gnu1987/gcc/cpp.texinfo

\input texinfo
@setfilename cpp
@settitle The C Preprocessor
@ifinfo
This file documents the GNU C Preprocessor.

Copyright (C) 1987 Richard M. Stallman.

Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
are preserved on all copies.

@ignore
Permission is granted to process this file through Tex and print the
results, provided the printed document carries copying permission
notice identical to this one except for the removal of this paragraph
(this paragraph not being relevant to the printed manual).

@end ignore
Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided also that
the entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for modified versions.
@end ifinfo

@titlepage
@sp 6
@center @titlefont{The C Preprocessor}
@sp 4
@center First Edition
@sp 1
@center January 1987
@sp 5
@center Richard M. Stallman
@page
@vskip 0pt plus 1filll
Copyright @copyright{} 1987 Richard M. Stallman.

Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
are preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided also that
the entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for modified versions.
@end titlepage
@page

@node Top, Global Actions,, (DIR)
@chapter The C Preprocessor

The C preprocessor is a @dfn{macro processor} that is used automatically by
the C compiler to transform your program before actual compilation.  It is
called a macro processor because it allows you to define @dfn{macros},
which are brief abbreviations for longer constructs.

The C preprocessor provides four separate facilities that you can use as
you see fit:

@itemize @bullet
@item
Inclusion of header files.  These are files of declarations that can be
substituted into your program.

@item
Macro expansion.  You can define @dfn{macros}, which are abbreviations
for arbitrary fragments of C code, and then the C preprocessor will
replace the macros with their definitions throughout the program.

@item
Conditional compilation.  Using special preprocessor commands, you
can include or exclude parts of the program according to various
conditions.

@item
Line control.  If you use a program to combine or rearrange source files into
an intermediate file which is then compiled, you can use line control
to inform the compiler of where each source line originally came from.
@end itemize

C preprocessors vary in some details.  This manual discusses the GNU C
preprocessor, the C Compatible Compiler Preprocessor.  The GNU C
preprocessor provides a superset of the features of ANSI Standard C.

ANSI Standard C requires the rejection of many harmless constructs commonly
used by today's C programs.  Such incompatibility would be inconvenient for
users, so the GNU C preprocessor is configured to accept these constructs
by default.  Strictly speaking, to get ANSI Standard C, you must use the
switches @samp{-T}, @samp{-undef} and @samp{-pedantic}, but in practice the
consequences of having strict ANSI Standard C make it undesirable to do
this.  @xref{Invocation}.

@menu
* Global Actions::    Actions made uniformly on all input files.
* Commands::          General syntax of preprocessor commands.
* Header Files::      How and why to use header files.
* Macros::            How and why to use macros.
* Conditionals::      How and why to use conditionals.
* Combining Sources:: Use of line control when you combine source files.
* Other Commands::    Miscellaneous preprocessor commands.
* Output::            Format of output from the C preprocessor.
* Invocation::        How to invoke the preprocessor; command switches.
* Concept Index::     Index of concepts and terms.
* Index::             Index of commands, predefined macros and switches.
@end menu

@node Global Actions, Commands, Top, Top
@section Transformations Made Globally

Most C preprocessor features are inactive unless you give specific commands
to request their use.  (Preprocessor commands are lines starting with
@samp{#}; @pxref{Commands}).  But there are three transformations that the
preprocessor always makes on all the input it receives, even in the absence
of commands.

@itemize @bullet
@item
All C comments are replaced with single spaces.

@item
Backslash-Newline sequences are deleted, no matter where.  This
feature allows you to break long lines for cosmetic purposes without
changing their meaning.

@item
Predefined macro names are replaced with their expansions
(@pxref{Predefined}).
@end itemize

The first two transformations are done @emph{before} nearly all other parsing
and before preprocessor commands are recognized.  Thus, for example, you
can split a line cosmetically with Backslash-Newline just about anywhere.
Even

@example
#defi\
ne FO\
O 10\
20
@end example

@noindent
is equivalent into @samp{#define FOO 1020}.

But there are a few exceptions to both transformations.

@itemize @bullet
@item
C comments and predefined macro names are not recognized inside a
@samp{#include} command in which the file name is delimited with
@samp{<} and @samp{>}.

@item
C comments and predefined macro names are never recognized within a
character or string constant.  (Strictly speaking, this is the rule,
not an exception, but it is worth noting here anyway.)

@item
Backslash-Newline is not discarded if preceded by an odd number of
additional Backslashes.  This is because the Backslashes quote each
other, leaving the Newline as an ordinary Newline.

@example
x\\
y
@end example

@noindent
is an example of a Backslash-Newline that is preserved from deletion for
this reason.
@end itemize

@node Commands, Header Files, Global Actions, Top
@section Preprocessor Commands

@cindex preprocessor commands
@cindex commands
Most preprocessor features are active only if you use preprocessor commands
to request their use.

Preprocessor commands are lines in your program that start with @samp{#}.
The @samp{#} is followed by an identifier that is the @dfn{command name}.
For example, @samp{#define} is the command that defines a macro.
Whitespace is also allowed before and after the @samp{#}.

The set of valid command names is fixed.  Programs cannot define new
preprocessor commands.

Some command names require arguments; these make up the rest of the command
line and must be separated from the command name by whitespace.  For example,
@samp{#define} must be followed by a macro name and the intended expansion
of the macro.

A preprocessor command cannot be more than one line in normal circumstances.
It may be split cosmetically with Backslash-Newline, but that has no effect
on its meaning.  Comments containing Newlines can also divide the command into
multiple lines, but the comments are changed to Spaces before the command
is interpreted.  The only way a significant Newline can occur in a preprocessor
command is within a string constant or character constant.  Note that
most C compilers that might be applied to the output from the preprocessor
do not accept string or character constants containing Newlines.

The @samp{#} and the command name cannot come from a macro expansion.  For
example, if @samp{foo} is defined as a macro expanding to @samp{define},
that does not make @samp{#foo} a valid preprocessor command.

@node Header Files, Macros, Commands, Top
@section Header Files

@cindex header file
A header file is a file containing C declarations and macro definitions
(@pxref{Macros}) to be shared between several source files.  You request
the use of a header file in your program with the C preprocessor command
@samp{#include}.

@node Header Uses, Include Syntax, Header Files, Header Files
@subsection Uses of Header Files

Header files serve two kinds of purposes.

@itemize @bullet
@item
@findex system header files
System header files declare the interfaces to parts of the operating
system.  You include them in your program to supply the definitions
you need to invoke system calls and libraries.

@item
Your own header files contain declarations for interfaces between the
source files of your program.  Each time you have a group of related
declarations and macro definitions all or most of which are needed in
several different source files, it is a good idea to create a header
file for them.
@end itemize

Including a header file produces the same results in C compilation as
copying the header file into each source file that needs it.  But such
copying would be time-consuming and error-prone.  With a header file, the
related declarations appear in only one place.  If they need to be changed,
they can be changed in one place, and programs that include the header file
will automatically use the new version when next recompiled.  The header
file eliminates the labor of finding and changing all the copies as well as
the risk that a failure to find one copy will result in inconsistencies
within a program.

The usual convention is to give header files names that end with @file{.h}.

@node Include Syntax, Include Operation, Header Uses, Header Files
@subsection The @samp{#include} Command

@findex #include
Both user and system header files are included using the preprocessor
command @samp{#include}.  It has three variants:

@table @code
@item #include <@var{file}>
This variant is used for system header files.  It searches for a file
named @var{file} in a standard list of system directories.

{{how can the user find out about/control this list?}}

The parsing of this form of @samp{#include} is slightly special
because comments are not recognized within the @samp{<@dots{}>}.
Thus, in @samp{#include <x/*y>} the @samp{/*} does not start a comment
and the command specifies inclusion of a system header file named
@file{x/*y}.  Of course, a header file with such a name is unlikely to
exist on Unix, where shell wildcard features would make it hard to
manipulate.

