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INFO-DIR-SECTION Programming
START-INFO-DIR-ENTRY
* Cpp: (cpp).		       The GNU C preprocessor.
END-INFO-DIR-ENTRY

   This file documents the GNU C Preprocessor.

   Copyright 1987, 1989, 1991, 1992, 1993, 1994, 1995, 1997, 1998 Free
Software Foundation, Inc.

   Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
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   Permission is granted to copy and distribute modified versions of
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that the entire resulting derived work is distributed under the terms
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   Permission is granted to copy and distribute translations of this
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File: cpp,  Node: Top,  Next: Global Actions,  Up: (DIR)

1 The C Preprocessor
********************

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

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

   * Inclusion of header files.  These are files of declarations that
     can be substituted into your program.

   * Macro expansion.  You can define "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.

   * Conditional compilation.  Using special preprocessing directives,
     you can include or exclude parts of the program according to
     various conditions.

   * 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.

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

   ANSI Standard C requires the rejection of many harmless constructs
commonly used by today's C programs.  Such incompatibility would be
inconvenient for users, so the GNU C preprocessor is configured to
accept these constructs by default.  Strictly speaking, to get ANSI
Standard C, you must use the options `-trigraphs', `-undef' and
`-pedantic', but in practice the consequences of having strict ANSI
Standard C make it undesirable to do this.  *Note Invocation::.

   The C preprocessor is designed for C-like languages; you may run into
problems if you apply it to other kinds of languages, because it assumes
that it is dealing with C.  For example, the C preprocessor sometimes
outputs extra white space to avoid inadvertent C token concatenation,
and this may cause problems with other languages.

* Menu:

* Global Actions::    Actions made uniformly on all input files.
* Directives::        General syntax of preprocessing directives.
* 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 Directives::  Miscellaneous preprocessing directives.
* Output::            Format of output from the C preprocessor.
* Invocation::        How to invoke the preprocessor; command options.
* Concept Index::     Index of concepts and terms.
* Index::             Index of directives, predefined macros and options.


File: cpp,  Node: Global Actions,  Next: Directives,  Prev: Top,  Up: Top

1.1 Transformations Made Globally
=================================

   Most C preprocessor features are inactive unless you give specific
directives to request their use.  (Preprocessing directives are lines
starting with `#'; *note Directives::).  But there are three
transformations that the preprocessor always makes on all the input it
receives, even in the absence of directives.

   * All C comments are replaced with single spaces.

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

   * Predefined macro names are replaced with their expansions (*note
     Predefined::).

   The first two transformations are done _before_ nearly all other
parsing and before preprocessing directives are recognized.  Thus, for
example, you can split a line cosmetically with Backslash-Newline
anywhere (except when trigraphs are in use; see below).

     /*
     */ # /*
     */ defi\
     ne FO\
     O 10\
     20

is equivalent into `#define FOO 1020'.  You can split even an escape
sequence with Backslash-Newline.  For example, you can split `"foo\bar"'
between the `\' and the `b' to get

     "foo\\
     bar"

This behavior is unclean: in all other contexts, a Backslash can be
inserted in a string constant as an ordinary character by writing a
double Backslash, and this creates an exception.  But the ANSI C
standard requires it.  (Strict ANSI C does not allow Newlines in string
constants, so they do not consider this a problem.)

   But there are a few exceptions to all three transformations.

   * C comments and predefined macro names are not recognized inside a
     `#include' directive in which the file name is delimited with `<'
     and `>'.

   * 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.)

   * Backslash-Newline may not safely be used within an ANSI "trigraph".
     Trigraphs are converted before Backslash-Newline is deleted.  If
     you write what looks like a trigraph with a Backslash-Newline
     inside, the Backslash-Newline is deleted as usual, but it is then
     too late to recognize the trigraph.

     This exception is relevant only if you use the `-trigraphs' option
     to enable trigraph processing.  *Note Invocation::.


File: cpp,  Node: Directives,  Next: Header Files,  Prev: Global Actions,  Up: Top

1.2 Preprocessing Directives
============================

   Most preprocessor features are active only if you use preprocessing
directives to request their use.

   Preprocessing directives are lines in your program that start with
`#'.  The `#' is followed by an identifier that is the "directive name".
For example, `#define' is the directive that defines a macro.
Whitespace is also allowed before and after the `#'.

   The set of valid directive names is fixed.  Programs cannot define
new preprocessing directives.

