@copying
@c man begin COPYRIGHT
Copyright @copyright{} 1987, 1989, 1991, 1992, 1993, 1994, 1995, 1996,
-1997, 1998, 1999, 2000, 2001, 2002, 2003
+1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004
Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document
Overview
+* Character sets::
* Initial processing::
* Tokenization::
* The preprocessing language::
@c man end
@menu
+* Character sets::
* Initial processing::
* Tokenization::
* The preprocessing language::
@end menu
+@node Character sets
+@section Character sets
+
+Source code character set processing in C and related languages is
+rather complicated. The C standard discusses two character sets, but
+there are really at least four.
+
+The files input to CPP might be in any character set at all. CPP's
+very first action, before it even looks for line boundaries, is to
+convert the file into the character set it uses for internal
+processing. That set is what the C standard calls the @dfn{source}
+character set. It must be isomorphic with ISO 10646, also known as
+Unicode. CPP uses the UTF-8 encoding of Unicode.
+
+At present, GNU CPP does not implement conversion from arbitrary file
+encodings to the source character set. Use of any encoding other than
+plain ASCII or UTF-8, except in comments, will cause errors. Use of
+encodings that are not strict supersets of ASCII, such as Shift JIS,
+may cause errors even if non-ASCII characters appear only in comments.
+We plan to fix this in the near future.
+
+All preprocessing work (the subject of the rest of this manual) is
+carried out in the source character set. If you request textual
+output from the preprocessor with the @option{-E} option, it will be
+in UTF-8.
+
+After preprocessing is complete, string and character constants are
+converted again, into the @dfn{execution} character set. This
+character set is under control of the user; the default is UTF-8,
+matching the source character set. Wide string and character
+constants have their own character set, which is not called out
+specifically in the standard. Again, it is under control of the user.
+The default is UTF-16 or UTF-32, whichever fits in the target's
+@code{wchar_t} type, in the target machine's byte
+order.@footnote{UTF-16 does not meet the requirements of the C
+standard for a wide character set, but the choice of 16-bit
+@code{wchar_t} is enshrined in some system ABIs so we cannot fix
+this.} Octal and hexadecimal escape sequences do not undergo
+conversion; @t{'\x12'} has the value 0x12 regardless of the currently
+selected execution character set. All other escapes are replaced by
+the character in the source character set that they represent, then
+converted to the execution character set, just like unescaped
+characters.
+
+GCC does not permit the use of characters outside the ASCII range, nor
+@samp{\u} and @samp{\U} escapes, in identifiers. We hope this will
+change eventually, but there are problems with the standard semantics
+of such ``extended identifiers'' which must be resolved through the
+ISO C and C++ committees first.
+
@node Initial processing
@section Initial processing
@enumerate
@item
-@cindex character sets
@cindex line endings
The input file is read into memory and broken into lines.
-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 @kbd{LF}, @kbd{@w{CR
-LF}} and @kbd{CR} as end-of-line markers. These
-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}} and @kbd{CR} as end-of-line markers. These 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.)
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
The nine trigraphs and their replacements are
-@example
+@smallexample
Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??-
Replacement: [ ] @{ @} # \ ^ | ~
-@end example
+@end smallexample
@item
@cindex continued lines
@samp{/*} and continue until the next @samp{*/}. Block comments do not
nest:
-@example
+@smallexample
/* @r{this is} /* @r{one comment} */ @r{text outside comment}
-@end example
+@end smallexample
@dfn{Line comments} begin with @samp{//} 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.
-@example
+@smallexample
// @r{this is} // @r{one comment}
@r{text outside comment}
-@end example
+@end smallexample
@end enumerate
It is safe to put line comments inside block comments, or vice versa.
-@example
+@smallexample
@group
/* @r{block comment}
// @r{contains line comment}
// @r{line comment} /* @r{contains block comment} */
@end group
-@end example
+@end smallexample
But beware of commenting out one end of a block comment with a line
comment.
-@example
+@smallexample
@group
// @r{l.c.} /* @r{block comment begins}
@r{oops! this isn't a comment anymore} */
@end group
-@end example
+@end smallexample
-Comments are not recognized within string literals. @t{@w{"/* blah
-*/"}} is the string constant @samp{@w{/* blah */}}, not an empty string.
+Comments are not recognized within string literals.
+@t{@w{"/* blah */"}} is the string constant @samp{@w{/* 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
@samp{*/}, and @samp{//} onto multiple lines with backslash-newline.
For example:
-@example
+@smallexample
@group
/\
*
O 10\
20
@end group
-@end example
+@end smallexample
@noindent
is equivalent to @code{@w{#define FOO 1020}}. All these tricks are
change, except when the @samp{##} preprocessing operator is used to paste
tokens together. @xref{Concatenation}. For example,
-@example
+@smallexample
@group
#define foo() bar
foo()baz
@emph{not}
@expansion{} barbaz
@end group
-@end example
+@end smallexample
The compiler does not re-tokenize the preprocessor's output. Each
preprocessing token becomes one compiler token.
unlike trigraphs, but does not cover as much ground. The digraphs and
their corresponding normal punctuators are:
-@example
+@smallexample
Digraph: <% %> <: :> %: %:%:
Punctuator: @{ @} [ ] # ##
-@end example
+@end smallexample
@cindex other tokens
Any other single character is considered ``other.'' It is passed on to
text, NUL is considered white space. For example, these two directives
have the same meaning.