@var{file} may not contain a @samp{>} character.  It may, however,
contain a @samp{<} character.

@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 current working
directory, and then in a specified list of directories for user
header files, and finally in the same directories used for system header
files.  The current working directory is tried first because it is
presumed to be the location of the files of the program being
compiled.  The other directories for user header files are specified
with the command switch @samp{-I} (@pxref{Invocation}).

@var{file} may not contain @samp{"} characters unless they are
preceded by Backslashes.  Note, however, that such Backslashes are
@emph{not} discarded when searching for the file.  None of the character
escape sequences appropriate to string constants in C is processed;
all Backslashes are significant.  Thus, @samp{#include "x\n\"y"}
specifies a filename containing two Backslashes.  It is not clear why
this behavior is ever useful, but the ANSI standard specifies it.

@item #include @var{anything else}
This variant is called a @dfn{computed #include}.  Any @samp{#include}
command whose argument does not fit the above two forms is a computed
include.  @var{anything else} is checked for macro calls, which are
expanded (@pxref{Macros}).  When this is done, the result must fit
one of the above two variants.

This feature allows you to define a macro which controls the file name
to be used at a later point in the program.  One application of this
is to allow a site-configuration file for your program to specify the
names of the system include files to be used.  This can help in
porting the program to various operating systems in which the
necessary system header files are found in different places.
@end table

@node Include Operation,, Include Syntax, Header Files
@subsection How @samp{#include} Works

The @samp{#include} command works by directing the C preprocessor to scan
the specified file as input before continuing with the rest of the current
file.  The output from the preprocessor contains the output already
generated, followed by the output resulting from the included file,
followed by the output that comes from the text after the @samp{#include}
command.  For example, given two files as follows:

@example
/* File program.c */
int x;
#include "header.h"

main ()
@{
  printf (test ());
@}


/* File header.h */
char *test ();
@end example

@noindent
the output generated by the C preprocessor for @file{program.c} as input
would be

@example
int x;
char *test ();

main ()
@{
  printf (test ());
@}
@end example

Included files are not limited to declarations and macro definitions; they
are merely the typical use.  Any fragment of a C program can be included
from another file.  The include file could even contain the beginning of a
statement that is concluded in the containing file, or the end of a
statement that was started in the including file.  However, a comment or a
string or character constant may not start in the included file and finish
in the including file.  An unterminated comment, string constant or
character constant in an included file is considered to end (with an error
message) at the end of the file.

The line following the @samp{#include} command is always treated as a
separate line by the C preprocessor even if the included file lacks a final
newline.

@node Macros, Conditionals, Header Files, Top
@section Macros

A macro is a sort of abbreviation which you can define once and then
use later.  There are many complicated features associated with macros
in the C preprocessor.

@menu
* Simple Macros::    Macros that always expand the same way.
* Argument Macros::  Macros that accept arguments that are substituted
                       into the macro expansion.
* Predefined::       Predefined macros that are always available.
* Stringification::  Macro arguments converted into string constants.
* Concatenation::    Building tokens from parts taken from macro arguments.
* Undefining::       Cancelling a macro's definition.
* Redefining::       Changing a macro's definition.
* Macro Pitfalls::   Macros can confuse the unwary.  Here we explain
                       several common problems and strange features.
@end menu

@node Simple Macros, Argument Macros, Macros, Macros
@subsection Simple Macros

A @dfn{simple macro} is a kind of abbreviation.  It is a name which stands
for a fragment of code.

Before you can use a macro, you must @dfn{define} it explicitly with the
@samp{#define} command.  @samp{#define} is followed by the name of the
macro and then the code it should be an abbreviation for.  For example,

@example
#define BUFFER_SIZE 1020
@end example

@noindent
defines a macro named @samp{BUFFER_SIZE} as an abbreviation for the text
@samp{1020}.  Therefore, if somewhere after this @samp{#define} command
there comes a C statement of the form

@example
foo = (char *) xmalloc (BUFFER_SIZE);
@end example

@noindent
then the C preprocessor will recognize and @dfn{expand} the macro
@samp{BUFFER_SIZE}, resulting in

@example
foo = (char *) xmalloc (1020);
@end example

@noindent
the definition must be a single line; however, it may not end in the
middle of a multi-line string constant or character constant.

The use of all upper case for macro names is a standard convention.
Programs are easier to read when it is possible to tell at a glance which
names are macros.

Normally, a macro definition must be a single line, like all C preprocessor
commands.  (You can split a long macro definition cosmetically with
Backslash-Newline.)  There is one exception: Newlines can be included in
the macro definition if within a string or character constant.  By the same
token, it is not possible for a macro definition to contain an unbalanced
quote character; the definition automatically extends to include the
matching quote character that ends the string or character constant.
Comments within a macro definition may contain Newlines, which make no
difference since the comments are entirely replaced with Spaces regardless
of their contents.

Aside from the above, there is no restriction on what can go in a macro
body.  Parentheses need not balance.  The body need not resemble valid C
code.  (Of course, you might get error messages from the C compiler when
you use the macro.)

The C preprocessor scans your program sequentially, so macro definitions
take effect at the place you write them.  Therefore, the following input to
the C preprocessor

@example
foo = X;
#define X 4
bar = X;
@end example

@noindent
produces as output

@example
foo = X;

bar = 4;
@end example

After the preprocessor expands a macro name, the macro's definition body is
appended to the front of the remaining input, and the check for macro calls
continues.  Therefore, the macro body can contain calls to other macros.
For example, after

@example
#define BUFSIZE 1020
#define TABLESIZE BUFSIZE
@end example

@noindent
the name @samp{TABLESIZE} when used in the program would go through two
stages of expansion, resulting ultimately in @samp{1020}.

This is not at all the same as defining @samp{TABLESIZE} to be @samp{1020}.
The @samp{#define} for @samp{TABLESIZE} uses exactly the body you
specify---in this case, @samp{BUFSIZE}---and does not check to see whether
it too is the name of a macro.  It's only when you @emph{use} @samp{TABLESIZE}
that the result of its expansion is checked for more macro names.
@xref{Cascaded Macros}.

@node Argument Macros, Predefined, Simple Macros, Macros
@subsection Macros with Arguments

A simple macro always stands for exactly the same text, each time it is
used.  Macros can be more flexible when they accept @dfn{arguments}.
Arguments are fragments of code that you supply each time the macro is
used.  These fragments are included in the expansion of the macro according
to the directions in the macro definition.

To define a macro that uses arguments, you write a @samp{#define} command
with a list of @dfn{argument names} in parentheses after the name of the
macro.  The argument names may be any valid C identifiers, separated by
commas and optionally whitespace.  The open-parenthesis must follow the
macro name immediately, with no space in between.

For example, here is a macro that computes the minimum of two numeric
values:

@example
#define min(X, Y)  ((X) < (Y) ? (X) : (Y))
@end example

To use a macro that expects arguments, you write the name of the macro
followed by a list of @dfn{actual arguments} in parentheses. separated by
commas.  The number of actual arguments you give must match the number of
arguments the macro expects.   Examples of use of the macro @samp{min}
include @samp{min (1, 2)} and @samp{min (x + 28, *p)}.

The expansion text of the macro depends on the arguments you use.
Each of the argument names of the macro is replaced, throughout the
macro definition, with the corresponding actual argument.  Using the
same macro @samp{min} defined above, @samp{min (1, 2)} expands into

@example
((1) < (2) ? (1) : (2))
@end example

@noindent
where @samp{1} has been substituted for @samp{X} and @samp{2} for @samp{Y}.

