The C preprocessor implements the macro language used to transform C, C++, and Objective-C programs before they are compiled. It can also be useful on its own.
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The C preprocessor, often known as cpp, is a macro processor that is used automatically by the C compiler to transform your program before compilation. It is called a macro processor because it allows you to define macros, which are brief abbreviations for longer constructs.
The C preprocessor is intended to be used only with C, C++, and Objective-C source code. In the past, it has been abused as a general text processor. It will choke on input which does not obey C's lexical rules. For example, apostrophes will be interpreted as the beginning of character constants, and cause errors. Also, you cannot rely on it preserving characteristics of the input which are not significant to C-family languages. If a Makefile is preprocessed, all the hard tabs will be removed, and the Makefile will not work.
Having said that, you can often get away with using cpp on things which
are not C. Other Algol-ish programming languages are often safe
(Pascal, Ada, etc.) So is assembly, with caution. -traditional-cpp
mode preserves more white space, and is otherwise more permissive. Many
of the problems can be avoided by writing C or C++ style comments
instead of native language comments, and keeping macros simple.
Wherever possible, you should use a preprocessor geared to the language you are writing in. Modern versions of the GNU assembler have macro facilities. Most high level programming languages have their own conditional compilation and inclusion mechanism. If all else fails, try a true general text processor, such as GNU M4.
C preprocessors vary in some details. This manual discusses the GNU C
preprocessor, which provides a small superset of the features of ISO
Standard C. In its default mode, the GNU C preprocessor does not do a
few things required by the standard. These are features which are
rarely, if ever, used, and may cause surprising changes to the meaning
of a program which does not expect them. To get strict ISO Standard C,
you should use the -std=c89 or -std=c99 options, depending
on which version of the standard you want. To get all the mandatory
diagnostics, you must also use -pedantic. See Invocation.
This manual describes the behavior of the ISO preprocessor. To minimize gratuitous differences, where the ISO preprocessor's behavior does not conflict with traditional semantics, the traditional preprocessor should behave the same way. The various differences that do exist are detailed in the section Traditional Mode.
For clarity, unless noted otherwise, references to CPP in this
manual refer to GNU CPP.
The preprocessor performs a series of textual transformations on its input. These happen before all other processing. Conceptually, they happen in a rigid order, and the entire file is run through each transformation before the next one begins. CPP actually does them all at once, for performance reasons. These transformations correspond roughly to the first three "phases of translation" described in the C standard.
CPP expects its input to be a text file, that is, an unstructured stream of ASCII characters, with some characters indicating the end of a line of text. Extended ASCII character sets, such as ISO Latin-1 or Unicode encoded in UTF-8, are also acceptable. Character sets that are not strict supersets of seven-bit ASCII will not work. We plan to add complete support for international character sets in a future release.
Different systems use different conventions to indicate the end of a line. GCC accepts the ASCII control sequences LF, CR LF, CR, and LF CR as end-of-line markers. The first three are the canonical sequences used by Unix, DOS and VMS, and the classic Mac OS (before OSX) respectively. You may therefore safely copy source code written on any of those systems to a different one and use it without conversion. (GCC may lose track of the current line number if a file doesn't consistently use one convention, as sometimes happens when it is edited on computers with different conventions that share a network file system.) LF CR is included because it has been reported as an end-of-line marker under exotic conditions.
If the last line of any input file lacks an end-of-line marker, the end of the file is considered to implicitly supply one. The C standard says that this condition provokes undefined behavior, so GCC will emit a warning message.
-std
option, or you specify the -trigraphs option, then it
converts them.
These are nine three-character sequences, all starting with ??,
that are defined by ISO C to stand for single characters. They permit
obsolete systems that lack some of C's punctuation to use C. For
example, ??/ stands for \, so '??/n' is a character
constant for a newline.
Trigraphs are not popular and many compilers implement them incorrectly.
Portable code should not rely on trigraphs being either converted or
ignored. If you use the -Wall or -Wtrigraphs options,
GCC will warn you when a trigraph would change the meaning of your
program if it were converted.
In a string constant, you can prevent a sequence of question marks from
being confused with a trigraph by inserting a backslash between the
question marks. "(??\?)" is the string (???), not
(?]. Traditional C compilers do not recognize this idiom.
The nine trigraphs and their replacements are
Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??-
Replacement: [ ] { } # \ ^ | ~
A continued line is a line which ends with a backslash, \. The
backslash is removed and the following line is joined with the current
one. No space is inserted, so you may split a line anywhere, even in
the middle of a word. (It is generally more readable to split lines
only at white space.)
The trailing backslash on a continued line is commonly referred to as a backslash-newline.
If there is white space between a backslash and the end of a line, that is still a continued line. However, as this is usually the result of an editing mistake, and many compilers will not accept it as a continued line, GCC will warn you about it.
There are two kinds of comments. Block comments begin with
/* and continue until the next */. Block comments do not
nest:
/* this is /* one comment */ text outside comment
Line comments begin with // and continue to the end of the
current line. Line comments do not nest either, but it does not matter,
because they would end in the same place anyway.
// this is // one comment
text outside comment
It is safe to put line comments inside block comments, or vice versa.
/* block comment
// contains line comment
yet more comment
*/ outside comment
// line comment /* contains block comment */
But beware of commenting out one end of a block comment with a line comment.
// l.c. /* block comment begins
oops! this isn't a comment anymore */
Comments are not recognized within string literals.
"/* blah */" is the string constant /* blah */, not
an empty string.
Line comments are not in the 1989 edition of the C standard, but they are recognized by GCC as an extension. In C++ and in the 1999 edition of the C standard, they are an official part of the language.
Since these transformations happen before all other processing, you can
split a line mechanically with backslash-newline anywhere. You can
comment out the end of a line. You can continue a line comment onto the
next line with backslash-newline. You can even split /*,
*/, and // onto multiple lines with backslash-newline.
For example:
/\
*
*/ # /*
*/ defi\
ne FO\
O 10\
20
is equivalent to #define FOO 1020. All these tricks are
extremely confusing and should not be used in code intended to be
readable.
There is no way to prevent a backslash at the end of a line from being interpreted as a backslash-newline. This cannot affect any correct program, however.
After the textual transformations are finished, the input file is converted into a sequence of preprocessing tokens. These mostly correspond to the syntactic tokens used by the C compiler, but there are a few differences. White space separates tokens; it is not itself a token of any kind. Tokens do not have to be separated by white space, but it is often necessary to avoid ambiguities.
When faced with a sequence of characters that has more than one possible
tokenization, the preprocessor is greedy. It always makes each token,
starting from the left, as big as possible before moving on to the next
token. For instance, a+++++b is interpreted as
a ++ ++ + b, not as a ++ + ++ b, even though the
latter tokenization could be part of a valid C program and the former
could not.
Once the input file is broken into tokens, the token boundaries never
change, except when the ## preprocessing operator is used to paste
tokens together. See Concatenation. For example,
#define foo() bar
foo()baz
==> bar baz
not
==> barbaz
The compiler does not re-tokenize the preprocessor's output. Each preprocessing token becomes one compiler token.
Preprocessing tokens fall into five broad classes: identifiers,
preprocessing numbers, string literals, punctuators, and other. An
identifier is the same as an identifier in C: any sequence of
letters, digits, or underscores, which begins with a letter or
underscore. Keywords of C have no significance to the preprocessor;
they are ordinary identifiers. You can define a macro whose name is a
keyword, for instance. The only identifier which can be considered a
preprocessing keyword is defined. See Defined.
This is mostly true of other languages which use the C preprocessor. However, a few of the keywords of C++ are significant even in the preprocessor. See C++ Named Operators.
In the 1999 C standard, identifiers may contain letters which are not
part of the "basic source character set," at the implementation's
discretion (such as accented Latin letters, Greek letters, or Chinese
ideograms). This may be done with an extended character set, or the
\u and \U escape sequences. GCC does not presently
implement either feature in the preprocessor or the compiler.
As an extension, GCC treats $ as a letter. This is for
compatibility with some systems, such as VMS, where $ is commonly
used in system-defined function and object names. $ is not a
letter in strictly conforming mode, or if you specify the -$
option. See Invocation.
A preprocessing number has a rather bizarre definition. The
category includes all the normal integer and floating point constants
one expects of C, but also a number of other things one might not
initially recognize as a number. Formally, preprocessing numbers begin
with an optional period, a required decimal digit, and then continue
with any sequence of letters, digits, underscores, periods, and
exponents. Exponents are the two-character sequences e+,
e-, E+, E-, p+, p-, P+, and
P-. (The exponents that begin with p or P are new
to C99. They are used for hexadecimal floating-point constants.)
The purpose of this unusual definition is to isolate the preprocessor
from the full complexity of numeric constants. It does not have to
distinguish between lexically valid and invalid floating-point numbers,
which is complicated. The definition also permits you to split an
identifier at any position and get exactly two tokens, which can then be
pasted back together with the ## operator.
It's possible for preprocessing numbers to cause programs to be
misinterpreted. For example, 0xE+12 is a preprocessing number
which does not translate to any valid numeric constant, therefore a
syntax error. It does not mean 0xE + 12, which is what you
might have intended.
String literals are string constants, character constants, and
header file names (the argument of #include).1 String constants and character
constants are straightforward: "..." or '...'. In
either case embedded quotes should be escaped with a backslash:
'\'' is the character constant for '. There is no limit on
the length of a character constant, but the value of a character
constant that contains more than one character is
implementation-defined. See Implementation Details.
Header file names either look like string constants, "...", or are written with angle brackets instead, <...>. In either case, backslash is an ordinary character. There is no way to escape the closing quote or angle bracket. The preprocessor looks for the header file in different places depending on which form you use. See Include Operation.
No string literal may extend past the end of a line. Older versions of GCC accepted multi-line string constants. You may use continued lines instead, or string constant concatenation. See Differences from previous versions.
Punctuators are all the usual bits of punctuation which are
meaningful to C and C++. All but three of the punctuation characters in
ASCII are C punctuators. The exceptions are @, $, and
`. In addition, all the two- and three-character operators are
punctuators. There are also six digraphs, which the C++ standard
calls alternative tokens, which are merely alternate ways to spell
other punctuators. This is a second attempt to work around missing
punctuation in obsolete systems. It has no negative side effects,
unlike trigraphs, but does not cover as much ground. The digraphs and
their corresponding normal punctuators are:
Digraph: <% %> <: :> %: %:%:
Punctuator: { } [ ] # ##
Any other single character is considered "other." It is passed on to
the preprocessor's output unmolested. The C compiler will almost
certainly reject source code containing "other" tokens. In ASCII, the
only other characters are @, $, `, and control
characters other than NUL (all bits zero). (Note that $ is
normally considered a letter.) All characters with the high bit set
(numeric range 0x7F-0xFF) are also "other" in the present
implementation. This will change when proper support for international
character sets is added to GCC.
NUL is a special case because of the high probability that its appearance is accidental, and because it may be invisible to the user (many terminals do not display NUL at all). Within comments, NULs are silently ignored, just as any other character would be. In running text, NUL is considered white space. For example, these two directives have the same meaning.
#define X^@1
#define X 1
(where ^@ is ASCII NUL). Within string or character constants,
NULs are preserved. In the latter two cases the preprocessor emits a
warning message.
After tokenization, the stream of tokens may simply be passed straight to the compiler's parser. However, if it contains any operations in the preprocessing language, it will be transformed first. This stage corresponds roughly to the standard's "translation phase 4" and is what most people think of as the preprocessor's job.
The preprocessing language consists of directives to be executed and macros to be expanded. Its primary capabilities are:
There are a few more, less useful, features.
