The C preprocessor is a macro processor that is used automatically by the C compiler to transform your program before actual compilation. It is called a macro processor because it allows you to define macros, which are brief abbreviations for longer constructs.
The C preprocessor provides four separate facilities that you can use as you see fit:
C preprocessors vary in some details. This manual discusses the GNU C preprocessor, the C Compatible Compiler Preprocessor. The GNU C preprocessor provides a superset of the features of ANSI Standard C.
ANSI Standard C requires the rejection of many harmless constructs commonly
used by today's C programs. Such incompatibility would be inconvenient for
users, so the GNU C preprocessor is configured to accept these constructs
by default. Strictly speaking, to get ANSI Standard C, you must use the
options -trigraphs, -undef and -pedantic, but in
practice the consequences of having strict ANSI Standard C make it
undesirable to do this. See Invocation.
The C preprocessor is designed for C-like languages; you may run into problems if you apply it to other kinds of languages, because it assumes that it is dealing with C. For example, the C preprocessor sometimes outputs extra white space to avoid inadvertent C token concatenation, and this may cause problems with other languages.
Most C preprocessor features are inactive unless you give specific directives
to request their use. (Preprocessing directives are lines starting with
#; see Directives). But there are three transformations that the
preprocessor always makes on all the input it receives, even in the absence
of directives.
The first two transformations are done before nearly all other parsing and before preprocessing directives are recognized. Thus, for example, you can split a line cosmetically with Backslash-Newline anywhere (except when trigraphs are in use; see below).
/* */ # /* */ defi\ ne FO\ O 10\ 20
is equivalent into #define FOO 1020. You can split even an escape
sequence with Backslash-Newline. For example, you can split "foo\bar"
between the \ and the b to get
"foo\\ bar"
This behavior is unclean: in all other contexts, a Backslash can be inserted in a string constant as an ordinary character by writing a double Backslash, and this creates an exception. But the ANSI C standard requires it. (Strict ANSI C does not allow Newlines in string constants, so they do not consider this a problem.)
But there are a few exceptions to all three transformations.
#include directive in which the file name is delimited with
< and >.
This exception is relevant only if you use the -trigraphs
option to enable trigraph processing. See Invocation.
Most preprocessor features are active only if you use preprocessing directives to request their use.
Preprocessing directives are lines in your program that start with #.
The # is followed by an identifier that is the directive name.
For example, #define is the directive that defines a macro.
Whitespace is also allowed before and after the #.
The set of valid directive names is fixed. Programs cannot define new preprocessing directives.
Some directive names require arguments; these make up the rest of the directive
line and must be separated from the directive name by whitespace. For example,
#define must be followed by a macro name and the intended expansion
of the macro. See Simple Macros.
A preprocessing directive cannot be more than one line in normal circumstances. It may be split cosmetically with Backslash-Newline, but that has no effect on its meaning. Comments containing Newlines can also divide the directive into multiple lines, but the comments are changed to Spaces before the directive is interpreted. The only way a significant Newline can occur in a preprocessing directive is within a string constant or character constant. Note that most C compilers that might be applied to the output from the preprocessor do not accept string or character constants containing Newlines.
The # and the directive name cannot come from a macro expansion. For
example, if foo is defined as a macro expanding to define,
that does not make #foo a valid preprocessing directive.
A header file is a file containing C declarations and macro definitions
(see Macros) to be shared between several source files. You request
the use of a header file in your program with the C preprocessing directive
#include.
#include directives.
#include does.
Header files serve two kinds of purposes.
Including a header file produces the same results in C compilation as copying the header file into each source file that needs it. But such copying would be time-consuming and error-prone. With a header file, the related declarations appear in only one place. If they need to be changed, they can be changed in one place, and programs that include the header file will automatically use the new version when next recompiled. The header file eliminates the labor of finding and changing all the copies as well as the risk that a failure to find one copy will result in inconsistencies within a program.
The usual convention is to give header files names that end with
.h. Avoid unusual characters in header file names, as they
reduce portability.
#include DirectiveBoth user and system header files are included using the preprocessing
directive #include. It has three variants:
#include <file>
-I
(see Invocation). The option -nostdinc inhibits searching
the standard system directories; in this case only the directories
you specify are searched.
The parsing of this form of #include is slightly special
because comments are not recognized within the <...>.
Thus, in #include <x/*y> the /* does not start a comment
and the directive specifies inclusion of a system header file named
x/*y. Of course, a header file with such a name is unlikely to
exist on Unix, where shell wildcard features would make it hard to
manipulate.
The argument file may not contain a > character. It may,
however, contain a < character.
#include "file"
-I- option is
used, the special treatment of the current directory is inhibited.)
The argument file may not contain " characters. If
backslashes occur within file, they are considered ordinary text
characters, not escape characters. None of the character escape
sequences appropriate to string constants in C are processed. Thus,
#include "x\n\\y" specifies a filename containing three
backslashes. It is not clear why this behavior is ever useful, but
the ANSI standard specifies it.
#include anything else
#include
directive whose argument does not fit the above two forms is a computed
include. The text anything else is checked for macro calls,
which are expanded (see Macros). When this is done, the result
must fit one of the above two variants--in particular, the expanded
text must in the end be surrounded by either quotes or angle braces.
This feature allows you to define a macro which controls the file name to be used at a later point in the program. One application of this is to allow a site-specific configuration file for your program to specify the names of the system include files to be used. This can help in porting the program to various operating systems in which the necessary system header files are found in different places.
#include WorksThe #include directive works by directing the C preprocessor to scan
the specified file as input before continuing with the rest of the current
file. The output from the preprocessor contains the output already
generated, followed by the output resulting from the included file,
followed by the output that comes from the text after the #include
directive. For example, given a header file header.h as follows,
char *test ();
and a main program called program.c that uses the header file,
like this,
int x;
#include "header.h"
main ()
{
printf (test ());
}
the output generated by the C preprocessor for program.c as input
would be
int x;
char *test ();
main ()
{
printf (test ());
}
Included files are not limited to declarations and macro definitions; those are merely the typical uses. Any fragment of a C program can be included from another file. The include file could even contain the beginning of a statement that is concluded in the containing file, or the end of a statement that was started in the including file. However, a comment or a string or character constant may not start in the included file and finish in the including file. An unterminated comment, string constant or character constant in an included file is considered to end (with an error message) at the end of the file.
It is possible for a header file to begin or end a syntactic unit such as a function definition, but that would be very confusing, so don't do it.
The line following the #include directive is always treated as a
separate line by the C preprocessor even if the included file lacks a final
newline.
Very often, one header file includes another. It can easily result that a certain header file is included more than once. This may lead to errors, if the header file defines structure types or typedefs, and is certainly wasteful. Therefore, we often wish to prevent multiple inclusion of a header file.
The standard way to do this is to enclose the entire real contents of the file in a conditional, like this:
#ifndef FILE_FOO_SEEN #define FILE_FOO_SEEN the entire file #endif /* FILE_FOO_SEEN */
The macro FILE_FOO_SEEN indicates that the file has been included
once already. In a user header file, the macro name should not begin
with _. In a system header file, this name should begin with
__ to avoid conflicts with user programs. In any kind of header
file, the macro name should contain the name of the file and some
additional text, to avoid conflicts with other header files.
The GNU C preprocessor is programmed to notice when a header file uses
this particular construct and handle it efficiently. If a header file
is contained entirely in a #ifndef conditional, then it records
that fact. If a subsequent #include specifies the same file,
and the macro in the #ifndef is already defined, then the file
is entirely skipped, without even reading it.
There is also an explicit directive to tell the preprocessor that it need
not include a file more than once. This is called #pragma once,
and was used in addition to the #ifndef conditional around
the contents of the header file. #pragma once is now obsolete
and should not be used at all.
In the Objective C language, there is a variant of #include
called #import which includes a file, but does so at most once.
If you use #import instead of #include, then you
don't need the conditionals inside the header file to prevent multiple
execution of the contents.
