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C Language manual
Copyright © COSMIC Software 1999, 2001
All rights reserved.
Table of Contents
Preface
Chapter 1
Historical Introduction
Chapter 2
C Language Overview
C Files ......................................................................................2-1
Lines ..................................................................................2-1
Comments..........................................................................2-2
Trigraphs............................................................................2-2
Lexical Tokens.........................................................................2-3
Identifiers...........................................................................2-3
Keywords...........................................................................2-3
Constants ...........................................................................2-4
Operators and Punctuators.................................................2-4
Declarations .............................................................................2-5
Integer Types .....................................................................2-5
Real Types .........................................................................2-7
Pointers ..............................................................................2-7
Arrays ................................................................................2-8
Structures...........................................................................2-9
Unions................................................................................2-9
Enumerations.....................................................................2-9
Functions ...........................................................................2-9
Chapter 3
Declarations
Integers.....................................................................................3-2
Pointers ....................................................................................3-2
Arrays.......................................................................................3-3
Modifiers..................................................................................3-5
Structures .................................................................................3-7
Unions......................................................................................3-9
Enumerations ...........................................................................3-9
Functions................................................................................3-10
Storage Class..........................................................................3-12
Typedef ..................................................................................3-14
Variable Scope.......................................................................3-14
Absolute Addressing..............................................................3-15
(i)
Chapter 4
Expressions
Variables .................................................................................. 4-2
Constants ................................................................................. 4-2
Strings ...................................................................................... 4-5
Sizeof ....................................................................................... 4-7
Operators ................................................................................. 4-7
Arithmetic operators.......................................................... 4-8
Bitwise operators ............................................................... 4-9
Boolean operators............................................................4-10
Assignment operators...................................................... 4-11
Addressing operators .......................................................4-13
Function call operator...................................................... 4-14
Conditional operator........................................................4-15
Sequence operator ........................................................... 4-16
Conversion operator ........................................................4-16
Priorities................................................................................. 4-18
Chapter 5
Statements
Block statement ....................................................................... 5-2
Expression statement ............................................................... 5-2
If statement .............................................................................. 5-3
While statement ....................................................................... 5-5
Do statement ............................................................................ 5-6
For statement ........................................................................... 5-7
Break statement ....................................................................... 5-9
Continue statement ................................................................5-10
Switch statement....................................................................5-11
Goto statement .......................................................................5-13
Return statement ....................................................................5-14
Chapter 6
Preprocessor
Macro Directives ..................................................................... 6-2
Hazardous Behaviours....................................................... 6-5
Predefined Symbols........................................................... 6-7
Conditional Directives............................................................. 6-8
Control Directives..................................................................6-10
#include ...........................................................................6-10
#error ...............................................................................6-10
(ii)
#line .................................................................................6-10
#pragma ...........................................................................6-11
(iii)
Preface
Chapter 1, “Historical Introduction”
Chapter 2, “C Language Overview”
- file structure, separate compile
- character set (trigraph, logical lines, preprocessor)
- naming, identifiers, keywords, operators, punctuation
- C objects (simple, aggregate)
- functions
Chapter 3, “Declarations”
- syntax for each kind of object
- modifiers const, volatile
- extra modifiers for pointers
- function declaration, arguments and local variables
- classes and scope
- type redeclaration (typedef)
Chapter 4, “Expressions”
- identifiers, operators and constants
- operators behaviour
- conversions, explicit, implicit
Chapter 5, “Statements”
- description of each statement
Chapter 6, “Preprocessor”
- description of each directives
- traps and solutions
- pragmas and their usage
© Copyright 2001 by COSMIC Software
Preface-1
CHAPTER
1
Historical Introduction
The C language was designed in 1972 at the Bell Lab’s by Denis
Ritchie, mainly to rewrite the UNIX operating system in a portable way.
UNIX was written originally in assembler and had to be completely
rewritten for each new machine. The C language was based on previous
languages named B and BCPL, and is logically called C to denote the
evolution.
The language is described in the well known book “The C Programming Language” published in 1978, whose “Appendix A” has been used
for many years as the de facto standard. Because of the growing popurality (The popularity) of UNIX and of the C language (growing), several companies started to provide C compilers outside of the UNIX
world, and for a wide range of processors, microprocessors, and even
micro-controllers.
The success of C for those small processors is due to the fact that it is
not a high level language, as PASCAL or ADA, but a highly portable
macro assembler, allowing with the same flexibility as assembler
almost the same efficiency.
The C language has been normalized from 1983 to 1990 and is now an
ANSI/ISO standard. It implements most of the usefull extentions added
to the original language in the previous decade, and a few extra features
allowing a more secured behaviour.
© Copyright 2001 by COSMIC Software
Historical Introduction 1-1
1
The ANSI/ISO standard distinguishes two different environments:
•
A hosted environment describing native compilers, whose target
processor uses a specific operating system providing disks file
handling, and execution environments.
•
A freestanding environment describing a cross compiler, whose
target processor is generally an embedded microprocessor or a
micro-controller without any operating system services. In other
words, everything has to be written to perform the simplest operation.
On a native system, the same machine is used to create, compile and
execute the C program. On an embedded system, the target application
has no disk, no operating system, no editor and cannot be used to create
and compile the C program. Another machine has to be used to create
and compile the application program, and then the result must be transferred and executed by the target system. The machine used to create
and compile the application is the Host system, and the machine used to
execute the application is the Target system.
COSMIC compilers are Cross compilers and conform to the Freestanding environment of the ANSI/ISO standard. They also implement
extensions to the standard to allow a better efficiency for each specific
target processor.
1-2 Historical Introduction
© Copyright 2001 by COSMIC Software
CHAPTER
2
C Language Overview
A C program is generally split in to several files, each containing a part
of the text describing the full application. Some parts may be already
written and used from libraries. Some parts may also be written in
assembler where the C compiler is not efficient enough, or does not
allow a direct access to some specific resources of the target processor.
C Files
Each of these C files has to be compiled, which translates the C text file
to a relocatable object file. Once all the files are compiled, the linker is
used to bind together all the object files and the required libraries, in
order to produce an executable file. Note that this result file is still in an
internal format and cannot be directly transferred to the target system. A
special translator is provided to convert the executable file to a downloadable format.
Lines
Each C text file contains a set of lines. A line contains characters and is
finished by a line terminator (line feed, carriage return). The C compiler
allows several physical lines to be concatenated in a single logical line,
whose length should not exceed 511 characters in order to strictly comply with the ANSI standard. The COSMIC compiler accepts up to 4095
characters in a logical line. Two physical lines are concatenated into a
single logical line if the first line ends with a backslash character ‘ \’
© Copyright 2001 by COSMIC Software
C Language Overview 2-1
2
C Files
just before the line terminator. This feature is important as the C language implements special directives, known as preprocessing directives, whose operands have to be located on the same logical line.
Comments
Comments are part of the text which are not meaningful for the compiler, but very important for the program readability and understanding.
They are removed from the original text and replaced by a single
whitespace character. A comment starts with the sequence /* and ends
with the sequence */. A comment may span over several lines but nesting comments is not allowed. As an extension from the ANSI standard,
the compiler also accepts C++ style comments, starting with the
sequence // and ending at the end of the same logical line.
Trigraphs
The C language uses almost all the ASCII character set to build the language components. Some terminals or workstations cannot display the
full ASCII set, and they need a specific mechanism to have access to all
the needed characters. These special characters are encoded using special sequences called trigraphs. A trigraph is a sequence of three characters beginning with two question marks ?? and followed by a
common character. These three characters are equivalent to a single one
from the following table:
??(
??/
??)
??’
??<
??!
??>
????=
[
\
]
^
{
|
}
~
#
All other sequences beginning with two question marks are left
unchanged.
2-2 C Language Overview
© Copyright 2001 by COSMIC Software
Lexical Tokens
Lexical Tokens
Characters on a logical line are grouped together to form lexical tokens.
These tokens are the basic entities of the language and consist of:
identifiers
keywords
constants
operators
punctuation
Identifiers
An identifier is used to give a name to an object. It begins with a letter
or the underscore character _, and is followed by any letter, digit or
underscore character. Uppercase and lowercase letters are not equivalent in C, so the two identifiers VAR1 and var1 do not describe the
same object. An identifier may have up to 255 characters. All the characters are significant for name comparisons.
Keywords
A keyword is a reserved identifier used by the language to describe a
special feature. It is used in declarations to describe the basic type of an
object, or in a function body to describe the statements executed.
A keyword name cannot be used as an object name. All C keywords are
lowercase names, so because lowercase and uppercase letters are different, the uppercase version of a keyword is available as an indentifier
although this is probably not a good programming practice.
The C keywords are:
auto
break
case
char
const
continue
default
do
double
else
enum
extern
float
for
goto
if
© Copyright 2001 by COSMIC Software
int
long
register
return
short
signed
sizeof
static
struct
switch
typedef
union
unsigned
void
volatile
while
C Language Overview 2-3
2
Lexical Tokens
Constants
A constant is used to describe a numerical value, or a character string.
Numerical constants may be expressed as real constants, integer constants or character constants. Integer constants may be expressed in
decimal, octal or hexadecimal base. The syntax for constants is
explained in the Expressions chapter.
Operators and Punctuators
An operator is used to describe an operation applied to one or several
objects. It is mainly meaningful in expressions, but also in declarations.
It is generally a short sequence using non alphanumeric characters. A
punctuator is used to separate or terminate a list of elements.
C operators and punctuators are:
...
<<=
>>=
!=
%=
##
&&
&=
*=
++
+=
--
-=
->
/=
<<
<=
==
>=
>>
^=
|=
||
!
~
%
&
(
)
*
+
,
.
/
:
;
<
=
>
?
[
]
^
{
|
}
#
Note that some sequences are used as operators and as punctuators,
such as *, =, :, # and ,.
Several punctuators have to be used by pairs, such as ( ), [ ], { }.
When parsing the input text, the compiler tries to build the longest
sequence as possible for a token, so when parsing:
a+++++b
the compiler will recognize:
a ++ ++ + b
which is not a valid construct
and not:
a ++ + ++ b
2-4 C Language Overview
which may be valid.
© Copyright 2001 by COSMIC Software
Declarations
Declarations
A C program is a set of tokens defining objects or variables, and functions to operate on these variables. The C language uses a declaration
to associate a type to a name. A type may be a simple type or a complex
type.
A simple type is numerical, and may be integer or real.
Integer Types
An integer type is one of:
char
short
int
long
1 byte
2 bytes
2 or 4 bytes
4 bytes
An object of a given type will occupy the corresponding size in memory, and will be able to hold a different range of values. Note that these
sizes are not defined exactly like that in the ANSI standard, but the
shown values are those commonly used with microprocessors. Note
that short may be written short int and that long may be written long
int.
The type int is equivalent either to type short or to type long depending
on the target processor capabilities. For most of the existing microprocessors, int is equivalent to short because their internal registers are at
most 16 bits wide. The type int is an important one because it is used as
a reference type for the expressions and for the function arguments. It is
a source of problems when adapting a program to another target processor because its size may also change.
An integer type may be prefixed by the keyword signed or unsigned to
give a more accurate definition of the range of possible values. The
keyword signed means that the values hold by the variable may be positive or negative. For instance, a signed char will hold values from -128
to +127 if numbers are represented using the two’s complement convention. This convention is used by all the common microprocessors.
The keyword unsigned means that the value held by the variable is positive only. An unsigned char will hold values from 0 to 255.
© Copyright 2001 by COSMIC Software
C Language Overview 2-5
2
Declarations
Note that these attributes do not change the object size in memory, but
alter the way the object is handled. For instance, comparing an unsigned
integer less than zero is meaningless, and will probably lead in a compiler error message. Other effects of these attributes will be discussed
with the expressions and conversions.
If these keywords are not used in an integer declaration, the object has a
default attribute depending on its type.
short
int
long
are signed by default
char
is implementation dependant
A plain char object may be either signed or unsigned, depending on
which attribute is simpler to implement. UNIX style compilers have
historically used chars signed by default, but this behaviour may not be
efficient on small microprocessors. If the operation of sign-extending a
char to an int costs more machine instructions than clearing the extra
bits, the default attribute for a plain char type will be unsigned. Otherwise, it will be signed. Note that it is possible to modify the default
behaviour of the COSMIC compilers by using a specific option on the
parser (-u).
There is another way to define an integer variable, by defining the range
of possible values. An enumeration is an integer type defined by a list
of identifiers. The compiler assigns an integer value to each of these
identifiers, or uses a specific value declared with the identifier.
