@c @cropmarks
@c @finalout
-@macro copyrightnotice
+@copying
@c man begin COPYRIGHT
Copyright @copyright{} 1987, 1989, 1991, 1992, 1993, 1994, 1995, 1996,
-1997, 1998, 1999, 2000, 2001, 2002
+1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004
Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document
man page gfdl(7).
@c man end
@end ignore
-@end macro
-@macro covertexts
@c man begin COPYRIGHT
This manual contains no Invariant Sections. The Front-Cover Texts are
(a) (see below), and the Back-Cover Texts are (b) (see below).
software. Copies published by the Free Software Foundation raise
funds for GNU development.
@c man end
-@end macro
+@end copying
@macro gcctabopt{body}
@code{\body\}
@c There is a fill at the bottom of the page, so we need a filll to
@c override it.
@vskip 0pt plus 1filll
-@copyrightnotice{}
-@covertexts{}
+@insertcopying
@end titlepage
@contents
@page
Overview
+* Character sets::
* Initial processing::
* Tokenization::
* The preprocessing language::
* Assertions::
* Obsolete once-only headers::
-* Miscellaneous obsolete features::
@end detailmenu
@end menu
-@copyrightnotice{}
-@covertexts{}
+@insertcopying
@end ifnottex
@node Overview
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. @option{-traditional}
+(Pascal, Ada, etc.) So is assembly, with caution. @option{-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.
you should use the @option{-std=c89} or @option{-std=c99} options, depending
on which version of the standard you want. To get all the mandatory
diagnostics, you must also use @option{-pedantic}. @xref{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 @ref{Traditional
+Mode}.
+
+For clarity, unless noted otherwise, references to @samp{CPP} in this
+manual refer to GNU CPP.
@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
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. GNU CPP actually does them
+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.
@enumerate
@item
-@cindex character sets
@cindex line endings
The input file is read into memory and broken into lines.
-GNU 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}}, @kbd{CR}, and @kbd{@w{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.) @kbd{@w{LF CR}} is included because it has been
-reported as an end-of-line marker under exotic conditions.
+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
@item
@cindex trigraphs
-If trigraphs are enabled, they are replaced by their corresponding
-single characters.
+@anchor{trigraphs}If trigraphs are enabled, they are replaced by their
+corresponding single characters. By default GCC ignores trigraphs,
+but if you request a strictly conforming mode with the @option{-std}
+option, or you specify the @option{-trigraphs} option, then it
+converts them.
These are nine three-character sequences, all starting with @samp{??},
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, @samp{??/} stands for @samp{\}, so @t{'??/n'} is a character
-constant for a newline. By default, GCC ignores trigraphs, but if you
-request a strictly conforming mode with the @option{-std} option, then
-it converts them.
+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 @option{-Wall} or @option{-Wtrigraphs} options,
-GCC will warn you when a trigraph would change the meaning of your
-program if it were converted.
+Trigraphs are not popular and many compilers implement them
+incorrectly. Portable code should not rely on trigraphs being either
+converted or ignored. With @option{-Wtrigraphs} GCC will warn you
+when a trigraph may change the meaning of your program if it were
+converted. @xref{Wtrigraphs}.
-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. @t{"(??\?)"} is the string @samp{(???)}, not
-@samp{(?]}. Traditional C compilers do not recognize this idiom.
+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, or by separating the string literal at the
+trigraph and making use of string literal concatenation. @t{"(??\?)"}
+is the string @samp{(???)}, not @samp{(?]}. Traditional C compilers
+do not recognize these idioms.
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
You can add to this list with the @option{-I@var{dir}} command line
option. All the directories named by @option{-I} are searched, in
-left-to-right order, @emph{before} the default directories. You can
-also prevent GCC from searching any of the default directories with the
-@option{-nostdinc} option. This is useful when you are compiling an
+left-to-right order, @emph{before} the default directories. The only
+exception is when @file{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 @option{-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.
+@option{-I} options are not ignored as described above when
+@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.
-If you name a search directory with @option{-I@var{dir}} that is also a
-system include directory, the @option{-I} wins; the directory will be
-searched according to the @option{-I} ordering, and it will not be
-treated as a system include directory. GCC will warn you when a system
-include directory is hidden in this way.