-@example
+@smallexample
#define X^@@1
#define X 1
-@end example
+@end smallexample
@noindent
(where @samp{^@@} is ASCII NUL)@. Within string or character constants,
directories to this list with the @option{-I} option (@pxref{Invocation}).
@item #include "@var{file}"
-This variant is used for header files of your own program. It searches
-for a file named @var{file} first in the directory containing the
-current file, then in the same directories used for @code{<@var{file}>}.
+This variant is used for header files of your own program. It
+searches for a file named @var{file} first in the directory containing
+the current file, then in the quote directories and then the same
+directories used for @code{<@var{file}>}. You can prepend directories
+to the list of quote directories with the @option{-iquote} option.
@end table
The argument of @samp{#include}, whether delimited with quote marks or
@samp{#include} directive. For example, if you have a header file
@file{header.h} as follows,
-@example
+@smallexample
char *test (void);
-@end example
+@end smallexample
@noindent
and a main program called @file{program.c} that uses the header file,
like this,
-@example
+@smallexample
int x;
#include "header.h"
@{
puts (test ());
@}
-@end example
+@end smallexample
@noindent
the compiler will see the same token stream as it would if
@file{program.c} read
-@example
+@smallexample
int x;
char *test (void);
@{
puts (test ());
@}
-@end example
+@end smallexample
Included files are not limited to declarations and macro definitions;
those are merely the typical uses. Any fragment of a C program can be
system, if you do not instruct it otherwise, it will look for headers
requested with @code{@w{#include <@var{file}>}} in:
-@example
+@smallexample
/usr/local/include
-/usr/lib/gcc-lib/@var{target}/@var{version}/include
+@var{libdir}/gcc/@var{target}/@var{version}/include
/usr/@var{target}/include
/usr/include
-@end example
+@end smallexample
For C++ programs, it will also look in @file{/usr/include/g++-v3},
first. In the above, @var{target} is the canonical name of the system
@option{-nostdinc} is in effect.
GCC looks for headers requested with @code{@w{#include "@var{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 @file{/usr/include/sys/stat.h} contains
+first in the directory containing the current file, then in the
+directories as specified by @option{-iquote} options, then in the same
+places it would have looked for a header requested with angle
+brackets. For example, if @file{/usr/include/sys/stat.h} contains
@code{@w{#include "types.h"}}, GCC looks for @file{types.h} first in
@file{/usr/include/sys}, then in its usual search path.
quote marks. Directories after @option{-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 @option{-I} switch.
+named by an @option{-I} switch. @option{-I-} is deprecated, -iquote
+should be used instead.
@option{-I. -I-} is not the same as no @option{-I} options at all, and does
not cause the same behavior for @samp{<>} includes that @samp{""}
The standard way to prevent this is to enclose the entire real contents
of the file in a conditional, like this:
-@example
+@smallexample
@group
/* File foo. */
#ifndef FILE_FOO_SEEN
#endif /* !FILE_FOO_SEEN */
@end group
-@end example
+@end smallexample
This construct is commonly known as a @dfn{wrapper #ifndef}.
When the header is included again, the conditional will be false,
configuration parameters to be used on different sorts of operating
systems, for instance. You could do this with a series of conditionals,
-@example
+@smallexample
#if SYSTEM_1
# include "system_1.h"
#elif SYSTEM_2
#elif SYSTEM_3
@dots{}
#endif
-@end example
+@end smallexample
That rapidly becomes tedious. Instead, the preprocessor offers the
ability to use a macro for the header name. This is called a
@dfn{computed include}. Instead of writing a header name as the direct
argument of @samp{#include}, you simply put a macro name there instead:
-@example
+@smallexample
#define SYSTEM_H "system_1.h"
@dots{}
#include SYSTEM_H
-@end example
+@end smallexample
@noindent
@code{SYSTEM_H} will be expanded, and the preprocessor will look for
string for embedded quotes, but neither does it process backslash
escapes in the string. Therefore
-@example
+@smallexample
#define HEADER "a\"b"
#include HEADER
-@end example
+@end smallexample
@noindent
looks for a file named @file{a\"b}. CPP searches for the file according
Headers}), it will recurse infinitely and cause a fatal error.