#import is obsolete because it is not a well designed feature.
It requires the users of a header file--the applications
programmers--to know that a certain header file should only be included
once. It is much better for the header file's implementor to write the
file so that users don't need to know this. Using #ifndef
accomplishes this goal.
Inheritance is what happens when one object or file derives some of its contents by virtual copying from another object or file. In the case of C header files, inheritance means that one header file includes another header file and then replaces or adds something.
If the inheriting header file and the base header file have different
names, then inheritance is straightforward: simply write #include
"base" in the inheriting file.
Sometimes it is necessary to give the inheriting file the same name as the base file. This is less straightforward.
For example, suppose an application program uses the system header
sys/signal.h, but the version of /usr/include/sys/signal.h
on a particular system doesn't do what the application program expects.
It might be convenient to define a "local" version, perhaps under the
name /usr/local/include/sys/signal.h, to override or add to the
one supplied by the system.
You can do this by compiling with the option -I., and
writing a file sys/signal.h that does what the application
program expects. But making this file include the standard
sys/signal.h is not so easy--writing #include
<sys/signal.h> in that file doesn't work, because it includes your own
version of the file, not the standard system version. Used in that file
itself, this leads to an infinite recursion and a fatal error in
compilation.
#include </usr/include/sys/signal.h> would find the proper file,
but that is not clean, since it makes an assumption about where the
system header file is found. This is bad for maintenance, since it
means that any change in where the system's header files are kept
requires a change somewhere else.
The clean way to solve this problem is to use
#include_next, which means, "Include the next file with
this name." This directive works like #include except in
searching for the specified file: it starts searching the list of header
file directories after the directory in which the current file
was found.
Suppose you specify -I /usr/local/include, and the list of
directories to search also includes /usr/include; and suppose
both directories contain sys/signal.h. Ordinary
#include <sys/signal.h> finds the file under
/usr/local/include. If that file contains #include_next
<sys/signal.h>, it starts searching after that directory, and finds the
file in /usr/include.
A macro is a sort of abbreviation which you can define once and then use later. There are many complicated features associated with macros in the C preprocessor.
A simple macro is a kind of abbreviation. It is a name which stands for a fragment of code. Some people refer to these as manifest constants.
Before you can use a macro, you must define it explicitly with the
#define directive. #define is followed by the name of the
macro and then the code it should be an abbreviation for. For example,
#define BUFFER_SIZE 1020
defines a macro named BUFFER_SIZE as an abbreviation for the text
1020. If somewhere after this #define directive there comes
a C statement of the form
foo = (char *) xmalloc (BUFFER_SIZE);
then the C preprocessor will recognize and expand the macro
BUFFER_SIZE, resulting in
foo = (char *) xmalloc (1020);
The use of all upper case for macro names is a standard convention. Programs are easier to read when it is possible to tell at a glance which names are macros.
Normally, a macro definition must be a single line, like all C preprocessing directives. (You can split a long macro definition cosmetically with Backslash-Newline.) There is one exception: Newlines can be included in the macro definition if within a string or character constant. This is because it is not possible for a macro definition to contain an unbalanced quote character; the definition automatically extends to include the matching quote character that ends the string or character constant. Comments within a macro definition may contain Newlines, which make no difference since the comments are entirely replaced with Spaces regardless of their contents.
Aside from the above, there is no restriction on what can go in a macro body. Parentheses need not balance. The body need not resemble valid C code. (But if it does not, you may get error messages from the C compiler when you use the macro.)
The C preprocessor scans your program sequentially, so macro definitions take effect at the place you write them. Therefore, the following input to the C preprocessor
foo = X; #define X 4 bar = X;
produces as output
foo = X; bar = 4;
After the preprocessor expands a macro name, the macro's definition body is appended to the front of the remaining input, and the check for macro calls continues. Therefore, the macro body can contain calls to other macros. For example, after
#define BUFSIZE 1020 #define TABLESIZE BUFSIZE
the name TABLESIZE when used in the program would go through two
stages of expansion, resulting ultimately in 1020.
This is not at all the same as defining TABLESIZE to be 1020.
The #define for TABLESIZE uses exactly the body you
specify--in this case, BUFSIZE--and does not check to see whether
it too is the name of a macro. It's only when you use TABLESIZE
that the result of its expansion is checked for more macro names.
See Cascaded Macros.
A simple macro always stands for exactly the same text, each time it is used. Macros can be more flexible when they accept arguments. Arguments are fragments of code that you supply each time the macro is used. These fragments are included in the expansion of the macro according to the directions in the macro definition. A macro that accepts arguments is called a function-like macro because the syntax for using it looks like a function call.
To define a macro that uses arguments, you write a #define directive
with a list of argument names in parentheses after the name of the
macro. The argument names may be any valid C identifiers, separated by
commas and optionally whitespace. The open-parenthesis must follow the
macro name immediately, with no space in between.
For example, here is a macro that computes the minimum of two numeric values, as it is defined in many C programs:
#define min(X, Y) ((X) < (Y) ? (X) : (Y))
(This is not the best way to define a "minimum" macro in GNU C. See Side Effects, for more information.)
To use a macro that expects arguments, you write the name of the macro
followed by a list of actual arguments in parentheses, separated by
commas. The number of actual arguments you give must match the number of
arguments the macro expects. Examples of use of the macro min
include min (1, 2) and min (x + 28, *p).
The expansion text of the macro depends on the arguments you use.
Each of the argument names of the macro is replaced, throughout the
macro definition, with the corresponding actual argument. Using the
same macro min defined above, min (1, 2) expands into
((1) < (2) ? (1) : (2))
where 1 has been substituted for X and 2 for Y.
Likewise, min (x + 28, *p) expands into
((x + 28) < (*p) ? (x + 28) : (*p))
Parentheses in the actual arguments must balance; a comma within parentheses does not end an argument. However, there is no requirement for brackets or braces to balance, and they do not prevent a comma from separating arguments. Thus,
macro (array[x = y, x + 1])
passes two arguments to macro: array[x = y and x +
1]. If you want to supply array[x = y, x + 1] as an argument,
you must write it as array[(x = y, x + 1)], which is equivalent C
code.
After the actual arguments are substituted into the macro body, the entire
result is appended to the front of the remaining input, and the check for
macro calls continues. Therefore, the actual arguments can contain calls
to other macros, either with or without arguments, or even to the same
macro. The macro body can also contain calls to other macros. For
example, min (min (a, b), c) expands into this text:
((((a) < (b) ? (a) : (b))) < (c) ? (((a) < (b) ? (a) : (b))) : (c))
(Line breaks shown here for clarity would not actually be generated.)
If a macro foo takes one argument, and you want to supply an
empty argument, you must write at least some whitespace between the
parentheses, like this: foo ( ). Just foo () is providing
no arguments, which is an error if foo expects an argument. But
foo0 () is the correct way to call a macro defined to take zero
arguments, like this:
#define foo0() ...
If you use the macro name followed by something other than an open-parenthesis (after ignoring any spaces, tabs and comments that follow), it is not a call to the macro, and the preprocessor does not change what you have written. Therefore, it is possible for the same name to be a variable or function in your program as well as a macro, and you can choose in each instance whether to refer to the macro (if an actual argument list follows) or the variable or function (if an argument list does not follow).
Such dual use of one name could be confusing and should be avoided
except when the two meanings are effectively synonymous: that is, when the
name is both a macro and a function and the two have similar effects. You
can think of the name simply as a function; use of the name for purposes
other than calling it (such as, to take the address) will refer to the
function, while calls will expand the macro and generate better but
equivalent code. For example, you can use a function named min in
the same source file that defines the macro. If you write &min with
no argument list, you refer to the function. If you write min (x,
bb), with an argument list, the macro is expanded. If you write
(min) (a, bb), where the name min is not followed by an
open-parenthesis, the macro is not expanded, so you wind up with a call to
the function min.
You may not define the same name as both a simple macro and a macro with arguments.