Except for expansion of predefined macros, all these operations are
triggered with preprocessing directives. Preprocessing directives
are lines in your program that start with #. Whitespace is
allowed before and after the #. The # is followed by an
identifier, the directive name. It specifies the operation to
perform. Directives are commonly referred to as #name
where name is the directive name. For example, #define is
the directive that defines a macro.
The # which begins a directive cannot come from a macro
expansion. Also, the directive name is not macro expanded. Thus, if
foo is defined as a macro expanding to define, that does
not make #foo a valid preprocessing directive.
The set of valid directive names is fixed. Programs cannot define new preprocessing directives.
Some directives 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.
A preprocessing directive cannot cover more than one line. The line may, however, be continued with backslash-newline, or by a block comment which extends past the end of the line. In either case, when the directive is processed, the continuations have already been merged with the first line to make one long line.
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 by including it, with the
C preprocessing directive #include.
Header files serve two purposes.
Including a header file produces the same results as copying the header file into each source file that needs it. 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.
In C, the usual convention is to give header files names that end with
.h. It is most portable to use only letters, digits, dashes, and
underscores in header file names, and at most one dot.
Both user and system header files are included using the preprocessing
directive #include. It has two variants:
#include <file>
-I option (see Invocation).
#include "file"
<file>.
The argument of #include, whether delimited with quote marks or
angle brackets, behaves like a string constant in that comments are not
recognized, and macro names are not expanded. Thus, #include <x/*y> specifies inclusion of a system header file named x/*y.
However, 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. (Some systems interpret \ as a pathname separator.
All of these also interpret / the same way. It is most portable
to use only /.)
It is an error if there is anything (other than comments) on the line after the file name.
The #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, if you have a header file
header.h as follows,
char *test (void);
and a main program called program.c that uses the header file,
like this,
int x;
#include "header.h"
int
main (void)
{
puts (test ());
}
the compiler will see the same token stream as it would if
program.c read
int x;
char *test (void);
int
main (void)
{
puts (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, an included file must consist of complete tokens. Comments and string literals which have not been closed by the end of an included file are invalid. For error recovery, they are considered to end at the end of the file.
To avoid confusion, it is best if header files contain only complete syntactic units--function declarations or definitions, type declarations, etc.
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.
GCC looks in several different places for headers. On a normal Unix
system, if you do not instruct it otherwise, it will look for headers
requested with #include <file> in:
/usr/local/include
/usr/lib/gcc-lib/target/version/include
/usr/target/include
/usr/include
For C++ programs, it will also look in /usr/include/g++-v3,
first. In the above, target is the canonical name of the system
GCC was configured to compile code for; often but not always the same as
the canonical name of the system it runs on. version is the
version of GCC in use.
You can add to this list with the -Idir command line
option. All the directories named by -I are searched, in
left-to-right order, before the default directories. The only
exception is when dir is already searched by default. In
this case, the option is ignored and the search order for system
directories remains unchanged.
Duplicate directories are removed from the quote and bracket search chains before the two chains are merged to make the final search chain. Thus, it is possible for a directory to occur twice in the final search chain if it was specified in both the quote and bracket chains.
You can prevent GCC from searching any of the default directories with
the -nostdinc option. This is useful when you are compiling an
operating system kernel or some other program that does not use the
standard C library facilities, or the standard C library itself.
-I options are not ignored as described above when
-nostdinc is in effect.
GCC looks for headers requested with #include "file"
first in the directory containing the current file, then in the same
places it would have looked for a header requested with angle brackets.
For example, if /usr/include/sys/stat.h contains
#include "types.h", GCC looks for types.h first in
/usr/include/sys, then in its usual search path.
#line (see Line Control) does not change GCC's idea of the
directory containing the current file.
You may put -I- at any point in your list of -I options.
This has two effects. First, directories appearing before the
-I- in the list are searched only for headers requested with
quote marks. Directories after -I- are searched for all
headers. Second, the directory containing the current file is not
searched for anything, unless it happens to be one of the directories
named by an -I switch.
-I. -I- is not the same as no -I options at all, and does
not cause the same behavior for <> includes that ""
includes get with no special options. -I. searches the
compiler's current working directory for header files. That may or may
not be the same as the directory containing the current file.
If you need to look for headers in a directory named -, write
-I./-.
There are several more ways to adjust the header search path. They are generally less useful. See Invocation.
If a header file happens to be included twice, the compiler will process its contents twice. This is very likely to cause an error, e.g. when the compiler sees the same structure definition twice. Even if it does not, it will certainly waste time.
The standard way to prevent this is to enclose the entire real contents of the file in a conditional, like this:
/* File foo. */
#ifndef FILE_FOO_SEEN
#define FILE_FOO_SEEN
the entire file
#endif /* !FILE_FOO_SEEN */
This construct is commonly known as a wrapper #ifndef.
When the header is included again, the conditional will be false,
because FILE_FOO_SEEN is defined. The preprocessor will skip
over the entire contents of the file, and the compiler will not see it
twice.
CPP optimizes even further. It remembers when a header file has a
wrapper #ifndef. If a subsequent #include specifies that
header, and the macro in the #ifndef is still defined, it does
not bother to rescan the file at all.
You can put comments outside the wrapper. They will not interfere with this optimization.
The macro FILE_FOO_SEEN is called the controlling macro or
guard macro. In a user header file, the macro name should not
begin with _. In a system header file, it 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.
Sometimes it is necessary to select one of several different header files to be included into your program. They might specify configuration parameters to be used on different sorts of operating systems, for instance. You could do this with a series of conditionals,
#if SYSTEM_1
# include "system_1.h"
#elif SYSTEM_2
# include "system_2.h"
#elif SYSTEM_3
...
#endif
That rapidly becomes tedious. Instead, the preprocessor offers the
ability to use a macro for the header name. This is called a
computed include. Instead of writing a header name as the direct
argument of #include, you simply put a macro name there instead:
#define SYSTEM_H "system_1.h"
...
#include SYSTEM_H
SYSTEM_H will be expanded, and the preprocessor will look for
system_1.h as if the #include had been written that way
originally. SYSTEM_H could be defined by your Makefile with a
-D option.
You must be careful when you define the macro. #define saves
tokens, not text. The preprocessor has no way of knowing that the macro
will be used as the argument of #include, so it generates
ordinary tokens, not a header name. This is unlikely to cause problems
if you use double-quote includes, which are close enough to string
constants. If you use angle brackets, however, you may have trouble.
The syntax of a computed include is actually a bit more general than the
above. If the first non-whitespace character after #include is
not " or <, then the entire line is macro-expanded
like running text would be.
If the line expands to a single string constant, the contents of that string constant are the file to be included. CPP does not re-examine the string for embedded quotes, but neither does it process backslash escapes in the string. Therefore
#define HEADER "a\"b"
#include HEADER
looks for a file named a\"b. CPP searches for the file according
to the rules for double-quoted includes.
If the line expands to a token stream beginning with a < token
and including a > token, then the tokens between the < and
the first > are combined to form the filename to be included.
Any whitespace between tokens is reduced to a single space; then any
space after the initial < is retained, but a trailing space
before the closing > is ignored. CPP searches for the file
according to the rules for angle-bracket includes.
In either case, if there are any tokens on the line after the file name, an error occurs and the directive is not processed. It is also an error if the result of expansion does not match either of the two expected forms.
These rules are implementation-defined behavior according to the C standard. To minimize the risk of different compilers interpreting your computed includes differently, we recommend you use only a single object-like macro which expands to a string constant. This will also minimize confusion for people reading your program.
Sometimes it is necessary to adjust the contents of a system-provided
header file without editing it directly. GCC's fixincludes
operation does this, for example. One way to do that would be to create
a new header file with the same name and insert it in the search path
before the original header. That works fine as long as you're willing
to replace the old header entirely. But what if you want to refer to
the old header from the new one?
You cannot simply include the old header with #include. That
will start from the beginning, and find your new header again. If your
header is not protected from multiple inclusion (see Once-Only Headers), it will recurse infinitely and cause a fatal error.
You could include the old header with an absolute pathname:
#include "/usr/include/old-header.h"
This works, but is not clean; should the system headers ever move, you would have to edit the new headers to match.
There is no way to solve this problem within the C standard, but you can
use the GNU extension #include_next. It 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 signal.h. Ordinary #include <signal.h> finds the file under /usr/local/include. If that
file contains #include_next <signal.h>, it starts searching
after that directory, and finds the file in /usr/include.
#include_next does not distinguish between <file>
and "file" inclusion, nor does it check that the file you
specify has the same name as the current file. It simply looks for the
file named, starting with the directory in the search path after the one
where the current file was found.
The use of #include_next can lead to great confusion. We
recommend it be used only when there is no other alternative. In
particular, it should not be used in the headers belonging to a specific
program; it should be used only to make global corrections along the
lines of fixincludes.
The header files declaring interfaces to the operating system and
runtime libraries often cannot be written in strictly conforming C.
Therefore, GCC gives code found in system headers special
treatment. All warnings, other than those generated by #warning
(see Diagnostics), are suppressed while GCC is processing a system
header. Macros defined in a system header are immune to a few warnings
wherever they are expanded. This immunity is granted on an ad-hoc
basis, when we find that a warning generates lots of false positives
because of code in macros defined in system headers.
Normally, only the headers found in specific directories are considered system headers. These directories are determined when GCC is compiled. There are, however, two ways to make normal headers into system headers.
The -isystem command line option adds its argument to the list of
directories to search for headers, just like -I. Any headers
found in that directory will be considered system headers.
All directories named by -isystem are searched after all
directories named by -I, no matter what their order was on the
command line. If the same directory is named by both -I and
-isystem, the -I option is ignored. GCC provides an
informative message when this occurs if -v is used.
There is also a directive, #pragma GCC system_header, which
tells GCC to consider the rest of the current include file a system
header, no matter where it was found. Code that comes before the
#pragma in the file will not be affected. #pragma GCC system_header has no effect in the primary source file.
On very old systems, some of the pre-defined system header directories
get even more special treatment. GNU C++ considers code in headers
found in those directories to be surrounded by an extern "C"
block. There is no way to request this behavior with a #pragma,
or from the command line.
A macro is a fragment of code which has been given a name. Whenever the name is used, it is replaced by the contents of the macro. There are two kinds of macros. They differ mostly in what they look like when they are used. Object-like macros resemble data objects when used, function-like macros resemble function calls.
You may define any valid identifier as a macro, even if it is a C
keyword. The preprocessor does not know anything about keywords. This
can be useful if you wish to hide a keyword such as const from an
older compiler that does not understand it. However, the preprocessor
operator defined (see Defined) can never be defined as a
macro, and C++'s named operators (see C++ Named Operators) cannot be
macros when you are compiling C++.
An object-like macro is a simple identifier which will be replaced by a code fragment. It is called object-like because it looks like a data object in code that uses it. They are most commonly used to give symbolic names to numeric constants.
You create macros with the #define directive. #define is
followed by the name of the macro and then the token sequence it should
be an abbreviation for, which is variously referred to as the macro's
body, expansion or replacement list. For example,
#define BUFFER_SIZE 1024
defines a macro named BUFFER_SIZE as an abbreviation for the
token 1024. If somewhere after this #define directive
there comes a C statement of the form
foo = (char *) malloc (BUFFER_SIZE);
then the C preprocessor will recognize and expand the macro
BUFFER_SIZE. The C compiler will see the same tokens as it would
if you had written
foo = (char *) malloc (1024);
By convention, macro names are written in upper case. Programs are easier to read when it is possible to tell at a glance which names are macros.