Likewise, @samp{min (x + 28, *p)} expands into

@example
((x + 28) < (*p) ? (x + 28) : (*p))
@end example

Parentheses in the actual arguments must balance; a comma within
parentheses does not end an argument.  However, there is no requirement for
brackets or braces to balance; thus, if you want to supply @samp{array[x =
y, x + 1]} as an argument, you must write it as @samp{array[(x = y, x +
1)]}, which is equivalent C code.

After the actual arguments are substituted into the macro body, the entire
result is appended to the front of the remaining input, and the check for
macro calls continues.  Therefore, the actual arguments can contain calls
to other macros, either with or without arguments, or even to the same
macro.  The macro body can also contain calls to other macros.  For
example, @samp{min (min (a, b), c)} expands into

@example
((((a) < (b) ? (a) : (b))) < (c)
 ? (((a) < (b) ? (a) : (b)))
 : (c))
@end example

@noindent
(Line breaks shown here for clarity would not actually be generated.)

If you use the macro name followed by something other than an
open-parenthesis, it is not a call to the macro, and the preprocessor does
not change what you have written.  Therefore, it is possible for the same
name to be a variable or function in your program as well as a macro,
and you can choose in each instance whether to refer to the macro
(if an actual argument list follows) or the variable or function (if
an argument list does not follow).

Such dual use of one name could be confusing and should be avoided
except when the two meanings are effectively synonymous: that is, when the
name is both a macro and a function and the two have similar effects.  You
can think of the name simply as as a function; use of the name for purposes
other than calling it (such as, to take the address) will refer to the
function, while calls will expand the macro and generate better but
equivalent code.  For example, you can use a function named @samp{min} in
the same source file that defines the macro.  If you write @samp{&min} with
no argument list, you refer to the function.  If you write @samp{min (x,
bb)}, with an argument list, the macro is expanded.  If you write
@samp{(min) (a, bb)}, where the name @samp{min} is not followed by an
open-parenthesis, the macro is not expanded, so you wind up with a call to
the function @samp{min}.

It is not allowed to define the same name as both a simple macro and
a macro with arguments.

In the definition of a macro with arguments, the list of argument names
must follow the macro name immediately with no space in between.  If there
is a space after the macro name, the macro is defined as taking no
arguments, and all the rest of the name is taken to be the expansion.
The reason for this is that it is often useful to define a macro that
takes no arguments and whose definition begins with an identifier in
parentheses.  This rule about spaces makes it possible for you to do either

@example
#define FOO(x) - 1 / (x)
@end example

@noindent
which defines @samp{FOO} to take an argument and expand into minus the
reciprocal of that argument, or

@example
#define BAR (x) - 1 / (x)
@end example

@noindent
which defines @samp{BAR} to take no argument and always expand into
@samp{(x) - 1 / (x)}.

@node Predefined, Stringification, Argument Macros, Macros
@subsection Predefined Macros

@cindex predefined macros
Several simple macros are predefined.  You can use them without giving
definitions for them.  They fall into two classes: standard macros and
system-specific macros.

@menu
* Standard Predefined::     Standard predefined macros.
* Nonstandard Predefined::  Nonstandard predefined macros.
@end menu

@node Standard Predefined, Nonstandard Predefined, Predefined, Predefined
@subsubsection Standard Predefined Macros

The standard predefined macros are available with the same meanings on all
operating systems and all kinds of machines.  Their names all start and end
with double underscores.  Here is a table of them.

@table @code
@item __FILE__
@findex __FILE__
This macro expands to the name of the current input file, in the form
of a C string constant.

@item __LINE__
@findex __LINE__
This macro expands to the current input line number, in the form of a
decimal integer constant.  While we call it a predefined macro, it's
a pretty strange macro, since its ``definition'' changes with each
new line of source code.

This and @samp{__FILE__} are useful in generating an error message to
report an inconsistency detected by the program; the message can state
the source line at which the inconsistency was detected.  For example,

@example
fprintf (stderr, "Internal error: negative string length "
                 "%d at %s, line %d.",
         length, __FILE__, __LINE__);
@end example

A @samp{#include} command changes the expansions of @samp{__FILE__}
and @samp{__LINE__} to correspond to the included file.  At the end of
that file, when processing resumes on the input file that contained
the @samp{#include} command, the expansions of @samp{__FILE__} and
@samp{__LINE__} revert to the values they had before the
@samp{#include} (but @samp{__LINE__} is then incremented by one as
processing moves to the line after the @samp{#include}).

The expansions of both @samp{__FILE__} and @samp{__LINE__} are altered
if a @samp{#line} command is used.  @xref{Combining Sources}.

@item __DATE__
@findex __DATE__
This macro expands to a string constant that describes the date on
which the preprocessor is being run.  The string constant contains
eleven characters and looks like @samp{"Jan 29 1987"} or @samp{"Apr
@ 1 1905"}.

@item __TIME__
@findex __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 @samp{"23:59:01"}.

@item __STDC__
@findex __STDC__
This macro expands to the constant 1, to signify that this is ANSI
Standard C.  (Whether that is actually true depends on what C compiler
will operate on the output from the preprocessor.)
@end table

@node Nonstandard Predefined,, Standard Predefined, Predefined
@subsubsection Nonstandard Predefined Macros

The C preprocessor normally has several predefined macros that vary between
machines because their purpose is to indicate what type of system and
machine is in use.  This manual, being for all systems and machines, cannot
tell you exactly what their names are; instead, we offer a list of some
typical ones.

Some nonstandard predefined macros describe the operating system in use,
with more or less specificity.  For example,

@table @code
@item unix
@findex unix
@samp{unix} is normally predefined on all Unix systems.

@item BSD
@findex BSD
@samp{BSD} is predefined on recent versions of Berkeley Unix
(perhaps only in version 4.3).
@end table

Other nonstandard predefined macros describe the kind of CPU, with more or
less specificity.  For example,

@table @code
@item vax
@findex vax
@samp{vax} is predefined on Vax computers.

@item mc68000
@findex mc68000
@samp{mc68000} is predefined on most computers whose CPU is a 68000,
68010 or 68020.

@item m68k
@findex m68k
@samp{m68k} is also predefined on most computers whose CPU is a 68000.
68010 or 68020; however, some makers use @samp{mc68000} and some use
@samp{m68k}.  Some predefine both names.

@item M68020
@findex M68020
@samp{M68020} has been observed to be predefined on some systems that
use 68020 CPUs---in addition to @samp{mc68000} and @samp{m68k} that
are less specific.
@end table

Yet other nonstandard predefined macros describe the manufacturer of
the system.  For example,

@table @code
@item sun
@findex sun
@samp{sun} is predefined on all models of Sun computers.

@item pyr
@findex pyr
@samp{sun} is predefined on all models of Pyramid computers.
@end table

These predefined symbols are not only nonstandard, they are contrary to the
ANSI standard because their names do not start with underscores.  However,
the GNU C preprocessor would be useless if it did not predefine the same
names that are normally predefined on the system and machine you are using.
Even system header files check the predefined names and will generate
incorrect declarations if they do not find the names that are expected.

The set of nonstandard predefined names in the GNU C preprocessor is controlled
by the macro @samp{PREDEFS}, which should be set with a @samp{-D} switch
in the @file{Makefile} to a string constant containing names separated
by commas.  The Doublequotes that delimit the string constant must be
escaped with Backslashes to get them through the shell.  For example,
@samp{\"unix,sun,m68k,mc68000\"} is used on Suns.

@node Stringification, Concatenation, Predefined, Macros
@subsection Stringification

@cindex stringification
@dfn{Stringification} means turning an code fragment into a string constant
whose contents are the text for the code fragment.  For example,
stringifying @samp{foo (z)} results in @samp{"foo (z)"}.