   Some directive names require arguments; these make up the rest of
the directive line and must be separated from the directive name by
whitespace.  For example, `#define' must be followed by a macro name
and the intended expansion of the macro.  *Note Simple Macros::.

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

   The `#' and the directive name cannot come from a macro expansion.
For example, if `foo' is defined as a macro expanding to `define', that
does not make `#foo' a valid preprocessing directive.


File: cpp,  Node: Header Files,  Next: Macros,  Prev: Directives,  Up: Top

1.3 Header Files
================

   A header file is a file containing C declarations and macro
definitions (*note Macros::) to be shared between several source files.
You request the use of a header file in your program with the C
preprocessing directive `#include'.

* Menu:

* Header Uses::         What header files are used for.
* Include Syntax::      How to write `#include' directives.
* Include Operation::   What `#include' does.
* Once-Only::		Preventing multiple inclusion of one header file.
* Inheritance::         Including one header file in another header file.


File: cpp,  Node: Header Uses,  Next: Include Syntax,  Prev: Header Files,  Up: Header Files

1.3.1 Uses of Header Files
--------------------------

   Header files serve two kinds of purposes.

   * System header files declare the interfaces to parts of the
     operating system.  You include them in your program to supply the
     definitions and declarations you need to invoke system calls and
     libraries.

   * 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.

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

   The usual convention is to give header files names that end with
`.h'.  Avoid unusual characters in header file names, as they reduce
portability.


File: cpp,  Node: Include Syntax,  Next: Include Operation,  Prev: Header Uses,  Up: Header Files

1.3.2 The `#include' Directive
------------------------------

   Both user and system header files are included using the
preprocessing directive `#include'.  It has three variants:

`#include <FILE>'
     This variant is used for system header files.  It searches for a
     file named FILE in a list of directories specified by you, then in
     a standard list of system directories.  You specify directories to
     search for header files with the command option `-I' (*note
     Invocation::).  The option `-nostdinc' inhibits searching the
     standard system directories; in this case only the directories you
     specify are searched.

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

     The argument FILE may not contain a `>' character.  It may,
     however, contain a `<' character.

`#include "FILE"'
     This variant is used for header files of your own program.  It
     searches for a file named FILE first in the current directory,
     then in the same directories used for system header files.  The
     current directory is the directory of the current input file.  It
     is tried first because it is presumed to be the location of the
     files that the current input file refers to.  (If the `-I-' option
     is used, the special treatment of the current directory is
     inhibited.)

     The argument FILE may not contain `"' characters.  If backslashes
     occur within FILE, they are considered ordinary text characters,
     not escape characters.  None of the character escape sequences
     appropriate to string constants in C are processed.  Thus,
     `#include "x\n\\y"' specifies a filename containing three
     backslashes.  It is not clear why this behavior is ever useful, but
     the ANSI standard specifies it.

`#include ANYTHING ELSE'
     This variant is called a "computed #include".  Any `#include'
     directive whose argument does not fit the above two forms is a
     computed include.  The text ANYTHING ELSE is checked for macro
     calls, which are expanded (*note Macros::).  When this is done,
     the result must fit one of the above two variants--in particular,
     the expanded text must in the end be surrounded by either quotes
     or angle braces.

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


File: cpp,  Node: Include Operation,  Next: Once-Only,  Prev: Include Syntax,  Up: Header Files

1.3.3 How `#include' Works
--------------------------

   The `#include' directive works by directing the C preprocessor to
scan the specified file as input before continuing with the rest of the
current file.  The output from the preprocessor contains the output
already generated, followed by the output resulting from the included
file, followed by the output that comes from the text after the
`#include' directive.  For example, given a header file `header.h' as
follows,

     char *test ();

and a main program called `program.c' that uses the header file, like
this,

     int x;
     #include "header.h"
     
     main ()
     {
       printf (test ());
     }

the output generated by the C preprocessor for `program.c' as input
would be

     int x;
     char *test ();
     
     main ()
     {
       printf (test ());
     }

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

   It is possible for a header file to begin or end a syntactic unit
such as a function definition, but that would be very confusing, so
don't do it.

   The line following the `#include' directive is always treated as a
separate line by the C preprocessor even if the included file lacks a
final newline.