You could include the old header with an absolute pathname:
-@example
+@smallexample
#include "/usr/include/old-header.h"
-@end example
+@end smallexample
@noindent
This works, but is not clean; should the system headers ever move, you
would have to edit the new headers to match.
be an abbreviation for, which is variously referred to as the macro's
@dfn{body}, @dfn{expansion} or @dfn{replacement list}. For example,
-@example
+@smallexample
#define BUFFER_SIZE 1024
-@end example
+@end smallexample
@noindent
defines a macro named @code{BUFFER_SIZE} as an abbreviation for the
token @code{1024}. If somewhere after this @samp{#define} directive
there comes a C statement of the form
-@example
+@smallexample
foo = (char *) malloc (BUFFER_SIZE);
-@end example
+@end smallexample
@noindent
then the C preprocessor will recognize and @dfn{expand} the macro
@code{BUFFER_SIZE}. The C compiler will see the same tokens as it would
if you had written
-@example
+@smallexample
foo = (char *) malloc (1024);
-@end example
+@end smallexample
-By convention, macro names are written in upper case. Programs are
+By convention, macro names are written in uppercase. Programs are
easier to read when it is possible to tell at a glance which names are
macros.
backslash-newline. When the macro is expanded, however, it will all
come out on one line. For example,
-@example
+@smallexample
#define NUMBERS 1, \
2, \
3
int x[] = @{ NUMBERS @};
@expansion{} int x[] = @{ 1, 2, 3 @};
-@end example
+@end smallexample
@noindent
The most common visible consequence of this is surprising line numbers
take effect at the place you write them. Therefore, the following input
to the C preprocessor
-@example
+@smallexample
foo = X;
#define X 4
bar = X;
-@end example
+@end smallexample
@noindent
produces
-@example
+@smallexample
foo = X;
bar = 4;
-@end example
+@end smallexample
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,
-@example
+@smallexample
@group
#define TABLESIZE BUFSIZE
#define BUFSIZE 1024
@expansion{} BUFSIZE
@expansion{} 1024
@end group
-@end example
+@end smallexample
@noindent
@code{TABLESIZE} is expanded first to produce @code{BUFSIZE}, then that
will always expand using the definition of @code{BUFSIZE} that is
currently in effect:
-@example
+@smallexample
#define BUFSIZE 1020
#define TABLESIZE BUFSIZE
#undef BUFSIZE
#define BUFSIZE 37
-@end example
+@end smallexample
@noindent
Now @code{TABLESIZE} expands (in two stages) to @code{37}.
you use the same @samp{#define} directive, but you put a pair of
parentheses immediately after the macro name. For example,
-@example
+@smallexample
#define lang_init() c_init()
lang_init()
@expansion{} c_init()
-@end example
+@end smallexample
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.
-@example
+@smallexample
extern void foo(void);
#define foo() /* optimized inline version */
@dots{}
foo();
funcptr = foo;
-@end example
+@end smallexample
Here the call to @code{foo()} will use the macro, but the function
pointer will get the address of the real function. If the macro were to
an object-like macro whose expansion happens to begin with a pair of
parentheses.
-@example
+@smallexample
#define lang_init () c_init()
lang_init()
@expansion{} () c_init()()
-@end example
+@end smallexample
The first two pairs of parentheses in this expansion come from the
macro. The third is the pair that was originally after the macro
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.
-@example
+@smallexample
#define min(X, Y) ((X) < (Y) ? (X) : (Y))
x = min(a, b); @expansion{} x = ((a) < (b) ? (a) : (b));
y = min(1, 2); @expansion{} y = ((1) < (2) ? (1) : (2));
z = min(a + 28, *p); @expansion{} z = ((a + 28) < (*p) ? (a + 28) : (*p));
-@end example
+@end smallexample
@noindent
(In this small example you can already see several of the dangers of
requirement for square brackets or braces to balance, and they do not
prevent a comma from separating arguments. Thus,
-@example
+@smallexample
macro (array[x = y, x + 1])
-@end example
+@end smallexample
@noindent
passes two arguments to @code{macro}: @code{array[x = y} and @code{x +
For example, @code{min (min (a, b), c)} is first expanded to
-@example
+@smallexample
min (((a) < (b) ? (a) : (b)), (c))
-@end example
+@end smallexample
@noindent
and then to
-@example
+@smallexample
@group
((((a) < (b) ? (a) : (b))) < (c)
? (((a) < (b) ? (a) : (b)))
: (c))
@end group
-@end example
+@end smallexample
@noindent
(Line breaks shown here for clarity would not actually be generated.)
there must be exactly one comma at the top level of its argument list.
Here are some silly examples using @code{min}:
-@example
+@smallexample
min(, b) @expansion{} (( ) < (b) ? ( ) : (b))
min(a, ) @expansion{} ((a ) < ( ) ? (a ) : ( ))
min(,) @expansion{} (( ) < ( ) ? ( ) : ( ))
min() @error{} macro "min" requires 2 arguments, but only 1 given
min(,,) @error{} macro "min" passed 3 arguments, but takes just 2
-@end example
+@end smallexample
Whitespace is not a preprocessing token, so if a macro @code{foo} takes
one argument, @code{@w{foo ()}} and @code{@w{foo ( )}} both supply it an
Macro parameters appearing inside string literals are not replaced by
their corresponding actual arguments.