Each of these identifiers becomes a new constant for the compiler, and
by examining the smallest and the largest values of the enumeration, the
compiler will choose the smallest integer type large enough to hold all
the values. The enumeration is a convenient way to define a set of codes
and at the same time the most efficient variable type to hold all of these
codes.
2-6 C Language Overview
© Copyright 2001 by COSMIC Software
Declarations
Real Types
A real type is one of
float
double
long double
4 bytes
8 bytes
more than 8 bytes
The type long double is generally used to describe internal types of
arithmetic coprocessors, handling reals on 9 or 10 bytes, in order to
avoid any loss of precision when storing an intermediate result in memory. The physical coding of a real number is not fixed by the ANSI
standard, but most compilers use the IEEE754 encoding mechanism. It
is probably not the most efficient for a small microprocessor, but it is a
convenient way to unify the encoding of reals over the various compilers and processors. With the IEEE754 coding, a float variable holds real
numbers with 7 digits precision and an exponent range of 10 to the
power + 38. A double variable holds real numbers with 14 digits precision and an exponent range of 10 to the power + 308.
For some small target processors, only the type float is supported by the
compiler. In that case, all the types are allowed in the program syntax,
but they are all mapped to the type float internally. This mechanism is
also available as an option for larger targets if the application does not
require a very accurate precision. It reduces the memory usage and the
time needed to perform the operations.
Pointers
The C language allows more complex types than the simple numerical
types. The first one is the pointer which allows for simple handling of
addresses. A pointer is a variable which contains an address. In order
to know what to do with that address, a type is associated with the
pointer.
A pointer takes the type of the object pointed at by the pointer value. A
pointer in C is associated with the operator *, which is used to declare a
pointer and to designate the object pointed at by the pointer. A pointer
allows access to any location of the processor memory without any control other than any hardware control mechanism. It means that a pointer
is as convenient as it is dangerous. In general, the C language does not
verify anything at execution time, producing good efficiency, but with
© Copyright 2001 by COSMIC Software
C Language Overview 2-7
2
Declarations
the same security as assembler, meaning none. All values are possible,
but the C language reserves the value zero to designate an invalid
pointer, although nothing stops a program accessing memory at address
zero with a pointer. The size of a pointer in memory depends on the target processor. Most of the small microprocessors address 64K of memory and use 16 bit addresses. A pointer is then equivalent to an
unsigned short. Some processors allow several memory spaces with
various addressing mechanisms. To allow the best efficiency, the compiler supports three different pointer types by defining a size attribute
along with the pointer:
a tiny pointer is a one byte address (unsigned char)
a near pointer is a two byte address (unsigned short)
a far pointer is a four byte address (unsigned long)
The compiler allows the user to select a memory model to choose a
default size when no attribute is explicitly specified. In most of the
cases, the near pointer will be used by default. When the addressing
space is not large enough to hold the full application, a possible solution
is to use a bank switched mechanism to enlarge the physical memory
space, while keeping the same microprocessor and its logical memory
addressing capabilities. In that case, far pointers are used to hold a two
component address, consisting of a bank number and a logical address.
Bank switching is mainly used for functions and sometimes allowed on
data accesses depending on the target processor.
Arrays
The next complex type is common to several languages. An array may
be defined as a collection of several objects of the same type. All these
objects are allocated contiguously in memory and they may be accessed
with an index, designating the rank of an object within an array.
An array is defined with the type of its elements and a dimension. An
index always starts at zero in C. An index is applied to an array with the
[] operators. No control is done to check that the index is in the proper
range. It is possible to define an array of all the existing data types of C.
An array in C has only one dimension. Multidimensional arrays are
possible by creating arrays of arrays.
2-8 C Language Overview
© Copyright 2001 by COSMIC Software
Declarations
Structures
A structure may be defined as a collection of several objects of different types. No index can be used as objects may have a different size.
Each object will be accessed individually by its name. Such a structure
member will be called a field. A structure is a convenient way of grouping together objects related to a specific feature. All the members are
allocated in memory in the order defined by the structure and contiguously. Note that some processors have addressing restrictions and need
sometimes to align data in memory on even boundaries for instance.
The C compiler will respect these restrictions and will create holes if
necessary to keep alignment. This means that a structure may not be
allocated exactly in the same way depending on the target processor.
Most small microprocessors do not require any alignment. The size of a
structure in memory will be the sum of the size of each of its fields, plus
if necessary the size of the padding holes. A structure may contain special fields called bitfields defining objects with a size smaller than a
byte. Such objects are defined with a bit size.
Unions
A union is a variant of a structure. In a structure, all the members are
individual objects allocated contiguously. In a union, all the members
refer to the same object, and are allocated at the same address. All the
members of a union are equivalent, and the size of a union will be the
size of its largest member. A union is a convenient way to save memory
space when a location may be used with different formats depending on
the context.
Enumerations
An enumeration is an integer object defined by the list of its possible
values. Each element is an integer constant and the compiler will
choose the smallest integer type to implement such an object.
Functions
The C language defines a function as an object. This allows the same
syntax to be used for a function declaration, and also allows a pointer to
point at a function. A function is a piece of code performing a transformation. It receives information by its arguments, it transforms these
input and the global information, and then produces a result, either by
returning a value, or by updating some global variables. To help the
© Copyright 2001 by COSMIC Software
C Language Overview 2-9
2
Declarations
function in its work, it is allowed to have local variables which are
accessed only by that function and exist only when the function is
active. Arguments and local variables will generally be allocated on the
stack to allow recursivity. For some very small microprocessors, the
stack is not easily addressable and the compiler will not use the processor stack. It will either simulate a stack in memory if the application
needs to use recursivity, or will allocate these variables once for ever at
link time, reducing the entry/exit time and code load of each function,
but stopping any usage of recursivity.
When a function call is executed the arguments are copied onto the
stack before the function is called. After the call, the return value is copied to its destination and the arguments are removed from the stack. An
argument or a return value may be any of the C data objects. Objects are
copied, with the exception of arrays, whose address is copied and not
the full content. So passing an array as argument will only move a
pointer, but passing a structure as argument will copy the full content of
the structure on to the stack, whatever its size.
A C program may be seen as a collection of objects, each of these
objects being a variable or a function. An application has to start with a
specific function. The C environment defines the function called main
as the first one to be called when an application is starting. This is only
a convention and this may be changed by modifying the compiler environment.
2-10 C Language Overview
© Copyright 2001 by COSMIC Software
CHAPTER
3
Declarations
An object declaration in C follows the global format:
<class> <type> <name> <initialization> ;
This global format is shared by all the C objects including functions.
Each field differs depending on the object type.
<class> is the storage class, and gives information about how the
object is allocated and where it is known and then accessible.
<type> gives the basic type of the object and is generally completed by
the name information.
<name> gives a name to the object and is generally an identifier. It
may be followed or preceded by additional information to declare complex objects.
<initialization> gives an initial value to the object. Depending on how
the object is allocated, it may be static information to be built by the
linker, or it may be some executable code to set up the variable when it
is created. This field is optional and may be omitted.
Each declaration is terminated by the semicolon character ;. To be
more convenient, a declaration may contain several occurrences of the
© Copyright 2001 by COSMIC Software
Declarations 3-1
3
Integers
pair <name> <initialization>, separated by commas. Each variable
shares the same storage class and the same basic type.
Integers
An integer variable is declared using the following basic types:
char
short or short int
int
long or long int
These basic types may be prefixed by the keyword signed or unsigned.
The type unsigned int may be shortened by writing only unsigned.
Types short, int and long are signed by default. Type char is defaulted
to either signed char or unsigned char depending on the target processor. For most of the small microprocessors, a plain char is defaulted to
unsigned char.
A real variable is declared using the following basic types:
float
double
long double
In most of the cases, type long double is equivalent to type double. For
small microprocessors, all real types are mapped to type float.
For these numerical variables, an initialization is written by an equal
sign = followed by a constant. Here are some examples:
char c;
short val = 1;
int a, b;
unsigned long l1 = 0, l2 = 3;
Pointers
A pointer is declared with two parameters. The first one indicates that
the variable is a pointer, and the second is the type of the pointed object.
In order to match the global syntax, the <type> field is used to declare
3-2 Declarations
© Copyright 2001 by COSMIC Software
Arrays
the type of the pointed object. The fact that the variable is a pointer will
be indicated by prefixing the variable name with the character *.
Beware that declaring a pointer does not allocate memory for the
pointed object, but only for the pointer itself. The initialization field is
written as for a numerical variable, but the constant is an address constant and not a numerical constant, except for the numerical value zero
which is conventionally representing the NULL pointer. Here are some
examples:
char *pc;
/* pointer to a char */
int *pv = &a; /* initialized to address of a */
short *ps = 0; /* initialized to NULL pointer */
A pointer can be declared with the special type void. Such a pointer is
handled by the compiler as a plain pointer regarding the assignement
operations, but the object pointed at by the pointer cannot be accessed
directly. Such a syntax is interesting when a pointer has to share different types.
Arrays
An array is declared with three parameters. The first one indicates that
the variable is an array, the second indicates how many elements are in
the array and the third is the type of one element. In order to match the
global syntax, the <type> field is used to declare the type of one element. The fact that the variable is an array and its dimension will be
indicated by adding the dimension written between square brackets
[10] after the name. The dimension is an integer constant which may
be omitted in some cases. An array initialization will be written by an
equal sign = followed by a list of values placed between curly braces,
and separated by commas. Here are some examples:
char tab[10];
/* array of 10 chars */
int values[3] = {1, 2, 3};
An initialization list may be smaller than the dimension, but never
larger. If the list is smaller than the specified dimension, the missing
elements are filled with zeroes. If an initialization list is specified, the
dimension may be omitted. In that case, the compiler gives the array the
same length than the specified list:
© Copyright 2001 by COSMIC Software
Declarations 3-3
3
Arrays
int values[]={1, 2, 3};/* array of 3 elements */
An array of char elements can be initialized with a text string. written
as a sequence of characters enclose by double quotes characters:
char string[10] = “hello”;
The missing characters are filled with zeroes. Because a text string is
conventionally ended by a NULL character, the following declaration:
char string[] = “hello”;
defines an array of 6 characters, 5 for the word hello itself, plus one
for the ending NULL which will be appended by the compiler. Note
that if you write the following declaration:
char string[5] = “hello”;
the compiler will declare an array of 5 characters, and will not complain, or add any NULL character at the end. Any smaller dimension
will cause an error.
All these declarations may be applied recursively to themselves, thus
declaring pointers to pointers, array of arrays, arrays of pointers, pointers to arrays, and so on. To declare an array of 10 pointers to chars, we
write:
char *ptab[10];
But if we need to declare a pointer to an array of 10 chars, we should
write:
char *tabp[10];
Unfortunately, this is the same declaration for two distinct objects. The
mechanism as described above is not enough to allow all the possible
declarations without ambiguity. The C syntax for declaration uses priorities to avoid ambiguities, and parenthesis to modify the order of priorities. The array indicators [] have a greater priority than the pointer
indicator *. Using this priority, the above example will always declare
an array of 10 pointers.
3-4 Declarations
© Copyright 2001 by COSMIC Software
Modifiers
To declare a pointer to an array, parentheses have to be used to apply
first the pointer indicator to the name:
char (*tabp)[10];
Modifiers
A declaration may be completed by using a modifier. A modifier is
either of the keywords const and volatile or any of the space modifiers
accepted by the compiler. A space modifier is written with an at sign @
followed by an identifier. The compiler accepts some predefined space
modifiers, available for all the target processors, plus several target specific space modifiers, available only for some target processors. The
COSMIC compiler provides three basic space modifiers for data
objects: @tiny, @near and @far. @tiny designates a memory space
for which a one byte address is needed. @near designates a memory
space for which a two byte address is needed. @far designates a memory space for which a four byte address is needed. The compilers are
provided with one or several different memory models implementing
various default behaviours, so if none of these space modifiers is specified, the selected memory model will enforce the proper one.
The const modifier means that the object to which it is applied is constant. The compiler will reject any attempt to modify directly its value
by an assignment. A cross compiler goes further and may decide to
locate such a constant variable in the code area, which is normally written in a PROM. A const object can be initialized only in its declaration.
