
If the quotes should be changed from, say, ‘[’ to ‘[[’, temporary quote characters have to
be defined. To achieve this, two calls of changequote must be made, one for the temporary
quotes and one for the new quotes.
Macros are recognized in preference to the begin-quote string, so if a prefix of start can be
recognized as part of a potential macro name, the quoting mechanism is effectively disabled.
Unless you use changeword (see Section 8.4 [Changeword], page 67), this means that start
should not begin with a letter, digit, or ‘_’ (underscore). However, even though quoted
strings are not recognized, the quote characters can still be discerned in macro expansion
and in trace output.
define(‘echo’, ‘$@’)
⇒
define(‘hi’, ‘HI’)
⇒
changequote(‘q’, ‘Q’)
⇒
q hi Q hi
⇒q HI Q HI
echo(hi)
⇒qHIQ
changequote
⇒
changequote(‘-’, ‘EOF’)
⇒
- hi EOF hi
⇒ hi HI
changequote
⇒
changequote(‘1’, ‘2’)
⇒
hi1hi2
⇒hi1hi2
hi 1hi2
⇒HI hi
Quotes are recognized in preference to argument collection. In particular, if start is
a single ‘(’, then argument collection is effectively disabled. For portability with other
implementations, it is a good idea to avoid ‘(’, ‘,’, and ‘)’ as the first character in start.
define(‘echo’, ‘$#:$@:’)
⇒
define(‘hi’, ‘HI’)
⇒
changequote(‘(’,‘)’)
⇒
echo(hi)
⇒0::hi
64
GNU M4 1.4.17 macro processor
changequote
⇒
changequote(‘((’, ‘))’)
⇒
echo(hi)
⇒1:HI:
echo((hi))
⇒0::hi
changequote
⇒
changequote(‘,’, ‘)’)
⇒
echo(hi,hi)bye)
⇒1:HIhibye:
However, if you are not worried about portability, using ‘(’ and ‘)’ as quoting characters
has an interesting property—you can use it to compute a quoted string containing the
expansion of any quoted text, as long as the expansion results in both balanced quotes
and balanced parentheses. The trick is realizing expand uses ‘$1’ unquoted, to trigger
its expansion using the normal quoting characters, but uses extra parentheses to group
unquoted commas that occur in the expansion without consuming whitespace following
those commas. Then _expand uses changequote to convert the extra parentheses back
into quoting characters. Note that it takes two more changequote invocations to restore
the original quotes. Contrast the behavior on whitespace when using ‘$*’, via quote, to
attempt the same task.
changequote(‘[’, ‘]’)dnl
define([a], [1, (b)])dnl
define([b], [2])dnl
define([quote], [[$*]])dnl
define([expand], [_$0(($1))])dnl
define([_expand],
[changequote([(], [)])$1changequote‘’changequote(‘[’, ‘]’)])dnl
expand([a, a, [a, a], [[a, a]]])
⇒1, (2), 1, (2), a, a, [a, a]
quote(a, a, [a, a], [[a, a]])
⇒1,(2),1,(2),a, a,[a, a]
If end is a prefix of start, the end-quote will be recognized in preference to a nested
begin-quote. In particular, changing the quotes to have the same string for start and end
disables nesting of quotes. When quote nesting is disabled, it is impossible to double-quote
strings across macro expansions, so using the same string is not done very often.
define(‘hi’, ‘HI’)
⇒
changequote(‘""’, ‘"’)
⇒
""hi"""hi"
⇒hihi
""hi" ""hi"
Chapter 8: Input control
65
⇒hi hi
""hi"" "hi"
⇒hi" "HI"
changequote
⇒
‘hi‘hi’hi’
⇒hi‘hi’hi
changequote(‘"’, ‘"’)
⇒
"hi"hi"hi"
⇒hiHIhi
It is an error if the end of file occurs within a quoted string.
‘hello world’
⇒hello world
‘dangling quote
^D
error m4:stdin:2: ERROR: end of file in string
ifelse(‘dangling quote
^D
error m4:stdin:1: ERROR: end of file in string
8.3 Changing the comment delimiters
The default comment delimiters can be changed with the builtin macro changecom:
changecom ([start], [end = ‘NL’])
[Builtin]
This sets start as the new begin-comment delimiter and end as the new end-comment
delimiter. If both arguments are missing, or start is void, then comments are disabled.
Otherwise, if end is missing or void, the default end-comment delimiter of newline is
used. The comment delimiters can be of any length.
The expansion of changecom is void.
define(‘comment’, ‘COMMENT’)
⇒
# A normal comment
⇒# A normal comment
changecom(‘/*’, ‘*/’)
⇒
# Not a comment anymore
⇒# Not a COMMENT anymore
But: /* this is a comment now */ while this is not a comment
⇒But: /* this is a comment now */ while this is not a COMMENT
Note how comments are copied to the output, much as if they were quoted strings. If
you want the text inside a comment expanded, quote the begin-comment delimiter.
Calling changecom without any arguments, or with start as the empty string, will effectively disable the commenting mechanism. To restore the original comment start of ‘#’,
you must explicitly ask for it. If start is not empty, then an empty end will use the default
66
GNU M4 1.4.17 macro processor
end-comment delimiter of newline, as otherwise, it would be impossible to end a comment.
However, this is not portable, as some other m4 implementations preserve the previous
non-empty delimiters instead.
define(‘comment’, ‘COMMENT’)
⇒
changecom
⇒
# Not a comment anymore
⇒# Not a COMMENT anymore
changecom(‘#’, ‘’)
⇒
# comment again
⇒# comment again
The comment strings can safely contain eight-bit characters. If no single character is
appropriate, start and end can be of any length. Other implementations cap the delimiter
length to five characters, but GNU has no inherent limit.
Comments are recognized in preference to macros. However, this is not compatible with
other implementations, where macros and even quoting takes precedence over comments,
so it may change in a future release. For portability, this means that start should not
begin with a letter, digit, or ‘_’ (underscore), and that neither the start-quote nor the
start-comment string should be a prefix of the other.
define(‘hi’, ‘HI’)
⇒
define(‘hi1hi2’, ‘hello’)
⇒
changecom(‘q’, ‘Q’)
⇒
q hi Q hi
⇒q hi Q HI
changecom(‘1’, ‘2’)
⇒
hi1hi2
⇒hello
hi 1hi2
⇒HI 1hi2
Comments are recognized in preference to argument collection. In particular, if start
is a single ‘(’, then argument collection is effectively disabled. For portability with other
implementations, it is a good idea to avoid ‘(’, ‘,’, and ‘)’ as the first character in start.
define(‘echo’, ‘$#:$*:$@:’)
⇒
define(‘hi’, ‘HI’)
⇒
changecom(‘(’,‘)’)
⇒
echo(hi)
Chapter 8: Input control
67
⇒0:::(hi)
changecom
⇒
changecom(‘((’, ‘))’)
⇒
echo(hi)
⇒1:HI:HI:
echo((hi))
⇒0:::((hi))
changecom(‘,’, ‘)’)
⇒
echo(hi,hi)bye)
⇒1:HI,hi)bye:HI,hi)bye:
changecom
⇒
echo(hi,‘,‘’hi’,hi)
⇒3:HI,,HI,HI:HI,,‘’hi,HI:
echo(hi,‘,‘’hi’,hi‘’changecom(‘,,’, ‘hi’))
⇒3:HI,,‘’hi,HI:HI,,‘’hi,HI:
It is an error if the end of file occurs within a comment.
changecom(‘/*’, ‘*/’)
⇒
/*dangling comment
^D
error m4:stdin:2: ERROR: end of file in comment
8.4 Changing the lexical structure of words
The macro changeword and all associated functionality is experimental. It is
only available if the --enable-changeword option was given to configure, at
GNU m4 installation time. The functionality will go away in the future, to be
replaced by other new features that are more efficient at providing the same
capabilities. Do not rely on it. Please direct your comments about it the same
way you would do for bugs.
A file being processed by m4 is split into quoted strings, words (potential macro names)
and simple tokens (any other single character). Initially a word is defined by the following
regular expression:
[_a-zA-Z][_a-zA-Z0-9]*
Using changeword, you can change this regular expression:
changeword (regex)
[Optional builtin]
Changes the regular expression for recognizing macro names to be regex. If regex is
empty, use ‘[_a-zA-Z][_a-zA-Z0-9]*’. regex must obey the constraint that every
prefix of the desired final pattern is also accepted by the regular expression. If regex
contains grouping parentheses, the macro invoked is the portion that matched the
first group, rather than the entire matching string.
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GNU M4 1.4.17 macro processor
The expansion of changeword is void. The macro changeword is recognized only with
parameters.
Relaxing the lexical rules of m4 might be useful (for example) if you wanted to apply
translations to a file of numbers:
ifdef(‘changeword’, ‘’, ‘errprint(‘ skipping: no changeword support
’)m4exit(‘77’)’)dnl
changeword(‘[_a-zA-Z0-9]+’)
⇒
define(‘1’, ‘0’)1
⇒0
Tightening the lexical rules is less useful, because it will generally make some of the
builtins unavailable. You could use it to prevent accidental call of builtins, for example:
ifdef(‘changeword’, ‘’, ‘errprint(‘ skipping: no changeword support
’)m4exit(‘77’)’)dnl
define(‘_indir’, defn(‘indir’))
⇒
changeword(‘_[_a-zA-Z0-9]*’)
⇒
esyscmd(‘foo’)
⇒esyscmd(foo)
_indir(‘esyscmd’, ‘echo hi’)
⇒hi
⇒
Because m4 constructs its words a character at a time, there is a restriction on the regular
expressions that may be passed to changeword. This is that if your regular expression
accepts ‘foo’, it must also accept ‘f’ and ‘fo’.
ifdef(‘changeword’, ‘’, ‘errprint(‘ skipping: no changeword support
’)m4exit(‘77’)’)dnl
define(‘foo
’, ‘bar
’)
⇒
dnl This example wants to recognize changeword, dnl, and ‘foo\n’.
dnl First, we check that our regexp will match.
regexp(‘changeword’, ‘[cd][a-z]*\|foo[
]’)
⇒0
regexp(‘foo
’, ‘[cd][a-z]*\|foo[
]’)
⇒0
regexp(‘f’, ‘[cd][a-z]*\|foo[
]’)
⇒-1
foo
Chapter 8: Input control
69
⇒foo
changeword(‘[cd][a-z]*\|foo[
]’)
⇒
dnl Even though ‘foo\n’ matches, we forgot to allow ‘f’.
foo
⇒foo
changeword(‘[cd][a-z]*\|fo*[
]?’)
⇒
dnl Now we can call ‘foo\n’.
foo
⇒bar
changeword has another function. If the regular expression supplied contains any
grouped subexpressions, then text outside the first of these is discarded before symbol
lookup. So:
ifdef(‘changeword’, ‘’, ‘errprint(‘ skipping: no changeword support
’)m4exit(‘77’)’)dnl
ifdef(‘__unix__’, ,
‘errprint(‘ skipping: syscmd does not have unix semantics
’)m4exit(‘77’)’)dnl
changecom(‘/*’, ‘*/’)dnl
define(‘foo’, ‘bar’)dnl
changeword(‘#\([_a-zA-Z0-9]*\)’)
⇒
#esyscmd(‘echo foo \#foo’)
⇒foo bar
⇒
m4 now requires a ‘#’ mark at the beginning of every macro invocation, so one can use
m4 to preprocess plain text without losing various words like ‘divert’.
In m4, macro substitution is based on text, while in TEX, it is based on tokens.
changeword can throw this difference into relief. For example, here is the same idea
represented in TEX and m4. First, the TEX version:
\def\a{\message{Hello}}
\catcode‘\@=0
\catcode‘\\=12
@a
@bye
⇒Hello
Then, the m4 version:
ifdef(‘changeword’, ‘’, ‘errprint(‘ skipping: no changeword support
’)m4exit(‘77’)’)dnl
define(‘a’, ‘errprint(‘Hello’)’)dnl
changeword(‘@\([_a-zA-Z0-9]*\)’)
⇒
70
GNU M4 1.4.17 macro processor
@a
⇒errprint(Hello)
In the TEX example, the first line defines a macro a to print the message ‘Hello’. The
second line defines @ to be usable instead of \ as an escape character. The third line defines
\ to be a normal printing character, not an escape. The fourth line invokes the macro a.
So, when TEX is run on this file, it displays the message ‘Hello’.
When the m4 example is passed through m4, it outputs ‘errprint(Hello)’. The reason
for this is that TEX does lexical analysis of macro definition when the macro is defined. m4
just stores the text, postponing the lexical analysis until the macro is used.
You should note that using changeword will slow m4 down by a factor of about seven,
once it is changed to something other than the default regular expression. You can invoke
changeword with the empty string to restore the default word definition, and regain the
parsing speed.
8.5 Saving text until end of input
It is possible to ‘save’ some text until the end of the normal input has been seen. Text can
be saved, to be read again by m4 when the normal input has been exhausted. This feature is
normally used to initiate cleanup actions before normal exit, e.g., deleting temporary files.
To save input text, use the builtin m4wrap:
m4wrap (string, . . . )
[Builtin]
Stores string in a safe place, to be reread when end of input is reached. As a GNU
extension, additional arguments are concatenated with a space to the string.
The expansion of m4wrap is void. The macro m4wrap is recognized only with parameters.
define(‘cleanup’, ‘This is the ‘cleanup’ action.
’)
⇒
m4wrap(‘cleanup’)
⇒
This is the first and last normal input line.
⇒This is the first and last normal input line.
^D
⇒This is the cleanup action.
The saved input is only reread when the end of normal input is seen, and not if m4exit
is used to exit m4.
It is safe to call m4wrap from saved text, but then the order in which the saved text is
reread is undefined. If m4wrap is not used recursively, the saved pieces of text are reread
in the opposite order in which they were saved (LIFO—last in, first out). However, this
behavior is likely to change in a future release, to match POSIX, so you should not depend
on this order.
It is possible to emulate POSIX behavior even with older versions of GNU M4 by including the file m4-1.4.17/examples/wrapfifo.m4 from the distribution:
Chapter 8: Input control
71
$ m4 -I examples
undivert(‘wrapfifo.m4’)dnl
⇒dnl Redefine m4wrap to have FIFO semantics.
⇒define(‘_m4wrap_level’, ‘0’)dnl
⇒define(‘m4wrap’,
⇒‘ifdef(‘m4wrap’_m4wrap_level,
⇒
‘define(‘m4wrap’_m4wrap_level,
⇒
defn(‘m4wrap’_m4wrap_level)‘$1’)’,
⇒
‘builtin(‘m4wrap’, ‘define(‘_m4wrap_level’,
⇒
incr(_m4wrap_level))dnl
⇒m4wrap’_m4wrap_level)dnl
⇒define(‘m4wrap’_m4wrap_level, ‘$1’)’)’)dnl
include(‘wrapfifo.m4’)
⇒
m4wrap(‘a‘’m4wrap(‘c
’, ‘d’)’)m4wrap(‘b’)
⇒
^D
⇒abc
It is likewise possible to emulate LIFO behavior without resorting to the GNU M4
extension of builtin, by including the file m4-1.4.17/examples/wraplifo.m4 from the
distribution. (Unfortunately, both examples shown here share some subtle bugs. See if you
can find and correct them; or see Section 17.5 [Answers], page 125).
$ m4 -I examples
undivert(‘wraplifo.m4’)dnl
⇒dnl Redefine m4wrap to have LIFO semantics.
⇒define(‘_m4wrap_level’, ‘0’)dnl
⇒define(‘_m4wrap’, defn(‘m4wrap’))dnl
⇒define(‘m4wrap’,
⇒‘ifdef(‘m4wrap’_m4wrap_level,
⇒
‘define(‘m4wrap’_m4wrap_level,
⇒
‘$1’defn(‘m4wrap’_m4wrap_level))’,
⇒
‘_m4wrap(‘define(‘_m4wrap_level’, incr(_m4wrap_level))dnl
⇒m4wrap’_m4wrap_level)dnl
⇒define(‘m4wrap’_m4wrap_level, ‘$1’)’)’)dnl
include(‘wraplifo.m4’)
⇒
m4wrap(‘a‘’m4wrap(‘c
’, ‘d’)’)m4wrap(‘b’)
⇒
^D
⇒bac
Here is an example of implementing a factorial function using m4wrap:
define(‘f’, ‘ifelse(‘$1’, ‘0’, ‘Answer: 0!=1
’, eval(‘$1>1’), ‘0’, ‘Answer: $2$1=eval(‘$2$1’)
’, ‘m4wrap(‘f(decr(‘$1’), ‘$2$1*’)’)’)’)
72
GNU M4 1.4.17 macro processor
⇒
f(‘10’)
⇒
^D
⇒Answer: 10*9*8*7*6*5*4*3*2*1=3628800
Invocations of m4wrap at the same recursion level are concatenated and rescanned as
usual:
define(‘aa’, ‘AA
’)
⇒
m4wrap(‘a’)m4wrap(‘a’)
⇒
^D
⇒AA
however, the transition between recursion levels behaves like an end of file condition between
two input files.
m4wrap(‘m4wrap(‘)’)len(abc’)
⇒
^D
error m4:stdin:1: ERROR: end of file in argument list
Chapter 9: File inclusion
73
9 File inclusion
m4 allows you to include named files at any point in the input.
9.1 Including named files
There are two builtin macros in m4 for including files:
include (file)
sinclude (file)
[Builtin]
[Builtin]
Both macros cause the file named file to be read by m4. When the end of the file is
reached, input is resumed from the previous input file.
The expansion of include and sinclude is therefore the contents of file.
If file does not exist, is a directory, or cannot otherwise be read, the expansion is
void, and include will fail with an error while sinclude is silent. The empty string
counts as a file that does not exist.
The macros include and sinclude are recognized only with parameters.
include(‘none’)
error m4:stdin:1: cannot open ‘none’: No such file or directory
⇒
include()
error m4:stdin:2: cannot open ‘’: No such file or directory
⇒
sinclude(‘none’)
⇒
sinclude()
⇒
The rest of this section assumes that m4 is invoked with the -I option (see Section 2.2
[Invoking m4], page 8) pointing to the m4-1.4.17/examples directory shipped as part of
the GNU m4 package. The file m4-1.4.17/examples/incl.m4 in the distribution contains
the lines:
$ cat examples/incl.m4
⇒Include file start
⇒foo
⇒Include file end
Normally file inclusion is used to insert the contents of a file into the input stream. The
contents of the file will be read by m4 and macro calls in the file will be expanded:
$ m4 -I examples
define(‘foo’, ‘FOO’)
⇒
include(‘incl.m4’)
⇒Include file start
⇒FOO
⇒Include file end
⇒
74
GNU M4 1.4.17 macro processor
The fact that include and sinclude expand to the contents of the file can be used to
define macros that operate on entire files. Here is an example, which defines ‘bar’ to expand
to the contents of incl.m4:
$ m4 -I examples
define(‘bar’, include(‘incl.m4’))
⇒
This is ‘bar’: >>bar<<
⇒This is bar: >>Include file start
⇒foo
⇒Include file end
⇒<<
This use of include is not trivial, though, as files can contain quotes, commas, and
parentheses, which can interfere with the way the m4 parser works. GNU m4 seamlessly
concatenates the file contents with the next character, even if the included file ended in the
middle of a comment, string, or macro call. These conditions are only treated as end of file
errors if specified as input files on the command line.
In GNU m4, an alternative method of reading files is using undivert (see Section 10.2
[Undivert], page 76) on a named file.
9.2 Searching for include files
GNU m4 allows included files to be found in other directories than the current working
directory.
If the --prepend-include or -B command-line option was provided (see Section 2.2
[Invoking m4], page 8), those directories are searched first, in reverse order that those
options were listed on the command line. Then m4 looks in the current working directory.
Next comes the directories specified with the --include or -I option, in the order found
on the command line. Finally, if the M4PATH environment variable is set, it is expected to
contain a colon-separated list of directories, which will be searched in order.
If the automatic search for include-files causes trouble, the ‘p’ debug flag (see Section 7.3
[Debug Levels], page 58) can help isolate the problem.
Chapter 10: Diverting and undiverting output
75
10 Diverting and undiverting output
Diversions are a way of temporarily saving output. The output of m4 can at any time be
diverted to a temporary file, and be reinserted into the output stream, undiverted, again at
a later time.
Numbered diversions are counted from 0 upwards, diversion number 0 being the normal
output stream. GNU m4 tries to keep diversions in memory. However, there is a limit to
the overall memory usable by all diversions taken together (512K, currently). When this
maximum is about to be exceeded, a temporary file is opened to receive the contents of the
biggest diversion still in memory, freeing this memory for other diversions. When creating
the temporary file, m4 honors the value of the environment variable TMPDIR, and falls back
to /tmp. Thus, the amount of available disk space provides the only real limit on the number
and aggregate size of diversions.
Diversions make it possible to generate output in a different order than the input was
read. It is possible to implement topological sorting dependencies. For example, GNU Autoconf makes use of diversions under the hood to ensure that the expansion of a prerequisite
macro appears in the output prior to the expansion of a dependent macro, regardless of
which order the two macros were invoked in the user’s input file.
10.1 Diverting output
Output is diverted using divert:
divert ([number = ‘0’])
[Builtin]
The current diversion is changed to number. If number is left out or empty, it is
assumed to be zero. If number cannot be parsed, the diversion is unchanged.
The expansion of divert is void.
When all the m4 input will have been processed, all existing diversions are automatically
undiverted, in numerical order.
divert(‘1’)
This text is diverted.
divert
⇒
This text is not diverted.