-@example
+@smallexample
#define foo(x) x, "x"
foo(bar) @expansion{} bar, "x"
-@end example
+@end smallexample
@node Stringification
@section Stringification
Here is an example of a macro definition that uses stringification:
-@example
+@smallexample
@group
#define WARN_IF(EXP) \
do @{ if (EXP) \
@expansion{} do @{ if (x == 0)
fprintf (stderr, "Warning: " "x == 0" "\n"); @} while (0);
@end group
-@end example
+@end smallexample
@noindent
The argument for @code{EXP} is substituted once, as-is, into the
If you want to stringify the result of expansion of a macro argument,
you have to use two levels of macros.
-@example
+@smallexample
#define xstr(s) str(s)
#define str(s) #s
#define foo 4
@expansion{} xstr (4)
@expansion{} str (4)
@expansion{} "4"
-@end example
+@end smallexample
@code{s} is stringified when it is used in @code{str}, so it is not
macro-expanded first. But @code{s} is an ordinary argument to
needs to be a table of commands, perhaps an array of structures declared
as follows:
-@example
+@smallexample
@group
struct command
@{
@dots{}
@};
@end group
-@end example
+@end smallexample
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
constant can be created with stringification, and the function name by
concatenating the argument with @samp{_command}. Here is how it is done:
-@example
+@smallexample
#define COMMAND(NAME) @{ #NAME, NAME ## _command @}
struct command commands[] =
COMMAND (help),
@dots{}
@};
-@end example
+@end smallexample
@node Variadic Macros
@section Variadic Macros
a function can. The syntax for defining the macro is similar to that of
a function. Here is an example:
-@example
+@smallexample
#define eprintf(@dots{}) fprintf (stderr, __VA_ARGS__)
-@end example
+@end smallexample
This kind of macro is called @dfn{variadic}. When the macro is invoked,
all the tokens in its argument list after the last named argument (this
@code{@w{__VA_ARGS__}} in the macro body wherever it appears. Thus, we
have this expansion:
-@example
+@smallexample
eprintf ("%s:%d: ", input_file, lineno)
@expansion{} fprintf (stderr, "%s:%d: ", input_file, lineno)
-@end example
+@end smallexample
The variable argument is completely macro-expanded before it is inserted
into the macro expansion, just like an ordinary argument. You may use
before the @samp{@dots{}}; that name is used for the variable argument.
The @code{eprintf} macro above could be written
-@example
+@smallexample
#define eprintf(args@dots{}) fprintf (stderr, args)
-@end example
+@end smallexample
@noindent
using this extension. You cannot use @code{@w{__VA_ARGS__}} and this
You can have named arguments as well as variable arguments in a variadic
macro. We could define @code{eprintf} like this, instead:
-@example
+@smallexample
#define eprintf(format, @dots{}) fprintf (stderr, format, __VA_ARGS__)
-@end example
+@end smallexample
@noindent
This formulation looks more descriptive, but unfortunately it is less
variable argument empty, you will get a syntax error, because
there will be an extra comma after the format string.
-@example
+@smallexample
eprintf("success!\n", );
@expansion{} fprintf(stderr, "success!\n", );
-@end example
+@end smallexample
GNU CPP has a pair of extensions which deal with this problem. First,
you are allowed to leave the variable argument out entirely:
-@example
+@smallexample
eprintf ("success!\n")
@expansion{} fprintf(stderr, "success!\n", );
-@end example
+@end smallexample
@noindent
Second, the @samp{##} token paste operator has a special meaning when
placed between a comma and a variable argument. If you write
-@example
+@smallexample
#define eprintf(format, @dots{}) fprintf (stderr, format, ##__VA_ARGS__)
-@end example
+@end smallexample
@noindent
and the variable argument is left out when the @code{eprintf} macro is
@emph{not} happen if you pass an empty argument, nor does it happen if
the token preceding @samp{##} is anything other than a comma.
-@example
+@smallexample
eprintf ("success!\n")
@expansion{} fprintf(stderr, "success!\n");
-@end example
+@end smallexample
@noindent
The above explanation is ambiguous about the case where the only macro
be a comma, and there must be white space between that comma and
whatever comes immediately before it:
-@example
+@smallexample
#define eprintf(format, args@dots{}) fprintf (stderr, format , ##args)
-@end example
+@end smallexample
@noindent
@xref{Differences from previous versions}, for the gory details.
can state the source line at which the inconsistency was detected. For
example,
-@example
+@smallexample
fprintf (stderr, "Internal error: "
"negative string length "
"%d at %s, line %d.",
length, __FILE__, __LINE__);
-@end example
+@end smallexample
An @samp{#include} directive changes the expansions of @code{__FILE__}
and @code{__LINE__} to correspond to the included file. At the end of
that it expands to a version number. A fully conforming implementation
of the 1998 C++ standard will define this macro to @code{199711L}. The
GNU C++ compiler is not yet fully conforming, so it uses @code{1}
-instead. We hope to complete our implementation in the near future.