The volatile modifier means that the value of the object to which it is
applied may change alone, meaning without an explicit action of the
program flow. This is the case with an input port, or with a variable
updated by an interrupt function. The effect of such a directive is to stop
the compiler optimizing the accesses to such a variable. In the following example:
char PORTA;
PORTA = 1; /*create a short pulse on bit 0 */
PORTA = 0;
© Copyright 2001 by COSMIC Software
Declarations 3-5
3
Modifiers
The first assigment will be optimized out by the compiler as the PORTA
variable is supposed to be a plain memory location which is not used
between the two assigments. If such a variable is matching a physical
output port, it must be declared as a volatile object:
volatile char PORTA;
A modifier applies to the current element being defined. When applied
to a pointer, a modifier may affect the pointer itself, or the pointed
object, depending on its position in the declaration. If the modifier is
placed before the * character, it affects the pointed object. If the modifier is place after the * character, it affects the pointer.
const char *pc; /* pointer to a constant char */
pc = qc;
/* OK */
*pc = 0;
/* ERROR */
The first assignment modifies the pointer itself and is allowed. The second assignment tries to modify the pointed const object, and is then not
allowed.
char * const pc;/* constant pointer to a char */
pc = qc;
/* ERROR */
*pc = 0;
/* OK */
The first assignment tries to modify a const pointer and is then not
allowed. The second assignment modifies the pointed object and is
allowed.
const char * const pc;
The pc object is declared as a const pointer to a const object, so no
assignment to the pointer itself or to the pointed object will be allowed.
Such an object probably needs to be initialized within the declaration to
have any useful meaning for the program.
The compiler also implements special modifiers whose usage depends
on the target processor:
The @packed modifier is used when the target processor requests an
even alignment on word or larger objects, in order to stop the alignment
3-6 Declarations
© Copyright 2001 by COSMIC Software
Structures
for the specified object, assuming that unaligned accesses are still supported by the processor. It can also be used on a function definition to
stop alignment on local variables thus shortening the local size.
The @nostack modifier is used to allocate a function stack frame
(locals and arguments) in static memory instead of using the physical
stack. This feature is interesting for small processors where the physical
stack is not easily accessible. In such a case, the memory used for those
local areas is allocated and optimized by the linker in order to consume
the smallest amount of memory as possible.
Structures
A structure is declared by declaring all of its fields. They are grouped
between curly braces and are preceded by the keyword struct. A structure, as a type, may be named to be reused later. This feature avoids
repeating the full content of the structure. Such a name is called a tag
name and is placed between the keyword struct and the opening curly
brace.
struct {
int a;
char b;
long c;
}
/*
/*
/*
/*
unnamed structure */
first field is an int */
second is a char */
third is a long */
This set will fill the <type> field of the global declaration syntax. There
is no modification of the <name> field.
struct node {
/* named structure */
int value;
struct node *left;
struct node *right;
} n1, n2;
declares two structures of type node. A reference to a not yet completed structure is possible as long as it is used to define pointers. Once
a tag name has been associated to a structure, any further declaration
does not need to repeat the structure content:
struct node n3, n4;
© Copyright 2001 by COSMIC Software
Declarations 3-7
3
Structures
A structure initialization will be written by an equal sign = followed by
a list of values placed between curly braces, and separated by commas.
Each field is initialized by the corresponding value, converted to the
corresponding type.
struct {
char a;
int b;
long c;
} st = {1, 2, 3};
field a is initialized with value 1
field b is initialized with value 2
field c is initialized with value 3
If the initialization list contains less values than structure fields, the
missing fields are initialized to zero.
A bitfield is declared by suffixing the field name with a colon followed
by the number of bits used by the bitfield. A bitfield can be declared in
any integer type. The reference type used is considered to be the allocation unit, meaning that an integer of that type is open and filled until
there is no more space available. Any bitfield overlapping an allocation
unit boundary is allocated in a new integer.
struct {
char a:4;
char b:3;
char c:2;
}
This structure defines 3 bitfields based on a char allocation unit. A first
char is open and bitfields a and b are allocated inside. There is not
enough space available to allocated bitfield c. A new char is then open
and bitfield c is allocated inside. This structure is thus occupying 2
bytes in memory.
A bitfield without name will be used to skip the corresponding amount
of bits in the allocation unit. A zero bitfield width is forcing the opening
of a new allocation unit.
3-8 Declarations
© Copyright 2001 by COSMIC Software
Unions
The ANSI standard does not define in which order bitfields are filled.
The COSMIC compiler fills bitfields from the less significant bit to the
most significant bit by default. This ordering can be reversed by using
the +rev compiler option. The ANSI standard also limits the allocation
unit type to int or unsigned int. The COSMIC compiler allows all the
integer types as an extension.
Unions
A union is declared like a structure, but the keyword union replaces the
keyword struct. A union may be initialized, but as all the fields are
located at the same address, it is seen as a single variable for the initialization, which will be done using the first field of the union.
union {
char a;
int b;
long c;
} u = 1;
field a is initialized with the value 1 on a char. It is then more convenient to define the largest field first to be sure to initialize all the union
byte.
A tag name may be specified on a union. Tag names are in the same
name space, so a union tag name cannot be the same as a structure tag
name.
Enumerations
An enumeration is declared with a syntax close to the structure/union
declaration. The list of fields is replaced by a list of identifiers. The keyword enum replaces the keyword struct or union. A tag name may also
be specified, sharing the same name space than the structure and union
tag names. Each identifier will be assigned a constant value by the compiler. An enumeration variable will be allocated as a char, a short or a
long depending on the range of all the idenfiers. This means that the
compiler always needs to know all the enum members before it allocates an enum variable. If the -pne option has been set, an enum variable is always allocated as an int and then there is no need to know the
enum members before to allocate the variable.
© Copyright 2001 by COSMIC Software
Declarations 3-9
3
Functions
Note that the COSMIC compiler allows long enumerations as an extension to the ANSI standard.
enum {blue, white, red} flag;
The names blue, white and red are three new constants. Values are
assigned to the names starting at zero, and incrementing by one for each
new name. So blue is zero, white is one and red is two. The variable
flag will be declared as a plain char as a byte is large enough to hold
values from zero to two.
It is possible to define directly the value of a name by adding the character = followed by a value to the name.
enum {blue, white = 10, red} flag;
blue is still zero, white is now ten, and red is eleven, as the internal
counter is incremented from the previous value. These names become
new constants and may be used in any expression, even if they are not
assigned to an enumeration variable. In the same way, an enumeration
variable may be assigned with any kind of integer expression. An enumeration may be initialized as an integer variable.
Functions
A function is declared with three parameters. The first one indicates
that the object is a function, the second gives the argument list, and the
third is the type of the returned value. In order to match the global syntax, the <type> field is used to declare the type of the returned value.
The fact that the object is a function will be indicated by adding the
argument list written between parenthesis () after the name. The
<type> field is used to declare the type of the returned value. If the
function has nothing to return, the <type> field is replaced by the keyword void meaning that nothing is returned from the function. This
syntax will also allow the compiler to detect any invalid usage of the
function, if it is used in an expression or an assignment.
The argument list may be specified in two different ways. The first one
which is the oldest is known as the Kernigan and Ritchie (K&R) syntax. The second has been introduced by the standardization and is
known as the prototyped syntax.
3-10 Declarations
© Copyright 2001 by COSMIC Software
Functions
The K&R syntax specifies the argument list as a list of identifiers separated by commas between the parenthesis. This list is immediately followed by the full declaration of the identifiers specified in the list. An
undefined identifier will be defaulted to an int argument.
int max(a, b)
int a;
int b;
The prototyped syntax specifies the argument list as a list of individual
declarations separated by commas between the parenthesis.
int max(int a, int b)
The prototyped syntax offers extra features compared with the K&R
syntax. When a function is called with parameters passing, the compiler
will check that the number of arguments passed matches the number in
the declaration, and that each argument is passed with the expected
type. This means that the compiler will try to convert the actual argument into the expected type. If it is not possible, the compiler will produce an error message.
None of these checks are done when the function is declared with the
K&R syntax. If a function has no arguments, there should be no difference between the two syntaxes. To force the compiler to check that no
argument is passed, the keyword void will replace the argument list.
int func(void)
The K&R syntax allows a variable number of arguments to be passed.
In order to keep this feature with the prototyped syntax, a special punctuator has been introduced to tell the compiler that other arguments may
be specified, with unknown types. This is written by adding the
sequence ... as last argument.
int max(int a, int b, ...)
The compiler will check that there are at least two arguments, which
will be converted to int if they have a compatible type. It will not complain if there are more than two arguments, and they will be passed
without explicit conversion.
© Copyright 2001 by COSMIC Software
Declarations 3-11
3
Storage Class
The prototyped syntax is preferred as it allows for more verification and
thus more safety. It is possible to force the COSMIC compiler to check
that a function has been properly prototyped by using the -pp or +strict
options.
The initialization of a function is the body of the function, meaning the
list of statements describing the function behaviour. These statements
are grouped in a block, and placed between curly braces {}. There is no
equal sign as for the other initializations, and the ending semicolon is
not needed.
int max(int a, int b)
{
/* body of the function */
}
A function may have local variables which will be declared at the
beginning of the function block. In order to allow the recursivity, such
local variables and the arguments have to be located in an area allocated
dynamically. For most of the target microprocessors, this area is the
stack. This means that these local variables are not allocated in the same
way as the other variables.
Storage Class
It is possible in C to have a partial control over the way variables are
allocated. This is done with the <class> field defining the storage
class. This information will also control the scope of the variable,
meaning the locations in the program where this variable is known and
then accessible. The storage class is one of the following keyword:
extern
static
auto
register
extern means that the object is defined somewhere else. It should not
be initialized, although such a practice is possible. The extern keyword
is merely ignored in that case. The definition may be incomplete. An
array may be defined with an unknown dimension, by just writing the
bracket pair without dimension inside.
3-12 Declarations
© Copyright 2001 by COSMIC Software
Storage Class
extern int tab[];
A function may be declared with only the type of its arguments and of
its return value.
extern int max(int, int);
Note that for a function, the extern class is implied if no function body
is specified. Note also that if a complete declaration is written, useless
information (array dimension or argument names) is merely ignored.
The COSMIC compiler may in fact use a dimension array to optimize
the code needed to compute the address of an array element, so it may
be useful to keep the dimension even on an extern array declaration.
static means that the object is not accessible by all parts of the program.
If the object is declared outside a function, it is accessible only by the
functions of the same file. It has a file scope. If an object with the same
name is declared in another file, they will not describe the same memory location, and the compiler will not complain. If the object is
declared inside a function, it is accessible only by that function, just like
a local variable, but it is not allocated on the stack, meaning that the
variable keeps its value over the function calls. It has function scope.
auto means that the object is allocated dynamically, and this implies
that it is a local variable. This class cannot be applied to an object
declared outside a function. This is also the default class for an object
declared inside a function, so this keyword is most of the time omitted.
register means that the object is allocated if possible in a physical register of the processor. This class cannot be applied to an object declared
outside a function. If the request cannot be satisfied, the class is ignored
and the variable is defaulted to an auto variable.
A register variable is more efficient, but generally only a few variables
can be mapped in registers. For most of the small microprocessors, all
the internal registers are used by the compiler, and the register class is
always ignored. The register class may be applied to an argument. In
that case, the argument is still passed on the stack, but is copied into a
register if possible when the function starts. A register object may be
used as a plain object, except that it is not possible to take its address,
even if it has been mapped in memory.
© Copyright 2001 by COSMIC Software
Declarations 3-13
3
Typedef
Typedef
The C language allows the definition of a new type as a combination of
existing types. The global declaration syntax is used with the special
keyword typedef used in place of the <class> field. The <name> field
describes a new type equivalent to the type described by the declaration.
typedef int *PINT;
declares the identifier PINT to be a new type equivalent to type int *. It
may be used now as a basic type of the C language.
PINT ptab[10];
declares the variable ptab as an array of 10 elements, each element is
an object of type PINT, meaning a pointer to an int. This declaration is
equivalent to:
int *ptab[10];
The typedef feature is a convenient way to redefine all the types used by
an application in a coherent and more verbose way than using only the
basic C types.
Variable Scope
Once declared, an object can be hidden locally by another declaration.
A global variable (declared outside a function) has an application
scope, meaning that it can be accessed by any part of the application,
assuming it is properly declared in each file. When using the static keyword on its declaration, a global variable has a file scope, meaning that
it can be accessed only inside the file where it is declared. In any other
file, the same declaration will not access the same object. It means that
a static declaration at file level is hiding a global level variable declared
with the same name in another file.