The macro's body ends at the end of the #define line. You may
continue the definition onto multiple lines, if necessary, using
backslash-newline. When the macro is expanded, however, it will all
come out on one line. For example,
#define NUMBERS 1, \
2, \
3
int x[] = { NUMBERS };
==> int x[] = { 1, 2, 3 };
The most common visible consequence of this is surprising line numbers in error messages.
There is no restriction on what can go in a macro body provided it decomposes into valid preprocessing tokens. Parentheses need not balance, and the body need not resemble valid C code. (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. 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
foo = X;
bar = 4;
When the preprocessor expands a macro name, the macro's expansion replaces the macro invocation, then the expansion is examined for more macros to expand. For example,
#define TABLESIZE BUFSIZE
#define BUFSIZE 1024
TABLESIZE
==> BUFSIZE
==> 1024
TABLESIZE is expanded first to produce BUFSIZE, then that
macro is expanded to produce the final result, 1024.
Notice that BUFSIZE was not defined when TABLESIZE was
defined. The #define for TABLESIZE uses exactly the
expansion you specify--in this case, BUFSIZE--and does not
check to see whether it too contains macro names. Only when you
use TABLESIZE is the result of its expansion scanned 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.
If the expansion of a macro contains its own name, either directly or via intermediate macros, it is not expanded again when the expansion is examined for more macros. This prevents infinite recursion. See Self-Referential Macros, for the precise details.
You can also define macros whose use looks like a function call. These
are called function-like macros. To define a function-like macro,
you use the same #define directive, but you put a pair of
parentheses immediately after the macro name. For example,
#define lang_init() c_init()
lang_init()
==> c_init()
A function-like macro is only expanded if its name appears with a pair of parentheses after it. If you write just the name, it is left alone. This can be useful when you have a function and a macro of the same name, and you wish to use the function sometimes.
extern void foo(void);
#define foo() /* optimized inline version */
...
foo();
funcptr = foo;
Here the call to foo() will use the macro, but the function
pointer will get the address of the real function. If the macro were to
be expanded, it would cause a syntax error.
If you put spaces between the macro name and the parentheses in the macro definition, that does not define a function-like macro, it defines an object-like macro whose expansion happens to begin with a pair of parentheses.
#define lang_init () c_init()
lang_init()
==> () c_init()()
The first two pairs of parentheses in this expansion come from the
macro. The third is the pair that was originally after the macro
invocation. Since lang_init is an object-like macro, it does not
consume those parentheses.
Function-like macros can take arguments, just like true functions. To define a macro that uses arguments, you insert parameters between the pair of parentheses in the macro definition that make the macro function-like. The parameters must be valid C identifiers, separated by commas and optionally whitespace.
To invoke a macro that takes arguments, you write the name of the macro followed by a list of actual arguments in parentheses, separated by commas. The invocation of the macro need not be restricted to a single logical line--it can cross as many lines in the source file as you wish. The number of arguments you give must match the number of parameters in the macro definition. When the macro is expanded, each use of a parameter in its body is replaced by the tokens of the corresponding argument. (You need not use all of the parameters in the macro body.)
As an example, here is a macro that computes the minimum of two numeric values, as it is defined in many C programs, and some uses.
#define min(X, Y) ((X) < (Y) ? (X) : (Y))
x = min(a, b); ==> x = ((a) < (b) ? (a) : (b));
y = min(1, 2); ==> y = ((1) < (2) ? (1) : (2));
z = min(a + 28, *p); ==> z = ((a + 28) < (*p) ? (a + 28) : (*p));
(In this small example you can already see several of the dangers of macro arguments. See Macro Pitfalls, for detailed explanations.)
Leading and trailing whitespace in each argument is dropped, and all whitespace between the tokens of an argument is reduced to a single space. Parentheses within each argument must balance; a comma within such parentheses does not end the argument. However, there is no requirement for square 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 can write it as array[(x = y, x + 1)], which is equivalent C
code.
All arguments to a macro are completely macro-expanded before they are substituted into the macro body. After substitution, the complete text is scanned again for macros to expand, including the arguments. This rule may seem strange, but it is carefully designed so you need not worry about whether any function call is actually a macro invocation. You can run into trouble if you try to be too clever, though. See Argument Prescan, for detailed discussion.
For example, min (min (a, b), c) is first expanded to
min (((a) < (b) ? (a) : (b)), (c))
and then to
((((a) < (b) ? (a) : (b))) < (c)
? (((a) < (b) ? (a) : (b)))
: (c))
(Line breaks shown here for clarity would not actually be generated.)
You can leave macro arguments empty; this is not an error to the
preprocessor (but many macros will then expand to invalid code).
You cannot leave out arguments entirely; if a macro takes two arguments,
there must be exactly one comma at the top level of its argument list.
Here are some silly examples using min:
min(, b) ==> (( ) < (b) ? ( ) : (b))
min(a, ) ==> ((a ) < ( ) ? (a ) : ( ))
min(,) ==> (( ) < ( ) ? ( ) : ( ))
min((,),) ==> (((,)) < ( ) ? ((,)) : ( ))
min() error--> macro "min" requires 2 arguments, but only 1 given
min(,,) error--> macro "min" passed 3 arguments, but takes just 2
Whitespace is not a preprocessing token, so if a macro foo takes
one argument, foo () and foo ( ) both supply it an
empty argument. Previous GNU preprocessor implementations and
documentation were incorrect on this point, insisting that a
function-like macro that takes a single argument be passed a space if an
empty argument was required.
Macro parameters appearing inside string literals are not replaced by their corresponding actual arguments.
#define foo(x) x, "x"
foo(bar) ==> bar, "x"
Sometimes you may want to convert a macro argument into a string
constant. Parameters are not replaced inside string constants, but you
can use the # preprocessing operator instead. When a macro
parameter is used with a leading #, the preprocessor replaces it
with the literal text of the actual argument, converted to a string
constant. Unlike normal parameter replacement, the argument is not
macro-expanded first. This is called stringification.
There is no way to combine an argument with surrounding text and stringify it all together. Instead, you can write a series of adjacent string constants and stringified arguments. The preprocessor will replace the stringified arguments with string constants. The C compiler will then combine all the adjacent string constants into one long string.
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)
WARN_IF (x == 0);
==> do { if (x == 0)
fprintf (stderr, "Warning: " "x == 0" "\n"); } while (0);
The argument for EXP is substituted once, as-is, into the
if statement, and once, stringified, into the argument to
fprintf. If x were a macro, it would be expanded in the
if statement, but not in the string.
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
Swallowing the Semicolon.
Stringification in C involves more than putting double-quote characters
around the fragment. The preprocessor backslash-escapes the quotes
surrounding embedded string constants, and all backslashes within 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 string
or character constants are not duplicated: \n by itself
stringifies to "\n".
All leading and trailing whitespace in text being stringified is ignored. Any sequence of whitespace in the middle of the text is converted to a single space in the stringified result. Comments are replaced by whitespace long before stringification happens, so they never appear in stringified text.
There is no way to convert a macro argument into a character constant.
If you want to stringify the result of expansion of a macro argument, you have to use two levels of macros.
#define xstr(s) str(s)
#define str(s) #s
#define foo 4
str (foo)
==> "foo"
xstr (foo)
==> xstr (4)
==> str (4)
==> "4"
s is stringified when it is used in str, so it is not
macro-expanded first. But s is an ordinary argument to
xstr, so it is completely macro-expanded before xstr
itself is expanded (see Argument Prescan). Therefore, by the time
str gets to its argument, it has already been macro-expanded.
It is often useful to merge two tokens into one while expanding macros.
This is called token pasting or token concatenation. The
## preprocessing operator performs token pasting. When a macro
is expanded, the two tokens on either side of each ## operator
are combined into a single token, which then replaces the ## and
the two original tokens in the macro expansion. Usually both will be
identifiers, or one will be an identifier and the other a preprocessing
number. When pasted, they make a longer identifier. 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
token pasting.
However, two tokens that don't together form a valid token cannot be
pasted together. For example, you cannot concatenate x with
+ in either order. If you try, the preprocessor issues a warning
and emits the two tokens. Whether it puts white space between the
tokens is undefined. It is common to find unnecessary uses of ##
in complex macros. If you get this warning, it is likely that you can
simply remove the ##.
Both the tokens combined by ## could come from the macro body,
but you could just as well write them as one token in the first place.
Token pasting is most useful when one or both of the tokens comes from a
macro argument. If either of the tokens next to an ## is a
parameter name, it is replaced by its actual argument before ##
executes. As with stringification, the actual argument is not
macro-expanded first. If the argument is empty, that ## has no
effect.
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 *. You can put as much
whitespace between ## and its operands as you like, including
comments, and you can put comments in arguments that will be
concatenated. However, it is an error if ## appears at either
end of a macro body.
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) (void);
};
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),
...
};
A macro can be declared to accept a variable number of arguments much as a function can. The syntax for defining the macro is similar to that of a function. Here is an example:
#define eprintf(...) fprintf (stderr, __VA_ARGS__)
This kind of macro is called variadic. When the macro is invoked,
all the tokens in its argument list after the last named argument (this
macro has none), including any commas, become the variable
argument. This sequence of tokens replaces the identifier
__VA_ARGS__ in the macro body wherever it appears. Thus, we
have this expansion:
eprintf ("%s:%d: ", input_file, lineno)
==> fprintf (stderr, "%s:%d: ", input_file, lineno)
The variable argument is completely macro-expanded before it is inserted
into the macro expansion, just like an ordinary argument. You may use
the # and ## operators to stringify the variable argument
or to paste its leading or trailing token with another token. (But see
below for an important special case for ##.)
If your macro is complicated, you may want a more descriptive name for
the variable argument than __VA_ARGS__. CPP permits
this, as an extension. You may write an argument name immediately
before the ...; that name is used for the variable argument.
The eprintf macro above could be written
#define eprintf(args...) fprintf (stderr, args)
using this extension. You cannot use __VA_ARGS__ and this
extension in the same macro.
You can have named arguments as well as variable arguments in a variadic
macro. We could define eprintf like this, instead:
#define eprintf(format, ...) fprintf (stderr, format, __VA_ARGS__)
This formulation looks more descriptive, but unfortunately it is less flexible: you must now supply at least one argument after the format string. In standard C, you cannot omit the comma separating the named argument from the variable arguments. Furthermore, if you leave the variable argument empty, you will get a syntax error, because there will be an extra comma after the format string.
eprintf("success!\n", );
==> fprintf(stderr, "success!\n", );
GNU CPP has a pair of extensions which deal with this problem. First, you are allowed to leave the variable argument out entirely:
eprintf ("success!\n")
==> fprintf(stderr, "success!\n", );
Second, the ## token paste operator has a special meaning when
placed between a comma and a variable argument. If you write
#define eprintf(format, ...) fprintf (stderr, format, ##__VA_ARGS__)
and the variable argument is left out when the eprintf macro is
used, then the comma before the ## will be deleted. This does
not happen if you pass an empty argument, nor does it happen if
the token preceding ## is anything other than a comma.
eprintf ("success!\n")
==> fprintf(stderr, "success!\n");
The above explanation is ambiguous about the case where the only macro parameter is a variable arguments parameter, as it is meaningless to try to distinguish whether no argument at all is an empty argument or a missing argument. In this case the C99 standard is clear that the comma must remain, however the existing GCC extension used to swallow the comma. So CPP retains the comma when conforming to a specific C standard, and drops it otherwise.
C99 mandates that the only place the identifier __VA_ARGS__
can appear is in the replacement list of a variadic macro. It may not
be used as a macro name, macro argument name, or within a different type
of macro. It may also be forbidden in open text; the standard is
ambiguous. We recommend you avoid using it except for its defined
purpose.