File: cpp,  Node: Once-Only,  Next: Inheritance,  Prev: Include Operation,  Up: Header Files

1.3.4 Once-Only Include Files
-----------------------------

   Very often, one header file includes another.  It can easily result
that a certain header file is included more than once.  This may lead
to errors, if the header file defines structure types or typedefs, and
is certainly wasteful.  Therefore, we often wish to prevent multiple
inclusion of a header file.

   The standard way to do this is to enclose the entire real contents
of the file in a conditional, like this:

     #ifndef FILE_FOO_SEEN
     #define FILE_FOO_SEEN
     
     THE ENTIRE FILE
     
     #endif /* FILE_FOO_SEEN */

   The macro `FILE_FOO_SEEN' indicates that the file has been included
once already.  In a user header file, the macro name should not begin
with `_'.  In a system header file, this name should begin with `__' to
avoid conflicts with user programs.  In any kind of header file, the
macro name should contain the name of the file and some additional
text, to avoid conflicts with other header files.

   The GNU C preprocessor is programmed to notice when a header file
uses this particular construct and handle it efficiently.  If a header
file is contained entirely in a `#ifndef' conditional, then it records
that fact.  If a subsequent `#include' specifies the same file, and the
macro in the `#ifndef' is already defined, then the file is entirely
skipped, without even reading it.

   There is also an explicit directive to tell the preprocessor that it
need not include a file more than once.  This is called `#pragma once',
and was used _in addition to_ the `#ifndef' conditional around the
contents of the header file.  `#pragma once' is now obsolete and should
not be used at all.

   In the Objective C language, there is a variant of `#include' called
`#import' which includes a file, but does so at most once.  If you use
`#import' _instead of_ `#include', then you don't need the conditionals
inside the header file to prevent multiple execution of the contents.

   `#import' is obsolete because it is not a well designed feature.  It
requires the users of a header file--the applications programmers--to
know that a certain header file should only be included once.  It is
much better for the header file's implementor to write the file so that
users don't need to know this.  Using `#ifndef' accomplishes this goal.


File: cpp,  Node: Inheritance,  Prev: Once-Only,  Up: Header Files

1.3.5 Inheritance and Header Files
----------------------------------

   "Inheritance" is what happens when one object or file derives some
of its contents by virtual copying from another object or file.  In the
case of C header files, inheritance means that one header file includes
another header file and then replaces or adds something.

   If the inheriting header file and the base header file have different
names, then inheritance is straightforward: simply write `#include
"BASE"' in the inheriting file.

   Sometimes it is necessary to give the inheriting file the same name
as the base file.  This is less straightforward.

   For example, suppose an application program uses the system header
`sys/signal.h', but the version of `/usr/include/sys/signal.h' on a
particular system doesn't do what the application program expects.  It
might be convenient to define a "local" version, perhaps under the name
`/usr/local/include/sys/signal.h', to override or add to the one
supplied by the system.

   You can do this by compiling with the option `-I.', and writing a
file `sys/signal.h' that does what the application program expects.
But making this file include the standard `sys/signal.h' is not so
easy--writing `#include <sys/signal.h>' in that file doesn't work,
because it includes your own version of the file, not the standard
system version.  Used in that file itself, this leads to an infinite
recursion and a fatal error in compilation.

   `#include </usr/include/sys/signal.h>' would find the proper file,
but that is not clean, since it makes an assumption about where the
system header file is found.  This is bad for maintenance, since it
means that any change in where the system's header files are kept
requires a change somewhere else.

   The clean way to solve this problem is to use `#include_next', which
means, "Include the _next_ file with this name."  This directive works
like `#include' except in searching for the specified file: it starts
searching the list of header file directories _after_ the directory in
which the current file was found.

   Suppose you specify `-I /usr/local/include', and the list of
directories to search also includes `/usr/include'; and suppose both
directories contain `sys/signal.h'.  Ordinary `#include <sys/signal.h>'
finds the file under `/usr/local/include'.  If that file contains
`#include_next <sys/signal.h>', it starts searching after that
directory, and finds the file in `/usr/include'.


File: cpp,  Node: Macros,  Next: Conditionals,  Prev: Header Files,  Up: Top

1.4 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.


File: cpp,  Node: Simple Macros,  Next: Argument Macros,  Prev: Macros,  Up: Macros

1.4.1 Simple Macros
-------------------

   A "simple macro" is a kind of abbreviation.  It is a name which
stands for a fragment of code.  Some people refer to these as "manifest
constants".