A local variable (declared inside a function) has a function scope,
meaning that it can be accessed only inside that function, even if
declared with the static keyword. A function level object will hide any
object declared at file or global level with the same name.
3-14 Declarations
© Copyright 2001 by COSMIC Software
Absolute Addressing
Absolute Addressing
The COSMIC compiler allows an object to be declared along with its
address when it is known at compile time (I/O registers). The address is
specified just after the declaration and is replacing the initialization
part. It is prefixed by the @ character:
volatile char PORTB @0x10;
Such a declaration is in fact equivalent to an extern declaration so it is
not possible to initialize such a variable. This can also be applied to a
function declaration if such a function already exists at a known address
in the application. This cannot be used to locate a function defined in
the application at a predefined address.
© Copyright 2001 by COSMIC Software
Declarations 3-15
CHAPTER
4
Expressions
An expression is a set of variables, constants and operators which are
combined together to provide a result. C expressions are more extensive
than in other languages because the notion of a result is included in
operations such as assignments and function calls.
Some special operations such as array indexing and pointer arithmetic
can be the result of expressions. This makes C expressions a very powerful feature of the language.
© Copyright 2001 by COSMIC Software
Expressions 4-1
4
Variables
Variables
Variables are simply expressed by their name, which is a C identifier
which should have been previously defined, otherwise the compiler
does not know the type of that variable and does not know how to
access it. The only exception is that you can call a function which has
not been declared. The compiler will define that unknown function as
an external function returning an int. If the function is declared later in
the same file and the actual return type does not match, or if strict
checking options are used (-pp, +strict), the compiler will complain.
Constants
Constants can be expressed in several formats. Integer constants may be
expressed in decimal, octal, hexadecimal or character format.
A decimal number is written with numerical characters in the set
0123456789 and does not begin with the character 0. For example:
125
2A58
012
is a valid decimal constant
is NOT a valid decimal constant
is NOT a decimal constant
An octal number is written with numerical characters in the set
01234567 and begins with the character 0.
For example:
0177
0A00
377
is a valid octal constant
is NOT a valid octal constant
is NOT an octal constant
An hexadecimal number begins with the characters 0x or 0X followed
by characters in the set 0123456789ABCDEFabcdef. Letters a to f or
A to F represent elements from 10 to 15. Lower case and upper case letters have the same meaning in a hexadecimal constant.
0x125
0Xf0A
0xZZZ
xABC
4-2 Expressions
is a valid hexadecimal constant
is a valid hexadecimal constant
is NOT a valid hexadecimal constant
is NOT a valid hexadecimal constant
© Copyright 2001 by COSMIC Software
Constants
A character constant is written as a sequence of printable characters
enclosed by single quotes. Although this definition allows several characters to be entered, a character constant is usually limited to one character. The resulting value of such a constant is equal to the actual code
of that character (ASCII in most of the cases, including COSMIC compilers, but may be EBCDIC or any other depending on the target environment). If more than one character is specified in a character
constant, the result value is built as if the sequence was a number
expressed in base 256, where each character is a digit whose value is
between 0 and 255.
For example:
‘A’ is equivalent to 0x41 or 65 (ASCII)
‘AB’ is equivalent to 0x4142 (not very useful)
Non printable characters may be entered with a special sequence called
escape sequence and beginning with the backslash \ character. Such a
character may be expressed by its numerical code written in octal or in
hexadecimal. There is no way to express it in decimal.
An octal value will be entered by up to three octal digits.
A hexadecimal value will be entered by the character x followed by
hexadecimal digits (upper or lower case, including the x character). For
example:
\11 is equivalent to 011
\xFF is equivalent to 0xFF
A few control characters can be entered without using their numerical
value. It is useful not only because you do not need to know their coding, but also because this gives you the ability to write portable code, as
the result will depend on the target environment. A backspace character
may have a different code if the code used is ASCII or EBCDIC.
Note that hexadecimal character constants are not limited to three digits
like the octal constants. This may be a problem when used in string constants as shown below.
© Copyright 2001 by COSMIC Software
Expressions 4-3
4
Constants
The control characters mapped (with their ASCII code) are the following:
\a
\b
\t
\n
\v
\f
\r
BELL
BACKSPACE
TAB
LINE FEED
VERTICAL TAB
FORM FEED
CARRIAGE RETURN
(0x07)
(0x08)
(0x09)
(0x0A)
(0x0B)
(0x0C)
(0x0D)
The C syntax allows a character constant to be immediatly preceded by
the L letter:
L’A’
Such a constant is a wide character whose coding may exceed the
capability of a char object. This is mainly used for Japanese character
sets. The COSMIC compiler accepts this syntax but ignores the prefix
and uses the same encoding as plain character constants.
All these integer constants have at least the type int. If the entered constant exceeds the resolution of an int (which is signed), the final type
will be the smallest one able to represent the constant. Note that for a
decimal constant, if it cannot be expressed by an int, but by an
unsigned int, the result will be of type long and not unsigned int, as a
decimal constant usually expresses signed values.
It is possible to modify the default type by using suffix characters. Suffix l or L forces the constant type to be long, and suffix u or U forces
the constant type to be unsigned int or unsigned long. Both suffixes
may be specified, but each suffix can be specified once only. The suffix
has to follow the constant immediately without any white space in
between. These suffixes allow portable code to be written avoiding different behaviour depending on the int type resolution.
For example, assuming that type int is 16 bits:
125
125U
4-4 Expressions
is type int
is type unsigned int
© Copyright 2001 by COSMIC Software
Strings
125L
125UL
0xffff
65535
is type long
is type unsigned long
is type unsigned int (int if 32 bits wide)
is type long (int if 32 bits wide)
Floating point constants are entered with commonly used exponential
notation. A floating point begins by a numerical digit, or the . character
if the integer part is omitted. The exponent is written by the letter e or E
followed by an exponent, which may be signed. The exponent may be
omitted.
For example:
1.5
1.23e3
.2e-3
3E+2
E-1
is a valid floating point constant
is a valid floating point constant
is a valid floating point constant
is a valid floating point constant
is NOT a valid floating point constant
A floating point constant has the type double by default. The suffix
character f or F forces the compiler to define the constant with the type
float. The suffix l or L forces the constant to the type long double.
Note that types double and long double are the same for most of the targets.
For example:
1.5
1.5F
1.5L
is type double
is type float
is type long double (mapped to double)
Strings
The C language also defines a string constant to ease character strings
handling. Such a constant is written by a sequence of printable or non
printable characters enclosed by double quote characters. Non printable
characters are expressed the same way as individual character constants. Such a constant is built in memory, and its value is in fact the
memory address of that location. A NULL character is appended to the
sequence of characters to conventionally end the text string.
© Copyright 2001 by COSMIC Software
Expressions 4-5
4
Strings
For example:
“hello”
is a pointer of type char * to a memory area containing the characters
h, e, l, l, o and \0
Non printable characters may be entered as octal or hexadecimal constants:
“\15\12”
or
“\x0d\0xa”
define a string containing characters carriage return and line feed.
These constants may interfere with the next character of the same
string:
“\3123”
defines a string containing the two characters ‘\312’ and ‘3’, and not
the characters ‘\3’ , ‘1’, ‘2’, ‘3’. Such a string can be entered by
filling the octal constant up to three digits:
“\003123”
This trick cannot be used with hexadecimal constants because they have
no length limit. In case of any conflict, the only solution is to use the
octal syntax.
In order to allow long text strings to be specified, a string constant may
be split in several contiguous substrings, with white spaces or line feeds
in between. All the substrings will be conatenated by the compiler to
obtain one long string which will be created in memory.
For example:
“hello “
“world\n” is equivalent to “hello world\n”
The C syntax allows a string constant to be immediatly preceded by the
L letter:
L”hello”
4-6 Expressions
© Copyright 2001 by COSMIC Software
Sizeof
Such a constant is a wide string and each character is encoded as a
wide character. This is mainly used for Japanese character sets. The
COSMIC compiler accepts this syntax but ignores the prefix and uses
the same encoding than plain character constants.
Sizeof
The C language also allows a special constant which is written:
sizeof expression
or
sizeof (type)
In both syntaxes, the result is an integer constant equal to the size in
bytes of the specified object. If an expression is specified, the compiler
evaluates it without producing any code. It just applies the conversion
rules as explained below to get the resulting type of the expression. And
then, as if a type was directly specified, the compiler replaces the full
construct by a constant equal to the amount of bytes needed to store in
memory an object of that type.
Operators
Operators are symbolic sequences which perform an operation from
one or two operands (three for one operator), and provide a result. The
C evaluation rules describe how the operands are prepared depending
on their type, and what is the type of the result. First of all, operands are
promoted to the int type. This means that if the type of any operand is
smaller than the type int, it is converted to type int. If the type of an
operand is larger or equal to type int, it is left unchanged. Then, if the
operator requires two operands, and if those two operands do not have
the same type, the operand with the smallest type is converted into the
type of the largest. Then the operation is performed, and the result type
is the same as the largest operand.
Converting an integer type into a larger one will keep the sign of the
original operand if the smallest type is signed. Otherwise, the original
value is zero extended to reach the size of the largest type. Converting
any type to a larger floating point type will keep the sign if the original
© Copyright 2001 by COSMIC Software
Expressions 4-7
4
Operators
type is a floating point or a signed integer type, or will produce a positive value otherwise.
These rules imply that the result of any expression is at least an int
value. If the application handles variables with types smaller than int,
the code needed to evaluate expressions with these rules will contain a
lot of implicit conversions which may appear to be useless. Hopefully,
the compiler will shorten the code each time it detects that the final
result is the same than if the full rules were applied.
Operators may have different behaviour depending on the way they are
used and their operand types. This is a difficulty when learning C.
Operands may be subexpressions. Any conflict between operators is
resolved by priority rules, or by enclosing subexpressions in parentheses. For operators with the same priority, the grouping order allows
grouping from left to right, or right to left. For most of the operators, the
evaluation order of the operands for that operator is undefined, meaning
that you cannot assume that left operand will be evaluated before the
right operand. In most of the cases, it does not matter, but there are a
few situations where the evaluation order is importance. In such a case,
the expression should be split into several independent expressions.
Arithmetic operators
a +
+a
a -a
a *
a /
a %
b
b
b
b
b
returns the addition of operands a and b
returns the (promoted) value of operand a
returns the difference of operands a and b
returns the negated value of operand a
returns the product of operands a and b
returns the division of operand a by operand b
returns the remainder of the division of operand a
by operand b
All these operators apply to integer types. For these types, the division
operator gives as result the integer quotient of the operands division.
All these operators except the remainder % apply to floating point types.
Operators + and - also apply to pointers, but special rules are applied.
The possible constructs are:
p + i
4-8 Expressions
add an integer to a pointer (i + p is allowed)
© Copyright 2001 by COSMIC Software
Operators
p - i
p - q
subtract an integer from a pointer
subtract a pointer from another pointer
When adding or subtracting an integer to a pointer, the value of the promoted integer is multiplied by the size in bytes of the object pointed to
by the pointer, before the addition or subtraction takes place. So adding
one to a pointer modifies it so that it points to the next element rather
than just the next byte.
For example:
short *p;
p + 1
actually adds the value 2 to the pointer value,
because a short has a size of 2 bytes in memory.
The result of such an operation is a pointer with the same type as the
pointer operand.
Subtracting a pointer from another first requires that both pointers have
exactly the same type. Then, after having computed the difference
between the two operands, the result is divided by the size in bytes of
the related object. The result is then the number of elements which separate the two pointers. The result is always of type int.
Bitwise operators
a
a
a
a
& b
| b
^ b
<< b
a >> b
~a
returns the bitwise and of operands a and b
returns the bitwise or of operands a and b
returns the bitwise exclusive or of operands a and b
returns the value of promoted operand a left
shifted by the number of bits specified by operand b
returns the value of promoted operand a right
shifted by the number of bits specified by operand b
returns the one’s complement of promoted operand a
All these operators apply to integer types only. The right shift operator
will perform arithmetic shifts if the promoted type of its left operand is
signed (thus keeping the sign of the operand). Otherwise, it will perform logical shifts.