Variadic macros are a new feature in C99. GNU CPP has supported them
for a long time, but only with a named variable argument
(args..., not ... and __VA_ARGS__). If you are
concerned with portability to previous versions of GCC, you should use
only named variable arguments. On the other hand, if you are concerned
with portability to other conforming implementations of C99, you should
use only __VA_ARGS__.
Previous versions of CPP implemented the comma-deletion extension
much more generally. We have restricted it in this release to minimize
the differences from C99. To get the same effect with both this and
previous versions of GCC, the token preceding the special ## must
be a comma, and there must be white space between that comma and
whatever comes immediately before it:
#define eprintf(format, args...) fprintf (stderr, format , ##args)
See Differences from previous versions, for the gory details.
Several object-like macros are predefined; you use them without supplying their definitions. They fall into three classes: standard, common, and system-specific.
In C++, there is a fourth category, the named operators. They act like predefined macros, but you cannot undefine them.
The standard predefined macros are specified by the relevant language standards, so they are available with all compilers that implement those standards. Older compilers may not provide all of them. Their names all start with double underscores.
__FILE__
#include or as the
input file name argument. For example,
"/usr/local/include/myheader.h" is a possible expansion of this
macro.
__LINE__
__FILE__ and __LINE__ 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__);
An #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).
A #line directive changes __LINE__, and may change
__FILE__ as well. See Line Control.
C99 introduces __func__, and GCC has provided __FUNCTION__
for a long time. Both of these are strings containing the name of the
current function (there are slight semantic differences; see the GCC
manual). Neither of them is a macro; the preprocessor does not know the
name of the current function. They tend to be useful in conjunction
with __FILE__ and __LINE__, though.
__DATE__
"Feb 12 1996". If the day of the
month is less than 10, it is padded with a space on the left.
If GCC cannot determine the current date, it will emit a warning message
(once per compilation) and __DATE__ will expand to
"??? ?? ????".
__TIME__
"23:59:01".
If GCC cannot determine the current time, it will emit a warning message
(once per compilation) and __TIME__ will expand to
"??:??:??".
__STDC__
-traditional-cpp option is used.
This macro is not defined if the -traditional-cpp option is used.
On some hosts, the system compiler uses a different convention, where
__STDC__ is normally 0, but is 1 if the user specifies strict
conformance to the C Standard. CPP follows the host convention when
processing system header files, but when processing user files
__STDC__ is always 1. This has been reported to cause problems;
for instance, some versions of Solaris provide X Windows headers that
expect __STDC__ to be either undefined or 1. See Invocation.
__STDC_VERSION__
L where yyyy and
mm are the year and month of the Standard version. This signifies
which version of the C Standard the compiler conforms to. Like
__STDC__, this is not necessarily accurate for the entire
implementation, unless GNU CPP is being used with GCC.
The value 199409L signifies the 1989 C standard as amended in
1994, which is the current default; the value 199901L signifies
the 1999 revision of the C standard. Support for the 1999 revision is
not yet complete.
This macro is not defined if the -traditional-cpp option is
used, nor when compiling C++ or Objective-C.
__STDC_HOSTED__
__cplusplus
__cplusplus to test whether a header is compiled by a C compiler
or a C++ compiler. This macro is similar to __STDC_VERSION__, in
that it expands to a version number. A fully conforming implementation
of the 1998 C++ standard will define this macro to 199711L. The
GNU C++ compiler is not yet fully conforming, so it uses 1
instead. We hope to complete our implementation in the near future.
__OBJC__
__OBJC__ to test whether a header is compiled
by a C compiler or a Objective-C compiler.
__ASSEMBLER__
The common predefined macros are GNU C extensions. They are available with the same meanings regardless of the machine or operating system on which you are using GNU C. Their names all start with double underscores.
__GNUC__
__GNUC_MINOR__
__GNUC_PATCHLEVEL__
__GNUC__ to 3,
__GNUC_MINOR__ to 2, and __GNUC_PATCHLEVEL__ to 1. They
are defined only when the entire compiler is in use; if you invoke the
preprocessor directly, they are not defined.
__GNUC_PATCHLEVEL__ is new to GCC 3.0; it is also present in the
widely-used development snapshots leading up to 3.0 (which identify
themselves as GCC 2.96 or 2.97, depending on which snapshot you have).
If all you need to know is whether or not your program is being compiled
by GCC, you can simply test __GNUC__. If you need to write code
which depends on a specific version, you must be more careful. Each
time the minor version is increased, the patch level is reset to zero;
each time the major version is increased (which happens rarely), the
minor version and patch level are reset. If you wish to use the
predefined macros directly in the conditional, you will need to write it
like this:
/* Test for GCC > 3.2.0 */
#if __GNUC__ > 3 || \
(__GNUC__ == 3 && (__GNUC_MINOR__ > 2 || \
(__GNUC_MINOR__ == 2 && \
__GNUC_PATCHLEVEL__ > 0))
Another approach is to use the predefined macros to calculate a single number, then compare that against a threshold:
#define GCC_VERSION (__GNUC__ * 10000 \
+ __GNUC_MINOR__ * 100 \
+ __GNUC_PATCHLEVEL__)
...
/* Test for GCC > 3.2.0 */
#if GCC_VERSION > 30200
Many people find this form easier to understand.
__GNUG__
(__GNUC__ && __cplusplus).
__STRICT_ANSI__
-ansi switch, or a
-std switch specifying strict conformance to some version of ISO C,
was specified when GCC was invoked. It is defined to 1.
This macro exists primarily to direct GNU libc's header files to
restrict their definitions to the minimal set found in the 1989 C
standard.
__BASE_FILE__
__INCLUDE_LEVEL__
#include directive and decremented at the
end of every included file. It starts out at 0, it's value within the
base file specified on the command line.
__ELF__
__VERSION__
__OPTIMIZE__
__OPTIMIZE_SIZE__
__NO_INLINE__
__OPTIMIZE__ is
defined in all optimizing compilations. __OPTIMIZE_SIZE__ is
defined if the compiler is optimizing for size, not speed.
__NO_INLINE__ is defined if no functions will be inlined into
their callers (when not optimizing, or when inlining has been
specifically disabled by -fno-inline).
These macros cause certain GNU header files to provide optimized
definitions, using macros or inline functions, of system library
functions. You should not use these macros in any way unless you make
sure that programs will execute with the same effect whether or not they
are defined. If they are defined, their value is 1.
__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 use this macro
yourself; instead, refer to the standard macros defined in limits.h.
__WCHAR_UNSIGNED__
__CHAR_UNSIGNED__, this macro is defined if and only if the
data type wchar_t is unsigned and the front-end is in C++ mode.
__REGISTER_PREFIX__
m68k-aout environment it
expands to nothing, but in the m68k-coff environment it expands
to a single %.
__USER_LABEL_PREFIX__
m68k-aout environment it expands to an _, but in the
m68k-coff environment it expands to nothing.
This macro will have the correct definition even if
-f(no-)underscores is in use, but it will not be correct if
target-specific options that adjust this prefix are used (e.g. the
OSF/rose -mno-underscores option).
__SIZE_TYPE__
__PTRDIFF_TYPE__
__WCHAR_TYPE__
__WINT_TYPE__
size_t, ptrdiff_t, wchar_t, and wint_t
typedefs, respectively. They exist to make the standard header files
stddef.h and wchar.h work correctly. You should not use
these macros directly; instead, include the appropriate headers and use
the typedefs.
__CHAR_BIT__
char data type. It exists to make the standard header given
numerical limits work correctly. You should not use
this macro directly; instead, include the appropriate headers.
__SCHAR_MAX__
__WCHAR_MAX__
__SHRT_MAX__
__INT_MAX__
__LONG_MAX__
__LONG_LONG_MAX__
signed char, wchar_t,
signed short,
signed int, signed long, and signed long long types
respectively. They exist to make the standard header given numerical limits
work correctly. You should not use these macros directly; instead, include
the appropriate headers.
__USING_SJLJ_EXCEPTIONS__
setjmp and longjmp for exception
handling.
__NEXT_RUNTIME__
-fnext-runtime) is in use for Objective-C. If the GNU
runtime is used, this macro is not defined, so that you can use this
macro to determine which runtime (NeXT or GNU) is being used.
__LP64__
_LP64
long int and pointer both use 64-bits and
int uses 32-bit.
The C preprocessor normally predefines several macros that indicate what
type of system and machine is in use. They are obviously different on
each target supported by GCC. This manual, being for all systems and
machines, cannot tell you what their names are, but you can use
cpp -dM to see them all. See Invocation. All system-specific
predefined macros expand to the constant 1, so you can test them with
either #ifdef or #if.
The C standard requires that all system-specific macros be part of the
reserved namespace. All names which begin with two underscores,
or an underscore and a capital letter, are reserved for the compiler and
library to use as they wish. However, historically system-specific
macros have had names with no special prefix; for instance, it is common
to find unix defined on Unix systems. For all such macros, GCC
provides a parallel macro with two underscores added at the beginning
and the end. If unix is defined, __unix__ will be defined
too. There will never be more than two underscores; the parallel of
_mips is __mips__.
When the -ansi option, or any -std option that
requests strict conformance, is given to the compiler, all the
system-specific predefined macros outside the reserved namespace are
suppressed. The parallel macros, inside the reserved namespace, remain
defined.
We are slowly phasing out all predefined macros which are outside the
reserved namespace. You should never use them in new programs, and we
encourage you to correct older code to use the parallel macros whenever
you find it. We don't recommend you use the system-specific macros that
are in the reserved namespace, either. It is better in the long run to
check specifically for features you need, using a tool such as
autoconf.
In C++, there are eleven keywords which are simply alternate spellings
of operators normally written with punctuation. These keywords are
treated as such even in the preprocessor. They function as operators in
#if, and they cannot be defined as macros or poisoned. In C, you
can request that those keywords take their C++ meaning by including
iso646.h. That header defines each one as a normal object-like
macro expanding to the appropriate punctuator.
These are the named operators and their corresponding punctuators:
| Named Operator | Punctuator
|
and | &&
|
and_eq | &=
|
bitand | &
|
bitor | |
|
compl | ~
|
not | !
|
not_eq | !=
|
or | ||
|
or_eq | |=
|
xor | ^
|
xor_eq | ^=
|
If a macro ceases to be useful, it may be undefined with the
#undef directive. #undef takes a single argument, the
name of the macro to undefine. You use the bare macro name, even if the
macro is function-like. It is an error if anything appears on the line
after the macro name. #undef has no effect if the name is not a
macro.
#define FOO 4
x = FOO; ==> x = 4;
#undef FOO
x = FOO; ==> x = FOO;
Once a macro has been undefined, that identifier may be redefined
as a macro by a subsequent #define directive. The new definition
need not have any resemblance to the old definition.
However, if an identifier which is currently a macro is redefined, then the new definition must be effectively the same as the old one. Two macro definitions are effectively the same if:
These definitions are effectively the same:
#define FOUR (2 + 2)
#define FOUR (2 + 2)
#define FOUR (2 /* two */ + 2)
but these are not:
#define FOUR (2 + 2)
#define FOUR ( 2+2 )
#define FOUR (2 * 2)
#define FOUR(score,and,seven,years,ago) (2 + 2)
If a macro is redefined with a definition that is not effectively the same as the old one, the preprocessor issues a warning and changes the macro to use the new definition. If the new definition is effectively the same, the redefinition is silently ignored. This allows, for instance, two different headers to define a common macro. The preprocessor will only complain if the definitions do not match.
Occasionally it is convenient to use preprocessor directives within the arguments of a macro. The C and C++ standards declare that behavior in these cases is undefined.