   Before you can use a macro, you must "define" it explicitly with the
`#define' directive.  `#define' is followed by the name of the macro
and then the code it should be an abbreviation for.  For example,

     #define BUFFER_SIZE 1020

defines a macro named `BUFFER_SIZE' as an abbreviation for the text
`1020'.  If somewhere after this `#define' directive there comes a C
statement of the form

     foo = (char *) xmalloc (BUFFER_SIZE);

then the C preprocessor will recognize and "expand" the macro
`BUFFER_SIZE', resulting in

     foo = (char *) xmalloc (1020);

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

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

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

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

     foo = X;
     #define X 4
     bar = X;

produces as output

     foo = X;
     
     bar = 4;

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

     #define BUFSIZE 1020
     #define TABLESIZE BUFSIZE

the name `TABLESIZE' when used in the program would go through two
stages of expansion, resulting ultimately in `1020'.

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


File: cpp,  Node: Argument Macros,  Next: Predefined,  Prev: Simple Macros,  Up: Macros

1.4.2 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 "arguments".
Arguments are fragments of code that you supply each time the macro is
used.  These fragments are included in the expansion of the macro
according to the directions in the macro definition.  A macro that
accepts arguments is called a "function-like macro" because the syntax
for using it looks like a function call.

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

   For example, here is a macro that computes the minimum of two numeric
values, as it is defined in many C programs:

     #define min(X, Y)  ((X) < (Y) ? (X) : (Y))

(This is not the best way to define a "minimum" macro in GNU C.  *Note
Side Effects::, for more information.)

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

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

     ((1) < (2) ? (1) : (2))

where `1' has been substituted for `X' and `2' for `Y'.

   Likewise, `min (x + 28, *p)' expands into

     ((x + 28) < (*p) ? (x + 28) : (*p))

   Parentheses in the actual arguments must balance; a comma within
parentheses does not end an argument.  However, there is no requirement
for brackets or braces to balance, and they do not prevent a comma from
separating arguments.  Thus,

     macro (array[x = y, x + 1])

passes two arguments to `macro': `array[x = y' and `x + 1]'.  If you
want to supply `array[x = y, x + 1]' as an argument, you must write it
as `array[(x = y, x + 1)]', which is equivalent C code.

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

     ((((a) < (b) ? (a) : (b))) < (c)
      ? (((a) < (b) ? (a) : (b)))
      : (c))

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

   If a macro `foo' takes one argument, and you want to supply an empty
argument, you must write at least some whitespace between the
parentheses, like this: `foo ( )'.  Just `foo ()' is providing no
arguments, which is an error if `foo' expects an argument.  But `foo0
()' is the correct way to call a macro defined to take zero arguments,
like this:

     #define foo0() ...

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

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

   You may not define the same name as both a simple macro and a macro
with arguments.

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

     #define FOO(x) - 1 / (x)

(which defines `FOO' to take an argument and expand into minus the
reciprocal of that argument) or this:

     #define BAR (x) - 1 / (x)

(which defines `BAR' to take no argument and always expand into `(x) -
1 / (x)').

   Note that the _uses_ of a macro with arguments can have spaces before
the left parenthesis; it's the _definition_ where it matters whether
there is a space.


File: cpp,  Node: Predefined,  Next: Stringification,  Prev: Argument Macros,  Up: Macros

1.4.3 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.


File: cpp,  Node: Standard Predefined,  Next: Nonstandard Predefined,  Prev: Predefined,  Up: Predefined

1.4.3.1 Standard Predefined Macros
..................................

   The standard predefined macros are available with the same meanings
regardless of the machine or operating system on which you are using
GNU C.  Their names all start and end with double underscores.  Those
preceding `__GNUC__' in this table are standardized by ANSI C; the rest
are GNU C extensions.

`__FILE__'
     This macro expands to the name of the current input file, in the
     form of a C string constant.  The precise name returned is the one
     that was specified in `#include' or as the input file name
     argument.

`__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 `__FILE__' are useful in generating an error message to
     report an inconsistency detected by the program; the message can
     state the source line at which the inconsistency was detected.
     For example,

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

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

     The expansions of both `__FILE__' and `__LINE__' are altered if a
     `#line' directive is used.  *Note Combining Sources::.

`__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 `"Feb  1 1996"'.

`__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 `"23:59:01"'.

`__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.)