⇒This text is not diverted.
^D
⇒
⇒This text is diverted.
Several calls of divert with the same argument do not overwrite the previous diverted
text, but append to it. Diversions are printed after any wrapped text is expanded.
define(‘text’, ‘TEXT’)
⇒
divert(‘1’)‘diverted text.’
divert
⇒
76
GNU M4 1.4.17 macro processor
m4wrap(‘Wrapped text precedes ’)
⇒
^D
⇒Wrapped TEXT precedes diverted text.
If output is diverted to a negative diversion, it is simply discarded. This can be used to
suppress unwanted output. A common example of unwanted output is the trailing newlines
after macro definitions. Here is a common programming idiom in m4 for avoiding them.
divert(‘-1’)
define(‘foo’, ‘Macro ‘foo’.’)
define(‘bar’, ‘Macro ‘bar’.’)
divert
⇒
Traditional implementations only supported ten diversions. But as a GNU extension,
diversion numbers can be as large as positive integers will allow, rather than treating a
multi-digit diversion number as a request to discard text.
divert(eval(‘1<<28’))world
divert(‘2’)hello
^D
⇒hello
⇒world
Note that divert is an English word, but also an active macro without arguments.
When processing plain text, the word might appear in normal text and be unintentionally
swallowed as a macro invocation. One way to avoid this is to use the -P option to rename
all builtins (see Section 2.1 [Invoking m4], page 7). Another is to write a wrapper that
requires a parameter to be recognized.
We decided to divert the stream for irrigation.
⇒We decided to the stream for irrigation.
define(‘divert’, ‘ifelse(‘$#’, ‘0’, ‘‘$0’’, ‘builtin(‘$0’, $@)’)’)
⇒
divert(‘-1’)
Ignored text.
divert(‘0’)
⇒
We decided to divert the stream for irrigation.
⇒We decided to divert the stream for irrigation.
10.2 Undiverting output
Diverted text can be undiverted explicitly using the builtin undivert:
undivert ([diversions...])
[Builtin]
Undiverts the numeric diversions given by the arguments, in the order given. If no
arguments are supplied, all diversions are undiverted, in numerical order.
As a GNU extension, diversions may contain non-numeric strings, which are treated
as the names of files to copy into the output without expansion. A warning is issued
if a file could not be opened.
Chapter 10: Diverting and undiverting output
77
The expansion of undivert is void.
divert(‘1’)
This text is diverted.
divert
⇒
This text is not diverted.
⇒This text is not diverted.
undivert(‘1’)
⇒
⇒This text is diverted.
⇒
Notice the last two blank lines. One of them comes from the newline following undivert,
the other from the newline that followed the divert! A diversion often starts with a blank
line like this.
When diverted text is undiverted, it is not reread by m4, but rather copied directly to
the current output, and it is therefore not an error to undivert into a diversion. Undiverting
the empty string is the same as specifying diversion 0; in either case nothing happens since
the output has already been flushed.
divert(‘1’)diverted text
divert
⇒
undivert()
⇒
undivert(‘0’)
⇒
undivert
⇒diverted text
⇒
divert(‘1’)more
divert(‘2’)undivert(‘1’)diverted text‘’divert
⇒
undivert(‘1’)
⇒
undivert(‘2’)
⇒more
⇒diverted text
When a diversion has been undiverted, the diverted text is discarded, and it is not
possible to bring back diverted text more than once.
divert(‘1’)
This text is diverted first.
divert(‘0’)undivert(‘1’)dnl
⇒
⇒This text is diverted first.
undivert(‘1’)
⇒
78
GNU M4 1.4.17 macro processor
divert(‘1’)
This text is also diverted but not appended.
divert(‘0’)undivert(‘1’)dnl
⇒
⇒This text is also diverted but not appended.
Attempts to undivert the current diversion are silently ignored. Thus, when the current
diversion is not 0, the current diversion does not get rearranged among the other diversions.
divert(‘1’)one
divert(‘2’)two
divert(‘3’)three
divert(‘2’)undivert‘’dnl
divert‘’undivert‘’dnl
⇒two
⇒one
⇒three
GNU m4 allows named files to be undiverted. Given a non-numeric argument, the contents of the file named will be copied, uninterpreted, to the current output. This complements the builtin include (see Section 9.1 [Include], page 73). To illustrate the difference,
assume the file foo contains:
$ cat foo
bar
then
define(‘bar’, ‘BAR’)
⇒
undivert(‘foo’)
⇒bar
⇒
include(‘foo’)
⇒BAR
⇒
If the file is not found (or cannot be read), an error message is issued, and the expansion
is void. It is possible to intermix files and diversion numbers.
divert(‘1’)diversion one
divert(‘2’)undivert(‘foo’)dnl
divert(‘3’)diversion three
divert‘’dnl
undivert(‘1’, ‘2’, ‘foo’, ‘3’)dnl
⇒diversion one
⇒bar
⇒bar
⇒diversion three
10.3 Diversion numbers
The current diversion is tracked by the builtin divnum:
Chapter 10: Diverting and undiverting output
divnum
79
[Builtin]
Expands to the number of the current diversion.
Initial divnum
⇒Initial 0
divert(‘1’)
Diversion one: divnum
divert(‘2’)
Diversion two: divnum
^D
⇒
⇒Diversion one: 1
⇒
⇒Diversion two: 2
10.4 Discarding diverted text
Often it is not known, when output is diverted, whether the diverted text is actually needed.
Since all non-empty diversion are brought back on the main output stream when the end
of input is seen, a method of discarding a diversion is needed. If all diversions should be
discarded, the easiest is to end the input to m4 with ‘divert(‘-1’)’ followed by an explicit
‘undivert’:
divert(‘1’)
Diversion one: divnum
divert(‘2’)
Diversion two: divnum
divert(‘-1’)
undivert
^D
No output is produced at all.
Clearing selected diversions can be done with the following macro:
cleardivert ([diversions...])
[Composite]
Discard the contents of each of the listed numeric diversions.
define(‘cleardivert’,
‘pushdef(‘_n’, divnum)divert(‘-1’)undivert($@)divert(_n)popdef(‘_n’)’)
⇒
It is called just like undivert, but the effect is to clear the diversions, given by the
arguments. (This macro has a nasty bug! You should try to see if you can find it and
correct it; or see Section 17.6 [Answers], page 126).
Chapter 11: Macros for text handling
81
11 Macros for text handling
There are a number of builtins in m4 for manipulating text in various ways, extracting
substrings, searching, substituting, and so on.
11.1 Calculating length of strings
The length of a string can be calculated by len:
len (string)
[Builtin]
Expands to the length of string, as a decimal number.
The macro len is recognized only with parameters.
len()
⇒0
len(‘abcdef’)
⇒6
11.2 Searching for substrings
Searching for substrings is done with index:
index (string, substring)
[Builtin]
Expands to the index of the first occurrence of substring in string. The first character
in string has index 0. If substring does not occur in string, index expands to ‘-1’.
The macro index is recognized only with parameters.
index(‘gnus, gnats, and armadillos’, ‘nat’)
⇒7
index(‘gnus, gnats, and armadillos’, ‘dag’)
⇒-1
Omitting substring evokes a warning, but still produces output; contrast this with an
empty substring.
index(‘abc’)
error m4:stdin:1: Warning: too few arguments to builtin ‘index’
⇒0
index(‘abc’, ‘’)
⇒0
index(‘abc’, ‘b’)
⇒1
11.3 Searching for regular expressions
Searching for regular expressions is done with the builtin regexp:
regexp (string, regexp, [replacement])
[Builtin]
Searches for regexp in string. The syntax for regular expressions is the same as in
GNU Emacs, which is similar to BRE, Basic Regular Expressions in POSIX. See
Section “Syntax of Regular Expressions” in The GNU Emacs Manual. Support for
82
GNU M4 1.4.17 macro processor
ERE, Extended Regular Expressions is not available, but will be added in GNU M4
2.0.
If replacement is omitted, regexp expands to the index of the first match of regexp
in string. If regexp does not match anywhere in string, it expands to -1.
If replacement is supplied, and there was a match, regexp changes the expansion to
this argument, with ‘\n’ substituted by the text matched by the nth parenthesized
sub-expression of regexp, up to nine sub-expressions. The escape ‘\&’ is replaced by
the text of the entire regular expression matched. For all other characters, ‘\’ treats
the next character literally. A warning is issued if there were fewer sub-expressions
than the ‘\n’ requested, or if there is a trailing ‘\’. If there was no match, regexp
expands to the empty string.
The macro regexp is recognized only with parameters.
regexp(‘GNUs not Unix’,
⇒5
regexp(‘GNUs not Unix’,
⇒-1
regexp(‘GNUs not Unix’,
⇒*** Unix *** nix ***
regexp(‘GNUs not Unix’,
⇒
‘\<[a-z]\w+’)
‘\<Q\w*’)
‘\w\(\w+\)$’, ‘*** \& *** \1 ***’)
‘\<Q\w*’, ‘*** \& *** \1 ***’)
Here are some more examples on the handling of backslash:
regexp(‘abc’, ‘\(b\)’, ‘\\\10\a’)
⇒\b0a
regexp(‘abc’, ‘b’, ‘\1\’)
error m4:stdin:2: Warning: sub-expression 1 not present
error m4:stdin:2: Warning: trailing \ ignored in replacement
⇒
regexp(‘abc’, ‘\(\(d\)?\)\(c\)’, ‘\1\2\3\4\5\6’)
error m4:stdin:3: Warning: sub-expression 4 not present
error m4:stdin:3: Warning: sub-expression 5 not present
error m4:stdin:3: Warning: sub-expression 6 not present
⇒c
Omitting regexp evokes a warning, but still produces output; contrast this with an empty
regexp argument.
regexp(‘abc’)
error m4:stdin:1: Warning: too few arguments to builtin ‘regexp’
⇒0
regexp(‘abc’, ‘’)
⇒0
regexp(‘abc’, ‘’, ‘\\def’)
⇒\def
11.4 Extracting substrings
Substrings are extracted with substr:
Chapter 11: Macros for text handling
83
substr (string, from, [length])
[Builtin]
Expands to the substring of string, which starts at index from, and extends for length
characters, or to the end of string, if length is omitted. The starting index of a string
is always 0. The expansion is empty if there is an error parsing from or length, if
from is beyond the end of string, or if length is negative.
The macro substr is recognized only with parameters.
substr(‘gnus, gnats, and armadillos’, ‘6’)
⇒gnats, and armadillos
substr(‘gnus, gnats, and armadillos’, ‘6’, ‘5’)
⇒gnats
Omitting from evokes a warning, but still produces output.
substr(‘abc’)
error m4:stdin:1: Warning: too few arguments to builtin ‘substr’
⇒abc
substr(‘abc’,)
error m4:stdin:2: empty string treated as 0 in builtin ‘substr’
⇒abc
11.5 Translating characters
Character translation is done with translit:
translit (string, chars, [replacement])
[Builtin]
Expands to string, with each character that occurs in chars translated into the character from replacement with the same index.
If replacement is shorter than chars, the excess characters of chars are deleted from
the expansion; if chars is shorter, the excess characters in replacement are silently
ignored. If replacement is omitted, all characters in string that are present in chars
are deleted from the expansion. If a character appears more than once in chars, only
the first instance is used in making the translation. Only a single translation pass is
made, even if characters in replacement also appear in chars.
As a GNU extension, both chars and replacement can contain character-ranges, e.g.,
‘a-z’ (meaning all lowercase letters) or ‘0-9’ (meaning all digits). To include a dash
‘-’ in chars or replacement, place it first or last in the entire string, or as the last
character of a range. Back-to-back ranges can share a common endpoint. It is not an
error for the last character in the range to be ‘larger’ than the first. In that case, the
range runs backwards, i.e., ‘9-0’ means the string ‘9876543210’. The expansion of a
range is dependent on the underlying encoding of characters, so using ranges is not
always portable between machines.
The macro translit is recognized only with parameters.
translit(‘GNUs not Unix’, ‘A-Z’)
⇒s not nix
translit(‘GNUs not Unix’, ‘a-z’, ‘A-Z’)
⇒GNUS NOT UNIX
translit(‘GNUs not Unix’, ‘A-Z’, ‘z-a’)
84
GNU M4 1.4.17 macro processor
⇒tmfs not fnix
translit(‘+,-12345’, ‘+--1-5’, ‘<;>a-c-a’)
⇒<;>abcba
translit(‘abcdef’, ‘aabdef’, ‘bcged’)
⇒bgced
In the ascii encoding, the first example deletes all uppercase letters, the second converts
lowercase to uppercase, and the third ‘mirrors’ all uppercase letters, while converting them
to lowercase. The two first cases are by far the most common, even though they are not
portable to ebcdic or other encodings. The fourth example shows a range ending in ‘-’,
as well as back-to-back ranges. The final example shows that ‘a’ is mapped to ‘b’, not ‘c’;
the resulting ‘b’ is not further remapped to ‘g’; the ‘d’ and ‘e’ are swapped, and the ‘f’ is
discarded.
Omitting chars evokes a warning, but still produces output.
translit(‘abc’)
error m4:stdin:1: Warning: too few arguments to builtin ‘translit’
⇒abc
11.6 Substituting text by regular expression
Global substitution in a string is done by patsubst:
patsubst (string, regexp, [replacement])
[Builtin]
Searches string for matches of regexp, and substitutes replacement for each match.
The syntax for regular expressions is the same as in GNU Emacs (see Section 11.3
[Regexp], page 81).
The parts of string that are not covered by any match of regexp are copied to the
expansion. Whenever a match is found, the search proceeds from the end of the
match, so a character from string will never be substituted twice. If regexp matches
a string of zero length, the start position for the search is incremented, to avoid infinite
loops.
When a replacement is to be made, replacement is inserted into the expansion, with
‘\n’ substituted by the text matched by the nth parenthesized sub-expression of
patsubst, for up to nine sub-expressions. The escape ‘\&’ is replaced by the text
of the entire regular expression matched. For all other characters, ‘\’ treats the next
character literally. A warning is issued if there were fewer sub-expressions than the
‘\n’ requested, or if there is a trailing ‘\’.
The replacement argument can be omitted, in which case the text matched by regexp
is deleted.
The macro patsubst is recognized only with parameters.
patsubst(‘GNUs not Unix’, ‘^’, ‘OBS: ’)
⇒OBS: GNUs not Unix
patsubst(‘GNUs not Unix’, ‘\<’, ‘OBS: ’)
⇒OBS: GNUs OBS: not OBS: Unix
patsubst(‘GNUs not Unix’, ‘\w*’, ‘(\&)’)
⇒(GNUs)() (not)() (Unix)()
patsubst(‘GNUs not Unix’, ‘\w+’, ‘(\&)’)
Chapter 11: Macros for text handling
85
⇒(GNUs) (not) (Unix)
patsubst(‘GNUs not Unix’, ‘[A-Z][a-z]+’)
⇒GN not
patsubst(‘GNUs not Unix’, ‘not’, ‘NOT\’)
error m4:stdin:6: Warning: trailing \ ignored in replacement
⇒GNUs NOT Unix
Here is a slightly more realistic example, which capitalizes individual words or whole
sentences, by substituting calls of the macros upcase and downcase into the strings.
upcase (text)
downcase (text)
capitalize (text)
[Composite]
[Composite]
[Composite]
Expand to text, but with capitalization changed: upcase changes all letters to upper
case, downcase changes all letters to lower case, and capitalize changes the first
character of each word to upper case and the remaining characters to lower case.
First, an example of their usage, using implementations distributed in m4-1.4.17/
examples/capitalize.m4.
$ m4 -I examples
include(‘capitalize.m4’)
⇒
upcase(‘GNUs not Unix’)
⇒GNUS NOT UNIX
downcase(‘GNUs not Unix’)
⇒gnus not unix
capitalize(‘GNUs not Unix’)
⇒Gnus Not Unix
Now for the implementation. There is a helper macro _capitalize which puts only its
first word in mixed case. Then capitalize merely parses out the words, and replaces them
with an invocation of _capitalize. (As presented here, the capitalize macro has some
subtle flaws. You should try to see if you can find and correct them; or see Section 17.7
[Answers], page 127).
$ m4 -I examples
undivert(‘capitalize.m4’)dnl
⇒divert(‘-1’)
⇒# upcase(text)
⇒# downcase(text)
⇒# capitalize(text)
⇒#
change case of text, simple version
⇒define(‘upcase’, ‘translit(‘$*’, ‘a-z’, ‘A-Z’)’)
⇒define(‘downcase’, ‘translit(‘$*’, ‘A-Z’, ‘a-z’)’)
⇒define(‘_capitalize’,
⇒
‘regexp(‘$1’, ‘^\(\w\)\(\w*\)’,
⇒
‘upcase(‘\1’)‘’downcase(‘\2’)’)’)
⇒define(‘capitalize’, ‘patsubst(‘$1’, ‘\w+’, ‘_$0(‘\&’)’)’)
⇒divert‘’dnl
86
GNU M4 1.4.17 macro processor
While regexp replaces the whole input with the replacement as soon as there is a match,
patsubst replaces each occurrence of a match and preserves non-matching pieces:
define(‘patreg’,
‘patsubst($@)
regexp($@)’)dnl
patreg(‘bar foo baz Foo’, ‘foo\|Foo’, ‘FOO’)
⇒bar FOO baz FOO
⇒FOO
patreg(‘aba abb 121’, ‘\(.\)\(.\)\1’, ‘\2\1\2’)
⇒bab abb 212
⇒bab
Omitting regexp evokes a warning, but still produces output; contrast this with an empty
regexp argument.
patsubst(‘abc’)
error m4:stdin:1: Warning: too few arguments to builtin ‘patsubst’
⇒abc
patsubst(‘abc’, ‘’)
⇒abc
patsubst(‘abc’, ‘’, ‘\\-’)
⇒\-a\-b\-c\-
11.7 Formatting strings (printf-like)
Formatted output can be made with format:
format (format-string, . . . )
[Builtin]
Works much like the C function printf. The first argument format-string can contain
‘%’ specifications which are satisfied by additional arguments, and the expansion of
format is the formatted string.
The macro format is recognized only with parameters.
Its use is best described by a few examples:
define(‘foo’, ‘The brown fox jumped over the lazy dog’)
⇒
format(‘The string "%s" uses %d characters’, foo, len(foo))
⇒The string "The brown fox jumped over the lazy dog" uses 38 characters
format(‘%*.*d’, ‘-1’, ‘-1’, ‘1’)
⇒1
format(‘%.0f’, ‘56789.9876’)
⇒56790
len(format(‘%-*X’, ‘5000’, ‘1’))
⇒5000
ifelse(format(‘%010F’, ‘infinity’), ‘
INF’, ‘success’,
format(‘%010F’, ‘infinity’), ‘ INFINITY’, ‘success’,
format(‘%010F’, ‘infinity’))
⇒success
ifelse(format(‘%.1A’, ‘1.999’), ‘0X1.0P+1’, ‘success’,
Chapter 11: Macros for text handling
87
format(‘%.1A’, ‘1.999’), ‘0X2.0P+0’, ‘success’,
format(‘%.1A’, ‘1.999’))
⇒success
format(‘%g’, ‘0xa.P+1’)
⇒20
Using the forloop macro defined earlier (see Section 6.4 [Forloop], page 46), this example
shows how format can be used to produce tabular output.
$ m4 -I examples
include(‘forloop.m4’)
⇒
forloop(‘i’, ‘1’, ‘10’, ‘format(‘%6d squared is %10d
’, i, eval(i**2))’)
⇒
1 squared is
1
⇒
2 squared is
4
⇒
3 squared is
9
⇒
4 squared is
16
⇒
5 squared is
25
⇒
6 squared is
36
⇒
7 squared is
49
⇒
8 squared is
64
⇒
9 squared is
81
⇒
10 squared is
100
⇒
The builtin format is modeled after the ANSI C ‘printf’ function, and supports these
‘%’ specifiers: ‘c’, ‘s’, ‘d’, ‘o’, ‘x’, ‘X’, ‘u’, ‘a’, ‘A’, ‘e’, ‘E’, ‘f’, ‘F’, ‘g’, ‘G’, and ‘%’; it supports
field widths and precisions, and the flags ‘+’, ‘-’, ‘ ’, ‘0’, ‘#’, and ‘’’. For integer specifiers,
the width modifiers ‘hh’, ‘h’, and ‘l’ are recognized, and for floating point specifiers, the
width modifier ‘l’ is recognized. Items not yet supported include positional arguments,
the ‘n’, ‘p’, ‘S’, and ‘C’ specifiers, the ‘z’, ‘t’, ‘j’, ‘L’ and ‘ll’ modifiers, and any platform
extensions available in the native printf. For more details on the functioning of printf, see
the C Library Manual, or the POSIX specification (for example, ‘%a’ is supported even on
platforms that haven’t yet implemented C99 hexadecimal floating point output natively).
Unrecognized specifiers result in a warning. It is anticipated that a future release of
GNU m4 will support more specifiers, and give better warnings when various problems such
as overflow are encountered. Likewise, escape sequences are not yet recognized.
format(‘%p’, ‘0’)
error m4:stdin:1: Warning: unrecognized specifier in ‘%p’
⇒
Chapter 12: Macros for doing arithmetic
89
12 Macros for doing arithmetic
Integer arithmetic is included in m4, with a C-like syntax. As convenient shorthands, there
are builtins for simple increment and decrement operations.