© Copyright 2001 by COSMIC Software
Expressions 4-9
4
Operators
Boolean operators
Boolean operators create or handle logical values true and false. There
is no special keyword for these values, numerical values are used
instead. False is value zero, whatever type it is. True is any value which
is not false, meaning any non zero value. This means that the result of
any expression may be used directly as a logical result. When producing a logical true, the compiler always produces the value 1.
a == b
a != b
a < b
a <= b
a > b
a >= b
a && b
a || b
!a
returns true if both operands are identical
returns true if both operands are different
returns true if operand a is strictly less than b
returns true if operand a is less or equal to b
returns true if operand a is strictly greater than b
returns true if operand a is greater or equal to b
returns true if operand a and operand b are true
returns true if operand a or operand b is true
returns true if operand a is false
Both operators && and || evaluate the left operand first. If the final
result is reached at that point, the right operand is NOT evaluated. This
is an important feature as it allows the second operand to rely on the
result of the first operand.
Boolean operators can be combined together and such a syntax is
accepted:
a < b < c
This is not behaving as if b was tested to be between a and c. The first
compare (a < b) is evaluated and produces a logical result equal to 0 or
1. This result is then compared with c. Such a syntax is correct for the
compiler which does not emit any error message, but it does not behave
as you might expect.
When comparing any object to a constant, the compiler is checking if
the constant does not exceed the maximum values possible for the
object type. In such a case, the compiler is able to decide if the test is
always true or always false and then optimizes the code by suppressing
the useless parts. When the +strict option is used, the compiler outputs
an error message when such an out of range compare is detected.
4-10 Expressions
© Copyright 2001 by COSMIC Software
Operators
Assignment operators
Assignment operators modify memory or registers with the result of
an expression. A memory location can be described by either its name,
or an expression involving pointers or arrays, and is always placed on
the left side of an assignment operator. Such an expression is called
L-value. The expression placed on the right side of an assignment operator is then called R-value. Conversion rules differ from the ones used
for other operators. The R-value is evaluated using the standard rules,
and the compiler does not consider the L-value type. When the R-value
has been evaluated, its resulting type is compared with the L-value type.
If both are identical, the result is copied without alteration. If the
L-value type is smaller than the R-value type, the result is converted
into the L-value type, by truncating integers, or converting floating
point types. If the L-value type is larger than the R-value type, the result
is extended to the L-value type, by either sign extension or zero extension for integers, depending on the R-value type, or by converting floating point types. When the +strict option is used, the compiler outputs
an error message when an assignment is truncating the R-value.
Assignment operators also return a value, which is equal to the L-value
content after the assignment. Consider the result as if the L-value was
read back after the assignment. In no case should the result be considered equal to the R-value, even if it is the case in some situations, for
instance if both types are equal.
a = b
transfer b into a, and returns a
Assignments are possible between pointers and between structures.
Both operands need to be of the same type. Note that for pointers, the
type pointer to void is compatible with all the other pointer types. The
integer constant 0 is also compatible with any pointer type, and is considered to be the NULL pointer. Pointers including modifiers are compatible if the left pointer has the same modifiers of the right pointer or
more. However, this constraint is too restrictive for embedded applications, and the COSMIC compiler will simply ignore the modifiers when
checking the pointers compatibility.
The COSMIC compiler allows pointers with different sizes, using the
special modifiers @tiny, @near and @far. By default, the compiler
© Copyright 2001 by COSMIC Software
Expressions 4-11
4
Operators
will widen the size of a pointer, but will not narrow it, unless authorized
by a parser option (-np).
Because the assignment operator returns a value, the following construct is possible in C:
a = b = c;
The behaviour of this expression is to evaluate c and to transfer the
result into b, then read b and transfer it into a. Both sub-expression a
and b must be L-values. Note that this expression is not identical to
a = c and b = c , because if a and b have different types, the implicit
conversions may alter the value transferred.
The C language allows several short-cuts in assignment expressions:
a
a
a
a
a
a
a
a
a
a
+= b
-= b
*= b
/= b
%= b
&= b
|= b
^= b
<<= b
>>= b
is equivalent to a
is equivalent to a
is equivalent to a
is equivalent to a
is equivalent to a
is equivalent to a
is equivalent to a
is equivalent to a
is equivalent to a
is equivalent to a
=
=
=
=
=
=
=
=
=
=
a
a
a
a
a
a
a
a
a
a
+ b
- b
* b
/ b
% b
& b
| b
^ b
<< b
>> b
These operators behave exactly as their equivalent constructs, with the
same type limitations as the simple operators. In addition to the source
code saving, the compiler knows that the L-value will be used twice,
and uses this clue to produce the more efficient code, by avoiding computing the same expression twice.
The C language also defines two special assignment operators allowing
pre or post incrementation or decrementation. These operators are
applied to L-value expressions, and can be placed before or after to
specify if the operation has to be done before or after using the L-value.
++a
a++
4-12 Expressions
equivalent to a += 1, returns a after increment
equivalent to a += 1, returns a before increment
© Copyright 2001 by COSMIC Software
Operators
--a
a--
equivalent to a -= 1, returns a after decrement
equivalent to a -= 1 , returns a before decrement
As for the previous operators, the type limitations are the same as for
the simple + and - operators.
The resulting expression involving an assignment operator is no longer
an L-value and cannot be re-assigned directly so the following expression
++ i --
is not valid in C, because if the i name is an L-value then ++i cannot be
an L-value so the -- operator cannot be applied to it.
Those operators should not be used several times on the same variable
in the same expression:
i++ - i++
will result 1 or -1 depending on the evaluation order.
Addressing operators
The C language defines pointers, and a set of operators which can be
applied to them, or which allows them to be built.
*ptr
&var
tab[i]
returns the value pointed by the pointer ptr
returns the address of variable var
returns an array element
The * operator applied to a pointer returns the value of the object
pointed by the pointer. It cannot be applied to a pointer to a function, or
to a pointer to void. The type of the result is the type provided in the
pointer declaration.
The & operator returns the address of its operand. The operand can be a
variable, or any legal L-value. It is not possible to compute the address
of a variable declared with the register class, even if the compiler did
not allocate it into a physical register, for portability reasons. The type
of the result is a pointer to the type of the operand.
© Copyright 2001 by COSMIC Software
Expressions 4-13
4
Operators
The [] operator is the indexing operator. It is applied to an array or a
pointer, and contains an integer expression between the square brackets.
Such an expression:
tab[i]
is equivalent to
*(tab + i)
where the + operator behaves as for a pointer plus an integer. This
equivalence gives some interesting results:
tab[0]
&tab[0]
is equivalent to
is equivalent to
*tab
&*tab, or simply tab
This last equivalence shows that an array name is equivalent to a
pointer to its first element. It also explains why the indexing operator
can be applied to an array or to a pointer. The index expression is not
checked against the boundary limits of an array. If the index goes
beyond an array limit, the program will probably not behave well, but
the compiler will not give any error or warning.
When used in a sizeof construct, an array name is not equivalent to a
pointer and the sizeof result is equal to the actual array size, and not to
the size of a pointer.
Because the + operator is commutative, *(tab + i) gives the same
result as *(i + tab) , so tab[i] can be written i[tab], even if such
a construct is not very familiar to computer language users.
Function call operator
The function call is an operator in C which produces a result value
computed from a user defined piece of code, and from a list of arguments. A function is called by writing its name followed by an expression list separated by commas, enclosed by parenthesis:
function ( exp_1, exp_2, exp_3 )
Each expression is an argument evaluated and passed to the function
according to the expected type if there is a prototype for that function.
The arguments are passed from the right to the left, but this does not
mean that they are always evaluated from the right to the left, although
this is true for most compilers.
4-14 Expressions
© Copyright 2001 by COSMIC Software
Operators
Each argument follows the standard evaluation rules and will be promoted if necessary. This means that a char variable will be passed as an
int, and a float variable passed as a double if both types are supported.
For small processors, this mechanism may consume too much time and
stack space, so the COSMIC compiler allows this default mechanism to
be stopped by an option (+nowiden). In such a case, any variable is
passed in its basic type, although expressions will be passed with their
expected promoted resulting type, unless it is cast. When the function is
declared with a prototype, each argument is cast to the declared type
before being passed. Note that the widening control option is not
always available, depending on the target processor’s ability to stack
single bytes or not.
The result of such a function call is the value returned by the function
with the expected type.
The K&R syntax does not require that a function is defined before it is
called. The return type is assumed to be int. Using the ANSI prototypes
or the strict checking options avoid most of the errors created by that
kind of situation.
The COSMIC compiler implements a special function call to produce
inline assembly code interfaced with C objects. The _asm function
receives a text string as a first argument containing the assembly source
code. This code will be produced instead of calling an actual function.
Extra arguments will behave as plain function arguments, and the compiler will expect to find any returned value in the expected location
(depending on the type and on the target processor).
Conditional operator
The conditional operator is equivalent to an if ... else sequence applied
to an expression. The expression:
test_exp ? true_exp : false_exp
is evaluated by computing the test_exp expression. If it is true, meaning not zero, then the true_exp expression is evaluated and the resulting value is the final result of the conditional expression. Otherwise, the
false_exp is evaluated and the resulting value is the final result of the
conditional expression. These two expressions must have a compatible
type and the type of the final expression follows the standard rules.
© Copyright 2001 by COSMIC Software
Expressions 4-15
4
Operators
Sequence operator
The sequence operator allows a single expression to be expressed as a
list of several expressions. The expression:
exp_1 , exp_2
is evaluated by computing the first expression, and then by computing
the second one. The final result of such an expression is the result of the
second one. The result of the first one is simply trashed. It means practically that such a construct will be interesting only if the first expression
does something, such as an assignment, a function call, an increment or
decrement operation. This operator is not allowed directly in a function
call sequence, because the comma is used as an argument separator. In
such a case, the sequence operator can be used only within expression
parenthesis.
Conversion operator
The cast operator allows the result of an expression to be converted into
a different type. The final type enclosed by parenthesis, is prepended to
the expression to be converted:
( new_type ) expression
The type specified between parenthesis is called an abstract type and is
written simply as a variable declaration where the object name is omitted. So basic types are simply written with their type name:
(char) 1
convert the constant 1 (whose default type is int)
into a constant 1 with char type.
(long) var convert the variable var whatever its type is to
type long.
The first operation does not produce any code, as it is just an internal
type change in the compiler. The second one will produce actual code,
unless var already has type long. Note that a conversion between
signed and unsigned objects of the same type does not produce any
code. The cast result will behave with its new type.
4-16 Expressions
© Copyright 2001 by COSMIC Software
Operators
The cast operator can be used to allow different pointers to be accepted
as compatible:
char *pc;
int *pi;
pi = (int *)pc;
When using the +strict option, the cast operator will be used to force
the compiler to accept a truncating assignment without producing any
error message:
char c;
int i;
c = (char)i;
The cast operator may be used to override the default evaluation rules.
Assuming both variables a and b are declared as unsigned char, the
following expression:
a == ~b
may not behave exactly as expected. The promotion rules force b to be
converted to an int before it is inverted, so if b is originaly equal to
0xff, it first becomes an int equal to 0x00ff (unsigned promotion)
and the inverted result is then 0xff00. This value is compare with the
promoted value of a and if a was originaly 0x00, the compare operator
returns a false result as it is comparing 0x0000 with 0xff00. In order
to force the compiler to evaluate the complement operator on a char
type, the expression has to be written:
a == (unsigned char)~b
In such a case, the promoted result of the complement operator 0xff00
is truncated to a char and the equality operator is comparing 0x00 with
0x00 and returns a true result.
© Copyright 2001 by COSMIC Software
Expressions 4-17
4
Priorities
Priorities
All these operators may be combined together in complex expressions.