-
@samp{#line} (@pxref{Line Control}) does not change GCC's idea of the
directory containing the current file.
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,
over the entire contents of the file, and the compiler will not see it
twice.
-GNU CPP optimizes even further. It remembers when a header file has a
+CPP optimizes even further. It remembers when a header file has a
wrapper @samp{#ifndef}. If a subsequent @samp{#include} specifies that
header, and the macro in the @samp{#ifndef} is still defined, it does
not bother to rescan the file at all.
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.
All directories named by @option{-isystem} are searched @emph{after} all
directories named by @option{-I}, no matter what their order was on the
command line. If the same directory is named by both @option{-I} and
-@option{-isystem}, @option{-I} wins; it is as if the @option{-isystem} option
-had never been specified at all. GCC warns you when this happens.
+@option{-isystem}, the @option{-I} option is ignored. GCC provides an
+informative message when this occurs if @option{-v} is used.
@findex #pragma GCC system_header
There is also a directive, @code{@w{#pragma GCC system_header}}, which
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
below for an important special case for @samp{##}.)
If your macro is complicated, you may want a more descriptive name for
-the variable argument than @code{@w{__VA_ARGS__}}. GNU CPP permits
+the variable argument than @code{@w{__VA_ARGS__}}. CPP permits
this, as an extension. You may write an argument name immediately
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{__VA_ARGS__} and this
+using this extension. You cannot use @code{@w{__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 @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
+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 @code{@w{__VA_ARGS__}}
can appear is in the replacement list of a variadic macro. It may not
with portability to other conforming implementations of C99, you should
use only @code{@w{__VA_ARGS__}}.
-Previous versions of GNU CPP implemented the comma-deletion extension
+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 @samp{##} must
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.
@subsection Standard Predefined Macros
@cindex standard predefined macros.
-The standard predefined macros are specified by the C and/or C++
+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.
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
characters and looks like @code{@w{"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 @code{__DATE__} will expand to
+@code{@w{"??? ?? ????"}}.
+
@item __TIME__
This macro expands to a string constant that describes the time at
which the preprocessor is being run. The string constant contains
eight characters and looks like @code{"23:59:01"}.
+If GCC cannot determine the current time, it will emit a warning message
+(once per compilation) and @code{__TIME__} will expand to
+@code{"??:??:??"}.
+
@item __STDC__
In normal operation, this macro expands to the constant 1, to signify
that this compiler conforms to ISO Standard C@. If GNU CPP is used with
a compiler other than GCC, this is not necessarily true; however, the
-preprocessor always conforms to the standard, unless the
-@option{-traditional} option is used.
+preprocessor always conforms to the standard unless the
+@option{-traditional-cpp} option is used.
-This macro is not defined if the @option{-traditional} option is used.
+This macro is not defined if the @option{-traditional-cpp} option is used.
On some hosts, the system compiler uses a different convention, where
@code{__STDC__} is normally 0, but is 1 if the user specifies strict
-conformance to the C Standard. GNU CPP follows the host convention when
+conformance to the C Standard. CPP follows the host convention when
processing system header files, but when processing user files
@code{__STDC__} is always 1. This has been reported to cause problems;
for instance, some versions of Solaris provide X Windows headers that
-expect @code{__STDC__} to be either undefined or 1. You may be able to
-work around this sort of problem by using an @option{-I} option to
-cancel treatment of those headers as system headers. @xref{Invocation}.
+expect @code{__STDC__} to be either undefined or 1. @xref{Invocation}.
@item __STDC_VERSION__
This macro expands to the C Standard's version number, a long integer
the 1999 revision of the C standard. Support for the 1999 revision is
not yet complete.
-This macro is not defined if the @option{-traditional} option is used, nor
-when compiling C++ or Objective-C@.
+This macro is not defined if the @option{-traditional-cpp} option is
+used, nor when compiling C++ or Objective-C@.
@item __STDC_HOSTED__
This macro is defined, with value 1, if the compiler's target is a
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
+use. You can use @code{__OBJC__} to test whether a header is compiled
+by a C compiler or a Objective-C compiler.