In the definition of a macro with arguments, the list of argument names must follow the macro name immediately with no space in between. If there is a space after the macro name, the macro is defined as taking no arguments, and all the rest of the line is taken to be the expansion. The reason for this is that it is often useful to define a macro that takes no arguments and whose definition begins with an identifier in parentheses. This rule about spaces makes it possible for you to do either this:
#define FOO(x) - 1 / (x)
(which defines FOO to take an argument and expand into minus the
reciprocal of that argument) or this:
#define BAR (x) - 1 / (x)
(which defines BAR to take no argument and always expand into
(x) - 1 / (x)).
Note that the uses of a macro with arguments can have spaces before the left parenthesis; it's the definition where it matters whether there is a space.
Several simple macros are predefined. You can use them without giving definitions for them. They fall into two classes: standard macros and system-specific macros.
The standard predefined macros are available with the same meanings
regardless of the machine or operating system on which you are using GNU C.
Their names all start and end with double underscores. Those preceding
__GNUC__ in this table are standardized by ANSI C; the rest are
GNU C extensions.
__FILE__
#include or as the input file name argument.
__LINE__
This and __FILE__ are useful in generating an error message to
report an inconsistency detected by the program; the message can state
the source line at which the inconsistency was detected. For example,
fprintf (stderr, "Internal error: "
"negative string length "
"%d at %s, line %d.",
length, __FILE__, __LINE__);
A #include directive changes the expansions of __FILE__
and __LINE__ to correspond to the included file. At the end of
that file, when processing resumes on the input file that contained
the #include directive, the expansions of __FILE__ and
__LINE__ revert to the values they had before the
#include (but __LINE__ is then incremented by one as
processing moves to the line after the #include).
The expansions of both __FILE__ and __LINE__ are altered
if a #line directive is used. See Combining Sources.
__DATE__
"Feb 1 1996".
__TIME__
"23:59:01".
__STDC__
On some hosts, system include files use a different convention, where
__STDC__ is normally 0, but is 1 if the user specifies strict
conformance to the C Standard. The preprocessor follows the host convention
when processing system include files, but when processing user files it follows
the usual GNU C convention.
This macro is not defined if the -traditional option is used.
__STDC_VERSION__
yyyymmL
where yyyy and mm are the year and month of the Standard version.
This signifies which version of the C Standard the preprocessor conforms to.
Like __STDC__, whether this version number is accurate
for the entire implementation depends on what C compiler
will operate on the output from the preprocessor.
This macro is not defined if the -traditional option is used.
__GNUC__
__GNUC__ is undefined. The value
identifies the major version number of GNU CC (1 for GNU CC
version 1, which is now obsolete, and 2 for version 2).
__GNUC_MINOR__
__GNUC__ > 2 || (__GNUC__ == 2 && __GNUC_MINOR__ >= 6)).
The last number, 3 in the
example above, denotes the bugfix level of the compiler; no macro
contains this value.
__GNUG__
__GNUG__ to distinguish between GNU C and GNU
C++.
__cplusplus
__cplusplus
to test whether a header is compiled by a C compiler or a C++ compiler.
__STRICT_ANSI__
-ansi switch was
specified when GNU C was invoked. Its definition is the null string.
This macro exists primarily to direct certain GNU header files not to
define certain traditional Unix constructs which are incompatible with
ANSI C.
__BASE_FILE__
__INCLUDE_LEVEL__
#include directive and decremented at every
end of file. For input files specified by command line arguments,
the nesting level is zero.
__VERSION__
"2.6.0".
__OPTIMIZE__
__CHAR_UNSIGNED__
char is
unsigned on the target machine. It exists to cause the standard header
file limits.h to work correctly. You should not refer to this
macro yourself; instead, refer to the standard macros defined in
limits.h. The preprocessor uses this macro to determine whether
or not to sign-extend large character constants written in octal; see
The #if Directive.
__REGISTER_PREFIX__
m68k-aout environment it expands to the null
string, but in the m68k-coff environment it expands to the string
%.
__USER_LABEL_PREFIX__
__REGISTER_PREFIX__, but describes the prefix applied
to user generated labels in assembler code. For example, in the
m68k-aout environment it expands to the string _, but in
the m68k-coff environment it expands to the null string. This
does not work with the -mno-underscores option that the i386
OSF/rose and m88k targets provide nor with the -mcall* options of
the rs6000 System V Release 4 target.
The C preprocessor normally has several predefined macros that vary between
machines because their purpose is to indicate what type of system and
machine is in use. This manual, being for all systems and machines, cannot
tell you exactly what their names are; instead, we offer a list of some
typical ones. You can use cpp -dM to see the values of
predefined macros; see Invocation.
Some nonstandard predefined macros describe the operating system in use, with more or less specificity. For example,
unix
unix is normally predefined on all Unix systems.
BSD
BSD is predefined on recent versions of Berkeley Unix
(perhaps only in version 4.3).
Other nonstandard predefined macros describe the kind of CPU, with more or less specificity. For example,
vax
vax is predefined on Vax computers.
mc68000
mc68000 is predefined on most computers whose CPU is a Motorola
68000, 68010 or 68020.
m68k
m68k is also predefined on most computers whose CPU is a 68000,
68010 or 68020; however, some makers use mc68000 and some use
m68k. Some predefine both names. What happens in GNU C
depends on the system you are using it on.
M68020
M68020 has been observed to be predefined on some systems that
use 68020 CPUs--in addition to mc68000 and m68k, which
are less specific.
_AM29K
_AM29000
_AM29K and _AM29000 are predefined for the AMD 29000
CPU family.
ns32000
ns32000 is predefined on computers which use the National
Semiconductor 32000 series CPU.
Yet other nonstandard predefined macros describe the manufacturer of the system. For example,
sun
sun is predefined on all models of Sun computers.
pyr
pyr is predefined on all models of Pyramid computers.
sequent
sequent is predefined on all models of Sequent computers.
These predefined symbols are not only nonstandard, they are contrary to the
ANSI standard because their names do not start with underscores.
Therefore, the option -ansi inhibits the definition of these
symbols.
This tends to make -ansi useless, since many programs depend on the
customary nonstandard predefined symbols. Even system header files check
them and will generate incorrect declarations if they do not find the names
that are expected. You might think that the header files supplied for the
Uglix computer would not need to test what machine they are running on,
because they can simply assume it is the Uglix; but often they do, and they
do so using the customary names. As a result, very few C programs will
compile with -ansi. We intend to avoid such problems on the GNU
system.
What, then, should you do in an ANSI C program to test the type of machine it will run on?
GNU C offers a parallel series of symbols for this purpose, whose names
are made from the customary ones by adding __ at the beginning
and end. Thus, the symbol __vax__ would be available on a Vax,
and so on.
The set of nonstandard predefined names in the GNU C preprocessor is
controlled (when cpp is itself compiled) by the macro
CPP_PREDEFINES, which should be a string containing -D
options, separated by spaces. For example, on the Sun 3, we use the
following definition:
#define CPP_PREDEFINES "-Dmc68000 -Dsun -Dunix -Dm68k"
This macro is usually specified in tm.h.
Stringification means turning a code fragment into a string constant
whose contents are the text for the code fragment. For example,
stringifying foo (z) results in "foo (z)".
In the C preprocessor, stringification is an option available when macro
arguments are substituted into the macro definition. In the body of the
definition, when an argument name appears, the character # before
the name specifies stringification of the corresponding actual argument
when it is substituted at that point in the definition. The same argument
may be substituted in other places in the definition without
stringification if the argument name appears in those places with no
#.
Here is an example of a macro definition that uses stringification:
#define WARN_IF(EXP) \
do { if (EXP) \
fprintf (stderr, "Warning: " #EXP "\n"); } \
while (0)
Here the actual argument for EXP is substituted once as given,
into the if statement, and once as stringified, into the
argument to fprintf. The do and while (0) are
a kludge to make it possible to write WARN_IF (arg);,
which the resemblance of WARN_IF to a function would make
C programmers want to do; see Swallow Semicolon.