Versions of CPP prior to 3.2 would reject such constructs with an
error message. This was the only syntactic difference between normal
functions and function-like macros, so it seemed attractive to remove
this limitation, and people would often be surprised that they could
not use macros in this way. Moreover, sometimes people would use
conditional compilation in the argument list to a normal library
function like printf, only to find that after a library upgrade
printf had changed to be a function-like macro, and their code
would no longer compile. So from version 3.2 we changed CPP to
successfully process arbitrary directives within macro arguments in
exactly the same way as it would have processed the directive were the
function-like macro invocation not present.
If, within a macro invocation, that macro is redefined, then the new definition takes effect in time for argument pre-expansion, but the original definition is still used for argument replacement. Here is a pathological example:
#define f(x) x x
f (1
#undef f
#define f 2
f)
which expands to
1 2 1 2
with the semantics described above.
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 counter-intuitive consequences that you must watch out for.
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 arguments. For example,
#define twice(x) (2*(x))
#define call_with_1(x) x(1)
call_with_1 (twice)
==> twice(1)
==> (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)
==> fprintf (stderr, "%s %d", p, 35)
The ability to piece together a macro call can be useful, but the use of unbalanced open parentheses in a macro body is just confusing, and should be avoided.
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));
==> a = (b & c + sizeof (int) - 1) / sizeof (int);
This 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);
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 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) \
{ 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 logical line, so that it resembles the way such 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. However, since it
looks like a function call, it minimizes confusion if you can use it
like a function call, writing a semicolon afterward, as in
SKIP_SPACES (p, lim);
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 { 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. The loop executes exactly once; most compilers generate no extra code for it.
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 extensions as
follows:
#define min(X, Y) \
({ typeof (X) x_ = (X); \
typeof (Y) y_ = (Y); \
(x_ < y_) ? x_ : y_; })
The ({ ... }) notation produces a compound statement that
acts as an expression. Its value is the value of its last statement.
This permits us to define local variables and assign each argument to
one. The local variables have underscores after their names to reduce
the risk of conflict with an identifier of wider scope (it is impossible
to avoid this entirely). Now each argument is evaluated exactly once.
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. Recall that all macro definitions are rescanned for more macros to replace. If the self-reference were considered a use of the macro, it would produce an infinitely large expansion. To prevent this, 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 the computer runs out of memory.
The self-reference rule 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.
One common, useful use of self-reference is to create a macro which expands to itself. If you write
#define EPERM EPERM
then the macro EPERM expands to EPERM. Effectively, it is
left alone by the preprocessor whenever it's used in running text. You
can tell that it's a macro with #ifdef. You might do this if you
want to define numeric constants with an enum, but have
#ifdef be true for each constant.
If a macro x expands to use a macro y, and the expansion of
y refers to the macro x, that is an indirect
self-reference of x. x is not expanded in this case
either. Thus, if we have
#define x (4 + y)
#define y (2 * x)
then x and y expand as follows:
x ==> (4 + y)
==> (4 + (2 * x))
y ==> (2 * x)
==> (2 * (4 + y))
Each macro is expanded when it appears in the definition of the other macro, but not when it indirectly appears in its own definition.
Macro arguments are completely macro-expanded before they are substituted into a macro body, unless they are stringified or pasted with other tokens. After substitution, the entire macro body, including the substituted arguments, is scanned again for macros to be expanded. The result is that the arguments are scanned twice to expand macro calls in them.
Most of the time, this has no effect. If the 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 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 argument of another macro (see Self-Referential Macros): the self-referential macro would be expanded once in the first scan, and a second time in the second scan. However, 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.
You might wonder, "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 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 argument, and the inner use of f would
appear during the main scan as an indirect self-reference and would not
be expanded.
If an argument is stringified or concatenated, the prescan does not occur. If you want to expand a macro, then stringify or concatenate its expansion, you can do that by causing one macro to call another macro that does the stringification or concatenation. For instance, if you have
#define AFTERX(x) X_ ## x
#define XAFTERX(x) AFTERX(x)
#define TABLESIZE 1024
#define BUFSIZE TABLESIZE
then AFTERX(BUFSIZE) expands to X_BUFSIZE, and
XAFTERX(BUFSIZE) expands to X_1024. (Not to
X_TABLESIZE. Prescan always does a complete expansion.)
This can cause a macro expanded on the second scan to be called with the wrong number of arguments. Here is an example:
#define foo a,b
#define bar(x) lose(x)
#define lose(x) (1 + (x))
We would like bar(foo) to turn into (1 + (foo)), which
would then turn into (1 + (a,b)). 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)
or
#define bar(x) lose((x))
The extra pair of parentheses prevents the comma in foo's
definition from being interpreted as an argument separator.
The invocation of a function-like macro can extend over many logical lines. However, in the present implementation, the entire expansion comes out on one line. Thus line numbers emitted by the compiler or debugger refer to the line the invocation started on, which might be different to the line containing the argument causing the problem.
Here is an example illustrating this:
#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 three--the line of ignore_second_arg--
even though the problematic code comes from line five.
We consider this a bug, and intend to fix it in the near future.
A conditional is a directive that instructs the preprocessor to
select whether or not to include a chunk of code in the final token
stream passed to the compiler. Preprocessor conditionals can test
arithmetic expressions, or whether a name is defined as a macro, or both
simultaneously using the special defined operator.
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.
However, the distinction is becoming less clear. Modern compilers often
do test if statements when a program is compiled, if their
conditions are known not to vary at run time, and eliminate code which
can never be executed. If you can count on your compiler to do this,
you may find that your program is more readable if you use if
statements with constant conditions (perhaps determined by macros). Of
course, you can only use this to exclude code, not type definitions or
other preprocessing directives, and you can only do it if the code
remains syntactically valid when it is not to be used.
GCC version 3 eliminates this kind of never-executed code even when not optimizing. Older versions did it only when optimizing.
There are three general reasons to use a conditional.
Simple programs that do not need system-specific logic or complex debugging hooks generally will not need to use preprocessing conditionals.
A conditional in the C preprocessor begins with a conditional
directive: #if, #ifdef or #ifndef.
The simplest sort of conditional is
#ifdef MACRO
controlled text
#endif /* MACRO */
This block is called a conditional group. controlled text will be included in the output of the preprocessor if and only if MACRO is defined. We say that the conditional succeeds if MACRO is defined, fails if it is not.
The controlled text inside of a conditional can include
preprocessing directives. They are executed only if the conditional
succeeds. You can nest conditional groups inside other conditional
groups, but they must be completely nested. In other words,
#endif always matches the nearest #ifdef (or
#ifndef, or #if). Also, you cannot start a conditional
group in one file and end it in another.
Even if a conditional fails, the controlled text inside it is still run through initial transformations and tokenization. Therefore, it must all be lexically valid C. Normally the only way this matters is that all comments and string literals inside a failing conditional group must still be properly ended.
The comment following the #endif is not required, but it is a
good practice if there is a lot of controlled text, because it
helps people match the #endif to the corresponding #ifdef.
Older programs sometimes put MACRO directly after the
#endif without enclosing it in a comment. This is invalid code
according to the C standard. CPP accepts it with a warning. It
never affects which #ifndef the #endif matches.
Sometimes you wish to use some code if a macro is not defined.
You can do this by writing #ifndef instead of #ifdef.
One common use of #ifndef is to include code only the first
time a header file is included. See Once-Only Headers.
Macro definitions can vary between compilations for several reasons. Here are some samples.
-D and -U
command line 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 command line options, perhaps set in the
Makefile. See Invocation.
config.h) that is adjusted when the program is compiled. It can
define or not define macros depending on the features of the system and
the desired capabilities of the program. The adjustment can be
automated by a tool such as autoconf, or done by hand.
The #if directive allows you to test the value of an arithmetic
expression, rather than the mere existence of one macro. Its syntax is
#if expression
controlled text
#endif /* expression */
expression is a C expression of integer type, subject to stringent restrictions. It may contain
&& and ||). The latter two obey the usual
short-circuiting rules of standard C.
defined operator, which lets you check whether macros
are defined in the middle of an #if.
#if MACRO instead of
#ifdef MACRO, if you know that MACRO, when defined, will
always have a nonzero value. Function-like macros used without their
function call parentheses are also treated as zero.
In some contexts this shortcut is undesirable. The -Wundef
option causes GCC to warn whenever it encounters an identifier which is
not a macro in an #if.
The preprocessor does not know anything about types in the language.
Therefore, sizeof operators are not recognized in #if, and
neither are enum constants. They will be taken as identifiers
which are not macros, and replaced by zero. In the case of
sizeof, this is likely to cause the expression to be invalid.
The preprocessor calculates the value of expression. It carries
out all calculations in the widest integer type known to the compiler;
on most machines supported by GCC this is 64 bits. This is not the same
rule as the compiler uses to calculate the value of a constant
expression, and may give different results in some cases. If the value
comes out to be nonzero, the #if succeeds and the controlled
text is included; otherwise it is skipped.
If expression is not correctly formed, GCC issues an error and treats the conditional as having failed.
The special operator defined is used in #if and
#elif expressions to test whether a certain name is defined as a
macro. defined name and defined (name) are
both expressions whose value is 1 if name is defined as a macro at
the current point in the program, and 0 otherwise. Thus, #if defined MACRO is precisely equivalent to #ifdef MACRO.
defined is useful when you wish to test more than one macro for
existence at once. For example,
#if defined (__vax__) || defined (__ns16000__)
would succeed if either of the names __vax__ or
__ns16000__ is defined as a macro.
Conditionals written like this:
#if defined BUFSIZE && BUFSIZE >= 1024
can generally be simplified to just #if BUFSIZE >= 1024,
since if BUFSIZE is not defined, it will be interpreted as having
the value zero.
If the defined operator appears as a result of a macro expansion,
the C standard says the behavior is undefined. GNU cpp treats it as a
genuine defined operator and evaluates it normally. It will warn
wherever your code uses this feature if you use the command-line option
-pedantic, since other compilers may handle it differently.
The #else directive can be added to a conditional to provide
alternative text to be used if the condition fails. This is what it
looks like:
#if expression
text-if-true
#else /* Not expression */
text-if-false
#endif /* Not expression */
If expression is nonzero, the text-if-true is included and the text-if-false is skipped. If expression is zero, the opposite happens.
You can use #else with #ifdef and #ifndef, too.
One 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 conditional group 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 conditional group. Then
the text after each #elif is processed only if the #elif
condition succeeds after the original #if and all previous
#elif directives within it have failed.
#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 for future reference, you often cannot simply comment it out. Block comments do not nest, so the first comment inside the old code will end the commenting-out. The probable result is a flood of syntax errors.
One way to avoid this problem is to use an always-false conditional
instead. For instance, put #if 0 before the deleted code and
#endif after it. This works even if the code being turned
off contains conditionals, but they must be entire conditionals
(balanced #if and #endif).
Some people use #ifdef notdef instead. This is risky, because
notdef might be accidentally defined as a macro, and then the
conditional would succeed. #if 0 can be counted on to fail.
Do not use #if 0 for comments which are not C code. Use a real
comment, instead. The interior of #if 0 must consist of complete
tokens; in particular, single-quote characters must balance. Comments
often contain unbalanced single-quote characters (known in English as
apostrophes). These confuse #if 0. They don't confuse
/*.
The directive #error causes the preprocessor to report a fatal
error. The tokens forming the rest of the line following #error
are used as the error message.
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
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 !defined(UNALIGNED_INT_ASM_OP) && defined(DWARF2_DEBUGGING_INFO)
#error "DWARF2_DEBUGGING_INFO requires UNALIGNED_INT_ASM_OP."