+
+@item __ASSEMBLER__
+This macro is defined with value 1 when preprocessing assembly
+language.
@end table
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.
-@item __OBJC__
-This macro is defined, with value 1, when the Objective-C compiler is in
-use. You can use @code{__OBJC__} to test whether a header is compiled
-by a C compiler or a Objective-C compiler.
-
@item __GNUG__
The GNU C++ compiler defines this. Testing it is equivalent to
testing @code{@w{(__GNUC__ && __cplusplus)}}.
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
file @file{limits.h} to work correctly. You should not use this macro
yourself; instead, refer to the standard macros defined in @file{limits.h}.
+@item __WCHAR_UNSIGNED__
+Like @code{__CHAR_UNSIGNED__}, this macro is defined if and only if the
+data type @code{wchar_t} is unsigned and the front-end is in C++ mode.
+
@item __REGISTER_PREFIX__
This macro expands to a single token (not a string constant) which is
the prefix applied to CPU register names in assembly language for this
these macros directly; instead, include the appropriate headers and use
the typedefs.
+@item __CHAR_BIT__
+Defined to the number of bits used in the representation of the
+@code{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.
+
+@item __SCHAR_MAX__
+@itemx __WCHAR_MAX__
+@itemx __SHRT_MAX__
+@itemx __INT_MAX__
+@itemx __LONG_MAX__
+@itemx __LONG_LONG_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
+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.
+
@item __USING_SJLJ_EXCEPTIONS__
This macro is defined, with value 1, if the compiler uses the old
mechanism based on @code{setjmp} and @code{longjmp} for exception
handling.
+
+@item __NEXT_RUNTIME__
+This macro is defined, with value 1, if (and only if) the NeXT runtime
+(as in @option{-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.
+
+@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.
@end table
@node System-specific Predefined Macros
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
the arguments of a macro. The C and C++ standards declare that
behavior in these cases is undefined.
-Versions of GNU CPP prior to 3.2 would reject such constructs with an
+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
f)
@end smallexample
-@noindent which expands to
+@noindent
+which expands to
@smallexample
1 2 1 2
@end smallexample
-@noindent with the semantics described above.
+@noindent
+with the semantics described above.
@node Macro Pitfalls
@section Macro Pitfalls
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}
helps people match the @samp{#endif} to the corresponding @samp{#ifdef}.
Older programs sometimes put @var{MACRO} directly after the
@samp{#endif} without enclosing it in a comment. This is invalid code
-according to the C standard. GNU CPP accepts it with a warning. It
+according to the C standard. CPP accepts it with a warning. It
never affects which @samp{#ifndef} the @samp{#endif} matches.
@findex #ifndef
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
constant: backslash escapes are interpreted. This is different from
@samp{#include}.
-Previous versions of GNU CPP did not interpret escapes in @samp{#line};
+Previous versions of CPP did not interpret escapes in @samp{#line};
we have changed it because the standard requires they be interpreted,
and most other compilers do.
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.
GNU extension either. We believe it came from System V@.
@findex #sccs
-The @samp{#sccs} directive is recognized on some systems, because it
-appears in their header files. It is a very old, obscure, extension
+The @samp{#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.
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
Traditional (pre-standard) C preprocessing is rather different from
the preprocessing specified by the standard. When GCC is given the
-@option{-traditional} option, it attempts to emulate a traditional
-preprocessor. We do not guarantee that GCC's behavior under
-@option{-traditional} matches any pre-standard preprocessor exactly.
+@option{-traditional-cpp} option, it attempts to emulate a traditional
+preprocessor.
-Traditional mode exists only for backward compatibility. We have no
-plans to augment it in any way nor will we change it except to fix
-catastrophic bugs. As of GCC 3.2, traditional mode is not supported for
-compilation, only preprocessing.
+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.
-This is a list of the differences. It may not be complete, and may not
-correspond exactly to the behavior of either GCC or a true traditional
-preprocessor.
+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.
-@itemize @bullet
-@item
-Traditional macro expansion pays no attention to single-quote or
-double-quote characters; macro argument symbols are replaced by the
-argument values even when they appear within apparent string or
-character constants.