The stringification feature is limited to transforming one macro argument
into one string constant: there is no way to combine the argument with
other text and then stringify it all together. But the example above shows
how an equivalent result can be obtained in ANSI Standard C using the
feature that adjacent string constants are concatenated as one string
constant. The preprocessor stringifies the actual value of EXP
into a separate string constant, resulting in text like
do { if (x == 0) \
fprintf (stderr, "Warning: " "x == 0" "\n"); } \
while (0)
but the C compiler then sees three consecutive string constants and concatenates them into one, producing effectively
do { if (x == 0) \
fprintf (stderr, "Warning: x == 0\n"); } \
while (0)
Stringification in C involves more than putting doublequote characters
around the fragment; it is necessary to put backslashes in front of all
doublequote characters, and all backslashes in string and character
constants, in order to get a valid C string constant with the proper
contents. Thus, stringifying p = "foo\n"; results in "p =
\"foo\\n\";". However, backslashes that are not inside of string or
character constants are not duplicated: \n by itself stringifies to
"\n".
Whitespace (including comments) in the text being stringified is handled according to precise rules. All leading and trailing whitespace is ignored. Any sequence of whitespace in the middle of the text is converted to a single space in the stringified result.
Concatenation means joining two strings into one. In the context of macro expansion, concatenation refers to joining two lexical units into one longer one. Specifically, an actual argument to the macro can be concatenated with another actual argument or with fixed text to produce a longer name. The longer name might be the name of a function, variable or type, or a C keyword; it might even be the name of another macro, in which case it will be expanded.
When you define a macro, you request concatenation with the special
operator ## in the macro body. When the macro is called,
after actual arguments are substituted, all ## operators are
deleted, and so is any whitespace next to them (including whitespace
that was part of an actual argument). The result is to concatenate
the syntactic tokens on either side of the ##.
Consider a C program that interprets named commands. There probably needs to be a table of commands, perhaps an array of structures declared as follows:
struct command
{
char *name;
void (*function) ();
};
struct command commands[] =
{
{ "quit", quit_command},
{ "help", help_command},
...
};
It would be cleaner not to have to give each command name twice, once in
the string constant and once in the function name. A macro which takes the
name of a command as an argument can make this unnecessary. The string
constant can be created with stringification, and the function name by
concatenating the argument with _command. Here is how it is done:
#define COMMAND(NAME) { #NAME, NAME ## _command }
struct command commands[] =
{
COMMAND (quit),
COMMAND (help),
...
};
The usual case of concatenation is concatenating two names (or a name and a
number) into a longer name. But this isn't the only valid case. It is
also possible to concatenate two numbers (or a number and a name, such as
1.5 and e3) into a number. Also, multi-character operators
such as += can be formed by concatenation. In some cases it is even
possible to piece together a string constant. However, two pieces of text
that don't together form a valid lexical unit cannot be concatenated. For
example, concatenation with x on one side and + on the other
is not meaningful because those two characters can't fit together in any
lexical unit of C. The ANSI standard says that such attempts at
concatenation are undefined, but in the GNU C preprocessor it is well
defined: it puts the x and + side by side with no particular
special results.
Keep in mind that the C preprocessor converts comments to whitespace before
macros are even considered. Therefore, you cannot create a comment by
concatenating / and *: the /* sequence that starts a
comment is not a lexical unit, but rather the beginning of a "long" space
character. Also, you can freely use comments next to a ## in a
macro definition, or in actual arguments that will be concatenated, because
the comments will be converted to spaces at first sight, and concatenation
will later discard the spaces.
To undefine a macro means to cancel its definition. This is done
with the #undef directive. #undef is followed by the macro
name to be undefined.
Like definition, undefinition occurs at a specific point in the source file, and it applies starting from that point. The name ceases to be a macro name, and from that point on it is treated by the preprocessor as if it had never been a macro name.
For example,
#define FOO 4 x = FOO; #undef FOO x = FOO;
expands into
x = 4; x = FOO;
In this example, FOO had better be a variable or function as well
as (temporarily) a macro, in order for the result of the expansion to be
valid C code.
The same form of #undef directive will cancel definitions with
arguments or definitions that don't expect arguments. The #undef
directive has no effect when used on a name not currently defined as a macro.
Redefining a macro means defining (with #define) a name that
is already defined as a macro.
A redefinition is trivial if the new definition is transparently identical to the old one. You probably wouldn't deliberately write a trivial redefinition, but they can happen automatically when a header file is included more than once (see Header Files), so they are accepted silently and without effect.
Nontrivial redefinition is considered likely to be an error, so
it provokes a warning message from the preprocessor. However, sometimes it
is useful to change the definition of a macro in mid-compilation. You can
inhibit the warning by undefining the macro with #undef before the
second definition.
In order for a redefinition to be trivial, the new definition must exactly match the one already in effect, with two possible exceptions:
Recall that a comment counts as whitespace.
In this section we describe some special rules that apply to macros and macro expansion, and point out certain cases in which the rules have counterintuitive consequences that you must watch out for.
Recall that when a macro is called with arguments, the arguments are substituted into the macro body and the result is checked, together with the rest of the input file, for more macro calls.
It is possible to piece together a macro call coming partially from the macro body and partially from the actual arguments. For example,
#define double(x) (2*(x)) #define call_with_1(x) x(1)
would expand call_with_1 (double) into (2*(1)).
Macro definitions do not have to have balanced parentheses. By writing an unbalanced open parenthesis in a macro body, it is possible to create a macro call that begins inside the macro body but ends outside of it. For example,
#define strange(file) fprintf (file, "%s %d", ... strange(stderr) p, 35)
This bizarre example expands to fprintf (stderr, "%s %d", p, 35)!
You may have noticed that in most of the macro definition examples shown above, each occurrence of a macro argument name had parentheses around it. In addition, another pair of parentheses usually surround the entire macro definition. Here is why it is best to write macros that way.
Suppose you define a macro as follows,
#define ceil_div(x, y) (x + y - 1) / y
whose purpose is to divide, rounding up. (One use for this operation is
to compute how many int objects are needed to hold a certain
number of char objects.) Then suppose it is used as follows:
a = ceil_div (b & c, sizeof (int));
This expands into
a = (b & c + sizeof (int) - 1) / sizeof (int);
which does not do what is intended. The operator-precedence rules of C make it equivalent to this:
a = (b & (c + sizeof (int) - 1)) / sizeof (int);
But what we want is this:
a = ((b & c) + sizeof (int) - 1)) / sizeof (int);
Defining the macro as
#define ceil_div(x, y) ((x) + (y) - 1) / (y)
provides the desired result.
Unintended grouping can result in another way. Consider
sizeof ceil_div(1, 2). That has the appearance of a C expression
that would compute the size of the type of ceil_div (1, 2), but in
fact it means something very different. Here is what it expands to:
sizeof ((1) + (2) - 1) / (2)
This would take the size of an integer and divide it by two. The precedence
rules have put the division outside the sizeof when it was intended
to be inside.
Parentheses around the entire macro definition can prevent such problems.
Here, then, is the recommended way to define ceil_div:
#define ceil_div(x, y) (((x) + (y) - 1) / (y))
Often it is desirable to define a macro that expands into a compound
statement. Consider, for example, the following macro, that advances a
pointer (the argument p says where to find it) across whitespace
characters:
#define SKIP_SPACES(p, limit) \
{ register char *lim = (limit); \
while (p != lim) { \
if (*p++ != ' ') { \
p--; break; }}}
Here Backslash-Newline is used to split the macro definition, which must be a single line, so that it resembles the way such C code would be laid out if not part of a macro definition.
A call to this macro might be SKIP_SPACES (p, lim). Strictly
speaking, the call expands to a compound statement, which is a complete
statement with no need for a semicolon to end it. But it looks like a
function call. So it minimizes confusion if you can use it like a function
call, writing a semicolon afterward, as in SKIP_SPACES (p, lim);
But this can cause trouble before else statements, because the
semicolon is actually a null statement. Suppose you write
if (*p != 0) SKIP_SPACES (p, lim); else ...