     On some hosts, system include files use a different convention,
     where `__STDC__' is normally 0, but is 1 if the user specifies
     strict conformance to the C Standard.  The preprocessor follows
     the host convention when processing system include files, but when
     processing user files it follows the usual GNU C convention.

     This macro is not defined if the `-traditional' option is used.

`__STDC_VERSION__'
     This macro expands to the C Standard's version number, a long
     integer constant of the form `YYYYMML' where YYYY and MM are the
     year and month of the Standard version.  This signifies which
     version of the C Standard the preprocessor conforms to.  Like
     `__STDC__', whether this version number is accurate for the entire
     implementation depends on what C compiler will operate on the
     output from the preprocessor.

     This macro is not defined if the `-traditional' option is used.

`__GNUC__'
     This macro is defined if and only if this is GNU C.  This macro is
     defined only when the entire GNU C compiler is in use; if you
     invoke the preprocessor directly, `__GNUC__' is undefined.  The
     value identifies the major version number of GNU CC (`1' for GNU CC
     version 1, which is now obsolete, and `2' for version 2).

`__GNUC_MINOR__'
     The macro contains the minor version number of the compiler.  This
     can be used to work around differences between different releases
     of the compiler (for example, if gcc 2.6.3 is known to support a
     feature, you can test for `__GNUC__ > 2 || (__GNUC__ == 2 &&
     __GNUC_MINOR__ >= 6)').  The last number, `3' in the example
     above, denotes the bugfix level of the compiler; no macro contains
     this value.

`__GNUG__'
     The GNU C compiler defines this when the compilation language is
     C++; use `__GNUG__' to distinguish between GNU C and GNU C++.

`__cplusplus'
     The draft ANSI standard for C++ used to require predefining this
     variable.  Though it is no longer required, GNU C++ continues to
     define it, as do other popular C++ compilers.  You can use
     `__cplusplus' to test whether a header is compiled by a C compiler
     or a C++ compiler.

`__STRICT_ANSI__'
     GNU C defines this macro if and only if the `-ansi' switch was
     specified when GNU C was invoked.  Its definition is the null
     string.  This macro exists primarily to direct certain GNU header
     files not to define certain traditional Unix constructs which are
     incompatible with ANSI C.

`__BASE_FILE__'
     This macro expands to the name of the main input file, in the form
     of a C string constant.  This is the source file that was specified
     as an argument when the C compiler was invoked.

`__INCLUDE_LEVEL__'
     This macro expands to a decimal integer constant that represents
     the depth of nesting in include files.  The value of this macro is
     incremented on every `#include' directive and decremented at every
     end of file.  For input files specified by command line arguments,
     the nesting level is zero.

`__VERSION__'
     This macro expands to a string constant which describes the
     version number of GNU C.  The string is normally a sequence of
     decimal numbers separated by periods, such as `"2.6.0"'.

`__OPTIMIZE__'
     GNU CC defines this macro in optimizing compilations.  It causes
     certain GNU header files to define alternative macro definitions
     for some system library functions.  You should not refer to or
     test the definition of this macro unless you make very sure that
     programs will execute with the same effect regardless.

`__CHAR_UNSIGNED__'
     GNU C defines this macro if and only if the data type `char' is
     unsigned on the target machine.  It exists to cause the standard
     header file `limits.h' to work correctly.  You should not refer to
     this macro yourself; instead, refer to the standard macros defined
     in `limits.h'.  The preprocessor uses this macro to determine
     whether or not to sign-extend large character constants written in
     octal; see *Note The `#if' Directive: #if Directive.

`__REGISTER_PREFIX__'
     This macro expands to a string (not a string constant) describing
     the prefix applied to CPU registers in assembler code.  You can
     use it to write assembler code that is usable in multiple
     environments.  For example, in the `m68k-aout' environment it
     expands to the null string, but in the `m68k-coff' environment it
     expands to the string `%'.

`__USER_LABEL_PREFIX__'
     Similar to `__REGISTER_PREFIX__', but describes the prefix applied
     to user generated labels in assembler code.  For example, in the
     `m68k-aout' environment it expands to the string `_', but in the
     `m68k-coff' environment it expands to the null string.  This does
     not work with the `-mno-underscores' option that the i386 OSF/rose
     and m88k targets provide nor with the `-mcall*' options of the
     rs6000 System V Release 4 target.