In the C preprocessor, stringification is an option available when macro
arguments are substituted into the macro definition.  In the body of the
definition, when an argument name appears, the character @samp{#} before
the name specifies stringification of the corresponding actual argument
when it is substituted at that point in the definition.  The same argument
may be substituted in other places in the definition without
stringification if the argument name appears in those places with no
@samp{#}.

Here is an example of a macro definition that uses stringification:

@example
#define WARN_IF(EXP) \
do @{ if (EXP) fprintf (stderr, "Warning: " #EXP "\n"); @} while (0)
@end example

@noindent
Here the actual argument for @samp{EXP} is substituted once as given,
into the @samp{if} statement, and once as stringified, into the
argument to @samp{fprintf}.  The @samp{do} and @samp{while (0)} are
a kludge to make it possible to write @samp{WARN_IF (@var{arg});},
which the resemblance of @samp{WARN_IF} to a function would make
C programmers want to do; @pxref{Swallow Semicolon}).

The stringification feature is limited to transforming one macro argument
into one string constant: there is no way to combine the argument with
other text and then stringify it all together.  But the example above shows
how an equivalent result can be obtained in ANSI Standard C using the
feature that adjacent string constants are concatenated as one string
constant.  The preprocessor stringifies @samp{EXP}'s actual argument
into a separate string constant, resulting in text like

@example
do @{ if (x == 0) fprintf (stderr, "Warning: " "x == 0" "\n"); @} while (0)
@end example

@noindent
but the C compiler then sees three consecutive string constants and
concatenates them into one, producing effectively

@example
do @{ if (x == 0) fprintf (stderr, "Warning: x == 0\n"); @} while (0)
@end example

Stringification in C involves more than putting doublequote characters
around the fragment; it is necessary to put backslashes in front of all
doublequote characters, and all backslashes in string and character
constants, in order to get a valid C string constant with the proper
contents.  Thus, stringifying @samp{p = "foo\n";} results in @samp{"p =
\"foo\\n\";"}.  However, backslashes that are not inside of string or
character constants are not duplicated: @samp{\n} by itself stringifies to
@samp{"\n"}.

Whitespace (including comments) in the text being stringified is handled
according to precise rules.  All leading and trailing whitespace is ignored.
Any sequence of whitespace in the middle of the text is converted to
a single space in the stringified result.

@node Concatenation, Undefining, Stringification, Macros
@subsection Concatenation

@cindex concatenation
@dfn{Concatenation} means joining two strings into one.  In the context
of macro expansion, concatenation refers to joining two lexical units
into one longer one.  Specifically, an actual argument to the macro can be
concatenated with another actual argument or with fixed text to produce
a longer name.  The longer name might be the name of a function,
variable or type, or a C keyword; it might even be the name of another
macro, in which case it will be expanded.

When you define a macro, you request concatenation with the special
operator @samp{##} in the macro body.  When the macro is called,
after actual arguments are substituted, all @samp{##} operators are
deleted, and so is any whitespace next to them (including whitespace
that was part of an actual argument).  The result is to concatenate
the syntactic tokens on either side of the @samp{##}.

Consider a C program that interprets named commands.  There probably needs
to be a table of commands, perhaps an array of structures declared as
follows:

@example
struct command
@{
  char *name;
  void (*function) ();
@};

struct command commands[] =
@{
  @{ "quit", quit_command@},
  @{ "help", help_command@},
  @dots{}
@};
@end example

It would be cleaner not to have to give each command name twice, once in
the string constant and once in the function name.  A macro which takes the
name of a command as an argument can make this unnecessary.  The string
constant can be created with stringification, and the function name by
concatenating the argument with @samp{_command}.  Here is how it is done:

@example
#define COMMAND(NAME)  @{ #NAME, NAME ## _command @}

struct command commands[] =
@{
  COMMAND (quit),
  COMMAND (help),
  @dots{}
@};
@end example

The usual case of concatenation is concatenating two names (or a name and a
number) into a longer name.  But this isn't the only valid case.  It is
also possible to concatenate two numbers (or a number and a name, such as
@samp{1.5} and @samp{e3}) into a number.  Also, multi-character operators
such as @samp{+=} can be formed by concatenation.  In some cases it is even
possible to piece together a string constant.  However, two pieces of text
that don't together form a valid lexical unit cannot be concatenated.  For
example, concatenation with @samp{x} on one side and @samp{+} on the other
is not meaningful because those two characters can't fit together in any
lexical unit of C.  The ANSI standard says that such attempts at
concatenation are undefined, but in the GNU C preprocessor it is well
defined: it puts the @samp{x} and @samp{+} side by side with no particular
special results.

Keep in mind that the C preprocessor converts comments to whitespace before
macros are even considered.  Therefore, you cannot create a comment by
concatenating @samp{/} and @samp{*}: the @samp{/*} sequence that starts a
comment is not a lexical unit, but rather the beginning of a ``long'' space
character.  Also, you can freely use comments next to a @samp{##} in a
macro definition, or in actual arguments that will be concatenated, because
the comments will be converted to spaces at first sight, and concatenation
will later discard the spaces.

@node Undefining, Redefining, Concatenation, Macros
@subsection Undefining Macros

@cindex undefining macros
To @dfn{undefine} a macro means to cancel its definition.  This is done
with the @samp{#undef} command.  @samp{#undef} is followed by the macro
name to be undefined.

Like definition, undefinition occurs at a specific point in the source
file, and it applies starting from that point.  The name ceases to be a
macro name, and from that point on it is treated by the preprocessor as if
it had never been a macro name.

For example,

@example
#define FOO 4
x = FOO;
#undef FOO
x = FOO;
@end example

@noindent
expands into

@example
x = 4;

x = FOO;
@end example

@noindent
In this example, @samp{FOO} had better be a variable or function as well
as (temporarily) a macro, in order for the result of the expansion to be
valid C code.

The same form of @samp{#undef} command will cancel definitions with
arguments or definitions that don't expect arguments.  The @samp{#undef}
command has no effect when used on a name not currently defined as a macro.

@node Redefining, Macro Pitfalls, Undefining, Macros
@subsection Redefining Macros

@cindex redefining macros
@dfn{Redefining} a macro means defining (with @samp{#define}) a name that
is already defined as a macro.

A redefinition is trivial if the new definition is transparently identical
to the old one.  You probably wouldn't deliberately write a trivial
redefinition, but they can happen automatically when a header file is
included more than once (@pxref{Header Files}), so they are accepted
silently and without effect.

Nontrivial redefinition is considered likely to be an error, so
it provokes a warning message from the preprocessor.  However, sometimes it
is useful to change the definition of a macro in mid-compilation.  You can
inhibit the warning by undefining the macro with @samp{#undef} before the
second definition.

In order for a reefinition to be trivial, the new definition must exactly match
the one already in effect, with two possible exceptions:

@itemize @bullet
@item
Whitespace may be added or deleted at the beginning or the end.

@item
Whitespace may be changed in the middle (but not inside strings).
However, it may not be eliminated entirely, and it may not be added
where there was no whitespace at all.
@end itemize

Recall that a comment counts as whitespace.

@node Macro Pitfalls,, Redefining, Macros
@subsection Pitfalls and Subtleties of Macros

In this section we describe some special rules that apply to macros and
macro expansion, and point out certain cases in which the rules have 
counterintuitive consequences that you must watch out for.

@menu
* Misnesting::        Macros can contain unmatched parentheses.
* Macro Parentheses:: Why apparently superfluous parentheses
                         may be necessary to avoid incorrect grouping.
* Swallow Semicolon:: Macros that look like functions
                         but expand into compound statements.
* Side Effects::      Unsafe macros that cause trouble when
                         arguments contain side effects.
* Self-Reference::    Macros whose definitions use the macros' own names.
* Argument Prescan::  Actual arguments are checked for macro calls
                         before they are substituted.
* Cascaded Macros::   Macros whose definitions use other macros.
@end menu

@node Misnesting, Macro Parentheses, Macro Pitfalls, Macro Pitfalls
@subsubsection Improperly Nested Constructs

Recall that when a macro is called with arguments, the arguments are
substituted into the macro body and the result is checked, together with
the rest of the input file, for more macro calls.