The presence of two statements--the compound statement and a null
statement--in between the if condition and the else
makes invalid C code.
The definition of the macro SKIP_SPACES can be altered to solve
this problem, using a do ... while statement. Here is how:
#define SKIP_SPACES(p, limit) \
do { register char *lim = (limit); \
while (p != lim) { \
if (*p++ != ' ') { \
p--; break; }}} \
while (0)
Now SKIP_SPACES (p, lim); expands into
do {...} while (0);
which is one statement.
Many C programs define a macro min, for "minimum", like this:
#define min(X, Y) ((X) < (Y) ? (X) : (Y))
When you use this macro with an argument containing a side effect, as shown here,
next = min (x + y, foo (z));
it expands as follows:
next = ((x + y) < (foo (z)) ? (x + y) : (foo (z)));
where x + y has been substituted for X and foo (z)
for Y.
The function foo is used only once in the statement as it appears
in the program, but the expression foo (z) has been substituted
twice into the macro expansion. As a result, foo might be called
two times when the statement is executed. If it has side effects or
if it takes a long time to compute, the results might not be what you
intended. We say that min is an unsafe macro.
The best solution to this problem is to define min in a way that
computes the value of foo (z) only once. The C language offers no
standard way to do this, but it can be done with GNU C extensions as
follows:
#define min(X, Y) \
({ typeof (X) __x = (X), __y = (Y); \
(__x < __y) ? __x : __y; })
If you do not wish to use GNU C extensions, the only solution is to be
careful when using the macro min. For example, you can
calculate the value of foo (z), save it in a variable, and use that
variable in min:
#define min(X, Y) ((X) < (Y) ? (X) : (Y))
...
{
int tem = foo (z);
next = min (x + y, tem);
}
(where we assume that foo returns type int).
A self-referential macro is one whose name appears in its definition. A special feature of ANSI Standard C is that the self-reference is not considered a macro call. It is passed into the preprocessor output unchanged.
Let's consider an example:
#define foo (4 + foo)
where foo is also a variable in your program.
Following the ordinary rules, each reference to foo will expand into
(4 + foo); then this will be rescanned and will expand into (4
+ (4 + foo)); and so on until it causes a fatal error (memory full) in the
preprocessor.
However, the special rule about self-reference cuts this process short
after one step, at (4 + foo). Therefore, this macro definition
has the possibly useful effect of causing the program to add 4 to
the value of foo wherever foo is referred to.
In most cases, it is a bad idea to take advantage of this feature. A
person reading the program who sees that foo is a variable will
not expect that it is a macro as well. The reader will come across the
identifier foo in the program and think its value should be that
of the variable foo, whereas in fact the value is four greater.
The special rule for self-reference applies also to indirect
self-reference. This is the case where a macro x expands to use a
macro y, and the expansion of y refers to the macro
x. The resulting reference to x comes indirectly from the
expansion of x, so it is a self-reference and is not further
expanded. Thus, after
#define x (4 + y) #define y (2 * x)
x would expand into (4 + (2 * x)). Clear?
But suppose y is used elsewhere, not from the definition of x.
Then the use of x in the expansion of y is not a self-reference
because x is not "in progress". So it does expand. However,
the expansion of x contains a reference to y, and that
is an indirect self-reference now because y is "in progress".
The result is that y expands to (2 * (4 + y)).
It is not clear that this behavior would ever be useful, but it is specified by the ANSI C standard, so you may need to understand it.
We have explained that the expansion of a macro, including the substituted actual arguments, is scanned over again for macro calls to be expanded.
What really happens is more subtle: first each actual argument text is scanned separately for macro calls. Then the results of this are substituted into the macro body to produce the macro expansion, and the macro expansion is scanned again for macros to expand.
The result is that the actual arguments are scanned twice to expand macro calls in them.
Most of the time, this has no effect. If the actual argument contained any macro calls, they are expanded during the first scan. The result therefore contains no macro calls, so the second scan does not change it. If the actual argument were substituted as given, with no prescan, the single remaining scan would find the same macro calls and produce the same results.
You might expect the double scan to change the results when a self-referential macro is used in an actual argument of another macro (see Self-Reference): the self-referential macro would be expanded once in the first scan, and a second time in the second scan. But this is not what happens. The self-references that do not expand in the first scan are marked so that they will not expand in the second scan either.
The prescan is not done when an argument is stringified or concatenated. Thus,
#define str(s) #s #define foo 4 str (foo)
expands to "foo". Once more, prescan has been prevented from
having any noticeable effect.
More precisely, stringification and concatenation use the argument as written, in un-prescanned form. The same actual argument would be used in prescanned form if it is substituted elsewhere without stringification or concatenation.
#define str(s) #s lose(s) #define foo 4 str (foo)
expands to "foo" lose(4).
You might now ask, "Why mention the prescan, if it makes no difference? And why not skip it and make the preprocessor faster?" The answer is that the prescan does make a difference in three special cases:
We say that nested calls to a macro occur when a macro's actual
argument contains a call to that very macro. For example, if f
is a macro that expects one argument, f (f (1)) is a nested
pair of calls to f. The desired expansion is made by
expanding f (1) and substituting that into the definition of
f. The prescan causes the expected result to happen.
Without the prescan, f (1) itself would be substituted as
an actual argument, and the inner use of f would appear
during the main scan as an indirect self-reference and would not
be expanded. Here, the prescan cancels an undesirable side effect
(in the medical, not computational, sense of the term) of the special
rule for self-referential macros.
But prescan causes trouble in certain other cases of nested macro calls. Here is an example:
#define foo a,b #define bar(x) lose(x) #define lose(x) (1 + (x)) bar(foo)
We would like bar(foo) to turn into (1 + (foo)), which
would then turn into (1 + (a,b)). But instead, bar(foo)
expands into lose(a,b), and you get an error because lose
requires a single argument. In this case, the problem is easily solved
by the same parentheses that ought to be used to prevent misnesting of
arithmetic operations:
#define foo (a,b) #define bar(x) lose((x))
The problem is more serious when the operands of the macro are not expressions; for example, when they are statements. Then parentheses are unacceptable because they would make for invalid C code:
#define foo { int a, b; ... }
In GNU C you can shield the commas using the ({...})
construct which turns a compound statement into an expression:
#define foo ({ int a, b; ... })
Or you can rewrite the macro definition to avoid such commas:
#define foo { int a; int b; ... }
There is also one case where prescan is useful. It is possible
to use prescan to expand an argument and then stringify it--if you use
two levels of macros. Let's add a new macro xstr to the
example shown above:
#define xstr(s) str(s) #define str(s) #s #define foo 4 xstr (foo)
This expands into "4", not "foo". The reason for the
difference is that the argument of xstr is expanded at prescan
(because xstr does not specify stringification or concatenation of
the argument). The result of prescan then forms the actual argument for
str. str uses its argument without prescan because it
performs stringification; but it cannot prevent or undo the prescanning
already done by xstr.
A cascade of macros is when one macro's body contains a reference to another macro. This is very common practice. For example,
#define BUFSIZE 1020 #define TABLESIZE BUFSIZE
This is not at all the same as defining TABLESIZE to be 1020.
The #define for TABLESIZE uses exactly the body you
specify--in this case, BUFSIZE--and does not check to see whether
it too is the name of a macro.
It's only when you use TABLESIZE that the result of its expansion
is checked for more macro names.
This makes a difference if you change the definition of BUFSIZE
at some point in the source file. TABLESIZE, defined as shown,
will always expand using the definition of BUFSIZE that is
currently in effect:
#define BUFSIZE 1020 #define TABLESIZE BUFSIZE #undef BUFSIZE #define BUFSIZE 37
Now TABLESIZE expands (in two stages) to 37. (The
#undef is to prevent any warning about the nontrivial
redefinition of BUFSIZE.)