File: cpp,  Node: Nonstandard Predefined,  Prev: Standard Predefined,  Up: Predefined

1.4.3.2 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.  You can use `cpp -dM' to see the
values of predefined macros; see *Note Invocation::.

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

`unix'
     `unix' is normally predefined on all Unix systems.

`BSD'
     `BSD' is predefined on recent versions of Berkeley Unix (perhaps
     only in version 4.3).

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

`vax'
     `vax' is predefined on Vax computers.

`mc68000'
     `mc68000' is predefined on most computers whose CPU is a Motorola
     68000, 68010 or 68020.

`m68k'
     `m68k' is also predefined on most computers whose CPU is a 68000,
     68010 or 68020; however, some makers use `mc68000' and some use
     `m68k'.  Some predefine both names.  What happens in GNU C depends
     on the system you are using it on.

`M68020'
     `M68020' has been observed to be predefined on some systems that
     use 68020 CPUs--in addition to `mc68000' and `m68k', which are
     less specific.

`_AM29K'
`_AM29000'
     Both `_AM29K' and `_AM29000' are predefined for the AMD 29000 CPU
     family.

`ns32000'
     `ns32000' is predefined on computers which use the National
     Semiconductor 32000 series CPU.

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

`sun'
     `sun' is predefined on all models of Sun computers.

`pyr'
     `pyr' is predefined on all models of Pyramid computers.

`sequent'
     `sequent' is predefined on all models of Sequent computers.

   These predefined symbols are not only nonstandard, they are contrary
to the ANSI standard because their names do not start with underscores.
Therefore, the option `-ansi' inhibits the definition of these symbols.

   This tends to make `-ansi' useless, since many programs depend on the
customary nonstandard predefined symbols.  Even system header files
check them and will generate incorrect declarations if they do not find
the names that are expected.  You might think that the header files
supplied for the Uglix computer would not need to test what machine
they are running on, because they can simply assume it is the Uglix;
but often they do, and they do so using the customary names.  As a
result, very few C programs will compile with `-ansi'.  We intend to
avoid such problems on the GNU system.

   What, then, should you do in an ANSI C program to test the type of
machine it will run on?

   GNU C offers a parallel series of symbols for this purpose, whose
names are made from the customary ones by adding `__' at the beginning
and end.  Thus, the symbol `__vax__' would be available on a Vax, and
so on.

   The set of nonstandard predefined names in the GNU C preprocessor is
controlled (when `cpp' is itself compiled) by the macro
`CPP_PREDEFINES', which should be a string containing `-D' options,
separated by spaces.  For example, on the Sun 3, we use the following
definition:

     #define CPP_PREDEFINES "-Dmc68000 -Dsun -Dunix -Dm68k"

This macro is usually specified in `tm.h'.


File: cpp,  Node: Stringification,  Next: Concatenation,  Prev: Predefined,  Up: Macros

1.4.4 Stringification
---------------------

   "Stringification" means turning a code fragment into a string
constant whose contents are the text for the code fragment.  For
example, stringifying `foo (z)' results in `"foo (z)"'.

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

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

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

Here the actual argument for `EXP' is substituted once as given, into
the `if' statement, and once as stringified, into the argument to
`fprintf'.  The `do' and `while (0)' are a kludge to make it possible
to write `WARN_IF (ARG);', which the resemblance of `WARN_IF' to a
function would make C programmers want to do; see *Note Swallow
Semicolon::.

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

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

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

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

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

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


File: cpp,  Node: Concatenation,  Next: Undefining,  Prev: Stringification,  Up: Macros

1.4.5 Concatenation
-------------------

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

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

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

     struct command
     {
       char *name;
       void (*function) ();
     };
     
     struct command commands[] =
     {
       { "quit", quit_command},
       { "help", help_command},
       ...
     };

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

     #define COMMAND(NAME)  { #NAME, NAME ## _command }
     
     struct command commands[] =
     {
       COMMAND (quit),
       COMMAND (help),
       ...
     };

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

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


File: cpp,  Node: Undefining,  Next: Redefining,  Prev: Concatenation,  Up: Macros

1.4.6 Undefining Macros
-----------------------

   To "undefine" a macro means to cancel its definition.  This is done
with the `#undef' directive.  `#undef' is followed by the macro name to
be undefined.