+instead. It is hoped to complete the implementation of standard C++
+in the near future.
@item __OBJC__
This macro is defined, with value 1, when the Objective-C compiler is in
predefined macros directly in the conditional, you will need to write it
like this:
-@example
+@smallexample
/* @r{Test for GCC > 3.2.0} */
#if __GNUC__ > 3 || \
(__GNUC__ == 3 && (__GNUC_MINOR__ > 2 || \
(__GNUC_MINOR__ == 2 && \
__GNUC_PATCHLEVEL__ > 0))
-@end example
+@end smallexample
@noindent
Another approach is to use the predefined macros to
calculate a single number, then compare that against a threshold:
-@example
+@smallexample
#define GCC_VERSION (__GNUC__ * 10000 \
+ __GNUC_MINOR__ * 100 \
+ __GNUC_PATCHLEVEL__)
@dots{}
/* @r{Test for GCC > 3.2.0} */
#if GCC_VERSION > 30200
-@end example
+@end smallexample
@noindent
Many people find this form easier to understand.
end of every included file. It starts out at 0, it's value within the
base file specified on the command line.
+@item __ELF__
+This macro is defined if the target uses the ELF object format.
+
@item __VERSION__
This macro expands to a string constant which describes the version of
the compiler in use. You should not rely on its contents having any
@itemx __PTRDIFF_TYPE__
@itemx __WCHAR_TYPE__
@itemx __WINT_TYPE__
+@itemx __INTMAX_TYPE__
+@itemx __UINTMAX_TYPE__
These macros are defined to the correct underlying types for the
-@code{size_t}, @code{ptrdiff_t}, @code{wchar_t}, and @code{wint_t}
+@code{size_t}, @code{ptrdiff_t}, @code{wchar_t}, @code{wint_t},
+@code{intmax_t}, and @code{uintmax_t}
typedefs, respectively. They exist to make the standard header files
@file{stddef.h} and @file{wchar.h} work correctly. You should not use
these macros directly; instead, include the appropriate headers and use
@itemx __INT_MAX__
@itemx __LONG_MAX__
@itemx __LONG_LONG_MAX__
-Defined to the maximum value of the @code{signed char}, @code{wchar_t},
+@itemx __INTMAX_MAX__
+Defined to the maximum value of the @code{signed char}, @code{wchar_t},
@code{signed short},
-@code{signed int}, @code{signed long}, and @code{signed long long} types
+@code{signed int}, @code{signed long}, @code{signed long long}, and
+@code{intmax_t} 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.
macro to determine which runtime (NeXT or GNU) is being used.
@item __LP64__
-@item _LP64
+@itemx _LP64
These macros are defined, with value 1, if (and only if) the compilation
is for a target where @code{long int} and pointer both use 64-bits and
@code{int} uses 32-bit.
after the macro name. @samp{#undef} has no effect if the name is not a
macro.
-@example
+@smallexample
#define FOO 4
x = FOO; @expansion{} x = 4;
#undef FOO
x = FOO; @expansion{} x = FOO;
-@end example
+@end smallexample
Once a macro has been undefined, that identifier may be @dfn{redefined}
as a macro by a subsequent @samp{#define} directive. The new definition
@noindent
These definitions are effectively the same:
-@example
+@smallexample
#define FOUR (2 + 2)
#define FOUR (2 + 2)
#define FOUR (2 /* two */ + 2)
-@end example
+@end smallexample
@noindent
but these are not:
-@example
+@smallexample
#define FOUR (2 + 2)
#define FOUR ( 2+2 )
#define FOUR (2 * 2)
#define FOUR(score,and,seven,years,ago) (2 + 2)
-@end example
+@end smallexample
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
a macro call coming partially from the macro body and partially from the
arguments. For example,
-@example
+@smallexample
#define twice(x) (2*(x))
#define call_with_1(x) x(1)
call_with_1 (twice)
@expansion{} twice(1)
@expansion{} (2*(1))
-@end example
+@end smallexample
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,
-@example
+@smallexample
#define strange(file) fprintf (file, "%s %d",
@dots{}
strange(stderr) p, 35)
@expansion{} fprintf (stderr, "%s %d", p, 35)
-@end example
+@end smallexample
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
Suppose you define a macro as follows,
-@example
+@smallexample
#define ceil_div(x, y) (x + y - 1) / y
-@end example
+@end smallexample
@noindent
whose purpose is to divide, rounding up. (One use for this operation is
to compute how many @code{int} objects are needed to hold a certain
number of @code{char} objects.) Then suppose it is used as follows:
-@example
+@smallexample
a = ceil_div (b & c, sizeof (int));
@expansion{} a = (b & c + sizeof (int) - 1) / sizeof (int);
-@end example
+@end smallexample
@noindent
This does not do what is intended. The operator-precedence rules of
C make it equivalent to this:
-@example
+@smallexample
a = (b & (c + sizeof (int) - 1)) / sizeof (int);
-@end example
+@end smallexample
@noindent
What we want is this:
-@example
+@smallexample
a = ((b & c) + sizeof (int) - 1)) / sizeof (int);
-@end example
+@end smallexample
@noindent
Defining the macro as
-@example
+@smallexample
#define ceil_div(x, y) ((x) + (y) - 1) / (y)
-@end example
+@end smallexample
@noindent
provides the desired result.
compute the size of the type of @code{ceil_div (1, 2)}, but in fact it
means something very different. Here is what it expands to:
-@example
+@smallexample
sizeof ((1) + (2) - 1) / (2)
-@end example
+@end smallexample
@noindent
This would take the size of an integer and divide it by two. The
Parentheses around the entire macro definition prevent such problems.