The order of evaluation depends on each operator priority. The expression:
a + b * c
will be evaluated as:
a + (b * c)
because in C (as in most of the computer languages), the multiplication
* has a higher priority than the addition +. Some operators have the
same priority, and in such a case, it is necessary to define in which order
they are evaluated. This is called grouping order and may be left to
right, or right to left. A left to right grouping is applied to the usual
arithmetic operators, meaning that:
a + b + c
is
((a + b) + c)
A right to left grouping is applied to the assignment operators, meaning
that:
a = b = c
is
(a = (b = c))
The C language defines 15 levels of priority, described here from the
highest to the lowest, with their grouping order:
4-18 Expressions
© Copyright 2001 by COSMIC Software
Priorities
1
Left to Right
post increment/decrement
array subscript
function call
structure/union member
pointer to a member
2
Right to Left
sizeof operator
pre increment/decrement
address of
content of
unary plus/minus
binary/logical not
cast
3
i + j
i - j
Left to Right
left shift
right shift
6
i * j
i / j
i % j
Left to Right
Addition
substract
5
sizeof i
++i --i
&i
*p
+i -i
~i !i
(type)i
Left to Right
multiply
divide
remainder
4
i++ i-tab[i]
func()
str.a
p->a
i << j
i >> j
Left to Right
less than
less than or equal
greater than
greater than or equal
© Copyright 2001 by COSMIC Software
i
i
i
i
< j
<= j
> j
>= j
Expressions 4-19
4
Priorities
7
Left to Right
equal to
not equal
8
i == j
i != j
Left to Right
binary and
9
i & j
Left to Right
binary exclusive or
10
Left to Right
binary or
11
i | j
Left to Right
logical and
12
i && j
Left to Right
logical or
13
i || j
Right to Left
conditional expression
14
i ? j : k
Right to Left
assignment
multiply assign
divide assign
remainder assign
plus assign
minus assign
left shift assign
right shift assign
and assign
exclusive or assign
or assign
15
i = j
i *= j
i /= j
i %= j
i += j
i -= j
i <<= j
i >>= j
i &= j
i ^= j
i |= j
Left to Right
comma
4-20 Expressions
i ^ j
i, j
© Copyright 2001 by COSMIC Software
Priorities
There are a few remarks about these levels. The shift operators have a
lower priority than the additive operators, although they are closer to
multiplication/division operations. This may produce an unexpected
result in the following expression:
word = high << 8 + low ;
Assuming that word is a short integer, high and low two unsigned
chars. This expression, which is supposed to combine two bytes concatenated into a word, will not produce the expected result. This is
because the addition has a higher priority than the left shift. The grouping is actually:
word = high << (8 + low) ;
This is clearly wrong. The result will be correct if the binary or operator
is used:
word = high << 8 | low ;
or if the shift operation is enclosed by parenthesis:
word = (high << 8) + low ;
Parenthesis should be used to avoid any ambiguity, without overloading
the expression and making it difficult to be read.
© Copyright 2001 by COSMIC Software
Expressions 4-21
CHAPTER
5
Statements
Statements are language instructions which can be entered only inside
a function body. They describe the function behaviour and they are
placed in a function declaration:
return_type function_name(argument_list)
{
local_declarations
statement_list
}
A list of statements comprises several statements placed one after the
other, without any separator. The C language uses a terminator character ; to mark the end of a statement if it is necessary to avoid any ambiguity in the understanding. A statement may or may not use a
semicolon as terminator, but two statements are never separated by
semicolons, even if it looks like that when reading C code.
The sequence entered between the two curly braces is called a block,
and a block is a statement. This will allow several statements to be
assembled together, and to behave syntactically as a single statement,
so in the following description, any indication of statement may be
replaced by any of the C statements, including a block. The statements
defined by the C language are as follows:
© Copyright 2001 by COSMIC Software
Statements 5-1
5
Block statement
Block statement
The syntax of a block statement is:
{
declaration_list
statement_list
}
The declaration part is a list of standard declarations mainly used to
declare local variables. The register class may be used here to get more
efficient code. These local variables are created when the block is
entered, and destroyed when the block is exited, so two contiguous
blocks will overlay their local variables in the same area (the stack in
most of the cases). Most of the compilers will compute the maximum
size needed by all the embedded blocks and will create the locals frame
once at the function entry. The overlapping will be implemented by
using same offsets for overlapping variables.
The statement part is a list of C statements as described below.
Expression statement
The syntax of an expression statement is:
expression;
using an expression as described before, and terminated be a semicolon. Any expression can be used, even if it does not perform any physical operation, but most of those expressions will be assignments,
function calls or increment/decrement operations. Note that it is possible to omit the expression, thus leaving a semicolon alone. This is the
empty statement, equivalent to a nop, but it is syntactically a valid
statement.
a = b + 1;
func(1, x);
++y;
5-2 Statements
© Copyright 2001 by COSMIC Software
If statement
are operative expressions, as they modify the program status by changing the value of a memory cell or a register, or by calling a function
which is supposed to do something.
A useless expression such as:
a;
is permitted by the syntax, and does not produce any code in most of the
cases. This is used when the variable is declared with the volatile
attribute to force the compiler to produce a load instruction. The variable is just read, which is important for some peripheral registers which
need to be read in order to clear interrupt flags for instance.
An empty statement will be simply:
;
and will be used each time a statement has to be entered, but where
nothing has to be done.
Note that when used alone, the ++ and -- operators have the same
behaviour on both sides of the modified objects:
x++;
++x;
are equivalent and produce the same code.
If statement
The syntax of an if statement is:
if ( expression )
statement
or
if ( expression )
statement
else
statement
© Copyright 2001 by COSMIC Software
Statements 5-3
5
If statement
In these two syntaxes, the parenthesis around the expression are part of
the syntax, and not subexpression parenthesis, so they need to be
entered. Note that there is no then keyword, as the closing brace
behaves exactly the same way. The behaviour of such a statement is as
follows. The expression is first evaluated, and the result is checked
against the two possible cases: true or false. If the expression is a comparison, or a set of combined comparisons with logical and/or operators
&&, ||, the result is the result of the comparison evaluation. If the
expression does not use any comparison, the result has to be a numerical value or an address (from a pointer). The result is true if the value is
not zero, and false is the result is zero. This is equivalent to an implied
comparison with zero :
if (a + b)
is equivalent to
if ((a + b) != 0)
In the first syntax, the following statement is executed only if the result
of the expression is true. Otherwise it is skipped.
if (a > b)
b = a;
In this example, variables a and b are compared, and if a is greater than
b, then b is set to the value of a.
In the second syntax, the following statement is executed only if the
result of the expression is true, and in this case, the statement following
the else keyword is skipped. Otherwise, the following statement is
skipped, and the statement following the else keyword is executed.
if (a < 10)
++a;
else
a = 0;
In this example, variable a is compared to the constant 10. If a is
smaller than 10, it is incremented. Otherwise, a is reset to zero thus
implementing a looping counter from zero to ten included.
If/else statements may be embedded. In such a case, an else statement
always completes the closest if statement, unless properly enclosed with
5-4 Statements
© Copyright 2001 by COSMIC Software
While statement
block braces. In this an example:
if (a < 10)
if (a > 5)
b = 1;
else
b = 0;
the text formatting has no influence on the compiler , and the program
will behave as if it were written:
if (a < 10)
{
if (a > 5)
b = 1;
else
b = 0;
}
because the else statement is associated with the closest if statement. To
achieve the behaviour suggested by the text formatting, the program
should be written:
if (a < 10)
{
if (a > 5)
b = 1;
}
else
b = 0;
Now, the second if is part of a full block which becomes the first statement of the first if. The else statement can only be associated with the
first if statement.
While statement
The syntax of a while statement is:
while ( expression )
statement
© Copyright 2001 by COSMIC Software
Statements 5-5
5
Do statement
This statement is used to implement a loop controlled by an expression.
If the result of the expression is true, the following statement is executed, and the expression is re-evaluated to decide if the iteration can
continue. When the expression is false, the following statement is
skipped, and the loop is exited. As the expression is evaluated first, the
loop statement will be executed zero or more times.
while (p < q)
*p++ = ‘\0’;
In this example, assuming that p and q are two pointers to char variables, the loop will clear all the characters between pointers p and q if at
the beginning, p is smaller than q. This can be expanded in such a way:
while (p < q)
{
*p = ‘\0’;
++p;
}
if the increment operator is not used combined with the indirection.
This time we have two different statements which need to be placed
into a block statement for both to be executed both while the condition
is true.
The empty statement can be used to implement a wait loop:
while (!(SCISR & READY))
;
Do statement
The syntax of a do statement is:
do
statement
while ( expression ) ;
This statement is also used to implement a loop controlled by an expression, but here the statement is executed first, and then the expression is
evaluated to decide if we loop again or if we stop there. This statement
5-6 Statements
© Copyright 2001 by COSMIC Software
For statement
is terminated by a semicolon. The loop statement will be executed one
or more times.
do
ok = do_it();
while (!ok);
In this example, the function do_it() is executed until its return value
is not zero.
For statement
The syntax of a for statement is:
for (expression_1 ; expression_2 ; expression_3)
statement
The for statement is also used to implement a loop, but in a more powerful way than the previous ones. This instruction is equivalent to the
following construct:
expression_1 ;
while ( expression_2 )
{
statement
expression_3 ;
}
The first expression is the loop initialization, the second expression
controls the loop iteration, and the third expression passes to the next
iteration. The statement is the loop body. This syntax, although functionally equivalent, has the advantage to include in one instruction all
the elements describing the loop:
for (i = 0; i < 10; ++i)
tab[i] = 0;
In this example, the loop initialization ( i = 0), the loop control ( i <
10) and the next element control (++i) are displayed on the same line
and the code reading and understanding is enhanced. More complex
controls may be implemented using this syntax, such as list walking:
© Copyright 2001 by COSMIC Software
Statements 5-7
5
For statement
for (p = list_head; p; p = p->next)
p->value = 0;
which walks though a linked list and resets a field, assuming for
instance the following declarations:
struct cell {
struct cell *next;
int value;
} *p, *list_head;
The for statement allows some variations. Any of the three expressions
may be omitted. The behaviour is simple for the first and the third
expressions. If they are omitted, they do not produce any code. This is
different for the second expression because it controls the loop iteration.
If the second expression is omitted, it is replaced by an always true condition, meaning that this creates an endless loop.
An embedded program which never returns can then be started by:
main()
{
for (;;)
operate();
}
The function operate will be repeated infinitely. This is sometimes written:
while (1)
operate();
which produces absolutely the same result, as 1 is always not zero,
meaning true. The first syntax just looks more aesthetic.
The sequence operator is useful when a for loop uses several control
variables:
for (i = 0, j = 10; i < j; ++i, --j)
x = tab[i], tab[j] = tab[i], tab[i] = x;
5-8 Statements
© Copyright 2001 by COSMIC Software
Break statement
Because the C syntax allows infinite loops, it also provides instructions
to exit such loops.
Break statement
The syntax of a break statement is simply:
break ;
This statement has to be placed inside the body statement (a block usually) of a while, do or for instruction. It stops the execution of the body
statement and jumps to the end of the statement, behaving as if the controlling expression was giving a false result. The remaining instructions
of the including block are simply skipped. The break statement is usually associated with an if statement to decide if the loop has to be exited
or not.
while (p < q)
{
if (!*p)
break;
*p++ = ‘A’;
}
In this example, the loop body sets a buffer to the character ‘A’ while the
p pointer is smaller than the q pointer. In the body statement, the break
instruction is executed if the current character is a zero. This will exit
the loop and the execution will continue from the statement following
the while block. The break statement in such a case can be considered
as a and condition combined with the while test, as the previous code
could have be written:
while (p < q && *p)
*p++ = ‘A’;
The not operator ! has been removed as the while condition is a continuation test, and not a termination test.
When applied to a for loop, the break statement exits the equivalent
body statement from the expanded while construct, meaning that the
third expression, if any was specified, is not evaluated.
© Copyright 2001 by COSMIC Software
Statements 5-9
5
Continue statement
When several loop statements are embedded together, a break statement
will be applied to the closest loop statement:
for (i = 0; i < 10; ++i)
{
while (valid(i))
{
if (tab[i] < 0)
break;
--tab[i];
}
tab[i] = 0;
}
In this example, the break instruction will stop the while loop only, thus
continuing the execution with the statement following the while block
(tab[i] = 0).
Continue statement
The syntax of a continue statement is simply:
continue ;
This statement has to be placed inside the body statement of a while, do
or for loop. Its behaviour is to abort the current iteration and to start a
new one. Practically, it means that the program execution continues by
re-evaluating the controlling expression. When applied to a for loop, the
third expression, if any specified, is evaluated before evaluating the second expression.
for (i = 0; i < 100; ++i)
{
if (tab[i] == 10)
continue;
++tab[i];
}
In this example, a for loop is used to increment all the elements of an
array up to a maximum value of 10. The continue statement is executed
if an element has already reached the value 10. In this case, the ++i
5-10 Statements
© Copyright 2001 by COSMIC Software
Switch statement
expression is executed, thus skipping to the next element, before reevaluating the test i < 100 and continue the loop.
This statement is useful to avoid a deep embedding of blocks when
dealing with complex control expressions inside a loop.
When used in embedded loop statements, a continue statement is
applied to the closest loop statement, as for the break instruction.
Switch statement
The syntax of a switch statement is:
switch ( expression )
{
case constant_1:
statement_list
case constant_2:
statement_list
default:
statement_list
}
A switch statement is followed by an integer expression and by a block
containing special case labels defining some entry points. A case label
is followed by a constant expression, defining an integer value known
at compile time. A case label defines only one value, but several different values may be associated together in defining several case labels
without any statement between them. All the values entered in case
labels must be distinct from the others. The optional default label has
no parameter.