#endif
The directive #warning is like #error, but causes the
preprocessor to issue a warning and continue preprocessing. The tokens
following #warning are 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.
Neither #error nor #warning macro-expands its argument.
Internal whitespace sequences are each replaced with a single space.
The line must consist of complete tokens. It is wisest to make the
argument of these directives be a single string constant; this avoids
problems with apostrophes and the like.
The C preprocessor informs the C compiler of the location in your source code where each token came from. Presently, this is just the file name and line number. All the tokens resulting from macro expansion are reported as having appeared on the line of the source file where the outermost macro was used. We intend to be more accurate in the future.
If you write a program which generates source code, such as the
bison parser generator, you may want to adjust the preprocessor's
notion of the current file name and line number by hand. Parts of the
output from bison are generated from scratch, other parts come
from a standard parser file. The rest are copied verbatim from
bison's input. You would like compiler error messages and
symbolic debuggers to be able to refer to bison's input file.
bison or any such program can arrange 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#include.
Previous versions of CPP did not interpret escapes in #line;
we have changed it because the standard requires they be interpreted,
and most other compilers do.
#line anything else#line directives alter the results of the __FILE__ and
__LINE__ predefined macros from that point on. See Standard Predefined Macros. They do not have any effect on #include's
idea of the directory containing the current file. This is a change
from GCC 2.95. Previously, a file reading
#line 1 "../src/gram.y"
#include "gram.h"
would search for gram.h in ../src, then the -I
chain; the directory containing the physical source file would not be
searched. In GCC 3.0 and later, the #include is not affected by
the presence of a #line referring to a different directory.
We made this change because the old behavior caused problems when
generated source files were transported between machines. For instance,
it is common practice to ship generated parsers with a source release,
so that people building the distribution do not need to have yacc or
Bison installed. These files frequently have #line directives
referring to the directory tree of the system where the distribution was
created. If GCC tries to search for headers in those directories, the
build is likely to fail.
The new behavior can cause failures too, if the generated file is not
in the same directory as its source and it attempts to include a header
which would be visible searching from the directory containing the
source file. However, this problem is easily solved with an additional
-I switch on the command line. The failures caused by the old
semantics could sometimes be corrected only by editing the generated
files, which is difficult and error-prone.
The #pragma directive is the method specified by the C standard
for providing additional information to the compiler, beyond what is
conveyed in the language itself. Three forms of this directive
(commonly known as pragmas) are specified by the 1999 C standard.
A C compiler is free to attach any meaning it likes to other pragmas.
GCC has historically preferred to use extensions to the syntax of the
language, such as __attribute__, for this purpose. However, GCC
does define a few pragmas of its own. These mostly have effects on the
entire translation unit or source file.
In GCC version 3, all GNU-defined, supported pragmas have been given a
GCC prefix. This is in line with the STDC prefix on all
pragmas defined by C99. For backward compatibility, pragmas which were
recognized by previous versions are still recognized without the
GCC prefix, but that usage is deprecated. Some older pragmas are
deprecated in their entirety. They are not recognized with the
GCC prefix. See Obsolete Features.
C99 introduces the _Pragma operator. This feature addresses a
major problem with #pragma: being a directive, it cannot be
produced as the result of macro expansion. _Pragma is an
operator, much like sizeof or defined, and can be embedded
in a macro.
Its syntax is _Pragma (string-literal), where
string-literal can be either a normal or wide-character string
literal. It is destringized, by replacing all \\ with a single
\ and all \" with a ". The result is then
processed as if it had appeared as the right hand side of a
#pragma directive. For example,
_Pragma ("GCC dependency \"parse.y\"")
has the same effect as #pragma GCC dependency "parse.y". The
same effect could be achieved using macros, for example
#define DO_PRAGMA(x) _Pragma (#x)
DO_PRAGMA (GCC dependency "parse.y")
The standard is unclear on where a _Pragma operator can appear.
The preprocessor does not accept it within a preprocessing conditional
directive like #if. To be safe, you are probably best keeping it
out of directives other than #define, and putting it on a line of
its own.
This manual documents the pragmas which are meaningful to the preprocessor itself. Other pragmas are meaningful to the C or C++ compilers. They are documented in the GCC manual.
#pragma GCC dependency
#pragma GCC dependency allows you to check the relative dates of
the current file and another file. If the other file is more recent than
the current file, a warning is issued. This is useful if the current
file is derived from the other file, and should be regenerated. The
other file is searched for using the normal include search path.
Optional trailing text can be used to give more information in the
warning message.
#pragma GCC dependency "parse.y"
#pragma GCC dependency "/usr/include/time.h" rerun fixincludes
#pragma GCC poison
#pragma GCC poison is followed by a list of identifiers to
poison. If any of those identifiers appears anywhere in the source
after the directive, it is a hard error. For example,
#pragma GCC poison printf sprintf fprintf
sprintf(some_string, "hello");
will produce an error.
If a poisoned identifier appears as part of the expansion of a macro which was defined before the identifier was poisoned, it will not cause an error. This lets you poison an identifier without worrying about system headers defining macros that use it.
For example,
#define strrchr rindex
#pragma GCC poison rindex
strrchr(some_string, 'h');
will not produce an error.
#pragma GCC system_header
The #ident directive takes one argument, a string constant. On
some systems, that string constant is copied into a special segment of
the object file. On other systems, the directive is ignored.
This directive is not part of the C standard, but it is not an official GNU extension either. We believe it came from System V.
The #sccs directive is recognized, because it appears in the
header files of some systems. It is a very old, obscure, extension
which we did not invent, and we have been unable to find any
documentation of what it should do, so GCC simply ignores it.
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.
When the C preprocessor is used with the C, C++, or Objective-C compilers, it is integrated into the compiler and communicates a stream of binary tokens directly to the compiler's parser. However, it can also be used in the more conventional standalone mode, where it produces textual output.
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. Long runs of blank lines are discarded.
The ISO standard specifies that it is implementation defined whether a preprocessor preserves whitespace between tokens, or replaces it with e.g. a single space. In GNU CPP, whitespace between tokens is collapsed to become a single space, with the exception that the first token on a non-directive line is preceded with sufficient spaces that it appears in the same column in the preprocessed output that it appeared in the original source file. This is so the output is easy to read. See Differences from previous versions. CPP does not insert any whitespace where there was none in the original source, except where necessary to prevent an accidental token paste.
Source file name and line number information is conveyed by lines of the form
# linenum filename flags
These are called linemarkers. They are inserted as needed into the output (but never within a string or character constant). They mean that the following line originated in file filename at line linenum. filename will never contain any non-printing characters; they are replaced with octal escape sequences.
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
extern "C" block.
As an extension, the preprocessor accepts linemarkers in non-assembler
input files. They are treated like the corresponding #line
directive, (see Line Control), except that trailing flags are
permitted, and are interpreted with the meanings described above. If
multiple flags are given, they must be in ascending order.
Some directives may be duplicated in the output of the preprocessor.
These are #ident (always), #pragma (only if the
preprocessor does not handle the pragma itself), and #define and
#undef (with certain debugging options). If this happens, the
# of the directive will always be in the first column, and there
will be no space between the # and the directive name. If macro
expansion happens to generate tokens which might be mistaken for a
duplicated directive, a space will be inserted between the # and
the directive name.
Traditional (pre-standard) C preprocessing is rather different from
the preprocessing specified by the standard. When GCC is given the
-traditional-cpp option, it attempts to emulate a traditional
preprocessor.
GCC versions 3.2 and later only support traditional mode semantics in the preprocessor, and not in the compiler front ends. This chapter outlines the traditional preprocessor semantics we implemented.
The implementation does not correspond precisely to the behavior of earlier versions of GCC, nor to any true traditional preprocessor. After all, inconsistencies among traditional implementations were a major motivation for C standardization. However, we intend that it should be compatible with true traditional preprocessors in all ways that actually matter.
The traditional preprocessor does not decompose its input into tokens the same way a standards-conforming preprocessor does. The input is simply treated as a stream of text with minimal internal form.
This implementation does not treat trigraphs (see trigraphs) specially since they were an invention of the standards committee. It handles arbitrarily-positioned escaped newlines properly and splices the lines as you would expect; many traditional preprocessors did not do this.
The form of horizontal whitespace in the input file is preserved in the output. In particular, hard tabs remain hard tabs. This can be useful if, for example, you are preprocessing a Makefile.
Traditional CPP only recognizes C-style block comments, and treats the
/* sequence as introducing a comment only if it lies outside
quoted text. Quoted text is introduced by the usual single and double
quotes, and also by an initial < in a #include
directive.
Traditionally, comments are completely removed and are not replaced with a space. Since a traditional compiler does its own tokenization of the output of the preprocessor, this means that comments can effectively be used as token paste operators. However, comments behave like separators for text handled by the preprocessor itself, since it doesn't re-lex its input. For example, in
#if foo/**/bar
foo and bar are distinct identifiers and expanded
separately if they happen to be macros. In other words, this
directive is equivalent to
#if foo bar
rather than
#if foobar
Generally speaking, in traditional mode an opening quote need not have a matching closing quote. In particular, a macro may be defined with replacement text that contains an unmatched quote. Of course, if you attempt to compile preprocessed output containing an unmatched quote you will get a syntax error.
However, all preprocessing directives other than #define
require matching quotes. For example:
#define m This macro's fine and has an unmatched quote
"/* This is not a comment. */
/* This is a comment. The following #include directive
is ill-formed. */
#include <stdio.h
Just as for the ISO preprocessor, what would be a closing quote can be escaped with a backslash to prevent the quoted text from closing.
The major difference between traditional and ISO macros is that the former expand to text rather than to a token sequence. CPP removes all leading and trailing horizontal whitespace from a macro's replacement text before storing it, but preserves the form of internal whitespace.
One consequence is that it is legitimate for the replacement text to contain an unmatched quote (see Traditional lexical analysis). An unclosed string or character constant continues into the text following the macro call. Similarly, the text at the end of a macro's expansion can run together with the text after the macro invocation to produce a single token.
Normally comments are removed from the replacement text after the
macro is expanded, but if the -CC option is passed on the
command line comments are preserved. (In fact, the current
implementation removes comments even before saving the macro
replacement text, but it careful to do it in such a way that the
observed effect is identical even in the function-like macro case.)
The ISO stringification operator # and token paste operator
## have no special meaning. As explained later, an effect
similar to these operators can be obtained in a different way. Macro
names that are embedded in quotes, either from the main file or after
macro replacement, do not expand.
CPP replaces an unquoted object-like macro name with its replacement text, and then rescans it for further macros to replace. Unlike standard macro expansion, traditional macro expansion has no provision to prevent recursion. If an object-like macro appears unquoted in its replacement text, it will be replaced again during the rescan pass, and so on ad infinitum. GCC detects when it is expanding recursive macros, emits an error message, and continues after the offending macro invocation.
#define PLUS +
#define INC(x) PLUS+x
INC(foo);
==> ++foo;
Function-like macros are similar in form but quite different in behavior to their ISO counterparts. Their arguments are contained within parentheses, are comma-separated, and can cross physical lines. Commas within nested parentheses are not treated as argument separators. Similarly, a quote in an argument cannot be left unclosed; a following comma or parenthesis that comes before the closing quote is treated like any other character. There is no facility for handling variadic macros.
This implementation removes all comments from macro arguments, unless
the -C option is given. The form of all other horizontal
whitespace in arguments is preserved, including leading and trailing
whitespace. In particular
f( )
is treated as an invocation of the macro f with a single
argument consisting of a single space. If you want to invoke a
function-like macro that takes no arguments, you must not leave any
whitespace between the parentheses.