12.1 Decrement and increment operators
Increment and decrement of integers are supported using the builtins incr and decr:
incr (number)
decr (number)
[Builtin]
[Builtin]
Expand to the numerical value of number, incremented or decremented, respectively,
by one. Except for the empty string, the expansion is empty if number could not be
parsed.
The macros incr and decr are recognized only with parameters.
incr(‘4’)
⇒5
decr(‘7’)
⇒6
incr()
error m4:stdin:3: empty string treated as 0 in builtin ‘incr’
⇒1
decr()
error m4:stdin:4: empty string treated as 0 in builtin ‘decr’
⇒-1
12.2 Evaluating integer expressions
Integer expressions are evaluated with eval:
eval (expression, [radix = ‘10’], [width])
[Builtin]
Expands to the value of expression. The expansion is empty if a problem is encountered while parsing the arguments. If specified, radix and width control the format
of the output.
Calculations are done with 32-bit signed numbers. Overflow silently results in wraparound. A warning is issued if division by zero is attempted, or if expression could
not be parsed.
Expressions can contain the following operators, listed in order of decreasing precedence.
‘()’
Parentheses
‘+ - ~ !’
Unary plus and minus, and bitwise and logical negation
‘**’
Exponentiation
‘* / %’
Multiplication, division, and modulo
‘+ -’
Addition and subtraction
‘<< >>’
Shift left or right
90
GNU M4 1.4.17 macro processor
‘> >= < <=’
Relational operators
‘== !=’
Equality operators
‘&’
Bitwise and
‘^’
Bitwise exclusive-or
‘|’
Bitwise or
‘&&’
Logical and
‘||’
Logical or
The macro eval is recognized only with parameters.
All binary operators, except exponentiation, are left associative. C operators that perform variable assignment, such as ‘+=’ or ‘--’, are not implemented, since eval only operates
on constants, not variables. Attempting to use them results in an error. However, since
traditional implementations treated ‘=’ as an undocumented alias for ‘==’ as opposed to
an assignment operator, this usage is supported as a special case. Be aware that a future
version of GNU M4 may support assignment semantics as an extension when POSIX mode
is not requested, and that using ‘=’ to check equality is not portable.
eval(‘2 = 2’)
error m4:stdin:1: Warning: recommend ==, not =, for equality operator
⇒1
eval(‘++0’)
error m4:stdin:2: invalid operator in eval: ++0
⇒
eval(‘0 |= 1’)
error m4:stdin:3: invalid operator in eval: 0 |= 1
⇒
Note that some older m4 implementations use ‘^’ as an alternate operator for the exponentiation, although POSIX requires the C behavior of bitwise exclusive-or. The precedence
of the negation operators, ‘~’ and ‘!’, was traditionally lower than equality. The unary operators could not be used reliably more than once on the same term without intervening
parentheses. The traditional precedence of the equality operators ‘==’ and ‘!=’ was identical instead of lower than the relational operators such as ‘<’, even through GNU M4 1.4.8.
Starting with version 1.4.9, GNU M4 correctly follows POSIX precedence rules. M4 scripts
designed to be portable between releases must be aware that parentheses may be required
to enforce C precedence rules. Likewise, division by zero, even in the unused branch of a
short-circuiting operator, is not always well-defined in other implementations.
Following are some examples where the current version of M4 follows C precedence rules,
but where older versions and some other implementations of m4 require explicit parentheses
to get the correct result:
eval(‘1 == 2 > 0’)
⇒1
eval(‘(1 == 2) > 0’)
⇒0
Chapter 12: Macros for doing arithmetic
91
eval(‘! 0 * 2’)
⇒2
eval(‘! (0 * 2)’)
⇒1
eval(‘1 | 1 ^ 1’)
⇒1
eval(‘(1 | 1) ^ 1’)
⇒0
eval(‘+ + - ~ ! ~ 0’)
⇒1
eval(‘2 || 1 / 0’)
⇒1
eval(‘0 || 1 / 0’)
error m4:stdin:9: divide by zero in eval: 0 || 1 / 0
⇒
eval(‘0 && 1 % 0’)
⇒0
eval(‘2 && 1 % 0’)
error m4:stdin:11: modulo by zero in eval: 2 && 1 % 0
⇒
As a GNU extension, the operator ‘**’ performs integral exponentiation. The operator
is right-associative, and if evaluated, the exponent must be non-negative, and at least one
of the arguments must be non-zero, or a warning is issued.
eval(‘2 ** 3 ** 2’)
⇒512
eval(‘(2 ** 3) ** 2’)
⇒64
eval(‘0 ** 1’)
⇒0
eval(‘2 ** 0’)
⇒1
eval(‘0 ** 0’)
⇒
error m4:stdin:5: divide by zero in eval: 0 ** 0
eval(‘4 ** -2’)
error m4:stdin:6: negative exponent in eval: 4 ** -2
⇒
Within expression, (but not radix or width), numbers without a special prefix are decimal. A simple ‘0’ prefix introduces an octal number. ‘0x’ introduces a hexadecimal number.
As GNU extensions, ‘0b’ introduces a binary number. ‘0r’ introduces a number expressed
in any radix between 1 and 36: the prefix should be immediately followed by the decimal
expression of the radix, a colon, then the digits making the number. For radix 1, leading
zeros are ignored, and all remaining digits must be ‘1’; for all other radices, the digits are
‘0’, ‘1’, ‘2’, . . . . Beyond ‘9’, the digits are ‘a’, ‘b’ . . . up to ‘z’. Lower and upper case
letters can be used interchangeably in numbers prefixes and as number digits.
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GNU M4 1.4.17 macro processor
Parentheses may be used to group subexpressions whenever needed. For the relational
operators, a true relation returns 1, and a false relation return 0.
Here are a few examples of use of eval.
eval(‘-3 * 5’)
⇒-15
eval(‘-99 / 10’)
⇒-9
eval(‘-99 % 10’)
⇒-9
eval(‘99 % -10’)
⇒9
eval(index(‘Hello world’, ‘llo’) >= 0)
⇒1
eval(‘0r1:0111 + 0b100 + 0r3:12’)
⇒12
define(‘square’, ‘eval(‘($1) ** 2’)’)
⇒
square(‘9’)
⇒81
square(square(‘5’)‘ + 1’)
⇒676
define(‘foo’, ‘666’)
⇒
eval(‘foo / 6’)
error m4:stdin:11: bad expression in eval: foo / 6
⇒
eval(foo / 6)
⇒111
As the last two lines show, eval does not handle macro names, even if they expand to
a valid expression (or part of a valid expression). Therefore all macros must be expanded
before they are passed to eval.
Some calculations are not portable to other implementations, since they have undefined
semantics in C, but GNU m4 has well-defined behavior on overflow. When shifting, an outof-range shift amount is implicitly brought into the range of 32-bit signed integers using an
implicit bit-wise and with 0x1f).
define(‘max_int’, eval(‘0x7fffffff’))
⇒
define(‘min_int’, incr(max_int))
⇒
eval(min_int‘ < 0’)
⇒1
eval(max_int‘ > 0’)
⇒1
ifelse(eval(min_int‘ / -1’), min_int, ‘overflow occurred’)
⇒overflow occurred
min_int
Chapter 12: Macros for doing arithmetic
93
⇒-2147483648
eval(‘0x80000000 % -1’)
⇒0
eval(‘-4 >> 1’)
⇒-2
eval(‘-4 >> 33’)
⇒-2
If radix is specified, it specifies the radix to be used in the expansion. The default radix
is 10; this is also the case if radix is the empty string. A warning results if the radix is
outside the range of 1 through 36, inclusive. The result of eval is always taken to be signed.
No radix prefix is output, and for radices greater than 10, the digits are lower case. The
width argument specifies the minimum output width, excluding any negative sign. The
result is zero-padded to extend the expansion to the requested width. A warning results if
the width is negative. If radix or width is out of bounds, the expansion of eval is empty.
eval(‘666’, ‘10’)
⇒666
eval(‘666’, ‘11’)
⇒556
eval(‘666’, ‘6’)
⇒3030
eval(‘666’, ‘6’, ‘10’)
⇒0000003030
eval(‘-666’, ‘6’, ‘10’)
⇒-0000003030
eval(‘10’, ‘’, ‘0’)
⇒10
‘0r1:’eval(‘10’, ‘1’, ‘11’)
⇒0r1:01111111111
eval(‘10’, ‘16’)
⇒a
eval(‘1’, ‘37’)
error m4:stdin:9: radix 37 in builtin ‘eval’ out of range
⇒
eval(‘1’, , ‘-1’)
error m4:stdin:10: negative width to builtin ‘eval’
⇒
eval()
error m4:stdin:11: empty string treated as 0 in builtin ‘eval’
⇒0
Chapter 13: Macros for running shell commands
95
13 Macros for running shell commands
There are a few builtin macros in m4 that allow you to run shell commands from within m4.
Note that the definition of a valid shell command is system dependent. On UNIX
systems, this is the typical /bin/sh. But on other systems, such as native Windows, the
shell has a different syntax of commands that it understands. Some examples in this chapter
assume /bin/sh, and also demonstrate how to quit early with a known exit value if this is
not the case.
13.1 Determining the platform
Sometimes it is desirable for an input file to know which platform m4 is running on. GNU
m4 provides several macros that are predefined to expand to the empty string; checking for
their existence will confirm platform details.
[Optional builtin]
[Optional builtin]
[Optional builtin]
[Optional builtin]
[Optional builtin]
[Optional builtin]
[Optional builtin]
Each of these macros is conditionally defined as needed to describe the environment of
m4. If defined, each macro expands to the empty string. For now, these macros silently
ignore all arguments, but in a future release of M4, they might warn if arguments are
present.
__gnu__
__os2__
os2
__unix__
unix
__windows__
windows
When GNU extensions are in effect (that is, when you did not use the -G option, see
Section 2.3 [Invoking m4], page 10), GNU m4 will define the macro __gnu__ to expand to
the empty string.
$ m4
__gnu__
⇒
__gnu__(‘ignored’)
⇒
Extensions are ifdef(‘__gnu__’, ‘active’, ‘inactive’)
⇒Extensions are active
$ m4 -G
__gnu__
⇒__gnu__
__gnu__(‘ignored’)
⇒__gnu__(ignored)
Extensions are ifdef(‘__gnu__’, ‘active’, ‘inactive’)
⇒Extensions are inactive
On UNIX systems, GNU m4 will define __unix__ by default, or unix when the -G option
is specified.
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GNU M4 1.4.17 macro processor
On native Windows systems, GNU m4 will define __windows__ by default, or windows
when the -G option is specified.
On OS/2 systems, GNU m4 will define __os2__ by default, or os2 when the -G option
is specified.
If GNU m4 does not provide a platform macro for your system, please report that as a
bug.
define(‘provided’, ‘0’)
⇒
ifdef(‘__unix__’, ‘define(‘provided’, incr(provided))’)
⇒
ifdef(‘__windows__’, ‘define(‘provided’, incr(provided))’)
⇒
ifdef(‘__os2__’, ‘define(‘provided’, incr(provided))’)
⇒
provided
⇒1
13.2 Executing simple commands
Any shell command can be executed, using syscmd:
syscmd (shell-command)
[Builtin]
Executes shell-command as a shell command.
The expansion of syscmd is void, not the output from shell-command! Output or
error messages from shell-command are not read by m4. See Section 13.3 [Esyscmd],
page 97, if you need to process the command output.
Prior to executing the command, m4 flushes its buffers. The default standard input,
output and error of shell-command are the same as those of m4.
By default, the shell-command will be used as the argument to the -c option of the
/bin/sh shell (or the version of sh specified by ‘command -p getconf PATH’, if your
system supports that). If you prefer a different shell, the configure script can be
given the option --with-syscmd-shell=location to set the location of an alternative
shell at GNU m4 installation; the alternative shell must still support -c.
The macro syscmd is recognized only with parameters.
define(‘foo’, ‘FOO’)
⇒
syscmd(‘echo foo’)
⇒foo
⇒
Note how the expansion of syscmd keeps the trailing newline of the command, as well
as using the newline that appeared after the macro.
The following is an example of shell-command using the same standard input as m4:
$ echo "m4wrap(\‘syscmd(\‘cat’)’)" | m4
⇒
Chapter 13: Macros for running shell commands
97
It tells m4 to read all of its input before executing the wrapped text, then hand a valid
(albeit emptied) pipe as standard input for the cat subcommand. Therefore, you should
be careful when using standard input (either by specifying no files, or by passing ‘-’ as a
file name on the command line, see Section 2.6 [Invoking m4], page 12), and also invoking
subcommands via syscmd or esyscmd that consume data from standard input. When
standard input is a seekable file, the subprocess will pick up with the next character not
yet processed by m4; when it is a pipe or other non-seekable file, there is no guarantee how
much data will already be buffered by m4 and thus unavailable to the child.
13.3 Reading the output of commands
If you want m4 to read the output of a shell command, use esyscmd:
esyscmd (shell-command)
[Builtin]
Expands to the standard output of the shell command shell-command.
Prior to executing the command, m4 flushes its buffers. The default standard input
and standard error of shell-command are the same as those of m4. The error output
of shell-command is not a part of the expansion: it will appear along with the error
output of m4.
By default, the shell-command will be used as the argument to the -c option of the
/bin/sh shell (or the version of sh specified by ‘command -p getconf PATH’, if your
system supports that). If you prefer a different shell, the configure script can be
given the option --with-syscmd-shell=location to set the location of an alternative
shell at GNU m4 installation; the alternative shell must still support -c.
The macro esyscmd is recognized only with parameters.
define(‘foo’, ‘FOO’)
⇒
esyscmd(‘echo foo’)
⇒FOO
⇒
Note how the expansion of esyscmd keeps the trailing newline of the command, as well
as using the newline that appeared after the macro.
Just as with syscmd, care must be exercised when sharing standard input between m4
and the child process of esyscmd.
13.4 Exit status
To see whether a shell command succeeded, use sysval:
[Builtin]
Expands to the exit status of the last shell command run with syscmd or esyscmd.
Expands to 0 if no command has been run yet.
sysval
sysval
⇒0
syscmd(‘false’)
⇒
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GNU M4 1.4.17 macro processor
ifelse(sysval, ‘0’, ‘zero’, ‘non-zero’)
⇒non-zero
syscmd(‘exit 2’)
⇒
sysval
⇒2
syscmd(‘true’)
⇒
sysval
⇒0
esyscmd(‘false’)
⇒
ifelse(sysval, ‘0’, ‘zero’, ‘non-zero’)
⇒non-zero
esyscmd(‘echo dnl && exit 127’)
⇒
sysval
⇒127
esyscmd(‘true’)
⇒
sysval
⇒0
sysval results in 127 if there was a problem executing the command, for example, if the
system-imposed argument length is exceeded, or if there were not enough resources to fork.
It is not possible to distinguish between failed execution and successful execution that had
an exit status of 127, unless there was output from the child process.
On UNIX platforms, where it is possible to detect when command execution is terminated by a signal, rather than a normal exit, the result is the signal number shifted left by
eight bits.
dnl This test assumes kill is a shell builtin, and that signals are
dnl recognizable.
ifdef(‘__unix__’, ,
‘errprint(‘ skipping: syscmd does not have unix semantics
’)m4exit(‘77’)’)dnl
syscmd(‘kill -9 $$’)
⇒
sysval
⇒2304
syscmd()
⇒
sysval
⇒0
esyscmd(‘kill -9 $$’)
⇒
sysval
⇒2304
Chapter 13: Macros for running shell commands
99
13.5 Making temporary files
Commands specified to syscmd or esyscmd might need a temporary file, for output or for
some other purpose. There is a builtin macro, mkstemp, for making a temporary file:
mkstemp (template)
maketemp (template)
[Builtin]
[Builtin]
Expands to the quoted name of a new, empty file, made from the string template,
which should end with the string ‘XXXXXX’. The six ‘X’ characters are then replaced
with random characters matching the regular expression ‘[a-zA-Z0-9._-]’, in order
to make the file name unique. If fewer than six ‘X’ characters are found at the end
of template, the result will be longer than the template. The created file will have
access permissions as if by chmod =rw,go=, meaning that the current umask of the m4
process is taken into account, and at most only the current user can read and write
the file.
The traditional behavior, standardized by POSIX, is that maketemp merely replaces
the trailing ‘X’ with the process id, without creating a file or quoting the expansion,
and without ensuring that the resulting string is a unique file name. In part, this
means that using the same template twice in the same input file will result in the
same expansion. This behavior is a security hole, as it is very easy for another process
to guess the name that will be generated, and thus interfere with a subsequent use of
syscmd trying to manipulate that file name. Hence, POSIX has recommended that
all new implementations of m4 provide the secure mkstemp builtin, and that users of
m4 check for its existence.
The expansion is void and an error issued if a temporary file could not be created.
The macros mkstemp and maketemp are recognized only with parameters.
If you try this next example, you will most likely get different output for the two file
names, since the replacement characters are randomly chosen:
$ m4
define(‘tmp’, ‘oops’)
⇒
maketemp(‘/tmp/fooXXXXXX’)
⇒/tmp/fooa07346
ifdef(‘mkstemp’, ‘define(‘maketemp’, defn(‘mkstemp’))’,
‘define(‘mkstemp’, defn(‘maketemp’))dnl
errprint(‘warning: potentially insecure maketemp implementation
’)’)
⇒
mkstemp(‘doc’)
⇒docQv83Uw
Unless you use the --traditional command line option (or -G, see Section 2.3 [Invoking
m4], page 10), the GNU version of maketemp is secure. This means that using the same
template to multiple calls will generate multiple files. However, we recommend that you use
the new mkstemp macro, introduced in GNU M4 1.4.8, which is secure even in traditional
mode. Also, as of M4 1.4.11, the secure implementation quotes the resulting file name,
so that you are guaranteed to know what file was created even if the random file name
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GNU M4 1.4.17 macro processor
happens to match an existing macro. Notice that this example is careful to use defn to
avoid unintended expansion of ‘foo’.
$ m4
define(‘foo’, ‘errprint(‘oops’)’)
⇒
syscmd(‘rm -f foo-??????’)sysval
⇒0
define(‘file1’, maketemp(‘foo-XXXXXX’))dnl
ifelse(esyscmd(‘echo \‘ foo-?????? \’’), ‘ foo-?????? ’,
‘no file’, ‘created’)
⇒created
define(‘file2’, maketemp(‘foo-XX’))dnl
define(‘file3’, mkstemp(‘foo-XXXXXX’))dnl
ifelse(len(defn(‘file1’)), len(defn(‘file2’)),
‘same length’, ‘different’)
⇒same length
ifelse(defn(‘file1’), defn(‘file2’), ‘same’, ‘different file’)
⇒different file
ifelse(defn(‘file2’), defn(‘file3’), ‘same’, ‘different file’)
⇒different file
ifelse(defn(‘file1’), defn(‘file3’), ‘same’, ‘different file’)
⇒different file
syscmd(‘rm ’defn(‘file1’) defn(‘file2’) defn(‘file3’))
⇒
sysval
⇒0
Chapter 14: Miscellaneous builtin macros
101
14 Miscellaneous builtin macros
This chapter describes various builtins, that do not really belong in any of the previous
chapters.
14.1 Printing error messages
You can print error messages using errprint:
errprint (message, . . . )
[Builtin]
Prints message and the rest of the arguments to standard error, separated by spaces.
Standard error is used, regardless of the --debugfile option (see Section 2.5 [Invoking
m4], page 11).
The expansion of errprint is void. The macro errprint is recognized only with
parameters.
errprint(‘Invalid arguments to forloop
’)
error Invalid arguments to forloop
⇒
errprint(‘1’)errprint(‘2’,‘3
’)
error 12 3
⇒
A trailing newline is not printed automatically, so it should be supplied as part of the
argument, as in the example. Unfortunately, the exact output of errprint is not very
portable to other m4 implementations: POSIX requires that all arguments be printed, but
some implementations of m4 only print the first. Furthermore, some BSD implementations
always append a newline for each errprint call, regardless of whether the last argument
already had one, and POSIX is silent on whether this is acceptable.
14.2 Printing current location
To make it possible to specify the location of an error, three utility builtins exist:
[Builtin]
[Builtin]
[Builtin]
Expand to the quoted name of the current input file, the current input line number
in that file, and the quoted name of the current invocation of m4.
__file__
__line__
__program__
errprint(__program__:__file__:__line__: ‘input error
’)
error m4:stdin:1: input error
⇒
Line numbers start at 1 for each file. If the file was found due to the -I option or
M4PATH environment variable, that is reflected in the file name. The syncline option (-s, see
Section 2.2 [Invoking m4], page 8), and the ‘f’ and ‘l’ flags of debugmode (see Section 7.3
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GNU M4 1.4.17 macro processor
[Debug Levels], page 58), also use this notion of current file and line. Redefining the three
location macros has no effect on syncline, debug, warning, or error message output.