+@menu
+* Traditional lexical analysis::
+* Traditional macros::
+* Traditional miscellany::
+* Traditional warnings::
+@end menu
-@item
-Traditionally, it is permissible for a macro expansion to end in the
-middle of a string or character constant. The constant continues into
-the text surrounding the macro call.
+@node Traditional lexical analysis
+@section Traditional lexical analysis
-@item
-However, the end of the line terminates a string or character constant,
-with no error. (This is a kluge. Traditional mode is commonly used to
-preprocess things which are not C, and have a different comment syntax.
-Single apostrophes often appear in comments. This kluge prevents the
-traditional preprocessor from issuing errors on such comments.)
+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.
-@item
-Preprocessing directives are recognized in traditional C only when their
-leading @samp{#} appears in the first column. There can be no
-whitespace between the beginning of the line and the @samp{#}.
+This implementation does not treat trigraphs (@pxref{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.
-@item
-In traditional C, a comment is equivalent to no text at all. (In ISO
-C, a comment counts as whitespace.) It can be used sort of the same way
-that @samp{##} is used in ISO C, to paste macro arguments together.
+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.
-@item
-Traditional C does not have the concept of a preprocessing number.
+Traditional CPP only recognizes C-style block comments, and treats the
+@samp{/*} 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 @samp{<} in a @code{#include}
+directive.
-@item
-A macro is not suppressed within its own definition, in traditional C@.
-Thus, any macro that is used recursively inevitably causes an error.
+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
-@item
-The @samp{#} and @samp{##} operators are not available in traditional
-C@.
+@smallexample
+#if foo/**/bar
+@end smallexample
+
+@noindent
+@samp{foo} and @samp{bar} are distinct identifiers and expanded
+separately if they happen to be macros. In other words, this
+directive is equivalent to
+
+@smallexample
+#if foo bar
+@end smallexample
+
+@noindent
+rather than
+
+@smallexample
+#if foobar
+@end smallexample
+
+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 @code{#define}
+require matching quotes. For example:
+
+@smallexample
+#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
+@end smallexample
+
+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.
+
+@node Traditional macros
+@section Traditional macros
+
+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 (@pxref{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 @option{-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 @samp{#} and token paste operator
+@samp{##} 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 @emph{ad infinitum}. GCC detects when it is expanding
+recursive macros, emits an error message, and continues after the
+offending macro invocation.
+
+@smallexample
+#define PLUS +
+#define INC(x) PLUS+x
+INC(foo);
+ @expansion{} ++foo;
+@end smallexample
+
+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 @option{-C} option is given. The form of all other horizontal
+whitespace in arguments is preserved, including leading and trailing
+whitespace. In particular
+
+@smallexample
+f( )
+@end smallexample
+
+@noindent
+is treated as an invocation of the macro @samp{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
+
+@smallexample
+#define str(x) "x"
+str(/* A comment */some text )
+ @expansion{} "some text "
+@end smallexample
+
+@noindent
+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.
+
+@smallexample
+#define suffix(x) foo_/**/x
+suffix(bar)
+ @expansion{} foo_bar
+@end smallexample
+@node Traditional miscellany
+@section Traditional miscellany
+
+Here are some things to be aware of when using the traditional
+preprocessor.
+
+@itemize @bullet
@item
-In traditional C, the text at the end of a macro expansion can run
-together with the text after the macro call, to produce a single token.
-This is impossible in ISO C@.
+Preprocessing directives are recognized only when their leading
+@samp{#} appears in the first column. There can be no whitespace
+between the beginning of the line and the @samp{#}, but whitespace can
+follow the @samp{#}.
@item
-None of the GNU extensions to the preprocessor are available in
-traditional mode, with the exception of a partial implementation of
-assertions, and those may be removed in the future.
+A true traditional C preprocessor does not recognize @samp{#error} or
+@samp{#pragma}, and may not recognize @samp{#elif}. CPP supports all
+the directives in traditional mode that it supports in ISO mode,
+including extensions, with the exception that the effects of
+@samp{#pragma GCC poison} are undefined.
@item
-A true traditional C preprocessor does not recognize @samp{#elif},
-@samp{#error}, or @samp{#pragma}. GCC supports @samp{#elif} and
-@samp{#error} even in traditional mode, but not @samp{#pragma}.