Here, then, is the recommended way to define @code{ceil_div}:
-@example
+@smallexample
#define ceil_div(x, y) (((x) + (y) - 1) / (y))
-@end example
+@end smallexample
@node Swallowing the Semicolon
@subsection Swallowing the Semicolon
pointer (the argument @code{p} says where to find it) across whitespace
characters:
-@example
+@smallexample
#define SKIP_SPACES(p, limit) \
@{ char *lim = (limit); \
while (p < lim) @{ \
if (*p++ != ' ') @{ \
p--; break; @}@}@}
-@end example
+@end smallexample
@noindent
Here backslash-newline is used to split the macro definition, which must
This can cause trouble before @code{else} statements, because the
semicolon is actually a null statement. Suppose you write
-@example
+@smallexample
if (*p != 0)
SKIP_SPACES (p, lim);
else @dots{}
-@end example
+@end smallexample
@noindent
The presence of two statements---the compound statement and a null
The definition of the macro @code{SKIP_SPACES} can be altered to solve
this problem, using a @code{do @dots{} while} statement. Here is how:
-@example
+@smallexample
#define SKIP_SPACES(p, limit) \
do @{ char *lim = (limit); \
while (p < lim) @{ \
if (*p++ != ' ') @{ \
p--; break; @}@}@} \
while (0)
-@end example
+@end smallexample
Now @code{SKIP_SPACES (p, lim);} expands into
-@example
+@smallexample
do @{@dots{}@} while (0);
-@end example
+@end smallexample
@noindent
which is one statement. The loop executes exactly once; most compilers
@cindex unsafe macros
Many C programs define a macro @code{min}, for ``minimum'', like this:
-@example
+@smallexample
#define min(X, Y) ((X) < (Y) ? (X) : (Y))
-@end example
+@end smallexample
When you use this macro with an argument containing a side effect,
as shown here,
-@example
+@smallexample
next = min (x + y, foo (z));
-@end example
+@end smallexample
@noindent
it expands as follows:
-@example
+@smallexample
next = ((x + y) < (foo (z)) ? (x + y) : (foo (z)));
-@end example
+@end smallexample
@noindent
where @code{x + y} has been substituted for @code{X} and @code{foo (z)}
no standard way to do this, but it can be done with GNU extensions as
follows:
-@example
+@smallexample
#define min(X, Y) \
(@{ typeof (X) x_ = (X); \
typeof (Y) y_ = (Y); \
(x_ < y_) ? x_ : y_; @})
-@end example
+@end smallexample
The @samp{(@{ @dots{} @})} notation produces a compound statement that
acts as an expression. Its value is the value of its last statement.
calculate the value of @code{foo (z)}, save it in a variable, and use
that variable in @code{min}:
-@example
+@smallexample
@group
#define min(X, Y) ((X) < (Y) ? (X) : (Y))
@dots{}
next = min (x + y, tem);
@}
@end group
-@end example
+@end smallexample
@noindent
(where we assume that @code{foo} returns type @code{int}).
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:
+the preprocessor output unchanged. Consider an example:
-@example
+@smallexample
#define foo (4 + foo)
-@end example
+@end smallexample
@noindent
where @code{foo} is also a variable in your program.
One common, useful use of self-reference is to create a macro which
expands to itself. If you write
-@example
+@smallexample
#define EPERM EPERM
-@end example
+@end smallexample
@noindent
then the macro @code{EPERM} expands to @code{EPERM}. Effectively, it is
self-reference} of @code{x}. @code{x} is not expanded in this case
either. Thus, if we have
-@example
+@smallexample
#define x (4 + y)
#define y (2 * x)
-@end example
+@end smallexample
@noindent
then @code{x} and @code{y} expand as follows:
-@example
+@smallexample
@group
x @expansion{} (4 + y)
@expansion{} (4 + (2 * x))
y @expansion{} (2 * x)
@expansion{} (2 * (4 + y))
@end group
-@end example
+@end smallexample
@noindent
Each macro is expanded when it appears in the definition of the other
another macro that does the stringification or concatenation. For
instance, if you have
-@example
+@smallexample
#define AFTERX(x) X_ ## x
#define XAFTERX(x) AFTERX(x)
#define TABLESIZE 1024
#define BUFSIZE TABLESIZE
-@end example
+@end smallexample
then @code{AFTERX(BUFSIZE)} expands to @code{X_BUFSIZE}, and
@code{XAFTERX(BUFSIZE)} expands to @code{X_1024}. (Not to
This can cause a macro expanded on the second scan to be called with the
wrong number of arguments. Here is an example:
-@example
+@smallexample
#define foo a,b
#define bar(x) lose(x)
#define lose(x) (1 + (x))
-@end example
+@end smallexample
We would like @code{bar(foo)} to turn into @code{(1 + (foo))}, which
would then turn into @code{(1 + (a,b))}. Instead, @code{bar(foo)}
by the same parentheses that ought to be used to prevent misnesting of
arithmetic operations:
-@example
+@smallexample
#define foo (a,b)
@exdent or
#define bar(x) lose((x))
-@end example
+@end smallexample
The extra pair of parentheses prevents the comma in @code{foo}'s
definition from being interpreted as an argument separator.