The behaviour of such a statement is as follows. The expression is evaluated and gives a numerical result. The program will then search for a
case label defined with a value equal to the expression result. If such a
label is found, the execution continues from the next statement following the label, until the end of the block statement. Execution will not
stop when crossing any other case or default label. This behaviour is not
common to the other high level languages and needs to be clearly
stated. It is possible to stop executing the block by entering a break
statement which will then make the switch behaviour look like the other
© Copyright 2001 by COSMIC Software
Statements 5-11
5
Switch statement
languages. In fact, the switch statement can be compared to a computed
jump, or computed goto in some languages (basic, fortran).
If no case label matches are found, there are two possible behaviours. If
a default label has been defined, the execution continues from the next
statement following it. Otherwise, the full block is skipped.
switch ( get_command() )
{
case ‘L’:
load();
break;
case ‘E’:
edit();
break;
case ‘X’:
save();
case ‘Q’:
quit();
break;
default:
bark();
break;
}
In this example, a switch statement is used to decide what to do from a
command letter. Each case statement activates one function and is followed by a break to avoid executing the other statements.
The case X shows a possible usage of the linear feature of a switch by
executing the save() function, then continuing by the following quit()
function, implementing the Q command as a direct exit, and the X command as a save and exit. The break statement following the last case or
default label is basically useless. It is nevertheless good to have it to
avoid forgetting it if the switch is extended later... Note that if all the
labels are associated with a break statement, the display order has no
importance, including the default label which can be placed anywhere.
Although the break statement has a meaning inside a switch statement,
the continue statement has no effect, and will be refused as not being
5-12 Statements
© Copyright 2001 by COSMIC Software
Goto statement
inside a loop statement. This may happen if the switch statement is
itself inside a loop statement:
for (i = 0; i < 100; ++i)
{
switch (tab[i])
{
case 0:
led = 1;
break;
case 1:
led = 0;
continue;
}
++tab[i];
}
In this example, the break statement will exit the switch statement, and
continue execution at statement ++tab[i], while the continue statement will be applied to the for statement, as it has no meaning for the
switch statement. The execution will continue at the ++i of the for loop.
In this case, it is not possible to use a break statement inside the switch
block to exit the for loop. This can be achieved simply only with a goto
statement, as explained below.
Goto statement
The syntax of a goto label is:
goto label ;
where label is a C identifier associated to a statement by the syntax:
label : statement
Despite all the high level language recommendations, a goto statement
can be used wherever it saves extra code or variables, without breaking
too much of the program’s readability. Goto’s may also decrease the
compiler optimization as it is more difficult to build a simple execution
path when too many goto’s are involved.
© Copyright 2001 by COSMIC Software
Statements 5-13
5
Return statement
A label is always followed by a statement, so it is not possible to jump
to the end of a block by such a syntax:
{
...
if (test)
goto exit;
...
exit:
}
Here, the label exit is followed by the closing curly brace, and this is a
syntax error. This has to be written by using the empty statement:
exit:
;
}
Return statement
The syntax of a return statement is:
return expression ;
or
return ;
The return statement is used to leave a function, and to return to the
expression which was calling that function. The first syntax is used to
return a value from the function. The expression is evaluated, converted
into the return type if necessary, and then placed in the return area (a
conventional register or predefined memory location) associated with
the called function. The second syntax is used when a function has
nothing to return, meaning that it should have been declared as a void
function. A return statement can be placed anywhere, usually associated with an if or a switch statement if it is not the last statement of a
block or a function. Note that if a function does not contain a return
statement at the end of the function block, the compiler automatically
inserts one to avoid the program continuing with the next function,
which is not very meaningful. This implicit return statement does not
5-14 Statements
© Copyright 2001 by COSMIC Software
Return statement
return any value. When the strict option is used (+strict), the compiler
also checks that a function which has a return type is actually returning
something.
There are no other statements in C. All the other features you can find
in some other languages (input/output, file control, mathematics, text
strings) are implemented by library routines. The C standard has also
normalized the basic libraries, thus guaranteeing that those features can
be used regardless of the compiler origin.
© Copyright 2001 by COSMIC Software
Statements 5-15
CHAPTER
6
Preprocessor
The C preprocessor is a text processor which operates on the C source
before it is actually parsed by the compiler. It provides macro and conditional features very closed to the ones available with most of the
existing assemblers.
The preprocessor modifies the C program source according to special
directives found in the program itself. Preprocessor directives start with
a sharp sign ‘#’ when found as the first significant character of a line.
Preprocessor directives are line based, and all the text of a directive
must be placed on a single logical line. Several physical lines can be
used if all of them but the last one end with the continuation character
backslash ‘\’.
There are three basic kinds of directives: macro directives, conditional
directives and control directives.
The macro directives allow text sequences to be replaced by some other
text sequences, depending on possible parameters.
The conditional directives allow selective compiling of the code
depending on conditions most of the time based on symbols defined by
some macro directives.
The control directives allow passing of information to the compiler in
order to configure or modify its behaviour.
© Copyright 2001 by COSMIC Software
Preprocessor 6-1
6
Macro Directives
Macro Directives
The three macro directives are:
#define IDENT rest_of_the_line
#define IDENT(parameter_list) rest_of_the_line
#undef IDENT
The two first syntaxes allow a macro to be defined, and the third syntax
allows a previous definition to be cancelled.
IDENT is a word following the rules for a C identifier, and may use low-
ercase or uppercase characters. For readability reasons, most macro
names are entered uppercase only.
rest_of_the_line represents all the characters from the first significant character immediately following IDENT (or the closing brace for
the second syntax) up to the last character of the line. This character
sequence will then replace the word IDENT each time it is found in the
C source after the definition.
The #undef directive will cancel the previous definition of a macro. No
error is reported is the #undef directive tries to cancel a macro which
has not been defined previously. However, you cannot redefine a macro
which has already be defined. In such a case, it has to be first cancelled
by a #undef directive before it is redefined.
The second syntax allows a replacement with parameters. Note that the
opening brace has to follow immediately the last character of the macro
name, without any whitespace. Otherwise, it is interpreted as the first
syntax and the parameter list along with the parenthesis will be part of
the replacement sequence. Each parameter is an identifier, separated
from the others by a comma.
#define SUM(a, b)
a + b
This macro defines the word SUM along with two parameters called a
and b. Parameters should appear in the replacement part, and the macro
should be used in the remaining C source with a matching number of
arguments.
6-2 Preprocessor
© Copyright 2001 by COSMIC Software
Macro Directives
An argument will replace any occurrence of its matching parameter in
the replacement list, before replacing the macro name with its arguments and the parenthesis in the C source. If the program contains the
following sequence:
x = SUM(y, z);
the final result will be:
x = y + z;
The preprocessor recognized SUM as a valid macro invocation, and successfully matched a with y, and b with z. The macro name and the
arguments with the parenthesis have been replaced by the replacement
con tent of the macro a + b where a and b were replaced by their values x and y.
Arguments are also simple text strings separated by commas, so if an
argument has to contain a comma, the full argument has to be enclosed
with extra parenthesis.
The preprocessor also allows two special operators in the replacement
list of a macro with parameters.
The operator # placed before a parameter name will turn it into a text
string by enclosing it by double quotes:
#define STRING(str)
# str
will transform:
ptr = STRING(hello);
into
ptr = “hello”;
This feature is interesting to use in conjunction with the string concatenation.
© Copyright 2001 by COSMIC Software
Preprocessor 6-3
6
Macro Directives
The operator ## placed between two words of the replacement sequence
will concatenate them into a single one. A word may be a parameter but
in such a case, the parameter will not be expanded before beeing concatenated.
#define BIT(var, bit)
var.b_ ## bit
will transform:
BIT(port, 3) = 1;
into
port.b_3 = 1;
Without this operator, it would have been impossible to get rid of the
white space between the base name and the bit number, and the compiler would have been unable to get the proper syntax.
Once a symbol has been completely replaced, the resulting string is
scanned again to look for subsequent replacements. Note that the original symbol will not be expanded again when rescanning the first result
thus avoiding recursive endless behaviour.
An ANSI macro cannot create a new preprocessor directive. The
COSMIC compiler allows such a feature by starting the replacement list
with a \# string. Once expanded, such a line will be re-executed as a
preprocessor directive.
#define INC(file) \#include #file “.h”
INC(stdio)
will expand to
\#include “stdio.h”
and will be re-executed after the removal of the prefixing \ thus including the file stdio.h.
6-4 Preprocessor
© Copyright 2001 by COSMIC Software
Macro Directives
Hazardous Behaviours
It is important to keep in mind that this replacement is done only on a
text basis, without any attempt to understand the result. This may lead
in a few unexpected side effects.
The following macro is used to get the absolute value of an expression:
#define ABS(x)
x > 0 ? x : -x
and as it is written, it is correct. In the following usage:
a = ABS(-b);
the replacement will produce:
a = -b > 0 ? -b : --b;
obtained directly by replacing a by -b. The last expression creates a
--b expression which decrements the b variable instead of loading its
direct value. To avoid such a situation, it is recommended to enclose
any occurrence of any parameter by parenthesis in the replacement list:
#define ABS(x) (x) > 0 ? (x) : -(x)
The replacement then becomes:
a = (-b) > 0 ? (-b) : -(-b);
and now, the last expression will be evaluated as b negated twice, and
optimized in a direct load of b.
Another side effect may be produced by an unexpected concatenation.
The macro:
#define SUM(a, b)
(a) + (b)
may be used in the following expression:
x = SUM(y, z) * 2;
© Copyright 2001 by COSMIC Software
Preprocessor 6-5
6
Macro Directives
When expanded, it becomes:
x = (y) + (z) * 2;
and now, the priority rules change the expected behaviour to:
x = (y) + ((z) * 2);
The solution is simply to use parenthesis around the whole definition:
#define SUM(a, b)
((a) + (b))
will produce as a result:
x = ((x) + (y)) * 2;
and the macro expansion is protected against any other operator.
A last example of an unexpected behaviour uses increment operators.
Assuming the previous definition for the ABS macro, the usage:
x = ABS(*p++);
will expand to:
x = (*p++) > 0 ? (*p++) : -(*p++);
In this expression, the pointer will be incremented twice and the result
is wrong coming from the element following the one tested. Unfortunately, there is no syntax trick to avoid this one.
Most of these errors are very difficult to find, because they do not produce errors at compile time, and because you do not see what is actually
expanded in reading the C source. By reading the example getting the
absolute value of *p++, there is only one increment seen. The extra one
is implied by the macro expansion, but you have to look at the macro
definition to find that. That is why it is important to immediately check
that a name is a macro, this is made easier using only uppercase names
for macros as a convention.
It is possible to have a look at the expanded source file by compiling it
with the -sp option, producing a result in a file with a .p extention.
6-6 Preprocessor
© Copyright 2001 by COSMIC Software
Macro Directives
Predefined Symbols
The preprocessor predefines a few symbols with a built-in behaviour.
Those symbols cannot be undefined by a #undef directive and then cannot be redefined to any other behaviour.
__FILE__ expands to a text string containing the name of the file
being compiled.
__LINE__ expands to a numerical value equal to the current line
number in the current source file.
__DATE__ expands to a text string containing the date you compiled
the program. The date format is “mmm dd yyyy”, where
mmm is the month abbreviated name, dd is the day and
yyyy the year.
__TIME__ expands to a text string containing the time you compiled
the program. The time format is “ hh:mm:ss”, where hh is
the hours, mm the minutes and ss the seconds.
__STDC__ expands to the numerical value 1 indicating that the com-
piler implements the ANSI features.
The COSMIC compiler also defines the following symbols:
__CSMC__ expands to a numerical value whose each bit indicates if a
specific option has been activated:
bit 0:
bit 1:
bit 2:
bit 3:
bit 4:
bit 5:
set if nowiden option specified (+nowiden)
set if single precision option specified (+sprec)
set if unsigned char option specified (-pu)
set if alignment option specified (+even)
set if reverse bitfield option specified (+rev)
set if no enum optimization specified (-pne)
This extra symbol may be used to select the proper behaviour depending on the compiler used.