+__STDC__ is not defined.
@item
-Traditional mode is text-based, not token-based, and comments are
-stripped after macro expansion. Therefore, @samp{/**/} can be used to
-paste tokens together provided that there is no whitespace between it
-and the tokens to be pasted.
+If you use digraphs the behavior is undefined.
@item
-Traditional mode preserves the amount and form of whitespace provided by
-the user. Hard tabs remain hard tabs. This can be useful, e.g.@: if you
-are preprocessing a Makefile (which we do not encourage).
+If a line that looks like a directive appears within macro arguments,
+the behavior is undefined.
+
@end itemize
+@node Traditional warnings
+@section Traditional warnings
You can request warnings about features that did not exist, or worked
differently, in traditional C with the @option{-Wtraditional} option.
-This works only if you do @emph{not} specify @option{-traditional}. GCC
-does not warn about features of ISO C which you must use when you are
-using a conforming compiler, such as the @samp{#} and @samp{##}
+GCC does not warn about features of ISO C which you must use when you
+are using a conforming compiler, such as the @samp{#} and @samp{##}
operators.
Presently @option{-Wtraditional} warns about:
suggests avoiding it altogether.
@item
-A function-like macro that appears without an argument list. In
-traditional C this was an error. In ISO C it merely means that the
-macro is not expanded.
+A function-like macro that appears without an argument list. In some
+traditional preprocessors this was an error. In ISO C it merely means
+that the macro is not expanded.
@item
The unary plus operator. This did not exist in traditional C@.
change subtly in future implementations.
Also documented here are obsolete features and changes from previous
-versions of GNU CPP@.
+versions of CPP@.
@menu
* Implementation-defined behavior::
@section Implementation-defined behavior
@cindex implementation-defined behavior
-This is how GNU CPP behaves in all the cases which the C standard
+This is how CPP behaves in all the cases which the C standard
describes as @dfn{implementation-defined}. This term means that the
implementation is free to do what it likes, but must document its choice
and stick to it.
@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}
+
+The C and C++ standards allow identifiers to be composed of @samp{_}
+and the alphanumeric characters. C++ and C99 also allow universal
+character names (not implemented in GCC), and C99 further permits
+implementation-defined characters.
+
+GCC allows the @samp{$} character in identifiers as an extension for
+most targets. This is true regardless of the @option{std=} switch,
+since this extension cannot conflict with standards-conforming
+programs. When preprocessing assembler, however, dollars are not
+identifier characters by default.
+
+Currently the targets that by default do not permit @samp{$} 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}.
@item Non-empty sequences of whitespace characters.
@item The numeric value of character constants in preprocessor expressions.
-The preprocessor and compiler interpret character constants in the same
-way; escape sequences such as @samp{\a} are given the values they would
-have on the target machine.
-
-Multi-character character constants are interpreted a character at a
-time, shifting the previous result left by the number of bits per
-character on the host, and adding the new character. For example, 'ab'
-on an 8-bit host would be interpreted as @w{'a' * 256 + 'b'}. If there
-are more characters in the constant than can fit in the widest native
-integer type on the host, usually a @code{long}, the excess characters
-are ignored and a diagnostic is given.
+The preprocessor and compiler interpret character constants in the
+same way; i.e.@: escape sequences such as @samp{\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 @code{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 @code{int} the
+compiler issues a warning, and the excess leading characters are
+ignored.
+
+For example, 'ab' for a target with an 8-bit @code{char} would be
+interpreted as @w{(int) ((unsigned char) 'a' * 256 + (unsigned char)
+'b')}, and '\234a' as @w{(int) ((unsigned char) '\234' * 256 + (unsigned
+char) 'a')}.
@item Source file inclusion.
@section Implementation limits
@cindex implementation limits
-GNU CPP has a small number of internal limits. This section lists the
+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
@item Nesting levels of conditional inclusion.
-The C standard mandates this be at least 63. GNU CPP is limited only by
+The C standard mandates this be at least 63. CPP is limited only by
available memory.
-@item Levels of parenthesised expressions within a full expression.
+@item Levels of parenthesized expressions within a full expression.