Traditional macro processing carries forward all newlines in macro arguments into the expansion of the macro. This means that, if some of the arguments are substituted more than once, or not at all, or out of order, newlines can be duplicated, lost, or moved around within the expansion. If the expansion consists of multiple statements, then the effect is to distort the line numbers of some of these statements. The result can be incorrect line numbers, in error messages or displayed in a debugger.
The GNU C preprocessor operating in ANSI C mode adjusts appropriately for multiple use of an argument--the first use expands all the newlines, and subsequent uses of the same argument produce no newlines. But even in this mode, it can produce incorrect line numbering if arguments are used out of order, or not used at all.
Here is an example illustrating this problem:
#define ignore_second_arg(a,b,c) a; c
ignore_second_arg (foo (),
ignored (),
syntax error);
The syntax error triggered by the tokens syntax error results
in an error message citing line four, even though the statement text
comes from line five.
In a macro processor, a conditional is a directive that allows a part of the program to be ignored during compilation, on some conditions. In the C preprocessor, a conditional can test either an arithmetic expression or whether a name is defined as a macro.
A conditional in the C preprocessor resembles in some ways an if
statement in C, but it is important to understand the difference between
them. The condition in an if statement is tested during the execution
of your program. Its purpose is to allow your program to behave differently
from run to run, depending on the data it is operating on. The condition
in a preprocessing conditional directive is tested when your program is compiled.
Its purpose is to allow different code to be included in the program depending
on the situation at the time of compilation.
Generally there are three kinds of reason to use a conditional.
Most simple programs that are intended to run on only one machine will not need to use preprocessing conditionals.
A conditional in the C preprocessor begins with a conditional
directive: #if, #ifdef or #ifndef.
See Conditionals-Macros, for information on #ifdef and
#ifndef; only #if is explained here.
#if and #endif.
#if DirectiveThe #if directive in its simplest form consists of
#if expression controlled text #endif /* expression */
The comment following the #endif is not required, but it is a good
practice because it helps people match the #endif to the
corresponding #if. Such comments should always be used, except in
short conditionals that are not nested. In fact, you can put anything at
all after the #endif and it will be ignored by the GNU C preprocessor,
but only comments are acceptable in ANSI Standard C.
expression is a C expression of integer type, subject to stringent restrictions. It may contain
long or
unsigned long.
char for these character constants; therefore, whether some
character codes are negative is determined by the C compiler used to
compile the preprocessor. If it treats char as signed, then
character codes large enough to set the sign bit will be considered
negative; otherwise, no character code is considered negative.
&& and ||).
Note that sizeof operators and enum-type values are not allowed.
enum-type values, like all other identifiers that are not taken
as macro calls and expanded, are treated as zero.
The controlled text inside of a conditional can include
preprocessing directives. Then the directives inside the conditional are
obeyed only if that branch of the conditional succeeds. The text can
also contain other conditional groups. However, the #if and
#endif directives must balance.
#else DirectiveThe #else directive can be added to a conditional to provide
alternative text to be used if the condition is false. This is what
it looks like:
#if expression text-if-true #else /* Not expression */ text-if-false #endif /* Not expression */
If expression is nonzero, and thus the text-if-true is
active, then #else acts like a failing conditional and the
text-if-false is ignored. Contrariwise, if the #if
conditional fails, the text-if-false is considered included.
#elif DirectiveOne common case of nested conditionals is used to check for more than two possible alternatives. For example, you might have
#if X == 1 ... #else /* X != 1 */ #if X == 2 ... #else /* X != 2 */ ... #endif /* X != 2 */ #endif /* X != 1 */
Another conditional directive, #elif, allows this to be abbreviated
as follows:
#if X == 1 ... #elif X == 2 ... #else /* X != 2 and X != 1*/ ... #endif /* X != 2 and X != 1*/
#elif stands for "else if". Like #else, it goes in the
middle of a #if-#endif pair and subdivides it; it does not
require a matching #endif of its own. Like #if, the
#elif directive includes an expression to be tested.
The text following the #elif is processed only if the original
#if-condition failed and the #elif condition succeeds.
More than one #elif can go in the same #if-#endif
group. Then the text after each #elif is processed only if the
#elif condition succeeds after the original #if and any
previous #elif directives within it have failed. #else is
equivalent to #elif 1, and #else is allowed after any
number of #elif directives, but #elif may not follow
#else.
If you replace or delete a part of the program but want to keep the old
code around as a comment for future reference, the easy way to do this
is to put #if 0 before it and #endif after it. This is
better than using comment delimiters /* and */ since those
won't work if the code already contains comments (C comments do not
nest).
This works even if the code being turned off contains conditionals, but
they must be entire conditionals (balanced #if and #endif).
Conversely, do not use #if 0 for comments which are not C code.
Use the comment delimiters /* and */ instead. The
interior of #if 0 must consist of complete tokens; in particular,
singlequote characters must balance. But comments often contain
unbalanced singlequote characters (known in English as apostrophes).
These confuse #if 0. They do not confuse /*.
Conditionals are useful in connection with macros or assertions, because
those are the only ways that an expression's value can vary from one
compilation to another. A #if directive whose expression uses no
macros or assertions is equivalent to #if 1 or #if 0; you
might as well determine which one, by computing the value of the
expression yourself, and then simplify the program.
For example, here is a conditional that tests the expression
BUFSIZE == 1020, where BUFSIZE must be a macro.
#if BUFSIZE == 1020
printf ("Large buffers!\n");
#endif /* BUFSIZE is large */
(Programmers often wish they could test the size of a variable or data
type in #if, but this does not work. The preprocessor does not
understand sizeof, or typedef names, or even the type keywords
such as int.)
The special operator defined is used in #if expressions to
test whether a certain name is defined as a macro. Either defined
name or defined (name) is an expression whose value
is 1 if name is defined as macro at the current point in the
program, and 0 otherwise. For the defined operator it makes no
difference what the definition of the macro is; all that matters is
whether there is a definition. Thus, for example,
#if defined (vax) || defined (ns16000)
would succeed if either of the names vax and ns16000 is
defined as a macro. You can test the same condition using assertions
(see Assertions), like this:
#if #cpu (vax) || #cpu (ns16000)
If a macro is defined and later undefined with #undef,
subsequent use of the defined operator returns 0, because
the name is no longer defined. If the macro is defined again with
another #define, defined will recommence returning 1.
Conditionals that test whether just one name is defined are very common, so there are two special short conditional directives for this case.
#ifdef name
#if defined (name).
#ifndef name
#if ! defined (name).
Macro definitions can vary between compilations for several reasons.
vax is a predefined macro. On other machines, it
would not be defined.
BUFSIZE might be defined in a configuration file for your
program that is included as a header file in each source file. You
would use BUFSIZE in a preprocessing conditional in order to
generate different code depending on the chosen configuration.
-D and -U
command options when you compile the program. You can arrange to
compile the same source file into two different programs by choosing
a macro name to specify which program you want, writing conditionals
to test whether or how this macro is defined, and then controlling
the state of the macro with compiler command options.
See Invocation.
Assertions are a more systematic alternative to macros in writing conditionals to test what sort of computer or system the compiled program will run on. Assertions are usually predefined, but you can define them with preprocessing directives or command-line options.
The macros traditionally used to describe the type of target are not classified in any way according to which question they answer; they may indicate a hardware architecture, a particular hardware model, an operating system, a particular version of an operating system, or specific configuration options. These are jumbled together in a single namespace. In contrast, each assertion consists of a named question and an answer. The question is usually called the predicate. An assertion looks like this:
#predicate (answer)
You must use a properly formed identifier for predicate. The
value of answer can be any sequence of words; all characters are
significant except for leading and trailing whitespace, and differences
in internal whitespace sequences are ignored. Thus, x + y is
different from x+y but equivalent to x + y. ) is
not allowed in an answer.