This example reuses the file incl.m4 mentioned earlier (see Section 9.1 [Include],
page 73):
$ m4 -I examples
define(‘foo’, ‘‘$0’ called at __file__:__line__’)
⇒
foo
⇒foo called at stdin:2
include(‘incl.m4’)
⇒Include file start
⇒foo called at examples/incl.m4:2
⇒Include file end
⇒
The location of macros invoked during the rescanning of macro expansion text corresponds to the location in the file where the expansion was triggered, regardless of how many
newline characters the expansion text contains. As of GNU M4 1.4.8, the location of text
wrapped with m4wrap (see Section 8.5 [M4wrap], page 70) is the point at which the m4wrap
was invoked. Previous versions, however, behaved as though wrapped text came from line
0 of the file “”.
define(‘echo’, ‘$@’)
⇒
define(‘foo’, ‘echo(__line__
__line__)’)
⇒
echo(__line__
__line__)
⇒4
⇒5
m4wrap(‘foo
’)
⇒
foo(errprint(__line__
__line__
))
error 8
error 9
⇒8
⇒8
__line__
⇒11
m4wrap(‘__line__
’)
⇒
^D
⇒12
Chapter 14: Miscellaneous builtin macros
103
⇒6
⇒6
The __program__ macro behaves like ‘$0’ in shell terminology. If you invoke m4 through
an absolute path or a link with a different spelling, rather than by relying on a PATH search
for plain ‘m4’, it will affect how __program__ expands. The intent is that you can use it to
produce error messages with the same formatting that m4 produces internally. It can also
be used within syscmd (see Section 13.2 [Syscmd], page 96) to pick the same version of m4
that is currently running, rather than whatever version of m4 happens to be first in PATH.
It was first introduced in GNU M4 1.4.6.
14.3 Exiting from m4
If you need to exit from m4 before the entire input has been read, you can use m4exit:
m4exit ([code = ‘0’])
[Builtin]
Causes m4 to exit, with exit status code. If code is left out, the exit status is zero. If
code cannot be parsed, or is outside the range of 0 to 255, the exit status is one. No
further input is read, and all wrapped and diverted text is discarded.
m4wrap(‘This text is lost due to ‘m4exit’.’)
⇒
divert(‘1’) So is this.
divert
⇒
m4exit And this is never read.
A common use of this is to abort processing:
fatal_error (message)
[Composite]
Abort processing with an error message and non-zero status. Prefix message with
details about where the error occurred, and print the resulting string to standard
error.
define(‘fatal_error’,
‘errprint(__program__:__file__:__line__‘: fatal error: $*
’)m4exit(‘1’)’)
⇒
fatal_error(‘this is a BAD one, buster’)
error m4:stdin:4: fatal error: this is a BAD one, buster
After this macro call, m4 will exit with exit status 1. This macro is only intended for error
exits, since the normal exit procedures are not followed, i.e., diverted text is not undiverted,
and saved text (see Section 8.5 [M4wrap], page 70) is not reread. (This macro could be
made more robust to earlier versions of m4. You should try to see if you can find weaknesses
and correct them; or see Section 17.8 [Answers], page 129).
Note that it is still possible for the exit status to be different than what was requested
by m4exit. If m4 detects some other error, such as a write error on standard output, the
exit status will be non-zero even if m4exit requested zero.
If standard input is seekable, then the file will be positioned at the next unread character.
If it is a pipe or other non-seekable file, then there are no guarantees how much data m4
might have read into buffers, and thus discarded.
Chapter 15: Fast loading of frozen state
105
15 Fast loading of frozen state
Some bigger m4 applications may be built over a common base containing hundreds of
definitions and other costly initializations. Usually, the common base is kept in one or more
declarative files, which files are listed on each m4 invocation prior to the user’s input file, or
else each input file uses include.
Reading the common base of a big application, over and over again, may be time consuming. GNU m4 offers some machinery to speed up the start of an application using lengthy
common bases.
15.1 Using frozen files
Suppose a user has a library of m4 initializations in base.m4, which is then used with
multiple input files:
$ m4 base.m4 input1.m4
$ m4 base.m4 input2.m4
$ m4 base.m4 input3.m4
Rather than spending time parsing the fixed contents of base.m4 every time, the user
might rather execute:
$ m4 -F base.m4f base.m4
once, and further execute, as often as needed:
$ m4 -R base.m4f input1.m4
$ m4 -R base.m4f input2.m4
$ m4 -R base.m4f input3.m4
with the varying input. The first call, containing the -F option, only reads and executes file
base.m4, defining various application macros and computing other initializations. Once the
input file base.m4 has been completely processed, GNU m4 produces in base.m4f a frozen
file, that is, a file which contains a kind of snapshot of the m4 internal state.
Later calls, containing the -R option, are able to reload the internal state of m4, from
base.m4f, prior to reading any other input files. This means instead of starting with a
virgin copy of m4, input will be read after having effectively recovered the effect of a prior
run. In our example, the effect is the same as if file base.m4 has been read anew. However,
this effect is achieved a lot faster.
Only one frozen file may be created or read in any one m4 invocation. It is not possible
to recover two frozen files at once. However, frozen files may be updated incrementally,
through using -R and -F options simultaneously. For example, if some care is taken, the
command:
$ m4 file1.m4 file2.m4 file3.m4 file4.m4
could be broken down in the following sequence, accumulating the same output:
$ m4 -F file1.m4f file1.m4
$ m4 -R file1.m4f -F file2.m4f file2.m4
$ m4 -R file2.m4f -F file3.m4f file3.m4
$ m4 -R file3.m4f file4.m4
Some care is necessary because not every effort has been made for this to work in all
cases. In particular, the trace attribute of macros is not handled, nor the current setting
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GNU M4 1.4.17 macro processor
of changeword. Currently, m4wrap and sysval also have problems. Also, interactions
for some options of m4, being used in one call and not in the next, have not been fully
analyzed yet. On the other end, you may be confident that stacks of pushdef definitions
are handled correctly, as well as undefined or renamed builtins, and changed strings for
quotes or comments. And future releases of GNU M4 will improve on the utility of frozen
files.
When an m4 run is to be frozen, the automatic undiversion which takes place at end of
execution is inhibited. Instead, all positively numbered diversions are saved into the frozen
file. The active diversion number is also transmitted.
A frozen file to be reloaded need not reside in the current directory. It is looked up the
same way as an include file (see Section 9.2 [Search Path], page 74).
If the frozen file was generated with a newer version of m4, and contains directives that
an older m4 cannot parse, attempting to load the frozen file with option -R will cause m4 to
exit with status 63 to indicate version mismatch.
15.2 Frozen file format
Frozen files are sharable across architectures. It is safe to write a frozen file on one machine
and read it on another, given that the second machine uses the same or newer version of
GNU m4. It is conventional, but not required, to give a frozen file the suffix of .m4f.
These are simple (editable) text files, made up of directives, each starting with a capital
letter and ending with a newline (NL). Wherever a directive is expected, the character ‘#’
introduces a comment line; empty lines are also ignored if they are not part of an embedded
string. In the following descriptions, each len refers to the length of the corresponding
strings str in the next line of input. Numbers are always expressed in decimal. There are
no escape characters. The directives are:
C len1 , len2 NL str1 str2 NL
Uses str1 and str2 as the begin-comment and end-comment strings. If omitted,
then ‘#’ and NL are the comment delimiters.
D number, len NL str NL
Selects diversion number, making it current, then copy str in the current diversion. number may be a negative number for a non-existing diversion. To
merely specify an active selection, use this command with an empty str. With
0 as the diversion number, str will be issued on standard output at reload time.
GNU m4 will not produce the ‘D’ directive with non-zero length for diversion 0,
but this can be done with manual edits. This directive may appear more than
once for the same diversion, in which case the diversion is the concatenation of
the various uses. If omitted, then diversion 0 is current.
F len1 , len2 NL str1 str2 NL
Defines, through pushdef, a definition for str1 expanding to the function whose
builtin name is str2. If the builtin does not exist (for example, if the frozen
file was produced by a copy of m4 compiled with changeword support, but the
version of m4 reloading was compiled without it), the reload is silent, but any
subsequent use of the definition of str1 will result in a warning. This directive
may appear more than once for the same name, and its order, along with ‘T’,
is important. If omitted, you will have no access to any builtins.
Chapter 15: Fast loading of frozen state
107
Q len1 , len2 NL str1 str2 NL
Uses str1 and str2 as the begin-quote and end-quote strings. If omitted, then
‘‘’ and ‘’’ are the quote delimiters.
T len1 , len2 NL str1 str2 NL
Defines, though pushdef, a definition for str1 expanding to the text given by
str2. This directive may appear more than once for the same name, and its
order, along with ‘F’, is important.
V number NL
Confirms the format of the file. m4 1.4.17 only creates and understands frozen
files where number is 1. This directive must be the first non-comment in the
file, and may not appear more than once.
Chapter 16: Compatibility with other versions of m4
109
16 Compatibility with other versions of m4
This chapter describes the many of the differences between this implementation of m4, and
of other implementations found under UNIX, such as System V Release 4, Solaris, and BSD
flavors. In particular, it lists the known differences and extensions to POSIX. However, the
list is not necessarily comprehensive.
At the time of this writing, POSIX 2001 (also known as IEEE Std 1003.1-2001) is
the latest standard, although a new version of POSIX is under development and includes
several proposals for modifying what m4 is required to do. The requirements for m4 are
shared between SUSv3 and POSIX, and can be viewed at http://www.opengroup.org/
onlinepubs/000095399/utilities/m4.html.
16.1 Extensions in GNU M4
This version of m4 contains a few facilities that do not exist in System V m4. These extra
facilities are all suppressed by using the -G command line option (see Section 2.3 [Invoking
m4], page 10), unless overridden by other command line options.
• In the $n notation for macro arguments, n can contain several digits, while the System
V m4 only accepts one digit. This allows macros in GNU m4 to take any number of
arguments, and not only nine (see Section 5.2 [Arguments], page 26).
This means that define(‘foo’, ‘$11’) is ambiguous between implementations. To
portably choose between grabbing the first parameter and appending 1 to the expansion, or grabbing the eleventh parameter, you can do the following:
define(‘a1’, ‘A1’)
⇒
dnl First argument, concatenated with 1
define(‘_1’, ‘$1’)define(‘first1’, ‘_1($@)1’)
⇒
dnl Eleventh argument, portable
define(‘_9’, ‘$9’)define(‘eleventh’, ‘_9(shift(shift($@)))’)
⇒
dnl Eleventh argument, GNU style
define(‘Eleventh’, ‘$11’)
⇒
first1(‘a’, ‘b’, ‘c’, ‘d’, ‘e’, ‘f’, ‘g’, ‘h’, ‘i’, ‘j’, ‘k’)
⇒A1
eleventh(‘a’, ‘b’, ‘c’, ‘d’, ‘e’, ‘f’, ‘g’, ‘h’, ‘i’, ‘j’, ‘k’)
⇒k
Eleventh(‘a’, ‘b’, ‘c’, ‘d’, ‘e’, ‘f’, ‘g’, ‘h’, ‘i’, ‘j’, ‘k’)
⇒k
Also see the argn macro (see Section 6.3 [Shift], page 41).
• The divert (see Section 10.1 [Divert], page 75) macro can manage more than 9 diversions. GNU m4 treats all positive numbers as valid diversions, rather than discarding
diversions greater than 9.
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GNU M4 1.4.17 macro processor
• Files included with include and sinclude are sought in a user specified search path,
if they are not found in the working directory. The search path is specified by the -I
option and the M4PATH environment variable (see Section 9.2 [Search Path], page 74).
• Arguments to undivert can be non-numeric, in which case the named file will be
included uninterpreted in the output (see Section 10.2 [Undivert], page 76).
• Formatted output is supported through the format builtin, which is modeled after the
C library function printf (see Section 11.7 [Format], page 86).
• Searches and text substitution through basic regular expressions are supported by the
regexp (see Section 11.3 [Regexp], page 81) and patsubst (see Section 11.6 [Patsubst],
page 84) builtins. Some BSD implementations use extended regular expressions instead.
• The output of shell commands can be read into m4 with esyscmd (see Section 13.3
[Esyscmd], page 97).
• There is indirect access to any builtin macro with builtin (see Section 5.8 [Builtin],
page 35).
• Macros can be called indirectly through indir (see Section 5.7 [Indir], page 34).
• The name of the program, the current input file, and the current input line number are
accessible through the builtins __program__, __file__, and __line__ (see Section 14.2
[Location], page 101).
• The format of the output from dumpdef and macro tracing can be controlled with
debugmode (see Section 7.3 [Debug Levels], page 58).
• The destination of trace and debug output can be controlled with debugfile (see
Section 7.4 [Debug Output], page 60).
• The maketemp (see Section 13.5 [Mkstemp], page 99) macro behaves like mkstemp,
creating a new file with a unique name on every invocation, rather than following the
insecure behavior of replacing the trailing ‘X’ characters with the m4 process id.
• POSIX only requires support for the command line options -s, -D, and -U, so all other
options accepted by GNU M4 are extensions. See Chapter 2 [Invoking m4], page 7, for
a description of these options.
The debugging and tracing facilities in GNU m4 are much more extensive than in most
other versions of m4.
16.2 Facilities in System V m4 not in GNU m4
The version of m4 from System V contains a few facilities that have not been implemented
in GNU m4 yet. Additionally, POSIX requires some behaviors that GNU m4 has not implemented yet. Relying on these behaviors is non-portable, as a future release of GNU m4 may
change.
• POSIX requires support for multiple arguments to defn, without any clarification on
how defn behaves when one of the multiple arguments names a builtin. System V m4
and some other implementations allow mixing builtins and text macros into a single
macro. GNU m4 only supports joining multiple text arguments, although a future implementation may lift this restriction to behave more like System V. The only portable
way to join text macros with builtins is via helper macros and implicit concatenation
of macro results.
Chapter 16: Compatibility with other versions of m4
111
• POSIX requires an application to exit with non-zero status if it wrote an error message
to stderr. This has not yet been consistently implemented for the various builtins that
are required to issue an error (such as eval (see Section 12.2 [Eval], page 89) when an
argument cannot be parsed).
• Some traditional implementations only allow reading standard input once, but GNU
m4 correctly handles multiple instances of ‘-’ on the command line.
• POSIX requires m4wrap (see Section 8.5 [M4wrap], page 70) to act in FIFO (first-in,
first-out) order, but GNU m4 currently uses LIFO order. Furthermore, POSIX states
that only the first argument to m4wrap is saved for later evaluation, but GNU m4 saves
and processes all arguments, with output separated by spaces.
• POSIX states that builtins that require arguments, but are called without arguments,
have undefined behavior. Traditional implementations simply behave as though empty
strings had been passed. For example, a‘’define‘’b would expand to ab. But GNU
m4 ignores certain builtins if they have missing arguments, giving adefineb for the
above example.
• Traditional implementations handle define(‘f’,‘1’) (see Section 5.1 [Define],
page 25) by undefining the entire stack of previous definitions, and if doing
undefine(‘f’) first. GNU m4 replaces just the top definition on the stack, as if doing
popdef(‘f’) followed by pushdef(‘f’,‘1’). POSIX allows either behavior.
• POSIX 2001 requires syscmd (see Section 13.2 [Syscmd], page 96) to evaluate command
output for macro expansion, but this was a mistake that is anticipated to be corrected
in the next version of POSIX. GNU m4 follows traditional behavior in syscmd where
output is not rescanned, and provides the extension esyscmd that does scan the output.
• At one point, POSIX required changequote(arg) (see Section 8.2 [Changequote],
page 62) to use newline as the close quote, but this was a bug, and the next version of POSIX is anticipated to state that using empty strings or just one argument
is unspecified. Meanwhile, the GNU m4 behavior of treating an empty end-quote delimiter as ‘’’ is not portable, as Solaris treats it as repeating the start-quote delimiter,
and BSD treats it as leaving the previous end-quote delimiter unchanged. For predictable results, never call changequote with just one argument, or with empty strings
for arguments.
• At one point, POSIX required changecom(arg,) (see Section 8.3 [Changecom],
page 65) to make it impossible to end a comment, but this is a bug, and the next
version of POSIX is anticipated to state that using empty strings is unspecified.
Meanwhile, the GNU m4 behavior of treating an empty end-comment delimiter
as newline is not portable, as BSD treats it as leaving the previous end-comment
delimiter unchanged. It is also impossible in BSD implementations to disable
comments, even though that is required by POSIX. For predictable results, never call
changecom with empty strings for arguments.
• Most implementations of m4 give macros a higher precedence than comments when parsing, meaning that if the start delimiter given to changecom (see Section 8.3 [Changecom], page 65) starts with a macro name, comments are effectively disabled. POSIX
does not specify what the precedence is, so this version of GNU m4 parser recognizes
comments, then macros, then quoted strings.
112
GNU M4 1.4.17 macro processor
• Traditional implementations allow argument collection, but not string and comment
processing, to span file boundaries. Thus, if a.m4 contains ‘len(’, and b.m4 contains
‘abc)’, m4 a.m4 b.m4 outputs ‘3’ with traditional m4, but gives an error message that
the end of file was encountered inside a macro with GNU m4. On the other hand,
traditional implementations do end of file processing for files included with include or
sinclude (see Section 9.1 [Include], page 73), while GNU m4 seamlessly integrates the
content of those files. Thus include(‘a.m4’)include(‘b.m4’) will output ‘3’ instead
of giving an error.
• Traditional m4 treats traceon (see Section 7.2 [Trace], page 55) without arguments as a
global variable, independent of named macro tracing. Also, once a macro is undefined,
named tracing of that macro is lost. On the other hand, when GNU m4 encounters
traceon without arguments, it turns tracing on for all existing definitions at the time,
but does not trace future definitions; traceoff without arguments turns tracing off
for all definitions regardless of whether they were also traced by name; and tracing by
name, such as with -tfoo at the command line or traceon(‘foo’) in the input, is an
attribute that is preserved even if the macro is currently undefined.
Additionally, while POSIX requires trace output, it makes no demands on the formatting of that output. Parsing trace output is not guaranteed to be reliable, even
between different releases of GNU M4; however, the intent is that any future changes
in trace output will only occur under the direction of additional debugmode flags (see
Section 7.3 [Debug Levels], page 58).
• POSIX requires eval (see Section 12.2 [Eval], page 89) to treat all operators with the
same precedence as C. However, earlier versions of GNU m4 followed the traditional
behavior of other m4 implementations, where bitwise and logical negation (‘~’ and
‘!’) have lower precedence than equality operators; and where equality operators (‘==’
and ‘!=’) had the same precedence as relational operators (such as ‘<’). Use explicit
parentheses to ensure proper precedence. As extensions to POSIX, GNU m4 gives welldefined semantics to operations that C leaves undefined, such as when overflow occurs,
when shifting negative numbers, or when performing division by zero. POSIX also
requires ‘=’ to cause an error, but many traditional implementations allowed it as an
alias for ‘==’.
• POSIX 2001 requires translit (see Section 11.5 [Translit], page 83) to treat each
character of the second and third arguments literally. However, it is anticipated that
the next version of POSIX will allow the GNU m4 behavior of treating ‘-’ as a range
operator.
• POSIX requires m4 to honor the locale environment variables of LANG, LC_ALL, LC_
CTYPE, LC_MESSAGES, and NLSPATH, but this has not yet been implemented in GNU
m4.
• POSIX states that only unquoted leading newlines and blanks (that is, space and tab)
are ignored when collecting macro arguments. However, this appears to be a bug in
POSIX, since most traditional implementations also ignore all whitespace (formfeed,
carriage return, and vertical tab). GNU m4 follows tradition and ignores all leading
unquoted whitespace.
• A strictly-compliant POSIX client is not allowed to use command-line arguments not
specified by POSIX. However, since this version of M4 ignores POSIXLY_CORRECT and
Chapter 16: Compatibility with other versions of m4
113
enables the option --gnu by default (see Section 2.3 [Invoking m4], page 10), a client
desiring to be strictly compliant has no way to disable GNU extensions that conflict
with POSIX when directly invoking the compiled m4. A future version of GNU M4 will
honor the environment variable POSIXLY_CORRECT, implicitly enabling --traditional
if it is set, in order to allow a strictly-compliant client. In the meantime, a client
needing strict POSIX compliance can use the workaround of invoking a shell script
wrapper, where the wrapper then adds --traditional to the arguments passed to the
compiled m4.
16.3 Other incompatibilities
There are a few other incompatibilities between this implementation of m4, and the System
V version.
• GNU m4 implements sync lines differently from System V m4, when text is being diverted. GNU m4 outputs the sync lines when the text is being diverted, and System V
m4 when the diverted text is being brought back.
The problem is which lines and file names should be attached to text that is being,
or has been, diverted. System V m4 regards all the diverted text as being generated
by the source line containing the undivert call, whereas GNU m4 regards the diverted
text as being generated at the time it is diverted.
The sync line option is used mostly when using m4 as a front end to a compiler. If a
diverted line causes a compiler error, the error messages should most probably refer to
the place where the diversion was made, and not where it was inserted again.
divert(2)2
divert(1)1
divert‘’0
⇒#line 3 "stdin"
⇒0
^D
⇒#line 2 "stdin"
⇒1
⇒#line 1 "stdin"
⇒2
The current m4 implementation has a limitation that the syncline output at the start of
each diversion occurs no matter what, even if the previous diversion did not end with a
newline. This goes contrary to the claim that synclines appear on a line by themselves,
so this limitation may be corrected in a future version of m4. In the meantime, when
using -s, it is wisest to make sure all diversions end with newline.
• GNU m4 makes no attempt at prohibiting self-referential definitions like:
define(‘x’, ‘x’)
⇒
define(‘x’, ‘x ’)
⇒
There is nothing inherently wrong with defining ‘x’ to return ‘x’. The wrong thing
is to expand ‘x’ unquoted, because that would cause an infinite rescan loop. In m4,
114
GNU M4 1.4.17 macro processor
one might use macros to hold strings, as we do for variables in other programming
languages, further checking them with:
ifelse(defn(‘holder’), ‘value’, ...)
In cases like this one, an interdiction for a macro to hold its own name would be
a useless limitation. Of course, this leaves more rope for the GNU m4 user to hang
himself! Rescanning hangs may be avoided through careful programming, a little like
for endless loops in traditional programming languages.