It is possible to piece together a macro call coming partially from the
macro body and partially from the actual arguments.  For example,

@example
#define double(x) (2*(x))
#define call_with_1(x) x(1)
@end example

@noindent
would expand @samp{call_with_1 (double)} into @samp{(2*(1))}.

Macro definitions do not have to have balanced parentheses.  By writing an
unbalanced open parenthesis in a macro body, it is possible to create a
macro call that begins inside the macro body but ends outside of it.  For
example,

@example
#define strange(file) fprintf (file, "%s %d",
@dots{}
strange(stderr) p, 35)
@end example

@noindent
This bizarre example expands to @samp{fprintf (stderr, "%s %d", p, 35)}!

@node Macro Parentheses, Swallow Semicolon, Misnesting, Macro Pitfalls
@subsubsection Unintended Grouping of Arithmetic

You may have noticed that in most of the macro definition examples shown
above, each occurrence of a macro argument name had parentheses around it.
In addition, another pair of parentheses usually surround the entire macro
definition.  Here is why it is best to write macros that way.

Suppose you define a macro as follows

@example
#define ceil_div(x, y) (x + y - 1) / y
@end example

@noindent
whose purpose is to divide, rounding up.  (One use for this
operation is to compute how many @samp{int}'s are needed to hold
a certain number of @samp{char}'s.)  Then suppose it is used as follows:

@example
a = ceil_div (b & c, sizeof (int));
@end example

@noindent
This expands into

@example
a = (b & c + sizeof (int) - 1) / sizeof (int);
@end example

@noindent
which is does not do what is intended the operator-precedence rules of
C make it equivalent to

@example
a = (b & (c + sizeof (int) - 1)) / sizeof (int);
@end example

What we want is

@example
a = ((b & c) + sizeof (int) - 1)) / sizeof (int);
@end example

@noindent
Defining the macro as

@example
#define ceil_div(x, y) ((x) + (y) - 1) / (y)
@end example

@noindent
provides the desired result.

However, unintended grouping can result in another way.  Consider
@samp{sizeof ceil_div(1, 2)}.  That has the appearance of a C expression
that would compute the size of the type of @samp{ceil_div (1, 2)}, but in
fact it means something very different.  Here is what it expands to:

@example
sizeof ((1) + (2) - 1) / (2)
@end example

@noindent
This would take the size of an integer and divide it by two.  The precedence
rules have put the division outside the @samp{sizeof} when it was intended
to be inside.

Parentheses around the entire macro definition can prevent such problems.
Here, then, is the recommended way to define @samp{ceil_div}:

@example
#define ceil_div(x, y) (((x) + (y) - 1) / (y))
@end example

@node Swallow Semicolon, Side Effects, Macro Parentheses, Macro Pitfalls
@subsubsection Swallowing the Semicolon

@cindex semicolons (after macro calls)
Often it is desirable to define a macro that expands into a compound
statement.  Consider, for example, the following macro, that advances a
pointer (the argument @samp{p} says where to find it) across whitespace
characters:

@example
#define SKIP_SPACES (p, limit)  \
@{ register char *lim = (limit); \
  while (p != lim) @{            \
    if (*p++ != ' ') @{          \
      p--; break; @}@}@}
@end example

@noindent
Here Backslash-Newline is used to split the macro definition, which must
be a single line, so that it resembles the way such C code would be
layed out if not part of a macro definition.

A call to this macro might be @samp{SKIP_SPACES (p, lim)}.  Strictly
speaking, the call expands to a compound statement, which is a complete
statement with no need for a semicolon to end it.  But it looks like a
function call.  So it minimizes confusion if you can use it like a function
call, writing a semicolon afterward, as in @samp{SKIP_SPACES (p, lim);}

But this can cause trouble before @samp{else} statements, because the
semicolon is actually a null statement.  Suppose you write

@example
if (*p != 0)
  SKIP_SPACES (p, lim);
else @dots{}
@end example

The presence of two statements---the compound statement and a null
statement---in between the @samp{if} condition and the @samp{else}
makes invalid C code.

The definition of the macro @samp{SKIP_SPACES} can be altered to solve
this problem, using a @samp{do @dots{} while} statement.  Here is how:
  
@example
#define SKIP_SPACES (p, limit)     \
do @{ register char *lim = (limit); \
     while (p != lim) @{            \
       if (*p++ != ' ') @{          \
         p--; break; @}@}@}           \
while (0)
@end example

Now @samp{SKIP_SPACES (p, lim);} expands into

@example
do @{@dots{}@} while (0);
@end example

which is one statement.

@node Side Effects, Self-Reference, Swallow Semicolon, Macro Pitfalls
@subsubsection Duplication of Side Effects

@cindex side effects (in macro arguments)
@cindex unsafe macros
Let's consider a call to the macro @samp{min}:

@example
#define min(X, Y)  ((X) < (Y) ? (X) : (Y))
@dots{}
next = min (x + y, foo (z));
@end example

expands into

@example
next = ((x + y) < (foo (z)) ? (x + y) : (foo (z)));
@end example

where @samp{x + y} has been substituted for @samp{X} and @samp{foo (z)}
for @samp{Y}.

The function @samp{foo} is used only once in the statement as it appears
in the program, but the expression @samp{foo (z)} has been substituted
twice into the macro expansion.  As a result, @samp{foo} might be called
two times when the statement is executed.  If it has side effects or
if it takes a long time to compute, the results might not be what you
intended.  We say that @samp{min} is an @dfn{unsafe} macro.

Solving this problem cleanly would involve defining @samp{min} in
a way that would compute the value of @samp{foo (z)} only once.
Unfortunately, the C language offers no way to do this.  So the only
solution is to be careful when @emph{using} the macro @samp{min}.
For example, you can calculate the value of @samp{foo (z)}, save it
in a variable, and use that variable in @samp{min}:

@example
#define min(X, Y)  ((X) < (Y) ? (X) : (Y))
@dots{}
@{
  int tem = foo (z);
  next = min (x + y, tem);
@}
@end example

@noindent
(where I assume that @samp{foo} returns type @samp{int}).

@node Self-Reference, Argument Prescan, Side Effects, Macro Pitfalls
@subsubsection Self-Referential Macros

@cindex self-reference
A @dfn{self-referential} macro is one whose name appears in its definition.
A special feature of ANSI Standard C is that the self-reference is not
considered a macro call.  It is passed into the preprocessor output
unchanged.

Let's consider an example:

@example
#define foo (4 + foo)
@end example

@noindent
where @samp{foo} is also a variable in your program.

Following the ordinary rules, each reference to @samp{foo} will expand into
@samp{(4 + foo)}; then this will be rescanned and will expand into @samp{(4
+ (4 + foo))}; and so on until it causes a fatal error (memory full) in the
preprocessor.

However, the special rule about self-reference cuts this process short
after one step, at @samp{(4 + foo)}.  Therefore, this macro definition
has the possibly useful effect of causing the program to add 4 to
the value of @samp{foo} wherever @samp{foo} is referred to.

In most cases, it is a bad idea to take advantage of this feature.
A person reading the program who sees that @samp{foo} is a variable will
not expect that it is a macro as well.  The reader will come across a
the identifier @samp{foo} in the program and think its value should be
that of the variable @samp{foo}, whereas in fact the value is four
greater.

The special rule for self-reference applies also to @dfn{indirect}
self-reference.  This is the case where a macro @var{x} expands to use a
macro @samp{y}, and @samp{y}'s expansion refers to the macro @samp{x}.  The
resulting reference to @samp{x} comes indirectly from the expansion of
@samp{x}, so it is a self-reference and is not further expanded.  Thus,
after

@example
#define x (4 + y)
#define y (2 * x)
@end example

@noindent
@samp{x} would expand into @samp{(4 + (2 * x))}.  Clear?