The C standard requires this to be at least 63. In preprocessor
conditional expressions, it is limited only by available memory.
@item Number of macros simultaneously defined in a single translation unit.
-The standard requires at least 4095 be possible. GNU CPP is limited only
+The standard requires at least 4095 be possible. CPP is limited only
by available memory.
@item Number of parameters in a macro definition and arguments in a macro call.
@item Number of characters on a logical source line.
-The C standard requires a minimum of 4096 be permitted. GNU CPP places
+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.
@node Obsolete Features
@section Obsolete Features
-GNU CPP has a number of features which are present mainly for
+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@.
@menu
* Assertions::
* Obsolete once-only headers::
-* Miscellaneous obsolete features::
@end menu
@node Assertions
@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
same predicate. All the answers for any given predicate are
simultaneously true.
-@cindex assertions, cancelling
+@cindex assertions, canceling
@findex #unassert
-Assertions can be cancelled with the @samp{#unassert} directive. It
+Assertions can be canceled with the @samp{#unassert} directive. It
has the same syntax as @samp{#assert}. In that form it cancels only the
answer which was specified on the @samp{#unassert} line; other answers
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
@node Obsolete once-only headers
@subsection Obsolete once-only headers
-GNU CPP supports two more ways of indicating that a header file should be
+CPP supports two more ways of indicating that a header file should be
read only once. Neither one is as portable as a wrapper @samp{#ifndef},
and we recommend you do not use them in new programs.
but it is not recognized by all preprocessors, so you cannot rely on it
in a portable program.
-@node Miscellaneous obsolete features
-@subsection Miscellaneous obsolete features
-
-Here are a few more obsolete features.
-
-@itemize @bullet
-@cindex invalid token paste
-@item Attempting to paste two tokens which together do not form a valid
-preprocessing token.
-
-The preprocessor currently warns about this and outputs the two tokens
-adjacently, which is probably the behavior the programmer intends. It
-may not work in future, though.
-
-Most of the time, when you get this warning, you will find that @samp{##}
-is being used superstitiously, to guard against whitespace appearing
-between two tokens. It is almost always safe to delete the @samp{##}.
-
-@cindex pragma poison
-@item @code{#pragma poison}
-
-This is the same as @code{#pragma GCC poison}. The version without the
-@code{GCC} prefix is deprecated. @xref{Pragmas}.
-
-@cindex multi-line string constants
-@item Multi-line string constants
-
-Older versions of GCC allowed string constants to extend across
-multiple logical lines of the source file. This ill-considered
-extension has now been removed. Instead, make use of ISO C
-concatenation of adjacent string literals, or use @samp{\n} followed
-by a backslash-newline.
-
-@end itemize
-
@node Differences from previous versions
@section Differences from previous versions
@cindex differences from previous versions
This section details behavior which has changed from previous versions
-of GNU CPP@. We do not plan to change it again in the near future, but
+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
@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
left to right. Older versions evaluated all @samp{#} operators first,
then all @samp{##} operators, in an unreliable order.
-@item The form of whitespace betwen tokens in preprocessor output
+@item The form of whitespace between tokens in preprocessor output
@xref{Preprocessor Output}, for the current textual format. This is
also the format used by stringification. Normally, the preprocessor
Formerly, in a macro expansion, if @samp{##} appeared before a variable
arguments parameter, and the set of tokens specified for that argument
-in the macro invocation was empty, previous versions of GNU CPP would
+in the macro invocation was empty, previous versions of CPP would
back up and remove the preceding sequence of non-whitespace characters
(@strong{not} the preceding token). This extension is in direct
conflict with the 1999 C standard and has been drastically pared back.
variable argument is empty, or the token before @samp{##} is not a
comma, then @samp{##} behaves as a normal token paste.
-@item Traditional mode and GNU extensions
-
-Traditional mode used to be implemented in the same program as normal
-preprocessing. Therefore, all the GNU extensions to the preprocessor
-were still available in traditional mode. It is now a separate program
-and does not implement any of the GNU extensions, except for a partial
-implementation of assertions. Even those may be removed in a future
-release.
-
@item @samp{#line} and @samp{#include}
The @samp{#line} directive used to change GCC's notion of the
@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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