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

   For example,

     #define FOO 4
     x = FOO;
     #undef FOO
     x = FOO;

expands into

     x = 4;
     
     x = FOO;

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

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


File: cpp,  Node: Redefining,  Next: Macro Pitfalls,  Prev: Undefining,  Up: Macros

1.4.7 Redefining Macros
-----------------------

   "Redefining" a macro means defining (with `#define') a name that is
already defined as a macro.

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

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

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

   * Whitespace may be added or deleted at the beginning or the end.

   * 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.

   Recall that a comment counts as whitespace.


File: cpp,  Node: Macro Pitfalls,  Prev: Redefining,  Up: Macros

1.4.8 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.
* Newlines in Args::  Sometimes line numbers get confused.


File: cpp,  Node: Misnesting,  Next: Macro Parentheses,  Prev: Macro Pitfalls,  Up: Macro Pitfalls

1.4.8.1 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,

     #define double(x) (2*(x))
     #define call_with_1(x) x(1)

would expand `call_with_1 (double)' into `(2*(1))'.

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

     #define strange(file) fprintf (file, "%s %d",
     ...
     strange(stderr) p, 35)

This bizarre example expands to `fprintf (stderr, "%s %d", p, 35)'!


File: cpp,  Node: Macro Parentheses,  Next: Swallow Semicolon,  Prev: Misnesting,  Up: Macro Pitfalls

1.4.8.2 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,

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

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

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

This expands into

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

which does not do what is intended.  The operator-precedence rules of C
make it equivalent to this:

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

But what we want is this:

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

Defining the macro as

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

provides the desired result.

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

     sizeof ((1) + (2) - 1) / (2)

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

   Parentheses around the entire macro definition can prevent such
problems.  Here, then, is the recommended way to define `ceil_div':

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


File: cpp,  Node: Swallow Semicolon,  Next: Side Effects,  Prev: Macro Parentheses,  Up: Macro Pitfalls

1.4.8.3 Swallowing the Semicolon
................................

   Often it is desirable to define a macro that expands into a compound
statement.  Consider, for example, the following macro, that advances a
pointer (the argument `p' says where to find it) across whitespace
characters:

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

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

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

   But this can cause trouble before `else' statements, because the
semicolon is actually a null statement.  Suppose you write

     if (*p != 0)
       SKIP_SPACES (p, lim);
     else ...

The presence of two statements--the compound statement and a null
statement--in between the `if' condition and the `else' makes invalid C
code.

   The definition of the macro `SKIP_SPACES' can be altered to solve
this problem, using a `do ... while' statement.  Here is how:

     #define SKIP_SPACES(p, limit)     \
     do { register char *lim = (limit); \
          while (p != lim) {            \
            if (*p++ != ' ') {          \
              p--; break; }}}           \
     while (0)

   Now `SKIP_SPACES (p, lim);' expands into

     do {...} while (0);

which is one statement.


File: cpp,  Node: Side Effects,  Next: Self-Reference,  Prev: Swallow Semicolon,  Up: Macro Pitfalls

1.4.8.4 Duplication of Side Effects
...................................

   Many C programs define a macro `min', for "minimum", like this:

     #define min(X, Y)  ((X) < (Y) ? (X) : (Y))

   When you use this macro with an argument containing a side effect,
as shown here,

     next = min (x + y, foo (z));

it expands as follows:

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

where `x + y' has been substituted for `X' and `foo (z)' for `Y'.

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

   The best solution to this problem is to define `min' in a way that
computes the value of `foo (z)' only once.  The C language offers no
standard way to do this, but it can be done with GNU C extensions as
follows:

     #define min(X, Y)                     \
     ({ typeof (X) __x = (X), __y = (Y);   \
        (__x < __y) ? __x : __y; })

   If you do not wish to use GNU C extensions, the only solution is to
be careful when _using_ the macro `min'.  For example, you can
calculate the value of `foo (z)', save it in a variable, and use that
variable in `min':

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

(where we assume that `foo' returns type `int').


File: cpp,  Node: Self-Reference,  Next: Argument Prescan,  Prev: Side Effects,  Up: Macro Pitfalls

1.4.8.5 Self-Referential Macros
...............................

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

   Let's consider an example:

     #define foo (4 + foo)

where `foo' is also a variable in your program.

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

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

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

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

     #define x (4 + y)
     #define y (2 * x)

`x' would expand into `(4 + (2 * x))'.  Clear?