But suppose @samp{y} is used elsewhere, not from the definition of @samp{x}.
Then the use of @samp{x} in the expansion of @samp{y} is not a self-reference
because @samp{x} is not ``in progress''.  So it does expand.  However,
the expansion of @samp{x} contains a reference to @samp{y}, and that
is an indirect self-reference now because @samp{y} is ``in progress''.
The result is that @samp{y} expands to @samp{(2 * (4 + y))}.

It is not clear that this behavior would ever be useful, but it is specified
by the ANSI C standard, so you need to understand it.

@node Argument Prescan, Cascaded Macros, Self-Reference, Macro Pitfalls
@subsubsection Separate Expansion of Macro Arguments

We have explained that the expansion of a macro, including the substituted
actual arguments, is scanned over again for macro calls to be expanded.

What really happens is more subtle: first each actual argument text is scanned
separately for macro calls.  Then the results of this are substituted into
the macro body to produce the macro expansion, and the macro expansion
is scanned again for macros to expand.

The result is that the actual arguments are scanned @emph{twice} to expand
macro calls in them.

Most of the time, this has no effect.  If the actual argument contained
any macro calls, they are expanded during the first scan.  The result
therefore contains no macro calls, so the second scan does not change it.
If the actual argument were substituted as given, with no prescan,
the single remaining scan would find the same macro calls and produce
the same results.

You might expect the double scan to change the results when a
self-referential macro is used in an actual argument of another macro
(@pxref{Self-Reference}): the self-referential macro would be expanded once
in the first scan, and a second time in the second scan.  But this is not
what happens.  The self-references that do not expand in the first scan are
marked so that they will not expand in the second scan either.

The prescan is not done when an argument is stringified or concatenated.
(More precisely, stringification and concatenation use the argument as
written, in un-prescanned form.  The same actual argument would be used in
prescanned form if it is substituted elsewhere without stringification or
concatenation.)  Thus,

@example
#define str(s) #s
#define foo 4
str (foo)
@end example

@noindent
expands to @samp{"foo"}.  Once more, prescan has been prevented from
having any noticeable effect.

You might now ask, ``Why mention the prescan, if it makes no difference?
Why not skip it and make the preprocessor faster?''  The answer is that the
prescan does make a difference in two special cases:

@itemize @bullet
@item
Nested calls to a macro.

@item
Macros that call other macros that stringify or concatenate.
@end itemize

We say that @dfn{nested} calls to a macro occur when a macro's actual
argument contains a call to that very macro.  For example, if @samp{f}
is a macro that expects one argument, @samp{f (f (1))} is a nested
pair of calls to @samp{f}.  The desired expansion is made by
expanding @samp{f (1)} and substituting that into the definition of
@samp{f}.  The prescan causes the expected result to happen.
Without the prescan, @samp{f (1)} itself would be substituted as
an actual argument, and the inner use of @samp{f} would appear
during the main scan as an indirect self-reference and would not
be expanded.  Here, the prescan cancels an undesirable side effect
(in the medical, not computational, sense of the term) of the special
rule for self-referential macros.

There is also one case where prescan is useful.  It is possible
to use prescan to expand an argument and then stringify it---if you use
two levels of macros.  Let's add a new macro @samp{xstr} to the
example shown above:

@example
#define xstr(s) str(s)
#define str(s) #s
#define foo 4
xstr (foo)
@end example

This expands into @samp{"4"}, not @samp{"foo"}.  The reason for the
difference is that the argument of @samp{xstr} is expanded at prescan
(because @samp{xstr} does not specify stringification or concatenation of
the argument).  The result of prescan then forms the actual argument for
@samp{str}.  @samp{str} uses its argument without prescan because it
performs strigification; but it cannot prevent or undo the prescanning
already done by @samp{xstr}.

@node Cascaded Macros,, Argument Prescan, Macro Pitfalls
@subsubsection Cascaded Use of Macros

@cindex cascaded macros
@cindex macro body uses macro
A @dfn{cascade} of macros is when one macro's body contains a reference
to another macro.  This is very common practice.  For example,

@example
#define BUFSIZE 1020
#define TABLESIZE BUFSIZE
@end example

This is not at all the same as defining @samp{TABLESIZE} to be @samp{1020}.
The @samp{#define} for @samp{TABLESIZE} uses exactly the body you
specify---in this case, @samp{BUFSIZE}---and does not check to see whether
it too is the name of a macro.

It's only when you @emph{use} @samp{TABLESIZE} that the result of its expansion
is checked for more macro names.

This makes a difference if you change the definition of @samp{BUFSIZE}
at some point in the source file.  @samp{TABLESIZE}, defined as shown,
will always expand using the definition of @samp{BUFSIZE} that is
currently in effect:

@example
#define BUFSIZE 1020
#define TABLESIZE BUFSIZE
#undef BUFSIZE
#define BUFSIZE 37
@end example

Now @samp{TABLESIZE} expands (in two stages) to @samp{37}.

@node Conditionals, Combining Sources, Macros, Top
@section Conditionals

@cindex conditionals
In a macro processor, a @dfn{conditional} is a command that allows a part
of the program to be ignored during compilation, on some conditions.
In the C preprocessor, a conditional can test either an arithmetic expression
or whether a name is defined as a macro.

A conditional in the C preprocessor resembles in some ways an @samp{if}
statement in C, but it is important to understand the difference between
them.  The condition in an @samp{if} statement is tested during the execution
of your program.  Its purpose is to allow your program to behave differently
from run to run, depending on the data it is operating on.  The condition
in a preprocessor conditional command is tested when your program is compiled.
Its purpose is to allow different code to be included in the program depending
on the situation at the time of compilation.

@menu
* Uses: Conditional Uses.       What conditionals are for.
* Syntax: Conditional Syntax.   How conditionals are written.
* Deletion: Deleted Code.       Making code into a comment.
* Macros: Conditionals-Macros.  Why conditionals are used with macros.
* Errors: #error Command.       Detecting inconsistent compilation parameters.
@end menu

@node Conditional Uses, Conditional Syntax, Conditionals, Conditionals
Generally there are three kinds of reason to use a conditional.

@itemize @bullet
@item
A program may need to use different code depending on the machine or
operating system it is to run on.  In some cases the code for one
operating system may be erroneous on another operating system; for
example, it might refer to library routines that do not exist on the
other system.  When this happens, it is not enough to avoid executing
the invalid code: merely having it in the program makes it impossible
to link the program and run it.  With a preprocessor conditional, the
offending code can be effectively excised from the program when it is
not valid.

@item
You may want to be able to compile the same source file into two
different programs.  Sometimes the difference between the programs is
that one makes frequent time-consuming consistency checks on its
intermediate data while the other does not.

@item
A conditional whose condition is always false is a good way to exclude
code from the program but keep it as a sort of comment for future
reference.
@end itemize

Most simple programs that are intended to run on only one machine will
not need to use preprocessor conditionals.

@node Conditional Syntax, Conditionals-Macros, Conditional Uses, Conditionals
@subsection Syntax of Conditionals

@findex #if
A conditional in the C preprocessor begins with a @dfn{conditional
command}: @samp{#if}, @samp{#ifdef} or @samp{#ifndef}.@xref{Conditionals},
for info on @samp{#ifdef} and @samp{#ifndef}; only @samp{#if} is explained
here.