Here is an example illustrating this:
-@example
+@smallexample
#define ignore_second_arg(a,b,c) a; c
ignore_second_arg (foo (),
ignored (),
syntax error);
-@end example
+@end smallexample
@noindent
The syntax error triggered by the tokens @code{syntax error} results in
The simplest sort of conditional is
-@example
+@smallexample
@group
#ifdef @var{MACRO}
#endif /* @var{MACRO} */
@end group
-@end example
+@end smallexample
@cindex conditional group
This block is called a @dfn{conditional group}. @var{controlled text}
The @samp{#if} directive allows you to test the value of an arithmetic
expression, rather than the mere existence of one macro. Its syntax is
-@example
+@smallexample
@group
#if @var{expression}
#endif /* @var{expression} */
@end group
-@end example
+@end smallexample
@var{expression} is a C expression of integer type, subject to stringent
restrictions. It may contain
comes out to be nonzero, the @samp{#if} succeeds and the @var{controlled
text} is included; otherwise it is skipped.
-If @var{expression} is not correctly formed, GCC issues an error and
-treats the conditional as having failed.
-
@node Defined
@subsection Defined
@code{defined} is useful when you wish to test more than one macro for
existence at once. For example,
-@example
+@smallexample
#if defined (__vax__) || defined (__ns16000__)
-@end example
+@end smallexample
@noindent
would succeed if either of the names @code{__vax__} or
Conditionals written like this:
-@example
+@smallexample
#if defined BUFSIZE && BUFSIZE >= 1024
-@end example
+@end smallexample
@noindent
can generally be simplified to just @code{@w{#if BUFSIZE >= 1024}},
alternative text to be used if the condition fails. This is what it
looks like:
-@example
+@smallexample
@group
#if @var{expression}
@var{text-if-true}
@var{text-if-false}
#endif /* Not @var{expression} */
@end group
-@end example
+@end smallexample
@noindent
If @var{expression} is nonzero, the @var{text-if-true} is included and
One common case of nested conditionals is used to check for more than two
possible alternatives. For example, you might have
-@example
+@smallexample
#if X == 1
@dots{}
#else /* X != 1 */
@dots{}
#endif /* X != 2 */
#endif /* X != 1 */
-@end example
+@end smallexample
Another conditional directive, @samp{#elif}, allows this to be
abbreviated as follows:
-@example
+@smallexample
#if X == 1
@dots{}
#elif X == 2
#else /* X != 2 and X != 1*/
@dots{}
#endif /* X != 2 and X != 1*/
-@end example
+@end smallexample
@samp{#elif} stands for ``else if''. Like @samp{#else}, it goes in the
middle of a conditional group and subdivides it; it does not require a
support. For example, if you know that the program will not run
properly on a VAX, you might write
-@example
+@smallexample
@group
#ifdef __vax__
#error "Won't work on VAXen. See comments at get_last_object."
#endif
@end group
-@end example
+@end smallexample
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 @samp{#error}. For example,
-@example
+@smallexample
#if !defined(UNALIGNED_INT_ASM_OP) && defined(DWARF2_DEBUGGING_INFO)
#error "DWARF2_DEBUGGING_INFO requires UNALIGNED_INT_ASM_OP."
#endif
-@end example
+@end smallexample
@findex #warning
The directive @samp{#warning} is like @samp{#error}, but causes the
processed as if it had appeared as the right hand side of a
@samp{#pragma} directive. For example,
-@example
+@smallexample
_Pragma ("GCC dependency \"parse.y\"")
-@end example
+@end smallexample
@noindent
has the same effect as @code{#pragma GCC dependency "parse.y"}. The
same effect could be achieved using macros, for example
-@example
+@smallexample
#define DO_PRAGMA(x) _Pragma (#x)
DO_PRAGMA (GCC dependency "parse.y")
-@end example
+@end smallexample
The standard is unclear on where a @code{_Pragma} operator can appear.
The preprocessor does not accept it within a preprocessing conditional
Optional trailing text can be used to give more information in the
warning message.