Chapter 17: Correct version of some examples
115
17 Correct version of some examples
Some of the examples in this manuals are buggy or not very robust, for demonstration
purposes. Improved versions of these composite macros are presented here.
17.1 Solution for exch
The exch macro (see Section 5.2 [Arguments], page 26) as presented requires clients to
double quote their arguments. A nicer definition, which lets clients follow the rule of thumb
of one level of quoting per level of parentheses, involves adding quotes in the definition of
exch, as follows:
define(‘exch’, ‘‘$2’, ‘$1’’)
⇒
define(exch(‘expansion text’, ‘macro’))
⇒
macro
⇒expansion text
17.2 Solution for forloop
The forloop macro (see Section 6.4 [Forloop], page 46) as presented earlier can go into
an infinite loop if given an iterator that is not parsed as a macro name. It does not do
any sanity checking on its numeric bounds, and only permits decimal numbers for bounds.
Here is an improved version, shipped as m4-1.4.17/examples/forloop2.m4; this version
also optimizes overhead by calling four macros instead of six per iteration (excluding those
in text), by not dereferencing the iterator in the helper _forloop.
$ m4 -d -I examples
undivert(‘forloop2.m4’)dnl
⇒divert(‘-1’)
⇒# forloop(var, from, to, stmt) - improved version:
⇒#
works even if VAR is not a strict macro name
⇒#
performs sanity check that FROM is larger than TO
⇒#
allows complex numerical expressions in TO and FROM
⇒define(‘forloop’, ‘ifelse(eval(‘($2) <= ($3)’), ‘1’,
⇒ ‘pushdef(‘$1’)_$0(‘$1’, eval(‘$2’),
⇒
eval(‘$3’), ‘$4’)popdef(‘$1’)’)’)
⇒define(‘_forloop’,
⇒ ‘define(‘$1’, ‘$2’)$4‘’ifelse(‘$2’, ‘$3’, ‘’,
⇒
‘$0(‘$1’, incr(‘$2’), ‘$3’, ‘$4’)’)’)
⇒divert‘’dnl
include(‘forloop2.m4’)
⇒
forloop(‘i’, ‘2’, ‘1’, ‘no iteration occurs’)
⇒
forloop(‘’, ‘1’, ‘2’, ‘ odd iterator name’)
⇒ odd iterator name odd iterator name
forloop(‘i’, ‘5 + 5’, ‘0xc’, ‘ 0x‘’eval(i, ‘16’)’)
116
GNU M4 1.4.17 macro processor
⇒ 0xa 0xb 0xc
forloop(‘i’, ‘a’, ‘b’, ‘non-numeric bounds’)
error m4:stdin:6: bad expression in eval (bad input): (a) <= (b)
⇒
One other change to notice is that the improved version used ‘_$0’ rather than ‘_foreach’
to invoke the helper routine. In general, this is a good practice to follow, because then the
set of macros can be uniformly transformed. The following example shows a transformation
that doubles the current quoting and appends a suffix ‘2’ to each transformed macro. If
foreach refers to the literal ‘_foreach’, then foreach2 invokes _foreach instead of the
intended _foreach2, and the mixing of quoting paradigms leads to an infinite recursion
loop in this example.
$ m4 -d -L 9 -I examples
define(‘arg1’, ‘$1’)include(‘forloop2.m4’)include(‘quote.m4’)
⇒
define(‘double’, ‘define(‘$1’‘2’,
arg1(patsubst(dquote(defn(‘$1’)), ‘[‘’]’, ‘\&\&’)))’)
⇒
double(‘forloop’)double(‘_forloop’)defn(‘forloop2’)
⇒ifelse(eval(‘‘($2) <= ($3)’’), ‘‘1’’,
⇒ ‘‘pushdef(‘‘$1’’)_$0(‘‘$1’’, eval(‘‘$2’’),
⇒
eval(‘‘$3’’), ‘‘$4’’)popdef(‘‘$1’’)’’)
forloop(i, 1, 5, ‘ifelse(’)forloop(i, 1, 5, ‘)’)
⇒
changequote(‘[’, ‘]’)changequote([‘‘], [’’])
⇒
forloop2(i, 1, 5, ‘‘ifelse(’’)forloop2(i, 1, 5, ‘‘)’’)
⇒
changequote‘’include(‘forloop.m4’)
⇒
double(‘forloop’)double(‘_forloop’)defn(‘forloop2’)
⇒pushdef(‘‘$1’’, ‘‘$2’’)_forloop($@)popdef(‘‘$1’’)
forloop(i, 1, 5, ‘ifelse(’)forloop(i, 1, 5, ‘)’)
⇒
changequote(‘[’, ‘]’)changequote([‘‘], [’’])
⇒
forloop2(i, 1, 5, ‘‘ifelse(’’)forloop2(i, 1, 5, ‘‘)’’)
error m4:stdin:12: recursion limit of 9 exceeded, use -L<N> to change it
One more optimization is still possible. Instead of repeatedly assigning a variable then
invoking or dereferencing it, it is possible to pass the current iterator value as a single argument. Coupled with curry if other arguments are needed (see Section 6.7 [Composition],
page 51), or with helper macros if the argument is needed in more than one place in the
expansion, the output can be generated with three, rather than four, macros of overhead
per iteration. Notice how the file m4-1.4.17/examples/forloop3.m4 rearranges the arguments of the helper _forloop to take two arguments that are placed around the current
value. By splitting a balanced set of parantheses across multiple arguments, the helper
macro can now be shared by forloop and the new forloop_arg.
Chapter 17: Correct version of some examples
117
$ m4 -I examples
include(‘forloop3.m4’)
⇒
undivert(‘forloop3.m4’)dnl
⇒divert(‘-1’)
⇒# forloop_arg(from, to, macro) - invoke MACRO(value) for
⇒#
each value between FROM and TO, without define overhead
⇒define(‘forloop_arg’, ‘ifelse(eval(‘($1) <= ($2)’), ‘1’,
⇒ ‘_forloop(‘$1’, eval(‘$2’), ‘$3(’, ‘)’)’)’)
⇒# forloop(var, from, to, stmt) - refactored to share code
⇒define(‘forloop’, ‘ifelse(eval(‘($2) <= ($3)’), ‘1’,
⇒ ‘pushdef(‘$1’)_forloop(eval(‘$2’), eval(‘$3’),
⇒
‘define(‘$1’,’, ‘)$4’)popdef(‘$1’)’)’)
⇒define(‘_forloop’,
⇒ ‘$3‘$1’$4‘’ifelse(‘$1’, ‘$2’, ‘’,
⇒
‘$0(incr(‘$1’), ‘$2’, ‘$3’, ‘$4’)’)’)
⇒divert‘’dnl
forloop(‘i’, ‘1’, ‘3’, ‘ i’)
⇒ 1 2 3
define(‘echo’, ‘$@’)
⇒
forloop_arg(‘1’, ‘3’, ‘ echo’)
⇒ 1 2 3
include(‘curry.m4’)
⇒
forloop_arg(‘1’, ‘3’, ‘curry(‘pushdef’, ‘a’)’)
⇒
a
⇒3
popdef(‘a’)a
⇒2
popdef(‘a’)a
⇒1
popdef(‘a’)a
⇒a
Of course, it is possible to make even more improvements, such as adding an optional
step argument, or allowing iteration through descending sequences. GNU Autoconf provides
some of these additional bells and whistles in its m4_for macro.
17.3 Solution for foreach
The foreach and foreachq macros (see Section 6.5 [Foreach], page 47) as presented earlier
each have flaws. First, we will examine and fix the quadratic behavior of foreachq:
$ m4 -I examples
include(‘foreachq.m4’)
⇒
traceon(‘shift’)debugmode(‘aq’)
118
GNU M4 1.4.17 macro processor
⇒
foreachq(‘x’, ‘‘1’, ‘2’, ‘3’,
’)dnl
⇒1
error m4trace: -3- shift(‘1’,
error m4trace: -2- shift(‘1’,
⇒2
error m4trace: -4- shift(‘1’,
error m4trace: -3- shift(‘2’,
error m4trace: -3- shift(‘1’,
error m4trace: -2- shift(‘2’,
⇒3
error m4trace: -5- shift(‘1’,
error m4trace: -4- shift(‘2’,
error m4trace: -3- shift(‘3’,
error m4trace: -4- shift(‘1’,
error m4trace: -3- shift(‘2’,
error m4trace: -2- shift(‘3’,
⇒4
error m4trace: -6- shift(‘1’,
error m4trace: -5- shift(‘2’,
error m4trace: -4- shift(‘3’,
error m4trace: -3- shift(‘4’)
‘4’’, ‘x
‘2’, ‘3’, ‘4’)
‘2’, ‘3’, ‘4’)
‘2’,
‘3’,
‘2’,
‘3’,
‘3’, ‘4’)
‘4’)
‘3’, ‘4’)
‘4’)
‘2’,
‘3’,
‘4’)
‘2’,
‘3’,
‘4’)
‘3’, ‘4’)
‘4’)
‘3’, ‘4’)
‘4’)
‘2’, ‘3’, ‘4’)
‘3’, ‘4’)
‘4’)
Each successive iteration was adding more quoted shift invocations, and the entire list
contents were passing through every iteration. In general, when recursing, it is a good idea
to make the recursion use fewer arguments, rather than adding additional quoted uses of
shift. By doing so, m4 uses less memory, invokes fewer macros, is less likely to run into
machine limits, and most importantly, performs faster. The fixed version of foreachq can
be found in m4-1.4.17/examples/foreachq2.m4:
$ m4 -I examples
include(‘foreachq2.m4’)
⇒
undivert(‘foreachq2.m4’)dnl
⇒include(‘quote.m4’)dnl
⇒divert(‘-1’)
⇒# foreachq(x, ‘item_1, item_2, ..., item_n’, stmt)
⇒#
quoted list, improved version
⇒define(‘foreachq’, ‘pushdef(‘$1’)_$0($@)popdef(‘$1’)’)
⇒define(‘_arg1q’, ‘‘$1’’)
⇒define(‘_rest’, ‘ifelse(‘$#’, ‘1’, ‘’, ‘dquote(shift($@))’)’)
⇒define(‘_foreachq’, ‘ifelse(‘$2’, ‘’, ‘’,
⇒ ‘define(‘$1’, _arg1q($2))$3‘’$0(‘$1’, _rest($2), ‘$3’)’)’)
⇒divert‘’dnl
traceon(‘shift’)debugmode(‘aq’)
⇒
foreachq(‘x’, ‘‘1’, ‘2’, ‘3’, ‘4’’, ‘x
Chapter 17: Correct version of some examples
119
’)dnl
⇒1
error m4trace: -3- shift(‘1’, ‘2’, ‘3’, ‘4’)
⇒2
error m4trace: -3- shift(‘2’, ‘3’, ‘4’)
⇒3
error m4trace: -3- shift(‘3’, ‘4’)
⇒4
Note that the fixed version calls unquoted helper macros in _foreachq to trim elements
immediately; those helper macros in turn must re-supply the layer of quotes lost in the macro
invocation. Contrast the use of _arg1q, which quotes the first list element, with _arg1 of the
earlier implementation that returned the first list element directly. Additionally, by calling
the helper method immediately, the ‘defn(‘iterator’)’ no longer contains unexpanded
macros.
The astute m4 programmer might notice that the solution above still uses more memory and macro invocations, and thus more time, than strictly necessary. Note that ‘$2’,
which contains an arbitrarily long quoted list, is expanded and rescanned three times per
iteration of _foreachq. Furthermore, every iteration of the algorithm effectively unboxes
then reboxes the list, which costs a couple of macro invocations. It is possible to rewrite
the algorithm for a bit more speed by swapping the order of the arguments to _foreachq
in order to operate on an unboxed list in the first place, and by using the fixed-length ‘$#’
instead of an arbitrary length list as the key to end recursion. The result is an overhead
of six macro invocations per loop (excluding any macros in text), instead of eight. This
alternative approach is available as m4-1.4.17/examples/foreach3.m4:
$ m4 -I examples
include(‘foreachq3.m4’)
⇒
undivert(‘foreachq3.m4’)dnl
⇒divert(‘-1’)
⇒# foreachq(x, ‘item_1, item_2, ..., item_n’, stmt)
⇒#
quoted list, alternate improved version
⇒define(‘foreachq’, ‘ifelse(‘$2’, ‘’, ‘’,
⇒ ‘pushdef(‘$1’)_$0(‘$1’, ‘$3’, ‘’, $2)popdef(‘$1’)’)’)
⇒define(‘_foreachq’, ‘ifelse(‘$#’, ‘3’, ‘’,
⇒ ‘define(‘$1’, ‘$4’)$2‘’$0(‘$1’, ‘$2’,
⇒
shift(shift(shift($@))))’)’)
⇒divert‘’dnl
traceon(‘shift’)debugmode(‘aq’)
⇒
foreachq(‘x’, ‘‘1’, ‘2’, ‘3’, ‘4’’, ‘x
’)dnl
⇒1
error m4trace: -4- shift(‘x’, ‘x
error ’, ‘’, ‘1’, ‘2’, ‘3’, ‘4’)
error m4trace: -3- shift(‘x
error ’, ‘’, ‘1’, ‘2’, ‘3’, ‘4’)
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GNU M4 1.4.17 macro processor
error m4trace: -2- shift(‘’, ‘1’, ‘2’, ‘3’, ‘4’)
⇒2
error
error
error
error
error
m4trace: -4’, ‘1’, ‘2’,
m4trace: -3’, ‘1’, ‘2’,
m4trace: -2-
shift(‘x’, ‘x
‘3’, ‘4’)
shift(‘x
‘3’, ‘4’)
shift(‘1’, ‘2’, ‘3’, ‘4’)
m4trace: -4’, ‘2’, ‘3’,
m4trace: -3’, ‘2’, ‘3’,
m4trace: -2-
shift(‘x’, ‘x
‘4’)
shift(‘x
‘4’)
shift(‘2’, ‘3’, ‘4’)
⇒3
error
error
error
error
error
⇒4
error
error
error
error
error
m4trace: -4- shift(‘x’, ‘x
’, ‘3’, ‘4’)
m4trace: -3- shift(‘x
’, ‘3’, ‘4’)
m4trace: -2- shift(‘3’, ‘4’)
In the current version of M4, every instance of ‘$@’ is rescanned as it is encountered.
Thus, the foreachq3.m4 alternative uses much less memory than foreachq2.m4, and executes as much as 10% faster, since each iteration encounters fewer ‘$@’. However, the implementation of rescanning every byte in ‘$@’ is quadratic in the number of bytes scanned
(for example, making the broken version in foreachq.m4 cubic, rather than quadratic, in
behavior). A future release of M4 will improve the underlying implementation by reusing
results of previous scans, so that both styles of foreachq can become linear in the number of bytes scanned. Notice how the implementation injects an empty argument prior to
expanding ‘$2’ within foreachq; the helper macro _foreachq then ignores the third argument altogether, and ends recursion when there are three arguments left because there was
nothing left to pass through shift. Thus, each iteration only needs one ifelse, rather
than the two conditionals used in the version from foreachq2.m4.
So far, all of the implementations of foreachq presented have been quadratic with M4
1.4.x. But forloop is linear, because each iteration parses a constant amount of arguments.
So, it is possible to design a variant that uses forloop to do the iteration, then uses ‘$@’
only once at the end, giving a linear result even with older M4 implementations. This implementation relies on the GNU extension that ‘$10’ expands to the tenth argument rather
than the first argument concatenated with ‘0’. The trick is to define an intermediate macro
that repeats the text m4_define(‘$1’, ‘$n’)$2‘’, with ‘n’ set to successive integers corresponding to each argument. The helper macro _foreachq_ is needed in order to generate
the literal sequences such as ‘$1’ into the intermediate macro, rather than expanding them
as the arguments of _foreachq. With this approach, no shift calls are even needed! Even
though there are seven macros of overhead per iteration instead of six in foreachq3.m4,
the linear scaling is apparent at relatively small list sizes. However, this approach will
need adjustment when a future version of M4 follows POSIX by no longer treating ‘$10’
as the tenth argument; the anticipation is that ‘${10}’ can be used instead, although that
alternative syntax is not yet supported.
Chapter 17: Correct version of some examples
121
$ m4 -I examples
include(‘foreachq4.m4’)
⇒
undivert(‘foreachq4.m4’)dnl
⇒include(‘forloop2.m4’)dnl
⇒divert(‘-1’)
⇒# foreachq(x, ‘item_1, item_2, ..., item_n’, stmt)
⇒#
quoted list, version based on forloop
⇒define(‘foreachq’,
⇒‘ifelse(‘$2’, ‘’, ‘’, ‘_$0(‘$1’, ‘$3’, $2)’)’)
⇒define(‘_foreachq’,
⇒‘pushdef(‘$1’, forloop(‘$1’, ‘3’, ‘$#’,
⇒ ‘$0_(‘1’, ‘2’, indir(‘$1’))’)‘popdef(
⇒
‘$1’)’)indir(‘$1’, $@)’)
⇒define(‘_foreachq_’,
⇒‘‘define(‘$$1’, ‘$$3’)$$2‘’’’)
⇒divert‘’dnl
traceon(‘shift’)debugmode(‘aq’)
⇒
foreachq(‘x’, ‘‘1’, ‘2’, ‘3’, ‘4’’, ‘x
’)dnl
⇒1
⇒2
⇒3
⇒4
For yet another approach, the improved version of foreach, available in m4-1.4.17/
examples/foreach2.m4, simply overquotes the arguments to _foreach to begin with, using
dquote_elt. Then _foreach can just use _arg1 to remove the extra layer of quoting that
was added up front:
$ m4 -I examples
include(‘foreach2.m4’)
⇒
undivert(‘foreach2.m4’)dnl
⇒include(‘quote.m4’)dnl
⇒divert(‘-1’)
⇒# foreach(x, (item_1, item_2, ..., item_n), stmt)
⇒#
parenthesized list, improved version
⇒define(‘foreach’, ‘pushdef(‘$1’)_$0(‘$1’,
⇒ (dquote(dquote_elt$2)), ‘$3’)popdef(‘$1’)’)
⇒define(‘_arg1’, ‘$1’)
⇒define(‘_foreach’, ‘ifelse(‘$2’, ‘(‘’)’, ‘’,
⇒ ‘define(‘$1’, _arg1$2)$3‘’$0(‘$1’, (dquote(shift$2)), ‘$3’)’)’)
⇒divert‘’dnl
traceon(‘shift’)debugmode(‘aq’)
⇒
foreach(‘x’, ‘(‘1’, ‘2’, ‘3’, ‘4’)’, ‘x
122
GNU M4 1.4.17 macro processor
’)dnl
error m4trace: -4- shift(‘1’, ‘2’, ‘3’, ‘4’)
error m4trace: -4- shift(‘2’, ‘3’, ‘4’)
error m4trace: -4- shift(‘3’, ‘4’)
⇒1
error m4trace: -3- shift(‘‘1’’, ‘‘2’’, ‘‘3’’, ‘‘4’’)
⇒2
error m4trace: -3- shift(‘‘2’’, ‘‘3’’, ‘‘4’’)
⇒3
error m4trace: -3- shift(‘‘3’’, ‘‘4’’)
⇒4
error m4trace: -3- shift(‘‘4’’)
It is likewise possible to write a variant of foreach that performs in linear time on M4
1.4.x; the easiest method is probably writing a version of foreach that unboxes its list,
then invokes _foreachq as previously defined in foreachq4.m4.
In summary, recursion over list elements is trickier than it appeared at first glance,
but provides a powerful idiom within m4 processing. As a final demonstration, both list
styles are now able to handle several scenarios that would wreak havoc on one or both of
the original implementations. This points out one other difference between the list styles.
foreach evaluates unquoted list elements only once, in preparation for calling _foreach,
similary for foreachq as provided by foreachq3.m4 or foreachq4.m4. But foreachq, as
provided by foreachq2.m4, evaluates unquoted list elements twice while visiting the first
list element, once in _arg1q and once in _rest. When deciding which list style to use,
one must take into account whether repeating the side effects of unquoted list elements will
have any detrimental effects.