@menu
* If: #if Command.     Basic conditionals using @samp{#if} and @samp{#endif}.
* Else: #else Command. Including some text if the condition fails.
* Elif: #elif Command. Testing several alternative possibilities.
@end menu

@node #if Command, #else Command, Conditional Syntax, Conditional Syntax
@subsubsection The @samp{#if} Command

The @samp{#if} command in its simplest form consists of

@example
#if @var{expression}
@var{controlled text}
#endif /* @var{expression} */
@end example

The comment following the @samp{#endif} is not required, but it is a good
practice because it helps people match the @samp{#endif} to the
corresponding @samp{#if}.  Such comments should always be used, except in
short conditionals that are not nested.  In fact, you can put anything at
all after the @samp{#endif} and it will be ignored by the GNU C preprocessor,
but only comments are acceptable in ANSI Standard C.

@var{expression} is a C expression of integer type, subject to stringent
restrictions.  It may contain

@itemize @bullet
@item
Integer constants, which are all regarded as @code{long} or
@code{unsigned long}.

@item
Character constants, which are interpreted according to the character
set and conventions of the machine and operating system on which the
preprocessor is running.  The GNU C preprocessor uses the C data type
@samp{char} for these character constants; therefore, whether some
character codes are negative is determined by the C compiler used to
compile the preprocessor.  If it treats @samp{char} as signed, then
character codes large enough to set the sign bit will be considered
negative; otherwise, no character code is considered negative.

@item
Arithmetic operators for addition, subtraction, multiplication,
division, bitwise operations, shifts, comparisons, and @samp{&&} and
@samp{||}.

@item
Identifiers that are not macros, which are all treated as zero(!).

@item
Macro calls.  All macro calls in the expression are expanded before
actual computation of the expression's value begins.
@end itemize

Note that @samp{sizeof} operators and @code{enum}-type values are not allowed.
@code{enum}-type values, like all other identifiers that are not taken
as macro calls and expanded, are treated as zero.

The text inside of a conditional can include preprocessor commands.  Then
the commands inside the conditional are obeyed only if that branch of the
conditional succeeds.  The text can also contain other conditional groups.
However, the @samp{#if}'s and @samp{#endif}'s must balance.

@node #else Command, #elif Command, #if Command, Conditional Syntax
@subsubsection The @samp{#else} Command

@findex #else
The @samp{#else} command can be added a conditional to provide alternative
text to be used if the condition is false.  This looks like

@example
#if @var{expression}
@var{text-if-true}
#else /* Not @var{expression} */
@var{text-if-false}
#endif /* Not @var{expression} */
@end example

If @var{expression} is nonzero, and the @var{text-if-true} is considered
included, then @samp{#else} acts like a failing conditional and the
@var{text-if-false} is ignored.  Contrariwise, if the @samp{#if}
conditional fails, the @var{text-if-false} is considered included.

@node #elif Command,, #else Command, Conditional Syntax
@subsubsection The @samp{#elif} Command

@findex #elif
One common case of nested conditionals is used to check for more than two
possible alternatives.  For example, you might have

@example
#if X == 1
@dots{}
#else /* X != 1 */
#if X == 2
@dots{}
#else /* X != 2 */
@dots{}
#endif /* X != 2 */
#endif /* X != 1 */
@end example

Another conditional command, @samp{#elif}, allows this to be abbreviated
as follows:

@example
#if X == 1
@dots{}
#elif X == 2
@dots{}
#else /* X != 2 and X != 1*/
@dots{}
#endif /* X != 2 and X != 1*/
@end example

@samp{#elif} stands for ``else if''.  Like @samp{#else}, it goes in the
middle of a @samp{#if}-@samp{#endif} pair and subdivides it; it does not
require a matching @samp{#endif} of its own.  Like @samp{#if}, the
@samp{#elif} command includes an expression to be tested.

The text following the @samp{#elif} is processed only if the original
@samp{#if}-condition failed and the @samp{#elif} condition succeeeds.  More
than one @samp{#elif} can go in the same @samp{#if}-@samp{#endif} group.
Then the text after each @samp{#elif} is processed only if the @samp{#elif}
condition succeeds after the original @samp{#if} and any previous
@samp{#elif}'s within it have failed.  @samp{#else} is equivalent to
@samp{#elif 1}, and @samp{#else} is allowed after any number of
@samp{#elif}'s, but @samp{#elif} may not follow a @samp{#else}.

@node Deleted Code, Conditionals-Macros, Conditional Syntax, Conditionals
@subsection Keeping Deleted Code for Future Reference

If you replace or delete a part of the program but want to keep the old
code around as a comment for future reference, the easy way to do this is
to put @samp{#if 0} before it and @samp{#endif} after it.

This works even if the code being turned off contains conditionals, but
they must be entire conditionals (balanced @samp{#if} and @samp{#endif}).

@node Conditionals-Macros, #error Command, Deleted Code, Conditionals
@subsection Conditionals and Macros

Conditionals are rarely useful except in connection with macros.  A
@samp{#if} command whose expression uses no macros is equivalent to
@samp{#if 1} or @samp{#if 0}; you might as well determine which one, by
computing the value of the expression yourself, and then simplify the
program.  But when the expression uses macros, its value can vary from
compilation to compilation.

For example, here is a conditional that tests the expression
@samp{BUFSIZE == 1020}, where @samp{BUFSIZE} must be a macro.

@example
#if BUFSIZE == 1020
  printf ("Large buffers!\n");
#endif /* BUFSIZE is large */
@end example

@findex defined
The special operator @samp{defined} may be used in @samp{#if} expressions
to test whether a certain name is defined as a macro.  Either
@samp{defined @var{name}} or @samp{defined (@var{name})}.

@noindent
is an expression whose value is 1 if @var{name} is defined as macro at
the current point in the program, and 0 otherwise.  For the
@samp{defined} operator it makes no difference what the definition of
the macro is; all that matters is whether there is a definition.  Thus,
for example,

@example
#if defined (vax) || defined (ns16000)
@end example

@noindent
would include the following code if either of the names @samp{vax} and
@samp{ns16000} is defined as a macro.

If a macro is defined and later undefined with @samp{#undef},
subsequent use of the @samp{defined} operator will return 0, because
the name is no longer defined.  If the macro is defined again with
another @samp{#define}, @samp{defined} will recommence returning 1.

@findex #ifdef
@findex #ifndef
Conditionals that test just the definedness of one name are very common, so
there are two special short conditional commands for this case.  They are

@table @code
@item #ifdef @var{name}
is equivalent to @samp{#if defined (@var{name})}.

@item #ifndef @var{name}
is equivalent to @samp{#if ! defined (@var{name})}.
@end table

Macro definitions can vary between compilations for several reasons.

@itemize @bullet
@item
Some macros are predefined on each kind of machine.  For example, on a
Vax, the name @samp{vax} is a predefined macro.  On other machines, it
would not be defined.

@item
Many more macros are defined by system header files.  Different
systems and machines define different macros, or give them different
values.  It is useful to test these macros with conditionals to avoid
using a system feature on a machine where it is not implemented.

@item
Macros are a common way of allowing users to customize a program for
different machines or applications.  For example, the macro
@samp{BUFSIZE} might be defined in a configuration file for your
program that is included as a header file in each source file.  You
would use @samp{BUFSIZE} in a preprocessor conditional in order to
generate different code depending on the chosen configuration.

@item
Macros can be defined or undefined with @samp{-D} and @samp{-U}
command switches when you compile the program.  You can arrange to
compile the same source file into two different programs by choosing
a macro name to specify which program you want, writing conditionals
to test whether or how this macro is defined, and then controlling
the state of the macro with compiler command switches.
@xref{Invocation}.
@end itemize

@node #error Command,, Conditionals-Macros, Conditionals
@subsection The @samp{#error} Command

@findex #error
The command @samp{#error} causes the preprocessor to report a fatal
error.  The rest of the line that follows @samp{#error} is used as the
error message.