-@example
+@smallexample
#pragma GCC dependency "parse.y"
#pragma GCC dependency "/usr/include/time.h" rerun fixincludes
-@end example
+@end smallexample
@item #pragma GCC poison
Sometimes, there is an identifier that you want to remove completely
poison. If any of those identifiers appears anywhere in the source
after the directive, it is a hard error. For example,
-@example
+@smallexample
#pragma GCC poison printf sprintf fprintf
sprintf(some_string, "hello");
-@end example
+@end smallexample
@noindent
will produce an error.
For example,
-@example
+@smallexample
#define strrchr rindex
#pragma GCC poison rindex
strrchr(some_string, 'h');
-@end example
+@end smallexample
@noindent
will not produce an error.
Source file name and line number information is conveyed by lines
of the form
-@example
+@smallexample
# @var{linenum} @var{filename} @var{flags}
-@end example
+@end smallexample
@noindent
These are called @dfn{linemarkers}. They are inserted as needed into
@item The mapping of physical source file multi-byte characters to the
execution character set.
-Currently, GNU cpp only supports character sets that are strict supersets
-of ASCII, and performs no translation of characters.
+Currently, CPP requires its input to be ASCII or UTF-8. The execution
+character set may be controlled by the user, with the
+@code{-ftarget-charset} and @code{-ftarget-wide-charset} options.
@item Identifier characters.
@anchor{Identifier characters}
GCC allows the @samp{$} character in identifiers as an extension for
most targets. This is true regardless of the @option{std=} switch,
since this extension cannot conflict with standards-conforming
-programs. Currently the targets that by default do not permit
-@samp{$} are AVR, IP2K, MMIX, MIPS Irix 3, ARM aout, and PowerPC
-targets for the AIX and BeOS operating systems.
+programs. When preprocessing assembler, however, dollars are not
+identifier characters by default.
+
+Currently the targets that by default do not permit @samp{$} are AVR,
+IP2K, MMIX, MIPS Irix 3, ARM aout, and PowerPC targets for the AIX and
+BeOS operating systems.
You can override the default with @option{-fdollars-in-identifiers} or
@option{fno-dollars-in-identifiers}. @xref{fdollars-in-identifiers}.
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
+and all the others known. It is intended that there should 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.)
+please report that as a bug. @xref{Bugs, , Reporting Bugs, gcc, Using
+the GNU Compiler Collection (GCC)}.
Where we say something is limited @dfn{only by available memory}, that
means that internal data structures impose no intrinsic limit, and space
@cindex predicates
An assertion looks like this:
-@example
+@smallexample
#@var{predicate} (@var{answer})
-@end example
+@end smallexample
@noindent
@var{predicate} must be a single identifier. @var{answer} can be any
conditional succeeds if either @code{vax} or @code{ns16000} has been
asserted as an answer for @code{machine}.
-@example
+@smallexample
#if #machine (vax) || #machine (ns16000)
-@end example
+@end smallexample
@noindent
You can test whether @emph{any} answer is asserted for a predicate by
omitting the answer in the conditional:
-@example
+@smallexample
#if #machine
-@end example
+@end smallexample
@findex #assert
Assertions are made with the @samp{#assert} directive. Its sole
argument is the assertion to make, without the leading @samp{#} that
identifies assertions in conditionals.
-@example
+@smallexample
#assert @var{predicate} (@var{answer})
-@end example
+@end smallexample
@noindent
You may make several assertions with the same predicate and different
for that predicate remain true. You can cancel an entire predicate by
leaving out the answer:
-@example
+@smallexample
#unassert @var{predicate}
-@end example
+@end smallexample
@noindent
In either form, if no such assertion has been made, @samp{#unassert} has
@itemize @bullet
+@item -I- deprecated
+
+This option has been deprecated in 3.5. @option{-iquote} is meant to
+replace the need for this option.
+
@item Order of evaluation of @samp{#} and @samp{##} operators
The standard does not specify the order of evaluation of a chain of
@ignore
@c man begin SYNOPSIS
cpp [@option{-D}@var{macro}[=@var{defn}]@dots{}] [@option{-U}@var{macro}]
- [@option{-I}@var{dir}@dots{}] [@option{-W}@var{warn}@dots{}]
+ [@option{-I}@var{dir}@dots{}] [@option{-iquote}@var{dir}@dots{}]
+ [@option{-W}@var{warn}@dots{}]
[@option{-M}|@option{-MM}] [@option{-MG}] [@option{-MF} @var{filename}]
- [@option{-MP}] [@option{-MQ} @var{target}@dots{}] [@option{-MT} @var{target}@dots{}]
+ [@option{-MP}] [@option{-MQ} @var{target}@dots{}]
+ [@option{-MT} @var{target}@dots{}]
+ [@option{-P}] [@option{-fno-working-directory}]
[@option{-x} @var{language}] [@option{-std=}@var{standard}]
@var{infile} @var{outfile}
@option{-M} (@pxref{Invocation}). These take precedence over
environment variables, which in turn take precedence over the
configuration of GCC@.
-
+
@include cppenv.texi
@c man end
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