If a macro argument crosses a new line, the new line is replaced with a space when forming the argument. If the previous line contained an unterminated quote, the following line inherits the quoted state.
Traditional preprocessors replace parameters in the replacement text with their arguments regardless of whether the parameters are within quotes or not. This provides a way to stringize arguments. For example
#define str(x) "x"
str(/* A comment */some text )
==> "some text "
Note that the comment is removed, but that the trailing space is preserved. Here is an example of using a comment to effect token pasting.
#define suffix(x) foo_/**/x
suffix(bar)
==> foo_bar
Here are some things to be aware of when using the traditional preprocessor.
# appears in the first column. There can be no whitespace
between the beginning of the line and the #, but whitespace can
follow the #.
#error or
#pragma, and may not recognize #elif. CPP supports all
the directives in traditional mode that it supports in ISO mode,
including extensions, with the exception that the effects of
#pragma GCC poison are undefined.
You can request warnings about features that did not exist, or worked
differently, in traditional C with the -Wtraditional option.
GCC does not warn about features of ISO C which you must use when you
are using a conforming compiler, such as the # and ##
operators.
Presently -Wtraditional warns about:
# appeared in column 1 on the line. Therefore
-Wtraditional warns about directives that traditional C
understands but would ignore because the # does not appear as the
first character on the line. It also suggests you hide directives like
#pragma not understood by traditional C by indenting them. Some
traditional implementations would not recognize #elif, so it
suggests avoiding it altogether.
U and LL integer constant suffixes, which were not
available in traditional C. (Traditional C does support the L
suffix for simple long integer constants.) You are not warned about
uses of these suffixes in macros defined in system headers. For
instance, UINT_MAX may well be defined as 4294967295U, but
you will not be warned if you use UINT_MAX.
You can usually avoid the warning, and the related warning about constants which are so large that they are unsigned, by writing the integer constant in question in hexadecimal, with no U suffix. Take care, though, because this gives the wrong result in exotic cases.
Here we document details of how the preprocessor's implementation affects its user-visible behavior. You should try to avoid undue reliance on behavior described here, as it is possible that it will change subtly in future implementations.
Also documented here are obsolete features and changes from previous versions of CPP.
This is how CPP behaves in all the cases which the C standard describes as implementation-defined. This term means that the implementation is free to do what it likes, but must document its choice and stick to it.
Currently, GNU cpp only supports character sets that are strict supersets of ASCII, and performs no translation of characters.
In textual output, each whitespace sequence is collapsed to a single space. For aesthetic reasons, the first token on each non-directive line of output is preceded with sufficient spaces that it appears in the same column as it did in the original source file.
The preprocessor and compiler interpret character constants in the
same way; i.e. escape sequences such as \a are given the
values they would have on the target machine.
The compiler values a multi-character character constant a character
at a time, shifting the previous value left by the number of bits per
target character, and then or-ing in the bit-pattern of the new
character truncated to the width of a target character. The final
bit-pattern is given type int, and is therefore signed,
regardless of whether single characters are signed or not (a slight
change from versions 3.1 and earlier of GCC). If there are more
characters in the constant than would fit in the target int the
compiler issues a warning, and the excess leading characters are
ignored.
For example, 'ab' for a target with an 8-bit char would be
interpreted as (int) ((unsigned char) 'a' * 256 + (unsigned char) 'b'), and '\234a' as (int) ((unsigned char) '\234' * 256 + (unsigned char) 'a').
For a discussion on how the preprocessor locates header files, Include Operation.
#include directive.
See Computed Includes.
#pragma directive that after macro-expansion
results in a standard pragma.
No macro expansion occurs on any #pragma directive line, so the
question does not arise.
Note that GCC does not yet implement any of the standard pragmas.
CPP has a small number of internal limits. This section lists the limits which the C standard requires to be no lower than some minimum, and all the others we are aware of. We intend there to be as few limits as possible. If you encounter an undocumented or inconvenient limit, please report that to us as a bug. (See the section on reporting bugs in the GCC manual.)
Where we say something is limited only by available memory, that
means that internal data structures impose no intrinsic limit, and space
is allocated with malloc or equivalent. The actual limit will
therefore depend on many things, such as the size of other things
allocated by the compiler at the same time, the amount of memory
consumed by other processes on the same computer, etc.
#include files.
We impose an arbitrary limit of 200 levels, to avoid runaway recursion. The standard requires at least 15 levels.
The C standard mandates this be at least 63. CPP is limited only by available memory.
The C standard requires this to be at least 63. In preprocessor conditional expressions, it is limited only by available memory.
The preprocessor treats all characters as significant. The C standard requires only that the first 63 be significant.
The standard requires at least 4095 be possible. CPP is limited only by available memory.
We allow USHRT_MAX, which is no smaller than 65,535. The minimum
required by the standard is 127.
The C standard requires a minimum of 4096 be permitted. CPP places no limits on this, but you may get incorrect column numbers reported in diagnostics for lines longer than 65,535 characters.
The standard does not specify any lower limit on the maximum size of a source file. GNU cpp maps files into memory, so it is limited by the available address space. This is generally at least two gigabytes. Depending on the operating system, the size of physical memory may or may not be a limitation.
CPP has a number of features which are present mainly for compatibility with older programs. We discourage their use in new code. In some cases, we plan to remove the feature in a future version of GCC.
Assertions are a deprecated 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.
Assertions were intended to provide a more systematic way to describe the compiler's target system. However, in practice they are just as unpredictable as the system-specific predefined macros. In addition, they are not part of any standard, and only a few compilers support them. Therefore, the use of assertions is less portable than the use of system-specific predefined macros. We recommend you do not use them at all.
An assertion looks like this:
#predicate (answer)
predicate must be a single identifier. answer can be any
sequence of tokens; all characters are significant except for leading
and trailing whitespace, and differences in internal whitespace
sequences are ignored. (This is similar to the rules governing macro
redefinition.) Thus, (x + y) is different from (x+y) but
equivalent to ( x + y ). Parentheses do not nest inside an
answer.
To test an assertion, you write it in an #if. For example, this
conditional succeeds if either vax or ns16000 has been
asserted as an answer for machine.
#if #machine (vax) || #machine (ns16000)
You can test whether any answer is asserted for a predicate by omitting the answer in the conditional:
#if #machine
Assertions are made with the #assert directive. Its sole
argument is the assertion to make, without the leading # that
identifies assertions in conditionals.
#assert predicate (answer)
You may make several assertions with the same predicate and different answers. Subsequent assertions do not override previous ones for the same predicate. All the answers for any given predicate are simultaneously true.
Assertions can be canceled with the #unassert directive. It
has the same syntax as #assert. In that form it cancels only the
answer which was specified on the #unassert line; other answers
for that predicate remain true. You can cancel an entire predicate by
leaving out the answer:
#unassert predicate
In either form, if no such assertion has been made, #unassert has
no effect.
You can also make or cancel assertions using command line options. See Invocation.
CPP supports two more ways of indicating that a header file should be
read only once. Neither one is as portable as a wrapper #ifndef,
and we recommend you do not use them in new programs.
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
inclusion of the contents. GCC permits the use of #import in C
and C++ as well as Objective-C. However, it is not in standard C or C++
and should therefore not be used by portable programs.
#import is not a well designed feature. It requires the users of
a header file to know that it 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 a wrapper #ifndef accomplishes
this goal.
In the present implementation, a single use of #import will
prevent the file from ever being read again, by either #import or
#include. You should not rely on this; do not use both
#import and #include to refer to the same header file.
Another way to prevent a header file from being included more than once
is with the #pragma once directive. If #pragma once is
seen when scanning a header file, that file will never be read again, no
matter what.
#pragma once does not have the problems that #import does,
but it is not recognized by all preprocessors, so you cannot rely on it
in a portable program.
This section details behavior which has changed from previous versions of CPP. We do not plan to change it again in the near future, but we do not promise not to, either.
The "previous versions" discussed here are 2.95 and before. The behavior of GCC 3.0 is mostly the same as the behavior of the widely used 2.96 and 2.97 development snapshots. Where there are differences, they generally represent bugs in the snapshots.
# and ## operators
The standard does not specify the order of evaluation of a chain of
## operators, nor whether # is evaluated before, after, or
at the same time as ##. You should therefore not write any code
which depends on any specific ordering. It is possible to guarantee an
ordering, if you need one, by suitable use of nested macros.
An example of where this might matter is pasting the arguments 1,
e and -2. This would be fine for left-to-right pasting,
but right-to-left pasting would produce an invalid token e-2.
GCC 3.0 evaluates # and ## at the same time and strictly
left to right. Older versions evaluated all # operators first,
then all ## operators, in an unreliable order.
See Preprocessor Output, for the current textual format. This is also the format used by stringification. Normally, the preprocessor communicates tokens directly to the compiler's parser, and whitespace does not come up at all.
Older versions of GCC preserved all whitespace provided by the user and inserted lots more whitespace of their own, because they could not accurately predict when extra spaces were needed to prevent accidental token pasting.
As an extension, GCC permits you to omit the variable arguments entirely when you use a variable argument macro. This is forbidden by the 1999 C standard, and will provoke a pedantic warning with GCC 3.0. Previous versions accepted it silently.
## swallowing preceding text in rest argument macros
Formerly, in a macro expansion, if ## appeared before a variable
arguments parameter, and the set of tokens specified for that argument
in the macro invocation was empty, previous versions of CPP would
back up and remove the preceding sequence of non-whitespace characters
(not the preceding token). This extension is in direct
conflict with the 1999 C standard and has been drastically pared back.
In the current version of the preprocessor, if ## appears between
a comma and a variable arguments parameter, and the variable argument is
omitted entirely, the comma will be removed from the expansion. If the
variable argument is empty, or the token before ## is not a
comma, then ## behaves as a normal token paste.
#line and #include
The #line directive used to change GCC's notion of the
"directory containing the current file," used by #include with
a double-quoted header file name. In 3.0 and later, it does not.
See Line Control, for further explanation.
#line
In GCC 2.95 and previous, the string constant argument to #line
was treated the same way as the argument to #include: backslash
escapes were not honored, and the string ended at the second ".
This is not compliant with the C standard. In GCC 3.0, an attempt was
made to correct the behavior, so that the string was treated as a real
string constant, but it turned out to be buggy. In 3.1, the bugs have
been fixed. (We are not fixing the bugs in 3.0 because they affect
relatively few people and the fix is quite invasive.)
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. All the options listed here are also acceptable to the C compiler and have the same meaning, except that the C compiler has different rules for specifying the output file.
Note: Whether you use the preprocessor by way of gcc
or cpp, the compiler driver is run first. This
program's purpose is to translate your command into invocations of the
programs that do the actual work. Their command line interfaces are
similar but not identical to the documented interface, and may change
without notice.
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 either file is omitted, it
means the same as if - had been specified for that file.
Unless otherwise noted, or the option ends in =, all options
which take an argument may have that argument appear either immediately
after the option, or with a space between option and argument:
-Ifoo and -I foo have the same effect.
Many options have multi-letter names; therefore multiple single-letter
options may not be grouped: -dM is very different from
-d -M.
-D name1.
-D name=definitionIf you wish to define a function-like macro on the command line, write
its argument list with surrounding parentheses before the equals sign
(if any). Parentheses are meaningful to most shells, so you will need
to quote the option. With sh and csh,
-D'name(args...)=definition' works.
-D and -U options are processed in the order they
are given on the command line. All -imacros file and
-include file options are processed after all
-D and -U options.
-U name-D option.
-undef
-I dir-I are searched before the standard
system include directories. If the directory dir is a standard
system include directory, the option is ignored to ensure that the
default search order for system directories and the special treatment
of system headers are not defeated
(see System Headers)
.