$ m4 -I examples
include(‘foreach2.m4’)
⇒
include(‘foreachq2.m4’)
⇒
dnl 0-element list:
foreach(‘x’, ‘’, ‘<x>’) / foreachq(‘x’, ‘’, ‘<x>’)
⇒ /
dnl 1-element list of empty element
foreach(‘x’, ‘()’, ‘<x>’) / foreachq(‘x’, ‘‘’’, ‘<x>’)
⇒<> / <>
dnl 2-element list of empty elements
foreach(‘x’, ‘(‘’,‘’)’, ‘<x>’) / foreachq(‘x’, ‘‘’,‘’’, ‘<x>’)
⇒<><> / <><>
dnl 1-element list of a comma
foreach(‘x’, ‘(‘,’)’, ‘<x>’) / foreachq(‘x’, ‘‘,’’, ‘<x>’)
⇒<,> / <,>
dnl 2-element list of unbalanced parentheses
foreach(‘x’, ‘(‘(’, ‘)’)’, ‘<x>’) / foreachq(‘x’, ‘‘(’, ‘)’’, ‘<x>’)
⇒<(><)> / <(><)>
define(‘ab’, ‘oops’)dnl using defn(‘iterator’)
Chapter 17: Correct version of some examples
foreach(‘x’, ‘(‘a’, ‘b’)’, ‘defn(‘x’)’) /dnl
foreachq(‘x’, ‘‘a’, ‘b’’, ‘defn(‘x’)’)
⇒ab / ab
define(‘active’, ‘ACT, IVE’)
⇒
traceon(‘active’)
⇒
dnl list of unquoted macros; expansion occurs before recursion
foreach(‘x’, ‘(active, active)’, ‘<x>
’)dnl
error m4trace: -4- active -> ‘ACT, IVE’
error m4trace: -4- active -> ‘ACT, IVE’
⇒<ACT>
⇒<IVE>
⇒<ACT>
⇒<IVE>
foreachq(‘x’, ‘active, active’, ‘<x>
’)dnl
error m4trace: -3- active -> ‘ACT, IVE’
error m4trace: -3- active -> ‘ACT, IVE’
⇒<ACT>
error m4trace: -3- active -> ‘ACT, IVE’
error m4trace: -3- active -> ‘ACT, IVE’
⇒<IVE>
⇒<ACT>
⇒<IVE>
dnl list of quoted macros; expansion occurs during recursion
foreach(‘x’, ‘(‘active’, ‘active’)’, ‘<x>
’)dnl
error m4trace: -1- active -> ‘ACT, IVE’
⇒<ACT, IVE>
error m4trace: -1- active -> ‘ACT, IVE’
⇒<ACT, IVE>
foreachq(‘x’, ‘‘active’, ‘active’’, ‘<x>
’)dnl
error m4trace: -1- active -> ‘ACT, IVE’
⇒<ACT, IVE>
error m4trace: -1- active -> ‘ACT, IVE’
⇒<ACT, IVE>
dnl list of double-quoted macro names; no expansion
foreach(‘x’, ‘(‘‘active’’, ‘‘active’’)’, ‘<x>
’)dnl
⇒<active>
⇒<active>
foreachq(‘x’, ‘‘‘active’’, ‘‘active’’’, ‘<x>
’)dnl
⇒<active>
123
124
GNU M4 1.4.17 macro processor
⇒<active>
17.4 Solution for copy
The macro copy presented above is unable to handle builtin tokens with M4 1.4.x, because
it tries to pass the builtin token through the macro curry, where it is silently flattened to an
empty string (see Section 6.7 [Composition], page 51). Rather than using the problematic
curry to work around the limitation that stack_foreach expects to invoke a macro that
takes exactly one argument, we can write a new macro that lets us form the exact twoargument pushdef call sequence needed, so that we are no longer passing a builtin token
through a text macro.
stack_foreach_sep (macro, pre, post, sep)
stack_foreach_sep_lifo (macro, pre, post, sep)
[Composite]
[Composite]
For each of the pushdef definitions associated with macro, expand the sequence
‘pre‘’definition‘’post’. Additionally, expand sep between definitions. stack_
foreach_sep visits the oldest definition first, while stack_foreach_sep_lifo visits
the current definition first. The expansion may dereference macro, but should not
modify it. There are a few special macros, such as defn, which cannot be used as the
macro parameter.
Note that stack_foreach(‘macro’, ‘action’) is equivalent to stack_foreach_
sep(‘macro’, ‘action(’, ‘)’). By supplying explicit parentheses, split among the pre
and post arguments to stack_foreach_sep, it is now possible to construct macro calls
with more than one argument, without passing builtin tokens through a macro call. It is
likewise possible to directly reference the stack definitions without a macro call, by leaving
pre and post empty. Thus, in addition to fixing copy on builtin tokens, it also executes
with fewer macro invocations.
The new macro also adds a separator that is only output after the first iteration of
the helper _stack_reverse_sep, implemented by prepending the original sep to pre and
omitting a sep argument in subsequent iterations. Note that the empty string that separates
sep from pre is provided as part of the fourth argument when originally calling _stack_
reverse_sep, and not by writing $4‘’$3 as the third argument in the recursive call; while
the other approach would give the same output, it does so at the expense of increasing the
argument size on each iteration of _stack_reverse_sep, which results in quadratic instead
of linear execution time. The improved stack walking macros are available in m4-1.4.17/
examples/stack_sep.m4:
$ m4 -I examples
include(‘stack_sep.m4’)
⇒
define(‘copy’, ‘ifdef(‘$2’, ‘errprint(‘$2 already defined
’)m4exit(‘1’)’,
‘stack_foreach_sep(‘$1’, ‘pushdef(‘$2’,’, ‘)’)’)’)dnl
pushdef(‘a’, ‘1’)pushdef(‘a’, defn(‘divnum’))
⇒
copy(‘a’, ‘b’)
⇒
b
Chapter 17: Correct version of some examples
125
⇒0
popdef(‘b’)
⇒
b
⇒1
pushdef(‘c’, ‘1’)pushdef(‘c’, ‘2’)
⇒
stack_foreach_sep_lifo(‘c’, ‘’, ‘’, ‘, ’)
⇒2, 1
undivert(‘stack_sep.m4’)dnl
⇒divert(‘-1’)
⇒# stack_foreach_sep(macro, pre, post, sep)
⇒# Invoke PRE‘’defn‘’POST with a single argument of each definition
⇒# from the definition stack of MACRO, starting with the oldest, and
⇒# separated by SEP between definitions.
⇒define(‘stack_foreach_sep’,
⇒‘_stack_reverse_sep(‘$1’, ‘tmp-$1’)’dnl
⇒‘_stack_reverse_sep(‘tmp-$1’, ‘$1’, ‘$2‘’defn(‘$1’)$3’, ‘$4‘’’)’)
⇒# stack_foreach_sep_lifo(macro, pre, post, sep)
⇒# Like stack_foreach_sep, but starting with the newest definition.
⇒define(‘stack_foreach_sep_lifo’,
⇒‘_stack_reverse_sep(‘$1’, ‘tmp-$1’, ‘$2‘’defn(‘$1’)$3’, ‘$4‘’’)’dnl
⇒‘_stack_reverse_sep(‘tmp-$1’, ‘$1’)’)
⇒define(‘_stack_reverse_sep’,
⇒‘ifdef(‘$1’, ‘pushdef(‘$2’, defn(‘$1’))$3‘’popdef(‘$1’)$0(
⇒ ‘$1’, ‘$2’, ‘$4$3’)’)’)
⇒divert‘’dnl
17.5 Solution for m4wrap
The replacement m4wrap versions presented above, designed to guarantee FIFO or LIFO
order regardless of the underlying M4 implementation, share a bug when dealing with
wrapped text that looks like parameter expansion. Note how the invocation of m4wrapn
interprets these parameters, while using the builtin preserves them for their intended use.
$ m4 -I examples
include(‘wraplifo.m4’)
⇒
m4wrap(‘define(‘foo’, ‘‘$0:’-$1-$*-$#-’)foo(‘a’, ‘b’)
’)
⇒
builtin(‘m4wrap’, ‘‘’define(‘bar’, ‘‘$0:’-$1-$*-$#-’)bar(‘a’, ‘b’)
’)
⇒
^D
⇒bar:-a-a,b-2⇒m4wrap0:---0-
126
GNU M4 1.4.17 macro processor
Additionally, the computation of _m4wrap_level and creation of multiple m4wrapn placeholders in the original examples is more expensive in time and memory than strictly necessary. Notice how the improved version grabs the wrapped text via defn to avoid parameter
expansion, then undefines _m4wrap_text, before stripping a level of quotes with _arg1 to
expand the text. That way, each level of wrapping reuses the single placeholder, which
starts each nesting level in an undefined state.
Finally, it is worth emulating the GNU M4 extension of saving all arguments to m4wrap,
separated by a space, rather than saving just the first argument. This is done with the
join macro documented previously (see Section 6.3 [Shift], page 41). The improved LIFO
example is shipped as m4-1.4.17/examples/wraplifo2.m4, and can easily be converted to
a FIFO solution by swapping the adjacent invocations of joinall and defn.
$ m4 -I examples
include(‘wraplifo2.m4’)
⇒
undivert(‘wraplifo2.m4’)dnl
⇒dnl Redefine m4wrap to have LIFO semantics, improved example.
⇒include(‘join.m4’)dnl
⇒define(‘_m4wrap’, defn(‘m4wrap’))dnl
⇒define(‘_arg1’, ‘$1’)dnl
⇒define(‘m4wrap’,
⇒‘ifdef(‘_$0_text’,
⇒
‘define(‘_$0_text’, joinall(‘ ’, $@)defn(‘_$0_text’))’,
⇒
‘_$0(‘_arg1(defn(‘_$0_text’)undefine(‘_$0_text’))’)dnl
⇒define(‘_$0_text’, joinall(‘ ’, $@))’)’)dnl
m4wrap(‘define(‘foo’, ‘‘$0:’-$1-$*-$#-’)foo(‘a’, ‘b’)
’)
⇒
m4wrap(‘lifo text
m4wrap(‘nested’, ‘’, ‘$@
’)’)
⇒
^D
⇒lifo text
⇒foo:-a-a,b-2⇒nested $@
17.6 Solution for cleardivert
The cleardivert macro (see Section 10.4 [Cleardivert], page 79) cannot, as it stands, be
called without arguments to clear all pending diversions. That is because using undivert
with an empty string for an argument is different than using it with no arguments at all.
Compare the earlier definition with one that takes the number of arguments into account:
define(‘cleardivert’,
‘pushdef(‘_n’, divnum)divert(‘-1’)undivert($@)divert(_n)popdef(‘_n’)’)
⇒
divert(‘1’)one
divert
Chapter 17: Correct version of some examples
127
⇒
cleardivert
⇒
undivert
⇒one
⇒
define(‘cleardivert’,
‘pushdef(‘_num’, divnum)divert(‘-1’)ifelse(‘$#’, ‘0’,
‘undivert‘’’, ‘undivert($@)’)divert(_num)popdef(‘_num’)’)
⇒
divert(‘2’)two
divert
⇒
cleardivert
⇒
undivert
⇒
17.7 Solution for capitalize
The capitalize macro (see Section 11.6 [Patsubst], page 84) as presented earlier does
not allow clients to follow the quoting rule of thumb. Consider the three macros active,
Active, and ACTIVE, and the difference between calling capitalize with the expansion of
a macro, expanding the result of a case change, and changing the case of a double-quoted
string:
$ m4 -I examples
include(‘capitalize.m4’)dnl
define(‘active’, ‘act1, ive’)dnl
define(‘Active’, ‘Act2, Ive’)dnl
define(‘ACTIVE’, ‘ACT3, IVE’)dnl
upcase(active)
⇒ACT1,IVE
upcase(‘active’)
⇒ACT3, IVE
upcase(‘‘active’’)
⇒ACTIVE
downcase(ACTIVE)
⇒act3,ive
downcase(‘ACTIVE’)
⇒act1, ive
downcase(‘‘ACTIVE’’)
⇒active
capitalize(active)
⇒Act1
capitalize(‘active’)
⇒Active
capitalize(‘‘active’’)
128
GNU M4 1.4.17 macro processor
⇒_capitalize(‘active’)
define(‘A’, ‘OOPS’)
⇒
capitalize(active)
⇒OOPSct1
capitalize(‘active’)
⇒OOPSctive
First, when capitalize is called with more than one argument, it was throwing away
later arguments, whereas upcase and downcase used ‘$*’ to collect them all. The fix is
simple: use ‘$*’ consistently.
Next, with single-quoting, capitalize outputs a single character, a set of quotes, then
the rest of the characters, making it impossible to invoke Active after the fact, and allowing
the alternate macro A to interfere. Here, the solution is to use additional quoting in the
helper macros, then pass the final over-quoted output string through _arg1 to remove the
extra quoting and finally invoke the concatenated portions as a single string.
Finally, when passed a double-quoted string, the nested macro _capitalize is never
invoked because it ended up nested inside quotes. This one is the toughest to fix. In short,
we have no idea how many levels of quotes are in effect on the substring being altered by
patsubst. If the replacement string cannot be expressed entirely in terms of literal text
and backslash substitutions, then we need a mechanism to guarantee that the helper macros
are invoked outside of quotes. In other words, this sounds like a job for changequote (see
Section 8.2 [Changequote], page 62). By changing the active quoting characters, we can
guarantee that replacement text injected by patsubst always occurs in the middle of a
string that has exactly one level of over-quoting using alternate quotes; so the replacement
text closes the quoted string, invokes the helper macros, then reopens the quoted string. In
turn, that means the replacement text has unbalanced quotes, necessitating another round
of changequote.
In the fixed version below, (also shipped as m4-1.4.17/examples/capitalize2.m4),
capitalize uses the alternate quotes of ‘<<[’ and ‘]>>’ (the longer strings are chosen so as
to be less likely to appear in the text being converted). The helpers _to_alt and _from_
alt merely reduce the number of characters required to perform a changequote, since the
definition changes twice. The outermost pair means that patsubst and _capitalize_alt
are invoked with alternate quoting; the innermost pair is used so that the third argument
to patsubst can contain an unbalanced ‘]>>’/‘<<[’ pair. Note that upcase and downcase
must be redefined as _upcase_alt and _downcase_alt, since they contain nested quotes
but are invoked with the alternate quoting scheme in effect.
$ m4 -I examples
include(‘capitalize2.m4’)dnl
define(‘active’, ‘act1, ive’)dnl
define(‘Active’, ‘Act2, Ive’)dnl
define(‘ACTIVE’, ‘ACT3, IVE’)dnl
define(‘A’, ‘OOPS’)dnl
capitalize(active; ‘active’; ‘‘active’’; ‘‘‘actIVE’’’)
⇒Act1,Ive; Act2, Ive; Active; ‘Active’
undivert(‘capitalize2.m4’)dnl
⇒divert(‘-1’)
Chapter 17: Correct version of some examples
129
⇒# upcase(text)
⇒# downcase(text)
⇒# capitalize(text)
⇒#
change case of text, improved version
⇒define(‘upcase’, ‘translit(‘$*’, ‘a-z’, ‘A-Z’)’)
⇒define(‘downcase’, ‘translit(‘$*’, ‘A-Z’, ‘a-z’)’)
⇒define(‘_arg1’, ‘$1’)
⇒define(‘_to_alt’, ‘changequote(‘<<[’, ‘]>>’)’)
⇒define(‘_from_alt’, ‘changequote(<<[‘]>>, <<[’]>>)’)
⇒define(‘_upcase_alt’, ‘translit(<<[$*]>>, <<[a-z]>>, <<[A-Z]>>)’)
⇒define(‘_downcase_alt’, ‘translit(<<[$*]>>, <<[A-Z]>>, <<[a-z]>>)’)
⇒define(‘_capitalize_alt’,
⇒ ‘regexp(<<[$1]>>, <<[^\(\w\)\(\w*\)]>>,
⇒
<<[_upcase_alt(<<[<<[\1]>>]>>)_downcase_alt(<<[<<[\2]>>]>>)]>>)’)
⇒define(‘capitalize’,
⇒ ‘_arg1(_to_alt()patsubst(<<[<<[$*]>>]>>, <<[\w+]>>,
⇒
_from_alt()‘]>>_$0_alt(<<[\&]>>)<<[’_to_alt())_from_alt())’)
⇒divert‘’dnl
17.8 Solution for fatal_error
The fatal_error macro (see Section 14.3 [M4exit], page 103) is not robust to versions
of GNU M4 earlier than 1.4.8, where invoking __file__ (see Section 14.2 [Location],
page 101) inside m4wrap would result in an empty string, and __line__ resulted in ‘0’
even though all files start at line 1. Furthermore, versions earlier than 1.4.6 did not support
the __program__ macro. If you want fatal_error to work across the entire 1.4.x release
series, a better implementation would be:
define(‘fatal_error’,
‘errprint(ifdef(‘__program__’, ‘__program__’, ‘‘m4’’)’dnl
‘:ifelse(__line__, ‘0’, ‘’,
‘__file__:__line__:’)‘ fatal error: $*
’)m4exit(‘1’)’)
⇒
m4wrap(‘divnum(‘demo of internal message’)
fatal_error(‘inside wrapped text’)’)
⇒
^D
error m4:stdin:6: Warning: excess arguments to builtin ‘divnum’ ignored
⇒0
error m4:stdin:6: fatal error: inside wrapped text
Appendix A: How to make copies of the overall M4 package
131
Appendix A How to make copies of the overall
M4 package
This appendix covers the license for copying the source code of the overall M4 package.
This manual is under a different set of restrictions, covered later (see Appendix B [Copying
This Manual], page 143).
A.1 License for copying the M4 package
Version 3, 29 June 2007
Copyright c 2007 Free Software Foundation, Inc. http://fsf.org/
Everyone is permitted to copy and distribute verbatim copies of this
license document, but changing it is not allowed.
Preamble
The GNU General Public License is a free, copyleft license for software and other kinds of
works.
The licenses for most software and other practical works are designed to take away your
freedom to share and change the works. By contrast, the GNU General Public License is
intended to guarantee your freedom to share and change all versions of a program—to make
sure it remains free software for all its users. We, the Free Software Foundation, use the
GNU General Public License for most of our software; it applies also to any other work
released this way by its authors. You can apply it to your programs, too.
When we speak of free software, we are referring to freedom, not price. Our General
Public Licenses are designed to make sure that you have the freedom to distribute copies
of free software (and charge for them if you wish), that you receive source code or can get
it if you want it, that you can change the software or use pieces of it in new free programs,
and that you know you can do these things.
To protect your rights, we need to prevent others from denying you these rights or asking
you to surrender the rights. Therefore, you have certain responsibilities if you distribute
copies of the software, or if you modify it: responsibilities to respect the freedom of others.
For example, if you distribute copies of such a program, whether gratis or for a fee, you
must pass on to the recipients the same freedoms that you received. You must make sure
that they, too, receive or can get the source code. And you must show them these terms so
they know their rights.
Developers that use the GNU GPL protect your rights with two steps: (1) assert copyright on the software, and (2) offer you this License giving you legal permission to copy,
distribute and/or modify it.
For the developers’ and authors’ protection, the GPL clearly explains that there is no
warranty for this free software. For both users’ and authors’ sake, the GPL requires that
modified versions be marked as changed, so that their problems will not be attributed
erroneously to authors of previous versions.
Some devices are designed to deny users access to install or run modified versions of the
software inside them, although the manufacturer can do so. This is fundamentally incompatible with the aim of protecting users’ freedom to change the software. The systematic
132
GNU M4 1.4.17 macro processor
pattern of such abuse occurs in the area of products for individuals to use, which is precisely where it is most unacceptable. Therefore, we have designed this version of the GPL
to prohibit the practice for those products. If such problems arise substantially in other
domains, we stand ready to extend this provision to those domains in future versions of the
GPL, as needed to protect the freedom of users.
Finally, every program is threatened constantly by software patents. States should not
allow patents to restrict development and use of software on general-purpose computers, but
in those that do, we wish to avoid the special danger that patents applied to a free program
could make it effectively proprietary. To prevent this, the GPL assures that patents cannot
be used to render the program non-free.
The precise terms and conditions for copying, distribution and modification follow.
TERMS AND CONDITIONS
0. Definitions.
“This License” refers to version 3 of the GNU General Public License.
“Copyright” also means copyright-like laws that apply to other kinds of works, such as
semiconductor masks.
“The Program” refers to any copyrightable work licensed under this License. Each
licensee is addressed as “you”. “Licensees” and “recipients” may be individuals or
organizations.
To “modify” a work means to copy from or adapt all or part of the work in a fashion
requiring copyright permission, other than the making of an exact copy. The resulting
work is called a “modified version” of the earlier work or a work “based on” the earlier
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A “covered work” means either the unmodified Program or a work based on the Program.
To “propagate” a work means to do anything with it that, without permission, would
make you directly or secondarily liable for infringement under applicable copyright law,
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To “convey” a work means any kind of propagation that enables other parties to make
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An interactive user interface displays “Appropriate Legal Notices” to the extent that it
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of user commands or options, such as a menu, a prominent item in the list meets this
criterion.
1. Source Code.
The “source code” for a work means the preferred form of the work for making modifications to it. “Object code” means any non-source form of a work.
Appendix A: How to make copies of the overall M4 package
133
A “Standard Interface” means an interface that either is an official standard defined
by a recognized standards body, or, in the case of interfaces specified for a particular
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The “System Libraries” of an executable work include anything, other than the work as
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The Corresponding Source need not include anything that users can regenerate automatically from other parts of the Corresponding Source.
The Corresponding Source for a work in source code form is that same work.
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All rights granted under this License are granted for the term of copyright on the
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Conveying under any other circumstances is permitted solely under the conditions
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GNU M4 1.4.17 macro processor
No covered work shall be deemed part of an effective technological measure under
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technological measures to the extent such circumvention is effected by exercising rights
under this License with respect to the covered work, and you disclaim any intention
to limit operation or modification of the work as a means of enforcing, against the
work’s users, your or third parties’ legal rights to forbid circumvention of technological
measures.
4. Conveying Verbatim Copies.
You may convey verbatim copies of the Program’s source code as you receive it, in any
medium, provided that you conspicuously and appropriately publish on each copy an
appropriate copyright notice; keep intact all notices stating that this License and any
non-permissive terms added in accord with section 7 apply to the code; keep intact all
notices of the absence of any warranty; and give all recipients a copy of this License
along with the Program.
You may charge any price or no price for each copy that you convey, and you may offer
support or warranty protection for a fee.
5. Conveying Modified Source Versions.
You may convey a work based on the Program, or the modifications to produce it from
the Program, in the form of source code under the terms of section 4, provided that
you also meet all of these conditions:
a. The work must carry prominent notices stating that you modified it, and giving a
relevant date.
b. The work must carry prominent notices stating that it is released under this License and any conditions added under section 7. This requirement modifies the
requirement in section 4 to “keep intact all notices”.
c. You must license the entire work, as a whole, under this License to anyone who
comes into possession of a copy. This License will therefore apply, along with any
applicable section 7 additional terms, to the whole of the work, and all its parts,
regardless of how they are packaged. This License gives no permission to license
the work in any other way, but it does not invalidate such permission if you have
separately received it.
d. If the work has interactive user interfaces, each must display Appropriate Legal
Notices; however, if the Program has interactive interfaces that do not display
Appropriate Legal Notices, your work need not make them do so.