__VERS__ expands to a text string containing the compiler version.
© Copyright 2001 by COSMIC Software
Preprocessor 6-7
6
Conditional Directives
Conditional Directives
The conditional directives are:
#ifdef IDENT
#ifndef IDENT
#if expression
and are associated with the ending directives
#else
#endif
#elif expression
A conditional directive is always followed by an ending directive. All
the C lines enclosed by the conditional directive and its ending directive
will be compiled or skipped depending on the result of the condition
test.
#ifdef IDENT
is true if there is the macro IDENT has been
previously defined
#ifndef IDENT
is true if IDENT is not the name of a macro
previously defined
#if expression
is true if the result of expression is not zero
The expression will be evaluated as a constant expression, so
after all the possible macro replacements inside the expression,
any word which has not been replaced by a number or an operator is replaced by the value zero before the evaluation. The
COSMIC compiler accepts the special operator sizeof and
enum members inside a #if expression, although this is not supported by the ANSI standard.
An ending directive may also become a conditional directive starting a
new conditional block, such as #else and #elif.
6-8 Preprocessor
© Copyright 2001 by COSMIC Software
Conditional Directives
Here are a few possible constructs:
#ifdef DEBUG
printf(“trace 1\n”);
#endif
If the symbol DEBUG has been defined previously, the line printf... will
be compiled. Otherwise, it is simply skipped.
#if TERM == 1
init_screen();
#else
init_printer();
#endif
If the symbol TERM has been previously defined equal to 1, the line
init_screen(); is compiled, and the line init_printer is
skipped. If TERM is define to anything else, or if TERM is not defined, the
behaviour is the opposite (if TERM is not defined, it is replaced by 0 and
the expression 0 == 1 is false).
The #elif directive is simply a contraction of a #else immediately
followed by a #if. It avoids too complex embedding in case of multiple
values:
#if TERM == 1
...
#elif TERM == 2
...
#elif TERM == 3
...
#else
...
#endif
© Copyright 2001 by COSMIC Software
Preprocessor 6-9
6
Control Directives
Control Directives
The control directives are:
#include
#include "filename"
or
#include <filename>
The preprocessor replaces such a line by the full content of the file
whose name is specified between double quotes or angle brackets. A
file specified between double quotes is searched first in the current
directory. A file specified between angle brackets is searched first in
some predefined system directories, or user specified directories. An
error will occur if the file is not found in any of the specified directories. An included file may contain other #include directives.
#error
#error rest_of_the_line
If this directive is encountered, the compiler outputs an error message
whose content is the rest_of_the_line. This directive is interesting
to force an error if something is detected wrong in the defined symbols:
#ifndef TERM
#error missing definition for TERM
#endif
If the symbol TERM is not defined, the compiler will output an error
message containing the text “missing definition for TERM”, and
will fail to compile the source file.
#line
#line number "filename"
This directive redefines the current line number to the specified
number, and the file name to the specified name in the text string. This
is mainly used by automatic code generators to allow an error to refer to
the input file name and line number rather than the intermediate C
source file produced. This is almost never used by a human written program. Note that this directive modifies the value of the predefined symbol __FILE__.
6-10 Preprocessor
© Copyright 2001 by COSMIC Software
Control Directives
#pragma
#pragma rest_of_the_line
This directive allows passing to the compiler any configuration directive useful for code generation. There is no standard or predefined syntax for the content of the directive, and each compiler may implement
whatever it needs. The only defined behaviour is that if the compiler
does not recognize the directive, it skips it without error message, thus
keeping this directive portable across different compilers.
The COSMIC compiler implements two pragmas to control the allocation of objects in memory spaces and in assembler sections.
#pragma space <class> <kind> <modifiers>
The #pragma space allows you to choose in which memory space C
objects are allocated. Such a directive will affect the objects declared
after it, until a new directive changes again the configuration.
The <class> field contains a keyword describing the object class:
extern
static
auto
*
const
for global objects
for static objects
for local objects
for pointed objects
for compiler constants
If this field is empty, all the classes are selected, except const.
The <kind> field specifies which kind of object is selected:
[]
()
specifies variables
specifies functions
If this field is empty, both kinds are selected.
The <modifier> field contains a list of modifiers to be applied by
default to the objects selected. Each modifier starts with the @ character
and must be a valid modifier supported by the compiler, as each target
supports a different set of modifiers. If the <modifier> field is empty,
all the attributes are turned off.
© Copyright 2001 by COSMIC Software
Preprocessor 6-11
6
Control Directives
The following directive:
#pragma space extern () @far
turns all the following global functions to be @far (bank switched).
Static functions are not affected by this directive.
#pragma space [] @eeprom
turns all the following variables of any class to be flagged as being in
eeprom. The effect of such a directive will be cancelled by the following:
#pragma space []
Refer to the specific compiler manual for the list of supported space
modifiers.
#pragma section <modifier> <kind_and_name>
The #pragma section directive allows the compiler to modify the section in which objects are allocated. The compiler splits the various program components in the following default sections:
executable code
constants
initialized variables
uninitialized variables
eeprom variables
.text
.const
.data or .bsct
.bss or .ubsct
.eeprom
Variables are allocated in the .bsct or .ubsct when flagged as zero
page. Those sections and the .eeprom section may not be defined
depending on the target capabilities.
Each of these sections can be renamed. The compiler then creates a new
assembler section with the proper name and attributes, and produces
there the matching objects. The compiler will prepend a dot . to the
provided name, and will check that the final name is not longer than 14
characters.
6-12 Preprocessor
© Copyright 2001 by COSMIC Software
Control Directives
The name is provided in the <kind_and_name> field along with the
kind of object in such a way:
( name ) defines a name for executable code
{ name } defines a name for initialized variables
[ name ] defines a name for uninitialized variables
The <modifier> allows chosing the right section by specifying the
proper attribute, depending on the object kind selected:
const { name }
changes the .const section name
@tiny { name }
changes the .bsct section name (or @dir depending on the actual target)
@eeprom { name } changes the .eeprom section name
Note that [] may be used instead of {} with the same effect for sections
which are not sensitive to the initialization or not (.const and .eeprom).
If the name part is omitted between the parenthesis, the matching section name turns back to its default value.
In the following example:
#pragma section {mor}
char MOR = 0x3c;
#pragma section {}
The compiler creates a section named .mor which replaces the default
.data section. The variable MOR is then created in the .mor section. The
compiler reverts to the original .data section for the next initialized variables.
The interrupt vectors can be located in a separate section with the following code:
#pragma section const {vector}
void (* const vectab[])(void) = {it1, it2, it3};
#pragma section const {}
© Copyright 2001 by COSMIC Software
Preprocessor 6-13
6
Control Directives
The vectab table will be produced in the created .vector section
instead of the default .const section.
The #pragma asm and #pragma endasm directives allow assembly
code to be directly inserted in a C program. Assembly code is entered
between those two directives as if they were written in an assembler
program. Note that there is no direct connection possible with existing
C objects. When used outside a function, such a block behaves syntactically as a declaration. When used inside a function, such a block
behaves as a block statement. It is not necessary to enclose it with curly
braces {} although it is more readable. The compiler accepts the directives #asm and #endasm as shortcuts for the #pragma ones.
6-14 Preprocessor
© Copyright 2001 by COSMIC Software
Index
Symbols
#asm 6-14
#elif directive 6-8
#else directive 6-8
#endasm 6-14
#endif directive 6-8
#error directive 6-10
#if directive 6-8
#ifdef directive 6-8
#ifndef directive 6-8
#line directive 6-10
#pragma asm 6-14
#pragma directive 6-11
#pragma endasm 6-14
#pragma section 6-12
#pragma space 6-11
#undef directive 6-2
@far modifier 4-11
@far space modifier 3-5
@near modifier 4-11
@near space modifier 3-5
@nostack modifier 3-7
@packed modifier 3-6
@tiny modifier 4-11
@tiny space modifier 3-5
\ character 2-1
_asm function 4-15
A
address 2-7, 3-15
address constant 3-3
address return 4-13
and operator 5-4
and, bitwise 4-9
argument list 3-10
array 2-8
array initialization 3-3
array keyword 3-3
array of arrays 3-4
arrays of pointers 3-4
Assignment operators 4-11
auto keyword 3-13
B
bank switch 2-8
bitfields 2-9
bitwise operators 4-9
block 3-12
block statement 5-2
body of a function 3-12
body statement 5-9
Boolean operators 4-10
break statement 5-9, 5-12
C
C expressions 4-1
C identifier 4-2
C keywords 2-3
C operators 2-4
C program 2-5
C punctuators 2-4
case label 5-11
cast operator 4-16
Index 1
comment end 2-2
comment start 2-2
Comments 2-2
conditional directives 6-1
conditional feature 6-1
conditional operator 4-15
configuration directive 6-11
const keyword 3-5
const modifier 3-5
constant 2-4, 3-5
Constants 4-2
constants 2-3
continue statement 5-10
control directives 6-1, 6-10
current line number 6-10
D
declaration 2-5
declaration, of an object 3-1
decrementation 4-12
default label 5-12
do statement 5-6
double 3-2
double type 4-5
E
eeprom 6-12
else keyword 5-4
else statement 5-4
enum keyword 3-9
enumeration 2-6, 2-9, 3-9
error message 6-10
escape sequence 4-3
exclusive or, bitwise 4-9
expression 4-1, 5-2
extern keyword 3-12
F
false, logical value 4-10
field 2-9
file scope 3-13
first expression 5-7
2 Index
float 2-7
float type 3-2, 4-5
for statement 5-7, 5-8
function 2-9, 3-10
function body 5-1
function call operator 4-14
G
global variable 3-14
goto label 5-13
goto statement 5-13
I
identifier 2-3
identifiers 2-3
if statement 5-3, 5-4
index 2-8
indexing operator 4-14
initial value 3-1
initialization of a function 3-12
initialization, of an array 3-3
int 2-5, 3-2
int type 4-4
integer type 2-5
integer variable 3-2
K
K&R syntax 3-11
Kernigan and Ritchie syntax 3-10
keyword 2-3
keyword name 2-3
keywords 2-3
L
label 5-14
leave, a function 5-14
line terminator 2-1
load instruction 5-3
local variable 3-14
local variables 3-12
logical true 4-10
long 2-5
long double 2-7, 3-2
long double type 4-5
long enumerations 3-10
long int 2-5
loop body 5-7
loop initialization 5-7
loop iteration 5-7
lowercase 2-3
L-value expression 4-11
M
macro 6-1
macro directives 6-1
main 2-10
memory model 2-8
memory modification 4-11
memory space 6-11
modifier 3-5
N
name 3-1
nop instruction 5-2
not operator 5-9
numerical constant 3-3
O
object class 6-11
operands 4-7
operator 2-4, 4-14
Operators 4-7
operators 2-3
or operator 5-4
or, bitwise 4-9
P
parameters 3-11
peripheral registers 5-3
physical register 3-13
pointer 2-7
pointer keyword 3-2
pointer return 4-13
pointer to void 4-11
pointers to arrays 3-4
pointers to pointers 3-4
post incrementation 4-12
pre incrementation 4-12
predefined space modifiers 3-5
preprocessing directives 2-2
preprocessor 6-1
preprocessor directive 6-1
prototyped syntax 3-10
punctuation 2-3
punctuators 2-4
R
real variable 3-2
register class 5-2
register keyword 3-13
register object 3-13
register variable 3-13
registers modification 4-11
return 5-14
return statement 5-14
returned value type 3-10
right shift operator 4-9
R-value expression 4-11
S
scope, of variable 3-12
second expression 5-7
semicolon 5-2
sequence operator 4-16
set of variables 4-1
short 2-5
short int 2-5
signed 2-5
signed keyword 3-2
size of a pointer 2-8
sizeof type 4-7
space modifiers 3-5
special ponctuator 3-11
Statements 5-1
Index 3
static keyword 3-13, 3-14
stop compiler optimizing 3-5
storage class 3-1, 3-12
string constant 4-5
struct keyword 3-7
structure 2-9, 3-7
structure initialization 3-8
switch statement 5-11
T
tag name 3-7
terminator character 5-1
then keyword 5-4
third expression 5-7
true, logical value 4-10
type 3-1
type equivalent 3-14
typedef keyword 3-14
U
union 2-9, 3-9
unsigned 2-5
unsigned char 3-2
unsigned int 3-2, 4-4
unsigned keyword 3-2
unsigned short 2-8
uppercase 2-3
useless expression 5-3
V
variable number of arguments 3-11
Variables 4-2
void 5-14
volatile attribute 5-3
volatile keyword 3-5
volatile modifier 3-5
W
while statement 5-5
wide character 4-4, 4-7
wide string 4-7
4 Index
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