You would use @samp{#error} inside of a conditional that detects a
combination of parameters which you know the program does not properly
support.  For example, if you know that the program will not run
properly on a Vax, you might write

@example
#ifdef vax
#error Won't work on Vaxen.  See comments at get_last_object.
#endif
@end example

@noindent
@xref{Nonstandard Predefined}, for why this works.

If you have several configuration parameters that must be set up by
the installation in a consistent way, you can use conditionals to detect
an inconsistency and report it with @samp{#error}.  For example,

@example
#if HASH_TABLE_SIZE % 2 == 0 || HASH_TABLE_SIZE % 3 == 0 \
    || HASH_TABLE_SIZE % 5 == 0
#error HASH_TABLE_SIZE should not be divisible by a small prime
#endif
@end example

@node Combining Sources, Other Commands, Conditionals, Top
@section Combining Source Files

@cindex line control
@findex #line
One of the jobs of the C preprocessor is to inform the C compiler of where
each line of C code came from: which source file and which line number.

C code can come from multiple source files if you use @samp{#include};
both @samp{#include} and the use of conditionals and macros can cause
the line number of a line in the preprocessor output to be different
from the line's number in the original source file.  You will appreciate
the value of making both the C compiler (in error messages) and symbolic
debuggers such as GDB use the line numbers in your source file.

The C preprocessor builds on this feature by offering a command by which
you can control the feature explicitly.  This is useful when a file for
input to the C preprocessor is the output from another program such as the
@code{bison} parser generator, which operates on another file that is the
true source file.  Parts of the output from @code{bison} are generated from
scratch, other parts come from a standard parser file.  The rest are copied
nearly verbatim from the source file, but their line numbers in the
@code{bison} output are not the same as their original line numbers.
Naturally you would like compiler error messages and symbolic debuggers to
know the original source file and line number of each line in the
@code{bison} output.

@code{bison} arranges this by writing @samp{#line} commands into the output
file.  @samp{#line} is a command that specifies the original line number
and source file name for subsequent input in the current preprocessor input
file.  @samp{#line} has three variants:

@table @code
@item #line @var{linenum}
Here @var{linenum} is a decimal integer constant.  This specifies that
the line number of the following line of input, in its original source file,
was @var{linenum}.

@item #line @var{linenum} @var{filename}
Here @var{linenum} is a decimal integer constant and @var{filename}
is a string constant.  This specifies that the following line of input
came originally from source file @var{filename} and its line number there
was @var{linenum}.  Keep in mind that @var{filename} is not just a
file name; it is surrounded by Doublequotes.

@item #line @var{anything else}
@var{anything else} is checked for macro calls, which are expanded.
The result should be a decimal integer constant followed optionally
by a string constant, as described above.
@end table

@samp{#line} commands alter the results of the @samp{__FILE__} and
@samp{__LINE__} predefined macros from that point on.  @xref{Standard
Predefined}.

@node Other Commands, Output, Combining Sources, Top
@section Miscellaneous Preprocessor Commands

@findex #pragma
@cindex null command
This section describes two additional preprocesor commands.  They are
not very useful, but are mentioned for completeness.

The @dfn{null command} consists of a @samp{#} followed by a Newline, with
only whitespace (including comments) in between.  A null command is
understood as a preprocessor command but has no effect on the preprocessor
output.  The primary significance of the existence of the null command is
that an input line consisting of just a @samp{#} will produce no output,
rather than a line of output containing just a @samp{#}.  Supposedly
some old C programs contain such lines.

The @samp{#pragma} command is specified in the ANSI standard to have an
arbitrary implementation-defined effect.  In the GNU C preprocessor,
@samp{#pragma} first attempts to run the game @code{rogue}; if that fails,
it tries to run the game @code{hack}; if that fails, it tries to run
GNU Emacs displaying the Tower of Hanoi; if that fails, it reports a
fatal error.  In any case, preprocessing does not continue.

@node Output, Invocation, Other Commands, Top
@section C Preprocessor Output

@cindex output format
The output from the C preprocessor looks much like the input, except
that all preprocessor command lines have been replaced with blank lines
and all comments with spaces.  Whitespace within a line is not altered;
however, a space is inserted after the expansions of most macro calls.

Source file name and line number information is conveyed by lines of
the form

@example
# @var{linenum} @var{filename}
@end example

@noindent
which are inserted as needed into the middle of the input (but never within
a string or character constant).  Such a line means that the following line
originated in file @var{filename} at line @var{linenum}.

@node Invocation,, Output, Top
@section Invoking the C Preprocessor

Most often when you use the C preprocessor you will not have to invoke it
explicitly: the C compiler will do so automatically.  However, the
preprocessor is sometimes useful individually.

The C preprocessor expects two file names as arguments, @var{infile} and
@var{outfile}.  The preprocessor reads @var{infile} together with any other
files it specifies with @samp{#include}.  All the output generated by the
combined input files is written in @var{outfile}.

Either @var{infile} or @var{outfile} may be @samp{-}, which as @var{infile}
means to read from standard input and as @var{outfile} means to write to
standard output.  Also, if @var{outfile} or both file names are omitted,
the standard output and standard input are used for the omitted file names.

@cindex switches
Here is a table of command switches accepted by the C preprocessor.  Most
of them can also be given when compiling a C program; they are passed along
automatically to the preprocessor when it is invoked by the compiler.

@table @samp
@item -P
@findex -P
Inhibit generation of @samp{#}-lines with line-number information in
the output from the preprocessor (@pxref{Output}).  This might be
useful when running the preprocessor on something that is not C code
and will be sent to a program which might be confused by the
@samp{#}-lines

@item -C
@findex -C
Do not discard comments: pass them through to the output file.
Comments appearing in arguments of a macro call will be copied to the
output before the expansion of the macro call.

@item -T
@findex -T
Process ANSI standard trigraph sequences.  These are three-character
sequences, all starting with @samp{??}, that are defined by ANSI C to
stand for single characters.  For example, @samp{??/} stands for
@samp{\}, so @samp{'??/n'} is a character constant for Newline.
Strictly speaking, the GNU C preprocessor does not support all
programs in ANSI Standard C unless @samp{-T} is used, but you if you
ever notice the difference it will be with relief.

You don't want to know any more about trigraphs.

@item -pedantic
@findex -pedantic
Issue warnings required by the ANSI C standard in certain cases such
as when text other than a comment follows @samp{#else} or @samp{#endif}.

@item -I @var{directory}
@findex -I
Add the directory @var{directory} to the end of the list of
directories to be searched for user header files (@pxref{Include
Syntax}).

@item -D @var{name}
@findex -D
Predefine @var{name} as a macro, with definition @samp{1}.

@item -D @var{name}=@var{definition}
Predefine @var{name} as a macro, with definition @var{definition}.
There are no restrictions on the contents of @var{definition}, but if
you are invoking the preprocessor from a shell or shell-like program
you may need to use the shell's quoting syntax to protect characters
such as spaces that have a meaning in the shell syntax.

@item -U @var{name}
@findex -U
Do not predefine @var{name}.  If both @samp{-U} and @samp{-D} are
specified for one name, the @samp{-U} beats the @samp{-D} and the name
is not predefined.

@item -undef
@findex -undef
Do not predefine any nonstandard macros.

@item -d
@findex -d
Instead of outputting the result of preprocessing, output a list of
@samp{#define} commands for all the macros defined during the
execution of the preprocessor.

@item -i @var{file}
@findex -i
Process @var{file} as input, discarding the resulting output, before
processing the regular input file.  Because the output generated from
@var{file} is discarded, the only effect of @samp{-i @var{file}} is to
make the macros defined in @var{file} available for use in the main
input.
@end table

@node Concept Index, Index, Invocation, Top
@unnumbered Concept Index
@printindex cp

@node Index,, Concept Index, Top
@unnumbered Index of Commands, Macros and Switches
@printindex fn

@contents
@bye