A compilation of a covered work with other separate and independent works, which
are not by their nature extensions of the covered work, and which are not combined
with it such as to form a larger program, in or on a volume of a storage or distribution
medium, is called an “aggregate” if the compilation and its resulting copyright are
not used to limit the access or legal rights of the compilation’s users beyond what the
individual works permit. Inclusion of a covered work in an aggregate does not cause
this License to apply to the other parts of the aggregate.
Appendix A: How to make copies of the overall M4 package
135
6. Conveying Non-Source Forms.
You may convey a covered work in object code form under the terms of sections 4 and
5, provided that you also convey the machine-readable Corresponding Source under
the terms of this License, in one of these ways:
a. Convey the object code in, or embodied in, a physical product (including a physical distribution medium), accompanied by the Corresponding Source fixed on a
durable physical medium customarily used for software interchange.
b. Convey the object code in, or embodied in, a physical product (including a physical distribution medium), accompanied by a written offer, valid for at least three
years and valid for as long as you offer spare parts or customer support for that
product model, to give anyone who possesses the object code either (1) a copy of
the Corresponding Source for all the software in the product that is covered by this
License, on a durable physical medium customarily used for software interchange,
for a price no more than your reasonable cost of physically performing this conveying of source, or (2) access to copy the Corresponding Source from a network
server at no charge.
c. Convey individual copies of the object code with a copy of the written offer to
provide the Corresponding Source. This alternative is allowed only occasionally
and noncommercially, and only if you received the object code with such an offer,
in accord with subsection 6b.
d. Convey the object code by offering access from a designated place (gratis or for
a charge), and offer equivalent access to the Corresponding Source in the same
way through the same place at no further charge. You need not require recipients
to copy the Corresponding Source along with the object code. If the place to
copy the object code is a network server, the Corresponding Source may be on
a different server (operated by you or a third party) that supports equivalent
copying facilities, provided you maintain clear directions next to the object code
saying where to find the Corresponding Source. Regardless of what server hosts
the Corresponding Source, you remain obligated to ensure that it is available for
as long as needed to satisfy these requirements.
e. Convey the object code using peer-to-peer transmission, provided you inform other
peers where the object code and Corresponding Source of the work are being offered
to the general public at no charge under subsection 6d.
A separable portion of the object code, whose source code is excluded from the Corresponding Source as a System Library, need not be included in conveying the object
code work.
A “User Product” is either (1) a “consumer product”, which means any tangible personal property which is normally used for personal, family, or household purposes, or
(2) anything designed or sold for incorporation into a dwelling. In determining whether
a product is a consumer product, doubtful cases shall be resolved in favor of coverage.
For a particular product received by a particular user, “normally used” refers to a
typical or common use of that class of product, regardless of the status of the particular user or of the way in which the particular user actually uses, or expects or is
expected to use, the product. A product is a consumer product regardless of whether
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the product has substantial commercial, industrial or non-consumer uses, unless such
uses represent the only significant mode of use of the product.
“Installation Information” for a User Product means any methods, procedures, authorization keys, or other information required to install and execute modified versions of a
covered work in that User Product from a modified version of its Corresponding Source.
The information must suffice to ensure that the continued functioning of the modified
object code is in no case prevented or interfered with solely because modification has
been made.
If you convey an object code work under this section in, or with, or specifically for
use in, a User Product, and the conveying occurs as part of a transaction in which
the right of possession and use of the User Product is transferred to the recipient in
perpetuity or for a fixed term (regardless of how the transaction is characterized),
the Corresponding Source conveyed under this section must be accompanied by the
Installation Information. But this requirement does not apply if neither you nor any
third party retains the ability to install modified object code on the User Product (for
example, the work has been installed in ROM).
The requirement to provide Installation Information does not include a requirement
to continue to provide support service, warranty, or updates for a work that has been
modified or installed by the recipient, or for the User Product in which it has been
modified or installed. Access to a network may be denied when the modification itself
materially and adversely affects the operation of the network or violates the rules and
protocols for communication across the network.
Corresponding Source conveyed, and Installation Information provided, in accord with
this section must be in a format that is publicly documented (and with an implementation available to the public in source code form), and must require no special password
or key for unpacking, reading or copying.
7. Additional Terms.
“Additional permissions” are terms that supplement the terms of this License by making exceptions from one or more of its conditions. Additional permissions that are
applicable to the entire Program shall be treated as though they were included in this
License, to the extent that they are valid under applicable law. If additional permissions apply only to part of the Program, that part may be used separately under those
permissions, but the entire Program remains governed by this License without regard
to the additional permissions.
When you convey a copy of a covered work, you may at your option remove any
additional permissions from that copy, or from any part of it. (Additional permissions
may be written to require their own removal in certain cases when you modify the
work.) You may place additional permissions on material, added by you to a covered
work, for which you have or can give appropriate copyright permission.
Notwithstanding any other provision of this License, for material you add to a covered
work, you may (if authorized by the copyright holders of that material) supplement
the terms of this License with terms:
a. Disclaiming warranty or limiting liability differently from the terms of sections 15
and 16 of this License; or
Appendix A: How to make copies of the overall M4 package
137
b. Requiring preservation of specified reasonable legal notices or author attributions
in that material or in the Appropriate Legal Notices displayed by works containing
it; or
c. Prohibiting misrepresentation of the origin of that material, or requiring that modified versions of such material be marked in reasonable ways as different from the
original version; or
d. Limiting the use for publicity purposes of names of licensors or authors of the
material; or
e. Declining to grant rights under trademark law for use of some trade names, trademarks, or service marks; or
f. Requiring indemnification of licensors and authors of that material by anyone who
conveys the material (or modified versions of it) with contractual assumptions
of liability to the recipient, for any liability that these contractual assumptions
directly impose on those licensors and authors.
All other non-permissive additional terms are considered “further restrictions” within
the meaning of section 10. If the Program as you received it, or any part of it, contains a notice stating that it is governed by this License along with a term that is a
further restriction, you may remove that term. If a license document contains a further
restriction but permits relicensing or conveying under this License, you may add to a
covered work material governed by the terms of that license document, provided that
the further restriction does not survive such relicensing or conveying.
If you add terms to a covered work in accord with this section, you must place, in the
relevant source files, a statement of the additional terms that apply to those files, or a
notice indicating where to find the applicable terms.
Additional terms, permissive or non-permissive, may be stated in the form of a separately written license, or stated as exceptions; the above requirements apply either
way.
8. Termination.
You may not propagate or modify a covered work except as expressly provided under this License. Any attempt otherwise to propagate or modify it is void, and will
automatically terminate your rights under this License (including any patent licenses
granted under the third paragraph of section 11).
However, if you cease all violation of this License, then your license from a particular
copyright holder is reinstated (a) provisionally, unless and until the copyright holder
explicitly and finally terminates your license, and (b) permanently, if the copyright
holder fails to notify you of the violation by some reasonable means prior to 60 days
after the cessation.
Moreover, your license from a particular copyright holder is reinstated permanently if
the copyright holder notifies you of the violation by some reasonable means, this is the
first time you have received notice of violation of this License (for any work) from that
copyright holder, and you cure the violation prior to 30 days after your receipt of the
notice.
Termination of your rights under this section does not terminate the licenses of parties
who have received copies or rights from you under this License. If your rights have
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GNU M4 1.4.17 macro processor
been terminated and not permanently reinstated, you do not qualify to receive new
licenses for the same material under section 10.
9. Acceptance Not Required for Having Copies.
You are not required to accept this License in order to receive or run a copy of the
Program. Ancillary propagation of a covered work occurring solely as a consequence of
using peer-to-peer transmission to receive a copy likewise does not require acceptance.
However, nothing other than this License grants you permission to propagate or modify
any covered work. These actions infringe copyright if you do not accept this License.
Therefore, by modifying or propagating a covered work, you indicate your acceptance
of this License to do so.
10. Automatic Licensing of Downstream Recipients.
Each time you convey a covered work, the recipient automatically receives a license
from the original licensors, to run, modify and propagate that work, subject to this
License. You are not responsible for enforcing compliance by third parties with this
License.
An “entity transaction” is a transaction transferring control of an organization, or
substantially all assets of one, or subdividing an organization, or merging organizations.
If propagation of a covered work results from an entity transaction, each party to that
transaction who receives a copy of the work also receives whatever licenses to the work
the party’s predecessor in interest had or could give under the previous paragraph, plus
a right to possession of the Corresponding Source of the work from the predecessor in
interest, if the predecessor has it or can get it with reasonable efforts.
You may not impose any further restrictions on the exercise of the rights granted or
affirmed under this License. For example, you may not impose a license fee, royalty, or
other charge for exercise of rights granted under this License, and you may not initiate
litigation (including a cross-claim or counterclaim in a lawsuit) alleging that any patent
claim is infringed by making, using, selling, offering for sale, or importing the Program
or any portion of it.
11. Patents.
A “contributor” is a copyright holder who authorizes use under this License of the
Program or a work on which the Program is based. The work thus licensed is called
the contributor’s “contributor version”.
A contributor’s “essential patent claims” are all patent claims owned or controlled by
the contributor, whether already acquired or hereafter acquired, that would be infringed
by some manner, permitted by this License, of making, using, or selling its contributor
version, but do not include claims that would be infringed only as a consequence of
further modification of the contributor version. For purposes of this definition, “control” includes the right to grant patent sublicenses in a manner consistent with the
requirements of this License.
Each contributor grants you a non-exclusive, worldwide, royalty-free patent license
under the contributor’s essential patent claims, to make, use, sell, offer for sale, import
and otherwise run, modify and propagate the contents of its contributor version.
In the following three paragraphs, a “patent license” is any express agreement or commitment, however denominated, not to enforce a patent (such as an express permission
to practice a patent or covenant not to sue for patent infringement). To “grant” such
Appendix A: How to make copies of the overall M4 package
139
a patent license to a party means to make such an agreement or commitment not to
enforce a patent against the party.
If you convey a covered work, knowingly relying on a patent license, and the Corresponding Source of the work is not available for anyone to copy, free of charge and under
the terms of this License, through a publicly available network server or other readily
accessible means, then you must either (1) cause the Corresponding Source to be so
available, or (2) arrange to deprive yourself of the benefit of the patent license for this
particular work, or (3) arrange, in a manner consistent with the requirements of this
License, to extend the patent license to downstream recipients. “Knowingly relying”
means you have actual knowledge that, but for the patent license, your conveying the
covered work in a country, or your recipient’s use of the covered work in a country,
would infringe one or more identifiable patents in that country that you have reason
to believe are valid.
If, pursuant to or in connection with a single transaction or arrangement, you convey,
or propagate by procuring conveyance of, a covered work, and grant a patent license
to some of the parties receiving the covered work authorizing them to use, propagate,
modify or convey a specific copy of the covered work, then the patent license you grant
is automatically extended to all recipients of the covered work and works based on it.
A patent license is “discriminatory” if it does not include within the scope of its coverage, prohibits the exercise of, or is conditioned on the non-exercise of one or more of the
rights that are specifically granted under this License. You may not convey a covered
work if you are a party to an arrangement with a third party that is in the business of
distributing software, under which you make payment to the third party based on the
extent of your activity of conveying the work, and under which the third party grants,
to any of the parties who would receive the covered work from you, a discriminatory
patent license (a) in connection with copies of the covered work conveyed by you (or
copies made from those copies), or (b) primarily for and in connection with specific
products or compilations that contain the covered work, unless you entered into that
arrangement, or that patent license was granted, prior to 28 March 2007.
Nothing in this License shall be construed as excluding or limiting any implied license or
other defenses to infringement that may otherwise be available to you under applicable
patent law.
12. No Surrender of Others’ Freedom.
If conditions are imposed on you (whether by court order, agreement or otherwise) that
contradict the conditions of this License, they do not excuse you from the conditions
of this License. If you cannot convey a covered work so as to satisfy simultaneously
your obligations under this License and any other pertinent obligations, then as a
consequence you may not convey it at all. For example, if you agree to terms that
obligate you to collect a royalty for further conveying from those to whom you convey
the Program, the only way you could satisfy both those terms and this License would
be to refrain entirely from conveying the Program.
13. Use with the GNU Affero General Public License.
Notwithstanding any other provision of this License, you have permission to link or
combine any covered work with a work licensed under version 3 of the GNU Affero
General Public License into a single combined work, and to convey the resulting work.
140
14.
15.
16.
17.
GNU M4 1.4.17 macro processor
The terms of this License will continue to apply to the part which is the covered work,
but the special requirements of the GNU Affero General Public License, section 13,
concerning interaction through a network will apply to the combination as such.
Revised Versions of this License.
The Free Software Foundation may publish revised and/or new versions of the GNU
General Public License from time to time. Such new versions will be similar in spirit
to the present version, but may differ in detail to address new problems or concerns.
Each version is given a distinguishing version number. If the Program specifies that
a certain numbered version of the GNU General Public License “or any later version”
applies to it, you have the option of following the terms and conditions either of that
numbered version or of any later version published by the Free Software Foundation.
If the Program does not specify a version number of the GNU General Public License,
you may choose any version ever published by the Free Software Foundation.
If the Program specifies that a proxy can decide which future versions of the GNU
General Public License can be used, that proxy’s public statement of acceptance of a
version permanently authorizes you to choose that version for the Program.
Later license versions may give you additional or different permissions. However, no
additional obligations are imposed on any author or copyright holder as a result of your
choosing to follow a later version.
Disclaimer of Warranty.
THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW. EXCEPT WHEN OTHERWISE STATED IN
WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES PROVIDE
THE PROGRAM “AS IS” WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
PURPOSE. THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE
OF THE PROGRAM IS WITH YOU. SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING, REPAIR OR
CORRECTION.
Limitation of Liability.
IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN
WRITING WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO
MODIFIES AND/OR CONVEYS THE PROGRAM AS PERMITTED ABOVE, BE
LIABLE TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR
INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO
LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM
TO OPERATE WITH ANY OTHER PROGRAMS), EVEN IF SUCH HOLDER OR
OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.
Interpretation of Sections 15 and 16.
If the disclaimer of warranty and limitation of liability provided above cannot be given
local legal effect according to their terms, reviewing courts shall apply local law that
Appendix A: How to make copies of the overall M4 package
141
most closely approximates an absolute waiver of all civil liability in connection with
the Program, unless a warranty or assumption of liability accompanies a copy of the
Program in return for a fee.
END OF TERMS AND CONDITIONS
How to Apply These Terms to Your New Programs
If you develop a new program, and you want it to be of the greatest possible use to the public,
the best way to achieve this is to make it free software which everyone can redistribute and
change under these terms.
To do so, attach the following notices to the program. It is safest to attach them to the
start of each source file to most effectively state the exclusion of warranty; and each file
should have at least the “copyright” line and a pointer to where the full notice is found.
one line to give the program’s name and a brief idea of what it does.
Copyright (C) year name of author
This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or (at
your option) any later version.
This program is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see http://www.gnu.org/licenses/.
Also add information on how to contact you by electronic and paper mail.
If the program does terminal interaction, make it output a short notice like this when it
starts in an interactive mode:
program Copyright (C) year name of author
This program comes with ABSOLUTELY NO WARRANTY; for details type ‘show w’.
This is free software, and you are welcome to redistribute it
under certain conditions; type ‘show c’ for details.
The hypothetical commands ‘show w’ and ‘show c’ should show the appropriate parts of
the General Public License. Of course, your program’s commands might be different; for a
GUI interface, you would use an “about box”.
You should also get your employer (if you work as a programmer) or school, if any, to
sign a “copyright disclaimer” for the program, if necessary. For more information on this,
and how to apply and follow the GNU GPL, see http://www.gnu.org/licenses/.
The GNU General Public License does not permit incorporating your program into
proprietary programs. If your program is a subroutine library, you may consider it more
useful to permit linking proprietary applications with the library. If this is what you want
to do, use the GNU Lesser General Public License instead of this License. But first, please
read http://www.gnu.org/philosophy/why-not-lgpl.html.
Appendix B: How to make copies of this manual
143
Appendix B How to make copies of this manual
This appendix covers the license for copying this manual. Note that some of the longer
examples in this manual are also distributed in the directory m4-1.4.17/examples/, where
a more permissive license is in effect when copying just the examples.
B.1 License for copying this manual
Version 1.3, 3 November 2008
Copyright c 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc.
http://fsf.org/
Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.
0. PREAMBLE
The purpose of this License is to make a manual, textbook, or other functional and
useful document free in the sense of freedom: to assure everyone the effective freedom
to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way
to get credit for their work, while not being considered responsible for modifications
made by others.
This License is a kind of “copyleft”, which means that derivative works of the document
must themselves be free in the same sense. It complements the GNU General Public
License, which is a copyleft license designed for free software.
We have designed this License in order to use it for manuals for free software, because
free software needs free documentation: a free program should come with manuals
providing the same freedoms that the software does. But this License is not limited to
software manuals; it can be used for any textual work, regardless of subject matter or
whether it is published as a printed book. We recommend this License principally for
works whose purpose is instruction or reference.
1. APPLICABILITY AND DEFINITIONS
This License applies to any manual or other work, in any medium, that contains a
notice placed by the copyright holder saying it can be distributed under the terms
of this License. Such a notice grants a world-wide, royalty-free license, unlimited in
duration, to use that work under the conditions stated herein. The “Document”,
below, refers to any such manual or work. Any member of the public is a licensee, and
is addressed as “you”. You accept the license if you copy, modify or distribute the work
in a way requiring permission under copyright law.
A “Modified Version” of the Document means any work containing the Document or
a portion of it, either copied verbatim, or with modifications and/or translated into
another language.
A “Secondary Section” is a named appendix or a front-matter section of the Document
that deals exclusively with the relationship of the publishers or authors of the Document
to the Document’s overall subject (or to related matters) and contains nothing that
could fall directly within that overall subject. (Thus, if the Document is in part a
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textbook of mathematics, a Secondary Section may not explain any mathematics.) The
relationship could be a matter of historical connection with the subject or with related
matters, or of legal, commercial, philosophical, ethical or political position regarding
them.
The “Invariant Sections” are certain Secondary Sections whose titles are designated, as
being those of Invariant Sections, in the notice that says that the Document is released
under this License. If a section does not fit the above definition of Secondary then it is
not allowed to be designated as Invariant. The Document may contain zero Invariant
Sections. If the Document does not identify any Invariant Sections then there are none.
The “Cover Texts” are certain short passages of text that are listed, as Front-Cover
Texts or Back-Cover Texts, in the notice that says that the Document is released under
this License. A Front-Cover Text may be at most 5 words, and a Back-Cover Text may
be at most 25 words.
A “Transparent” copy of the Document means a machine-readable copy, represented
in a format whose specification is available to the general public, that is suitable for
revising the document straightforwardly with generic text editors or (for images composed of pixels) generic paint programs or (for drawings) some widely available drawing
editor, and that is suitable for input to text formatters or for automatic translation to
a variety of formats suitable for input to text formatters. A copy made in an otherwise
Transparent file format whose markup, or absence of markup, has been arranged to
thwart or discourage subsequent modification by readers is not Transparent. An image
format is not Transparent if used for any substantial amount of text. A copy that is
not “Transparent” is called “Opaque”.
Examples of suitable formats for Transparent copies include plain ASCII without
markup, Texinfo input format, LaTEX input format, SGML or XML using a publicly
available DTD, and standard-conforming simple HTML, PostScript or PDF designed
for human modification. Examples of transparent image formats include PNG, XCF
and JPG. Opaque formats include proprietary formats that can be read and edited
only by proprietary word processors, SGML or XML for which the DTD and/or processing tools are not generally available, and the machine-generated HTML, PostScript
or PDF produced by some word processors for output purposes only.
The “Title Page” means, for a printed book, the title page itself, plus such following
pages as are needed to hold, legibly, the material this License requires to appear in the
title page. For works in formats which do not have any title page as such, “Title Page”
means the text near the most prominent appearance of the work’s title, preceding the
beginning of the body of the text.
The “publisher” means any person or entity that distributes copies of the Document
to the public.
A section “Entitled XYZ” means a named subunit of the Document whose title either
is precisely XYZ or contains XYZ in parentheses following text that translates XYZ in
another language. (Here XYZ stands for a specific section name mentioned below, such
as “Acknowledgements”, “Dedications”, “Endorsements”, or “History”.) To “Preserve
the Title” of such a section when you modify the Document means that it remains a
section “Entitled XYZ” according to this definition.
Appendix B: How to make copies of this manual
145
The Document may include Warranty Disclaimers next to the notice which states that
this License applies to the Document. These Warranty Disclaimers are considered to
be included by reference in this License, but only as regards disclaiming warranties:
any other implication that these Warranty Disclaimers may have is void and has no
effect on the meaning of this License.
2. VERBATIM COPYING
You may copy and distribute the Document in any medium, either commercially or
noncommercially, provided that this License, the copyright notices, and the license
notice saying this License applies to the Document are reproduced in all copies, and
that you add no other conditions whatsoever to those of this License. You may not use
technical measures to obstruct or control the reading or further copying of the copies
you make or distribute. However, you may accept compensation in exchange for copies.