-o filecpp. gcc has a
different interpretation of a second non-option argument, so you must
use -o to specify the output file.
-Wall
-Wcomment and -Wtrigraphs. Note that
many of the preprocessor's warnings are on by default and have no
options to control them.
-Wcomment
-Wcomments
/* appears in a /*
comment, or whenever a backslash-newline appears in a // comment.
(Both forms have the same effect.)
-Wtrigraphs
-trigraphs was also specified, but now works
independently. Warnings are not given for trigraphs within comments, as
they do not affect the meaning of the program.
-Wtraditional
-Wimport
#import is used.
-Wundef
#if directive, outside of defined. Such identifiers are
replaced with zero.
-Wunused-macros
Built-in macros, macros defined on the command line, and macros defined in include files are not warned about.
Note: If a macro is actually used, but only used in skipped conditional blocks, then CPP will report it as unused. To avoid the warning in such a case, you might improve the scope of the macro's definition by, for example, moving it into the first skipped block. Alternatively, you could provide a dummy use with something like:
#if defined the_macro_causing_the_warning
#endif
-Wendif-labels
#else or an #endif are followed by text.
This usually happens in code of the form
#if FOO
...
#else FOO
...
#endif FOO
The second and third FOO should be in comments, but often are not
in older programs. This warning is on by default.
-Werror
-Wsystem-headers
-w
-pedantic
-pedantic-errors
-pedantic but treats as warnings.
-M
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, including those coming from -include or
-imacros command line options.
Unless specified explicitly (with -MT or -MQ), the
object file name consists of the basename of the source file with any
suffix replaced with object file suffix. If there are many included
files then the rule is split into several lines using \-newline.
The rule has no commands.
This option does not suppress the preprocessor's debug output, such as
-dM. To avoid mixing such debug output with the dependency
rules you should explicitly specify the dependency output file with
-MF, or use an environment variable like
DEPENDENCIES_OUTPUT (see Environment Variables). Debug output
will still be sent to the regular output stream as normal.
Passing -M to the driver implies -E, and suppresses
warnings with an implicit -w.
-MM
-M but do not mention header files that are found in
system header directories, nor header files that are included,
directly or indirectly, from such a header.
This implies that the choice of angle brackets or double quotes in an
#include directive does not in itself determine whether that
header will appear in -MM dependency output. This is a
slight change in semantics from GCC versions 3.0 and earlier.
-MF file-M or -MM, specifies a
file to write the dependencies to. If no -MF switch is given
the preprocessor sends the rules to the same place it would have sent
preprocessed output.
When used with the driver options -MD or -MMD,
-MF overrides the default dependency output file.
-MG
-M requesting
dependency generation, -MG assumes missing header files are
generated files and adds them to the dependency list without raising
an error. The dependency filename is taken directly from the
#include directive without prepending any path. -MG
also suppresses preprocessed output, as a missing header file renders
this useless.
This feature is used in automatic updating of makefiles.
-MP
make gives if you remove header
files without updating the Makefile to match.
This is typical output:
test.o: test.c test.h
test.h:
-MT targetChange the target of the rule emitted by dependency generation. By
default CPP takes the name of the main input file, including any path,
deletes any file suffix such as .c, and appends the platform's
usual object suffix. The result is the target.
An -MT option will set the target to be exactly the string you
specify. If you want multiple targets, you can specify them as a single
argument to -MT, or use multiple -MT options.
For example, -MT '$(objpfx)foo.o' might give
$(objpfx)foo.o: foo.c
-MQ targetSame as -MT, but it quotes any characters which are special to
Make. -MQ '$(objpfx)foo.o' gives
$$(objpfx)foo.o: foo.c
The default target is automatically quoted, as if it were given with
-MQ.
-MD
-MD is equivalent to -M -MF file-E is not implied. The driver determines file based on
whether an -o option is given. If it is, the driver uses its
argument but with a suffix of .d, otherwise it take the
basename of the input file and applies a .d suffix.
If -MD is used in conjunction with -E, any
-o switch is understood to specify the dependency output file
(but see -MF), but if used without -E, each -o
is understood to specify a target object file.
Since -E is not implied, -MD can be used to generate
a dependency output file as a side-effect of the compilation process.
-MMD
-MD except mention only user header files, not system
-header files.
-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
c89
c89 is the customary shorthand for
this version of the standard.
The -ansi option is equivalent to -std=c89.
iso9899:199409
iso9899:1999
c99
iso9899:199x
c9x
gnu89
gnu99
gnu9x
c++98
gnu++98
-std=c++98 plus GNU extensions. This is the
default for C++ code.
-I-
-I
options before -I- are searched only for headers requested with
#include "file"; they are not searched for
#include <file>. If additional directories are
specified with -I options after the -I-, those
directories are searched for all #include directives.
In addition, -I- inhibits the use of the directory of the current
file directory as the first search directory for #include "file".
See Search Path.
-nostdinc
-I options
(and the directory of the current file, if appropriate) are searched.
-nostdinc++
-include file#include "file" appeared as the first
line of the primary source file. However, the first directory searched
for file is the preprocessor's working directory instead of
the directory containing the main source file. If not found there, it
is searched for in the remainder of the #include "..." search
chain as normal.
If multiple -include options are given, the files are included
in the order they appear on the command line.
-imacros file-include, except that any output produced by
scanning file is thrown away. Macros it defines remain defined.
This allows you to acquire all the macros from a header without also
processing its declarations.
All files specified by -imacros are processed before all files
specified by -include.
-idirafter dir-I and the standard system directories
have been exhausted. dir is treated as a system include directory.
-iprefix prefix-iwithprefix
options. If the prefix represents a directory, you should include the
final /.
-iwithprefix dir-iwithprefixbefore dir-iprefix, and add the resulting directory to the include search
path. -iwithprefixbefore puts it in the same place -I
would; -iwithprefix puts it where -idirafter would.
Use of these options is discouraged.
-isystem dir-I but before the standard system directories. Mark it
as a system directory, so that it gets the same special treatment as
is applied to the standard system directories.
See System Headers.
-fpreprocessed
-C to the compiler without
problems. In this mode the integrated preprocessor is little more than
a tokenizer for the front ends.
-fpreprocessed is implicit if the input file has one of the
extensions .i, .ii or .mi. These are the
extensions that GCC uses for preprocessed files created by
-save-temps.
-ftabstop=width-fno-show-column
dejagnu.
-A predicate=answer-A
predicate(answer), which is still supported, because
it does not use shell special characters.
See Assertions.
-A -predicate=answer-dCHARS
M
#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 all the predefined macros.
D
M 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.
N
D, but emit only the macro names, not their expansions.
I
#include directives in addition to the result of
preprocessing.
-P
-C
You should be prepared for side effects when using -C; it
causes the preprocessor to treat comments as tokens in their own right.
For example, comments appearing at the start of what would be a
directive line have the effect of turning that line into an ordinary
source line, since the first token on the line is no longer a #.
-CC
-C, except that comments contained within macros are
also passed through to the output file where the macro is expanded.
In addition to the side-effects of the -C option, the
-CC option causes all C++-style comments inside a macro
to be converted to C-style comments. This is to prevent later use
of that macro from inadvertently commenting out the remainder of
the source line.
The -CC option is generally used to support lint comments.
-traditional-cpp
-trigraphs
-remap
--help
--target-help
-v
-H
#include stack it is.
-version
--version
This section describes the environment variables that affect how CPP operates. You can use them to specify directories or prefixes to use when searching for include files, or to control dependency output.
Note that you can also specify places to search using options such as
-I, and control dependency output with options like
-M (see Invocation). These take precedence over
environment variables, which in turn take precedence over the
configuration of GCC.
CPATH
C_INCLUDE_PATH
CPLUS_INCLUDE_PATH
OBJC_INCLUDE_PATH
PATH, in which to look for header files.
The special character, PATH_SEPARATOR, is target-dependent and
determined at GCC build time. For Windows-based targets it is a
semicolon, and for almost all other targets it is a colon.
CPATH specifies a list of directories to be searched as if
specified with -I, but after any paths given with -I
options on the command line. This environment variable is used
regardless of which language is being preprocessed.
The remaining environment variables apply only when preprocessing the
particular language indicated. Each specifies a list of directories
to be searched as if specified with -isystem, but after any
paths given with -isystem options on the command line.
In all these variables, an empty element instructs the compiler to
search its current working directory. Empty elements can appear at the
beginning or end of a path. For instance, if the value of
CPATH is :/special/include, that has the same
effect as -I. -I/special/include.
See also Search Path.
DEPENDENCIES_OUTPUT
The value of DEPENDENCIES_OUTPUT can be just a file name, in
which case the Make rules are written to that file, guessing the target
name from the source file name. Or the value can have the form
file target, in which case the rules are written to
file file using target as the target name.
In other words, this environment variable is equivalent to combining
the options -MM and -MF
(see Invocation),
with an optional -MT switch too.
SUNPRO_DEPENDENCIES
DEPENDENCIES_OUTPUT (see above),
except that system header files are not ignored, so it implies
-M rather than -MM. However, the dependence on the
main input file is omitted.
See Invocation.
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#assert: Assertions
#define: Object-like Macros
#elif: Elif
#else: Else
#endif: Ifdef
#error: Diagnostics
#ident: Other Directives
#if: Conditional Syntax
#ifdef: Ifdef
#ifndef: Ifdef
#import: Obsolete once-only headers
#include: Include Syntax
#include_next: Wrapper Headers
#line: Line Control
#pragma GCC dependency: Pragmas
#pragma GCC poison: Pragmas
#pragma GCC system_header: System Headers, Pragmas
#sccs: Other Directives
#unassert: Assertions
#undef: Undefining and Redefining Macros
#warning: Diagnostics
CPP's command line options and environment variables are indexed here
without any initial - or --.
A: Invocation
ansi: Invocation
C: Invocation
C_INCLUDE_PATH: Environment Variables
CPATH: Environment Variables
CPLUS_INCLUDE_PATH: Environment Variables
D: Invocation
dD: Invocation
DEPENDENCIES_OUTPUT: Environment Variables
dI: Invocation
dM: Invocation
dN: Invocation
fno-show-column: Invocation
fpreprocessed: Invocation
ftabstop: Invocation
H: Invocation
help: Invocation
I: Invocation
I-: Invocation
idirafter: Invocation
imacros: Invocation
include: Invocation
iprefix: Invocation
isystem: Invocation
iwithprefix: Invocation
iwithprefixbefore: Invocation
M: Invocation
MD: Invocation
MF: Invocation
MG: Invocation
MM: Invocation
MMD: Invocation
MP: Invocation
MQ: Invocation
MT: Invocation
nostdinc: Invocation
nostdinc++: Invocation
o: Invocation
OBJC_INCLUDE_PATH: Environment Variables
P: Invocation
pedantic: Invocation
pedantic-errors: Invocation
remap: Invocation
std=: Invocation
SUNPRO_DEPENDENCIES: Environment Variables
target-help: Invocation
traditional-cpp: Invocation
trigraphs: Invocation
U: Invocation
undef: Invocation
v: Invocation
version: Invocation
w: Invocation
Wall: Invocation
Wcomment: Invocation
Wcomments: Invocation
Wendif-labels: Invocation
Werror: Invocation
Wimport: Invocation
Wsystem-headers: Invocation
Wtraditional: Invocation
Wtrigraphs: Invocation
Wundef: Invocation
Wunused-macros: Invocation
x: Invocation
# operator: Stringification
## operator: Concatenation
_Pragma: Pragmas
defined: Defined
#ifndef: Once-Only Headers