Here is a conditional to test whether the answer answer is asserted for the predicate predicate:
#if #predicate (answer)
There may be more than one answer asserted for a given predicate. If you omit the answer, you can test whether any answer is asserted for predicate:
#if #predicate
Most of the time, the assertions you test will be predefined assertions.
GNU C provides three predefined predicates: system, cpu,
and machine. system is for assertions about the type of
software, cpu describes the type of computer architecture, and
machine gives more information about the computer. For example,
on a GNU system, the following assertions would be true:
#system (gnu) #system (mach) #system (mach 3) #system (mach 3.subversion) #system (hurd) #system (hurd version)
and perhaps others. The alternatives with more or less version information let you ask more or less detailed questions about the type of system software.
On a Unix system, you would find #system (unix) and perhaps one of:
#system (aix), #system (bsd), #system (hpux),
#system (lynx), #system (mach), #system (posix),
#system (svr3), #system (svr4), or #system (xpg4)
with possible version numbers following.
Other values for system are #system (mvs)
and #system (vms).
Portability note: Many Unix C compilers provide only one answer
for the system assertion: #system (unix), if they support
assertions at all. This is less than useful.
An assertion with a multi-word answer is completely different from several
assertions with individual single-word answers. For example, the presence
of system (mach 3.0) does not mean that system (3.0) is true.
It also does not directly imply system (mach), but in GNU C, that
last will normally be asserted as well.
The current list of possible assertion values for cpu is:
#cpu (a29k), #cpu (alpha), #cpu (arm), #cpu
(clipper), #cpu (convex), #cpu (elxsi), #cpu
(tron), #cpu (h8300), #cpu (i370), #cpu (i386),
#cpu (i860), #cpu (i960), #cpu (m68k), #cpu
(m88k), #cpu (mips), #cpu (ns32k), #cpu (hppa),
#cpu (pyr), #cpu (ibm032), #cpu (rs6000),
#cpu (sh), #cpu (sparc), #cpu (spur), #cpu
(tahoe), #cpu (vax), #cpu (we32000).
You can create assertions within a C program using #assert, like
this:
#assert predicate (answer)
(Note the absence of a # before predicate.)
Each time you do this, you assert a new true answer for predicate.
Asserting one answer does not invalidate previously asserted answers;
they all remain true. The only way to remove an assertion is with
#unassert. #unassert has the same syntax as
#assert. You can also remove all assertions about
predicate like this:
#unassert predicate
You can also add or cancel assertions using command options
when you run gcc or cpp. See Invocation.
#error and #warning DirectivesThe directive #error causes the preprocessor to report a fatal
error. The rest of the line that follows #error is used as the
error message. The line must consist of complete tokens.
You would use #error inside of a conditional that detects a
combination of parameters which you know the program does not properly
support. For example, if you know that the program will not run
properly on a Vax, you might write
#ifdef __vax__ #error "Won't work on Vaxen. See comments at get_last_object." #endif
See Nonstandard Predefined, for why this works.
If you have several configuration parameters that must be set up by
the installation in a consistent way, you can use conditionals to detect
an inconsistency and report it with #error. For example,
#if HASH_TABLE_SIZE % 2 == 0 || HASH_TABLE_SIZE % 3 == 0 \
|| HASH_TABLE_SIZE % 5 == 0
#error HASH_TABLE_SIZE should not be divisible by a small prime
#endif
The directive #warning is like the directive #error, but causes
the preprocessor to issue a warning and continue preprocessing. The rest of
the line that follows #warning is used as the warning message.
You might use #warning in obsolete header files, with a message
directing the user to the header file which should be used instead.
One of the jobs of the C preprocessor is to inform the C compiler of where each line of C code came from: which source file and which line number.
C code can come from multiple source files if you use #include;
both #include and the use of conditionals and macros can cause
the line number of a line in the preprocessor output to be different
from the line's number in the original source file. You will appreciate
the value of making both the C compiler (in error messages) and symbolic
debuggers such as GDB use the line numbers in your source file.
The C preprocessor builds on this feature by offering a directive by which
you can control the feature explicitly. This is useful when a file for
input to the C preprocessor is the output from another program such as the
bison parser generator, which operates on another file that is the
true source file. Parts of the output from bison are generated from
scratch, other parts come from a standard parser file. The rest are copied
nearly verbatim from the source file, but their line numbers in the
bison output are not the same as their original line numbers.
Naturally you would like compiler error messages and symbolic debuggers to
know the original source file and line number of each line in the
bison input.
bison arranges this by writing #line directives into the output
file. #line is a directive that specifies the original line number
and source file name for subsequent input in the current preprocessor input
file. #line has three variants:
#line linenum
#line linenum filename
#line anything else
#line directives alter the results of the __FILE__ and
__LINE__ predefined macros from that point on. See Standard Predefined.
The output of the preprocessor (which is the input for the rest of the
compiler) contains directives that look much like #line directives.
They start with just # instead of #line, but this is
followed by a line number and file name as in #line. See Output.
This section describes three additional preprocessing directives. They are not very useful, but are mentioned for completeness.
The null directive consists of a # followed by a Newline, with
only whitespace (including comments) in between. A null directive is
understood as a preprocessing directive but has no effect on the preprocessor
output. The primary significance of the existence of the null directive is
that an input line consisting of just a # will produce no output,
rather than a line of output containing just a #. Supposedly
some old C programs contain such lines.
The ANSI standard specifies that the effect of the #pragma
directive is implementation-defined. In the GNU C preprocessor,
#pragma directives are not used, except for #pragma once
(see Once-Only). However, they are left in the preprocessor output,
so they are available to the compilation pass.
The #ident directive is supported for compatibility with certain
other systems. It is followed by a line of text. On some systems, the
text is copied into a special place in the object file; on most systems,
the text is ignored and this directive has no effect. Typically
#ident is only used in header files supplied with those systems
where it is meaningful.
The output from the C preprocessor looks much like the input, except
that all preprocessing directive lines have been replaced with blank lines
and all comments with spaces. Whitespace within a line is not altered;
however, unless -traditional is used, spaces may be inserted into
the expansions of macro calls to prevent tokens from being concatenated.
Source file name and line number information is conveyed by lines of the form
# linenum filename flags
which are inserted as needed into the middle of the input (but never within a string or character constant). Such a line means that the following line originated in file filename at line linenum.
After the file name comes zero or more flags, which are 1,
2, 3, or 4. If there are multiple flags, spaces separate
them. Here is what the flags mean:
1
2
3
4
Most often when you use the C preprocessor you will not have to invoke it explicitly: the C compiler will do so automatically. However, the preprocessor is sometimes useful on its own.
The C preprocessor expects two file names as arguments, infile and
outfile. The preprocessor reads infile together with any other
files it specifies with #include. All the output generated by the
combined input files is written in outfile.
Either infile or outfile may be -, which as infile
means to read from standard input and as outfile means to write to
standard output. Also, if outfile or both file names are omitted,
the standard output and standard input are used for the omitted file names.
Here is a table of command options accepted by the C preprocessor. These options can also be given when compiling a C program; they are passed along automatically to the preprocessor when it is invoked by the compiler.
-P
#-lines with line-number information in
the output from the preprocessor (see Output). This might be
useful when running the preprocessor on something that is not C code
and will be sent to a program which might be confused by the
#-lines.
-C
-traditional
1.0e+4 to be three tokens: 1.0e, +,
and 4.
# has no special meaning within a macro definition
in traditional C.
\ inside a macro argument suppresses the syntactic
significance of the following character.
Use the -traditional option when preprocessing Fortran code,
so that singlequotes and doublequotes
within Fortran comment lines
(which are generally not recognized as such by the preprocessor)
do not cause diagnostics
about unterminated character or string constants.
However, this option does not prevent diagnostics about unterminated comments when a C-style comment appears to start, but not end, within Fortran-style commentary.
So, the following Fortran comment lines are accepted with
-traditional:
C This isn't an unterminated character constant C Neither is "20000000000, an octal constant C in some dialects of Fortran
However, this type of comment line will likely produce a diagnostic, or at least unexpected output from the preprocessor, due to the unterminated comment:
C Some Fortran compilers accept /* as starting C an inline comment.