If you distribute a large enough number of copies you must also follow the conditions
in section 3.
You may also lend copies, under the same conditions stated above, and you may publicly
display copies.
3. COPYING IN QUANTITY
If you publish printed copies (or copies in media that commonly have printed covers) of
the Document, numbering more than 100, and the Document’s license notice requires
Cover Texts, you must enclose the copies in covers that carry, clearly and legibly, all
these Cover Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on
the back cover. Both covers must also clearly and legibly identify you as the publisher
of these copies. The front cover must present the full title with all words of the title
equally prominent and visible. You may add other material on the covers in addition.
Copying with changes limited to the covers, as long as they preserve the title of the
Document and satisfy these conditions, can be treated as verbatim copying in other
respects.
If the required texts for either cover are too voluminous to fit legibly, you should put
the first ones listed (as many as fit reasonably) on the actual cover, and continue the
rest onto adjacent pages.
If you publish or distribute Opaque copies of the Document numbering more than 100,
you must either include a machine-readable Transparent copy along with each Opaque
copy, or state in or with each Opaque copy a computer-network location from which
the general network-using public has access to download using public-standard network
protocols a complete Transparent copy of the Document, free of added material. If
you use the latter option, you must take reasonably prudent steps, when you begin
distribution of Opaque copies in quantity, to ensure that this Transparent copy will
remain thus accessible at the stated location until at least one year after the last time
you distribute an Opaque copy (directly or through your agents or retailers) of that
edition to the public.
It is requested, but not required, that you contact the authors of the Document well
before redistributing any large number of copies, to give them a chance to provide you
with an updated version of the Document.
4. MODIFICATIONS
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You may copy and distribute a Modified Version of the Document under the conditions
of sections 2 and 3 above, provided that you release the Modified Version under precisely
this License, with the Modified Version filling the role of the Document, thus licensing
distribution and modification of the Modified Version to whoever possesses a copy of
it. In addition, you must do these things in the Modified Version:
A. Use in the Title Page (and on the covers, if any) a title distinct from that of the
Document, and from those of previous versions (which should, if there were any,
be listed in the History section of the Document). You may use the same title as
a previous version if the original publisher of that version gives permission.
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Texts given in the Document’s license notice.
H. Include an unaltered copy of this License.
I. Preserve the section Entitled “History”, Preserve its Title, and add to it an item
stating at least the title, year, new authors, and publisher of the Modified Version
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J. Preserve the network location, if any, given in the Document for public access to
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K. For any section Entitled “Acknowledgements” or “Dedications”, Preserve the Title
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L. Preserve all the Invariant Sections of the Document, unaltered in their text and
in their titles. Section numbers or the equivalent are not considered part of the
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M. Delete any section Entitled “Endorsements”. Such a section may not be included
in the Modified Version.
Appendix B: How to make copies of this manual
147
N. Do not retitle any existing section to be Entitled “Endorsements” or to conflict in
title with any Invariant Section.
O. Preserve any Warranty Disclaimers.
If the Modified Version includes new front-matter sections or appendices that qualify
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You may add a passage of up to five words as a Front-Cover Text, and a passage of up
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The author(s) and publisher(s) of the Document do not by this License give permission
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5. COMBINING DOCUMENTS
You may combine the Document with other documents released under this License,
under the terms defined in section 4 above for modified versions, provided that you
include in the combination all of the Invariant Sections of all of the original documents,
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notice, and that you preserve all their Warranty Disclaimers.
The combined work need only contain one copy of this License, and multiple identical
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to the section titles in the list of Invariant Sections in the license notice of the combined
work.
In the combination, you must combine any sections Entitled “History” in the various original documents, forming one section Entitled “History”; likewise combine any
sections Entitled “Acknowledgements”, and any sections Entitled “Dedications”. You
must delete all sections Entitled “Endorsements.”
6. COLLECTIONS OF DOCUMENTS
You may make a collection consisting of the Document and other documents released
under this License, and replace the individual copies of this License in the various
documents with a single copy that is included in the collection, provided that you
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GNU M4 1.4.17 macro processor
follow the rules of this License for verbatim copying of each of the documents in all
other respects.
You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted
document, and follow this License in all other respects regarding verbatim copying of
that document.
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an “aggregate” if the copyright resulting from the compilation is not used to limit the
legal rights of the compilation’s users beyond what the individual works permit. When
the Document is included in an aggregate, this License does not apply to the other
works in the aggregate which are not themselves derivative works of the Document.
If the Cover Text requirement of section 3 is applicable to these copies of the Document,
then if the Document is less than one half of the entire aggregate, the Document’s Cover
Texts may be placed on covers that bracket the Document within the aggregate, or the
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must appear on printed covers that bracket the whole aggregate.
8. TRANSLATION
Translation is considered a kind of modification, so you may distribute translations
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include translations of some or all Invariant Sections in addition to the original versions
of these Invariant Sections. You may include a translation of this License, and all the
license notices in the Document, and any Warranty Disclaimers, provided that you
also include the original English version of this License and the original versions of
those notices and disclaimers. In case of a disagreement between the translation and
the original version of this License or a notice or disclaimer, the original version will
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If a section in the Document is Entitled “Acknowledgements”, “Dedications”, or “History”, the requirement (section 4) to Preserve its Title (section 1) will typically require
changing the actual title.
9. TERMINATION
You may not copy, modify, sublicense, or distribute the Document except as expressly
provided under this License. Any attempt otherwise to copy, modify, sublicense, or
distribute it is void, and will automatically terminate your rights under this License.
However, if you cease all violation of this License, then your license from a particular
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explicitly and finally terminates your license, and (b) permanently, if the copyright
holder fails to notify you of the violation by some reasonable means prior to 60 days
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Moreover, your license from a particular copyright holder is reinstated permanently if
the copyright holder notifies you of the violation by some reasonable means, this is the
first time you have received notice of violation of this License (for any work) from that
Appendix B: How to make copies of this manual
149
copyright holder, and you cure the violation prior to 30 days after your receipt of the
notice.
Termination of your rights under this section does not terminate the licenses of parties
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same material does not give you any rights to use it.
10. FUTURE REVISIONS OF THIS LICENSE
The Free Software Foundation may publish new, revised versions of the GNU Free
Documentation License from time to time. Such new versions will be similar in spirit
to the present version, but may differ in detail to address new problems or concerns.
See http://www.gnu.org/copyleft/.
Each version of the License is given a distinguishing version number. If the Document
specifies that a particular numbered version of this License “or any later version”
applies to it, you have the option of following the terms and conditions either of that
specified version or of any later version that has been published (not as a draft) by
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Software Foundation. If the Document specifies that a proxy can decide which future
versions of this License can be used, that proxy’s public statement of acceptance of a
version permanently authorizes you to choose that version for the Document.
11. RELICENSING
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Web server that publishes copyrightable works and also provides prominent facilities
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such a server. A “Massive Multiauthor Collaboration” (or “MMC”) contained in the
site means any set of copyrightable works thus published on the MMC site.
“CC-BY-SA” means the Creative Commons Attribution-Share Alike 3.0 license published by Creative Commons Corporation, a not-for-profit corporation with a principal
place of business in San Francisco, California, as well as future copyleft versions of that
license published by that same organization.
“Incorporate” means to publish or republish a Document, in whole or in part, as part
of another Document.
An MMC is “eligible for relicensing” if it is licensed under this License, and if all works
that were first published under this License somewhere other than this MMC, and
subsequently incorporated in whole or in part into the MMC, (1) had no cover texts
or invariant sections, and (2) were thus incorporated prior to November 1, 2008.
The operator of an MMC Site may republish an MMC contained in the site under
CC-BY-SA on the same site at any time before August 1, 2009, provided the MMC is
eligible for relicensing.
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ADDENDUM: How to use this License for your documents
To use this License in a document you have written, include a copy of the License in the
document and put the following copyright and license notices just after the title page:
Copyright (C) year your name.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3
or any later version published by the Free Software Foundation;
with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
Texts. A copy of the license is included in the section entitled ‘‘GNU
Free Documentation License’’.
If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the
“with. . . Texts.” line with this:
with the Invariant Sections being list their titles, with
the Front-Cover Texts being list, and with the Back-Cover Texts
being list.
If you have Invariant Sections without Cover Texts, or some other combination of the
three, merge those two alternatives to suit the situation.
If your document contains nontrivial examples of program code, we recommend releasing
these examples in parallel under your choice of free software license, such as the GNU
General Public License, to permit their use in free software.
Appendix C: Indices of concepts and macros
151
Appendix C Indices of concepts and macros
C.1 Index for all m4 macros
This index covers all m4 builtins, as well as several useful composite macros. References are
exclusively to the places where a macro is introduced the first time.
E
__file__ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
__gnu__ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
__line__ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
__os2__ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
__program__ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
__unix__ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
__windows__ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
A
argn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
array . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
array_set . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
builtin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
C
85
65
62
67
79
41
53
52
fatal_error . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
foreach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
foreachq . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
forloop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
ifdef . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ifelse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
include . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
incr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
indir . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
39
39
73
89
81
34
J
join . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
joinall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
L
len . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
D
debugfile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
debugmode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
decr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
define . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
define_blind . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
defn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
divert . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
divnum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
dnl . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
downcase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
dquote . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
dquote_elt. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
dumpdef . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
F
I
B
capitalize. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
changecom . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
changequote . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
changeword. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
cleardivert . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
cond . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
copy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
curry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
errprint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
esyscmd . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
eval . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
exch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
60
59
89
25
51
31
75
79
61
85
44
44
55
M
m4exit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
m4wrap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
maketemp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
mkstemp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
N
nargs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
O
os2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
152
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patsubst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
popdef . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
pushdef . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
stack_foreach_sep_lifo . . . . . . . . . . . . . . . . . . . . . 124
substr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
syscmd . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
sysval . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
Q
T
quote . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
traceoff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
traceon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
translit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
R
regexp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
rename . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
reverse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
S
shift . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
sinclude . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
stack_foreach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
stack_foreach_lifo . . . . . . . . . . . . . . . . . . . . . . . . . . 49
stack_foreach_sep . . . . . . . . . . . . . . . . . . . . . . . . . . 124
U
undefine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
undivert . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
unix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
upcase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
30
76
95
85
W
windows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
C.2 Index for many concepts
A
argument currying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
arguments to macros . . . . . . . . . . . . . . . . . . . . . . . 21, 26
arguments to macros, special . . . . . . . . . . . . . . . . . . 27
arguments, joining . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
arguments, more than nine . . . . . . . . . . . . 27, 45, 120
arguments, quoted macro . . . . . . . . . . . . . . . . . . . . . . 23
arguments, reversing . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
arithmetic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
avoiding quadratic behavior . . . . . . . . . . . . . . . . . . 118
B
basic regular expressions . . . . . . . . . . . . . . . . . . . 81, 84
blind macro . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19, 40, 51
bug reports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
builtins, indirect call of . . . . . . . . . . . . . . . . . . . . . . . . 35
builtins, special tokens . . . . . . . . . . . . . . . . . . . . . . . . . 32
C
call of builtins, indirect . . . . . . . . . . . . . . . . . . . . . . . . 35
call of macros, indirect . . . . . . . . . . . . . . . . . . . . . . . . . 34
case statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
changing comment delimiters . . . . . . . . . . . . . . . . . . 65
changing quote delimiters . . . . . . . . . . . . . . . . . . . . . . 62
changing syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
characters, translating . . . . . . . . . . . . . . . . . . . . . . . . . 83
command line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
command line, file names on the . . . . . . . . . . . . . . . 12
command line, macro definitions on the . . . . . . . . . 8
command line, options . . . . . . . . . . . . . . . . . . . . . . . . . . 7
commands, exit status from shell . . . . . . . . . . . . . . 97
commands, running shell. . . . . . . . . . . . . . . . . . . . . . . 95
comment delimiters, changing . . . . . . . . . . . . . . . . . . 65
comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
comments, copied to output . . . . . . . . . . . . . . . . . . . 65
comparing strings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
compatibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
composing macros . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
concatenating arguments . . . . . . . . . . . . . . . . . . . . . . 42
conditional, short-circuiting . . . . . . . . . . . . . . . . . . . . 41
conditionals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
controlling debugging output . . . . . . . . . . . . . . . . . . 58
copying macros . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
counting loops . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
currying arguments . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
D
debugging macros . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
debugging output, controlling . . . . . . . . . . . . . . . . . .
debugging output, saving . . . . . . . . . . . . . . . . . . . . . .
decrement operator . . . . . . . . . . . . . . . . . . . . . . . . . . . .
deferring expansion . . . . . . . . . . . . . . . . . . . . . . . . . . . .
deferring output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
defining new macros . . . . . . . . . . . . . . . . . . . . . . . . . . .
definition stack . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33,
definitions, displaying macro . . . . . . . . . . . . . . . 31,
deleting macros . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
55
58
60
89
70
75
25
49
55
30
Appendix C: Indices of concepts and macros
deleting whitespace in input . . . . . . . . . . . . . . . . . . . 61
delimiters, changing . . . . . . . . . . . . . . . . . . . . . . . . 62, 65
discarding diverted text . . . . . . . . . . . . . . . . . . . . . . . . 79
discarding input . . . . . . . . . . . . . . . . . . . . . . . . 39, 61, 76
displaying macro definitions. . . . . . . . . . . . . . . . . . . . 55
diversion numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
diverted text, discarding . . . . . . . . . . . . . . . . . . . . . . . 79
diverting output to files . . . . . . . . . . . . . . . . . . . . . . . . 75
dumping into frozen file . . . . . . . . . . . . . . . . . . . . . . 105
E
error messages, printing . . . . . . . . . . . . . . . . . . . . . . 101
errors, fatal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
evaluation, of integer expressions . . . . . . . . . . . . . . . 89
examples, understanding . . . . . . . . . . . . . . . . . . . . . . . . 5
executing shell commands . . . . . . . . . . . . . . . . . . . . . 95
exit status from shell commands . . . . . . . . . . . . . . . 97
exiting from m4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
expansion of macros . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
expansion, deferring . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
expansion, tracing macro . . . . . . . . . . . . . . . . . . . . . . 55
expressions, evaluation of integer . . . . . . . . . . . . . . . 89
expressions, regular . . . . . . . . . . . . . . . . . . . . . . . . 81, 84
extracting substrings . . . . . . . . . . . . . . . . . . . . . . . . . . 82
F
fast loading of frozen files . . . . . . . . . . . . . . . . . . . . . 105
fatal errors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
FDL, GNU Free Documentation License . . . . . . 143
file format, frozen file . . . . . . . . . . . . . . . . . . . . . . . . . 106
file inclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . 73, 76, 78
file names, on the command line . . . . . . . . . . . . . . . 12
files, diverting output to . . . . . . . . . . . . . . . . . . . . . . . 75
files, names of temporary . . . . . . . . . . . . . . . . . . . . . . 99
for each loops . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
for loops. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
formatted output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
Free Documentation License (FDL), GNU . . . . 143
frozen file format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
frozen files for fast loading . . . . . . . . . . . . . . . . . . . . 105
G
General Public License (GPL), GNU . . . . . . . . . . 131
GNU extensions . . 19, 25, 27, 34, 35, 59, 60, 74, 76,
78, 81, 84, 86, 91, 97, 99, 105, 109
GNU Free Documentation License . . . . . . . . . . . . 143
GNU General Public License . . . . . . . . . . . . . . . . . 131
GNU M4, history of . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
GPL, GNU General Public License . . . . . . . . . . . 131
H
history of m4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
153
I
included files, search path for . . . . . . . . . . . . . . . . . . 74
inclusion, of files . . . . . . . . . . . . . . . . . . . . . . . 73, 76, 78
increment operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
indirect call of builtins . . . . . . . . . . . . . . . . . . . . . . . . . 35
indirect call of macros . . . . . . . . . . . . . . . . . . . . . . . . . 34
initialization, frozen state . . . . . . . . . . . . . . . . . . . . . 105
input location . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9, 101
input tokens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
input, discarding . . . . . . . . . . . . . . . . . . . . . . . 39, 61, 76
input, saving . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
integer arithmetic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
integer expression evaluation . . . . . . . . . . . . . . . . . . . 89
invoking m4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
invoking macros . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
iterating over lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
J
joining arguments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
L
length of strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
lexical structure of words . . . . . . . . . . . . . . . . . . . . . . 67
License, code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
License, manual . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
limit, nesting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
literal output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
local variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
location, input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9, 101
loops . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
loops, counting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
loops, list iteration . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
M
M4PATH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
macro composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
macro definitions, on the command line . . . . . . . . . 8
macro expansion, tracing . . . . . . . . . . . . . . . . . . . . . . 55
macro invocation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
macro, blind . . . . . . . . . . . . . . . . . . . . . . . . . . . 19, 40, 51
macros, arguments to . . . . . . . . . . . . . . . . . . . . . . 21, 26
macros, copying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
macros, debugging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
macros, displaying definitions . . . . . . . . . . . . . . 31, 55
macros, expansion of. . . . . . . . . . . . . . . . . . . . . . . . . . . 23
macros, how to define new . . . . . . . . . . . . . . . . . . . . . 25
macros, how to delete . . . . . . . . . . . . . . . . . . . . . . . . . . 30
macros, how to rename . . . . . . . . . . . . . . . . . . . . . . . . 31
macros, indirect call of . . . . . . . . . . . . . . . . . . . . . . . . . 34
macros, quoted arguments to . . . . . . . . . . . . . . . . . . 23
macros, recursive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
macros, special arguments to . . . . . . . . . . . . . . . . . . 27
macros, temporary redefinition of . . . . . . . . . . . . . . 33
manipulating quotes . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
154
messages, printing error . . . . . . . . . . . . . . . . . . . . . . 101
more than nine arguments . . . . . . . . . . . . . 27, 45, 120
multibranches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
N
names . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
nesting limit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
nine arguments, more than . . . . . . . . . . . . 27, 45, 120
numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
O
options, command line . . . . . . . . . . . . . . . . . . . . . . . . . . 7
output, diverting to files . . . . . . . . . . . . . . . . . . . . . . . 75
output, formatted . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
output, literal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
output, saving debugging . . . . . . . . . . . . . . . . . . . . . . 60
overview of m4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
P
pattern substitution . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
platform macros . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
positional parameters, more than nine . . . . . . . . . 27
POSIX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
POSIXLY_CORRECT . . . . . . . . . . . . . . . . . . . . . . . . . . 7, 112
preprocessor features . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
printing error messages . . . . . . . . . . . . . . . . . . . . . . . 101
pushdef stack . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33, 49
Q
quadratic behavior, avoiding . . . . . . . . . . . . . . . . . . 118
quote delimiters, changing . . . . . . . . . . . . . . . . . . . . . 62
quote manipulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
quoted macro arguments . . . . . . . . . . . . . . . . . . . . . . . 23
quoted string . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
quoting rule of thumb . . . . . . . . . . . . . . . . . . . . . . . . . 23
R
recursive macros . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
redefinition of macros, temporary . . . . . . . . . . . . . . 33
regular expressions . . . . . . . . . . . . . . . . . . . . . 67, 81, 84
reloading a frozen file . . . . . . . . . . . . . . . . . . . . . . . . . 105
renaming macros . . . . . . . . . . . . . . . . . . . . . . . . . . . 31, 53
reporting bugs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
rescanning . . . . . . . . . . . . . . . . . . . . . 11, 20, 29, 31, 113
reversing arguments . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
rule of thumb, quoting . . . . . . . . . . . . . . . . . . . . . . . . . 23
running shell commands . . . . . . . . . . . . . . . . . . . . . . . 95
GNU M4 1.4.17 macro processor
S
saving debugging output . . . . . . . . . . . . . . . . . . . . . . . 60
saving input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
search path for included files . . . . . . . . . . . . . . . . . . . 74
shell commands, exit status from . . . . . . . . . . . . . . 97
shell commands, running. . . . . . . . . . . . . . . . . . . . . . . 95
short-circuiting conditional . . . . . . . . . . . . . . . . . . . . 41
special arguments to macros . . . . . . . . . . . . . . . . . . . 27
stack, macro definition . . . . . . . . . . . . . . . . . . . . . 33, 49
standard error, output to . . . . . . . . . . . . . . . . . 55, 101
status of shell commands . . . . . . . . . . . . . . . . . . . . . . 97
status, setting m4 exit. . . . . . . . . . . . . . . . . . . . . . . . . 103
string, quoted . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
strings, length of . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
substitution by regular expression . . . . . . . . . . . . . . 84
substrings, extracting . . . . . . . . . . . . . . . . . . . . . . . . . . 82
substrings, locating . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
suggestions, reporting . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
suppressing warnings . . . . . . . . . . . . . . . . . . . . . . . . . . 22
switch statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
synchronization lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
syntax, changing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
T
temporary file names . . . . . . . . . . . . . . . . . . . . . . . . . .
temporary redefinition of macros . . . . . . . . . . . . . . .
TMPDIR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
tokens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
tokens, builtin macro . . . . . . . . . . . . . . . . . . . . . . . . . .
tokens, special . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
tracing macro expansion . . . . . . . . . . . . . . . . . . . . . . .
translating characters . . . . . . . . . . . . . . . . . . . . . . . . . .
99
33
75
15
32
16
55
83
U
undefining macros . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
UNIX commands, exit status from . . . . . . . . . . . . . 97
UNIX commands, running . . . . . . . . . . . . . . . . . . . . . 95
V
variables, local. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
W
warnings, suppressing . . . . . . . . . . . . . . . . . . . . . . . . . . 22
words . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
words, lexical structure of . . . . . . . . . . . . . . . . . . . . . . 67
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