Note that g77 automatically supplies
the -traditional option
when it invokes the preprocessor.
However, a future version of g77
might use a different, more-Fortran-aware preprocessor
in place of cpp.
-trigraphs
??, that are defined by ANSI C to
stand for single characters. For example, ??/ stands for
\, so '??/n' is a character constant for a newline.
Strictly speaking, the GNU C preprocessor does not support all
programs in ANSI Standard C unless -trigraphs is used, but if
you ever notice the difference it will be with relief.
You don't want to know any more about trigraphs.
-pedantic
#else or #endif.
-pedantic-errors
-pedantic, except that errors are produced rather than
warnings.
-Wtrigraphs
-trigraphs or -ansi; in the
future this restriction will be removed.
-Wcomment
/* appears in a /*
comment, or whenever a Backslash-Newline appears in a // comment.
-Wall
-Wtrigraphs and -Wcomment (but not
-Wtraditional or -Wundef).
-Wtraditional
-Wundef
#if directive.
-I directory
-I option,
the directories are scanned in left-to-right order; the standard
system directories come after.
-I-
-I options before the -I-
option are searched only for the case of #include "file";
they are not searched for #include <file>.
If additional directories are specified with -I options after
the -I-, these directories are searched for all #include
directives.
In addition, the -I- option inhibits the use of the current
directory as the first search directory for #include "file".
Therefore, the current directory is searched only if it is requested
explicitly with -I.. Specifying both -I- and -I.
allows you to control precisely which directories are searched before
the current one and which are searched after.
-nostdinc
-I options
(and the current directory, if appropriate) are searched.
-nostdinc++
-remap
header.gcc exists in that directory. This can be used
to work around limitations of file systems with file name restrictions.
The header.gcc file should contain a series of lines with two
tokens on each line: the first token is the name to map, and the second
token is the actual name to use.
-D name
1.
-D name=definition
-D for the same name, the rightmost definition takes
effect.
-U name
-U and -D are
specified for one name, the -U beats the -D and the name
is not predefined.
-undef
-gcc
gcc -E; you can turn them off
in that case with -no-gcc.
-A predicate(answer)
You can use -A- to disable all predefined assertions; it also
undefines all predefined macros and all macros that preceded it on the
command line.
-dM
#define directives for all the macros defined during the
execution of the preprocessor, including predefined macros. This gives
you a way of finding out what is predefined in your version of the
preprocessor; assuming you have no file foo.h, the command
touch foo.h; cpp -dM foo.h
will show the values of any predefined macros.
-dD
-dM except in two respects: it does not include the
predefined macros, and it outputs both the #define
directives and the result of preprocessing. Both kinds of output go to
the standard output file.
-dI
#include directives in addition to the result of preprocessing.
-M [-MG]
make describing the dependencies of the main
source file. The preprocessor outputs one make rule containing
the object file name for that source file, a colon, and the names of
all the included files. If there are many included files then the
rule is split into several lines using \-newline.
-MG says to treat missing header files as generated files and assume
they live in the same directory as the source file. It must be specified
in addition to -M.
This feature is used in automatic updating of makefiles.
-MM [-MG]
-M but mention only the files included with #include
"file". System header files included with #include
<file> are omitted.
-MD file
-M but the dependency information is written to file.
This is in addition to compiling the file as specified---MD does
not inhibit ordinary compilation the way -M does.
When invoking gcc, do not specify the file argument.
gcc will create file names made by replacing ".c" with ".d" at
the end of the input file names.
In Mach, you can use the utility md to merge multiple dependency
files into a single dependency file suitable for using with the make
command.
-MMD file
-MD except mention only user header files, not system
header files.
-H
-imacros file
-imacros file
is to make the macros defined in file available for use in the
main input.
-include file
-idirafter dir
-I adds to).
-iprefix prefix
-iwithprefix
options.
-iwithprefix dir
-iprefix.
-isystem dir
-x c
-x c++
-x objective-c
-x assembler-with-cpp
.c, .cc, .m, or .S. Some other common
extensions for C++ and assembly are also recognized. If cpp does not
recognize the extension, it will treat the file as C; this is the most
generic mode.
Note: Previous versions of cpp accepted a -lang option
which selected both the language and the standards conformance level.
This option has been removed, because it conflicts with the -l
option.
-std=standard
-ansi
standard may be one of:
iso9899:1990
iso9899:199409
c89
c89 is the customary
shorthand for this version of the standard.
The -ansi option is equivalent to -std=c89.
iso9899:199x
c9x
x.
gnu89
gnu9x
-Wp,-lint
lint embedded in
comments, and emit them preceded by #pragma lint. For example,
the comment /* NOTREACHED */ becomes #pragma lint
NOTREACHED.
Because of the clash with -l, you must use the awkward syntax
above. In a future release, this option will be replaced by
-flint or -Wlint; we are not sure which yet.
-$
$ in identifiers. The C standard does not
permit this, but it is a common extension.
##: Concatenation
#include: Include Syntax
#assert: Assertions
#cpu: Assertions
#define: Argument Macros
#elif: #elif Directive
#else: #else Directive
#error: #error Directive
#ident: Other Directives
#if: Conditional Syntax
#ifdef: Conditionals-Macros
#ifndef: Conditionals-Macros
#import: Once-Only
#include: Include Syntax
#include_next: Inheritance
#line: Combining Sources
#machine: Assertions
#pragma: Other Directives
#pragma once: Once-Only
#system: Assertions
#unassert: Assertions
#warning: #error Directive
-$: Invocation
-A: Invocation
-ansi: Invocation
-C: Invocation
-D: Invocation
-dD: Invocation
-dI: Invocation
-dM: Invocation
-gcc: Invocation
-H: Invocation
-I: Invocation
-idirafter: Invocation
-imacros: Invocation
-include: Invocation
-iprefix: Invocation
-isystem: Invocation
-iwithprefix: Invocation
-lint: Invocation
-M: Invocation
-MD: Invocation
-MM: Invocation
-MMD: Invocation
-nostdinc: Invocation
-nostdinc++: Invocation
-P: Invocation
-pedantic: Invocation
-pedantic-errors: Invocation
-remap: Invocation
-std: Invocation
-traditional: Invocation
-trigraphs: Invocation
-U: Invocation
-undef: Invocation
-Wall: Invocation
-Wcomment: Invocation
-Wtraditional: Invocation
-Wtrigraphs: Invocation
-Wundef: Invocation
-x assembler-with-cpp: Invocation
-x c: Invocation
-x objective-c: Invocation
__BASE_FILE__: Standard Predefined
__CHAR_UNSIGNED__: Standard Predefined
__cplusplus: Standard Predefined
__DATE__: Standard Predefined
__FILE__: Standard Predefined
__GNUC__: Standard Predefined
__GNUC_MINOR__: Standard Predefined
__GNUG__: Standard Predefined
__INCLUDE_LEVEL_: Standard Predefined
__LINE__: Standard Predefined
__OPTIMIZE__: Standard Predefined
__REGISTER_PREFIX__: Standard Predefined
__STDC__: Standard Predefined
__STDC_VERSION__: Standard Predefined
__STRICT_ANSI__: Standard Predefined
__TIME__: Standard Predefined
__USER_LABEL_PREFIX__: Standard Predefined
__VERSION__: Standard Predefined
_AM29000: Nonstandard Predefined
_AM29K: Nonstandard Predefined
BSD: Nonstandard Predefined
defined: Conditionals-Macros
M68020: Nonstandard Predefined
m68k: Nonstandard Predefined
mc68000: Nonstandard Predefined
ns32000: Nonstandard Predefined
pyr: Nonstandard Predefined
sequent: Nonstandard Predefined
sun: Nonstandard Predefined
system header files: Header Uses
unix: Nonstandard Predefined
vax: Nonstandard Predefined