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

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


File: cpp,  Node: Argument Prescan,  Next: Cascaded Macros,  Prev: Self-Reference,  Up: Macro Pitfalls

1.4.8.6 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 _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
(*note Self-Reference::): the self-referential macro would be expanded
once in the first scan, and a second time in the second scan.  But this
is not what happens.  The self-references that do not expand in the
first scan are marked so that they will not expand in the second scan
either.

   The prescan is not done when an argument is stringified or
concatenated.  Thus,

     #define str(s) #s
     #define foo 4
     str (foo)

expands to `"foo"'.  Once more, prescan has been prevented from having
any noticeable effect.

   More precisely, stringification and concatenation use the argument as
written, in un-prescanned form.  The same actual argument would be used
in prescanned form if it is substituted elsewhere without
stringification or concatenation.

     #define str(s) #s lose(s)
     #define foo 4
     str (foo)

   expands to `"foo" lose(4)'.

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

   * Nested calls to a macro.

   * Macros that call other macros that stringify or concatenate.

   * Macros whose expansions contain unshielded commas.

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

   But prescan causes trouble in certain other cases of nested macro
calls.  Here is an example:

     #define foo  a,b
     #define bar(x) lose(x)
     #define lose(x) (1 + (x))
     
     bar(foo)

We would like `bar(foo)' to turn into `(1 + (foo))', which would then
turn into `(1 + (a,b))'.  But instead, `bar(foo)' expands into
`lose(a,b)', and you get an error because `lose' requires a single
argument.  In this case, the problem is easily solved by the same
parentheses that ought to be used to prevent misnesting of arithmetic
operations:

     #define foo (a,b)
     #define bar(x) lose((x))

   The problem is more serious when the operands of the macro are not
expressions; for example, when they are statements.  Then parentheses
are unacceptable because they would make for invalid C code:

     #define foo { int a, b; ... }

In GNU C you can shield the commas using the `({...})' construct which
turns a compound statement into an expression:

     #define foo ({ int a, b; ... })

   Or you can rewrite the macro definition to avoid such commas:

     #define foo { int a; int b; ... }

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

     #define xstr(s) str(s)
     #define str(s) #s
     #define foo 4
     xstr (foo)

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


File: cpp,  Node: Cascaded Macros,  Next: Newlines in Args,  Prev: Argument Prescan,  Up: Macro Pitfalls

1.4.8.7 Cascaded Use of Macros
..............................

   A "cascade" of macros is when one macro's body contains a reference
to another macro.  This is very common practice.  For example,

     #define BUFSIZE 1020
     #define TABLESIZE BUFSIZE

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

   It's only when you _use_ `TABLESIZE' that the result of its expansion
is checked for more macro names.

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

     #define BUFSIZE 1020
     #define TABLESIZE BUFSIZE
     #undef BUFSIZE
     #define BUFSIZE 37

Now `TABLESIZE' expands (in two stages) to `37'.  (The `#undef' is to
prevent any warning about the nontrivial redefinition of `BUFSIZE'.)


File: cpp,  Node: Newlines in Args,  Prev: Cascaded Macros,  Up: Macro Pitfalls

1.4.9 Newlines in Macro Arguments
---------------------------------

   Traditional macro processing carries forward all newlines in macro
arguments into the expansion of the macro.  This means that, if some of
the arguments are substituted more than once, or not at all, or out of
order, newlines can be duplicated, lost, or moved around within the
expansion.  If the expansion consists of multiple statements, then the
effect is to distort the line numbers of some of these statements.  The
result can be incorrect line numbers, in error messages or displayed in
a debugger.

   The GNU C preprocessor operating in ANSI C mode adjusts appropriately
for multiple use of an argument--the first use expands all the
newlines, and subsequent uses of the same argument produce no newlines.
But even in this mode, it can produce incorrect line numbering if
arguments are used out of order, or not used at all.

   Here is an example illustrating this problem:

     #define ignore_second_arg(a,b,c) a; c
     
     ignore_second_arg (foo (),
                        ignored (),
                        syntax error);

The syntax error triggered by the tokens `syntax error' results in an
error message citing line four, even though the statement text comes
from line five.


File: cpp,  Node: Conditionals,  Next: Combining Sources,  Prev: Macros,  Up: Top

1.5 Conditionals
================

   In a macro processor, a "conditional" is a directive that allows a
part of the program to be ignored during compilation, on some
conditions.  In the C preprocessor, a conditional can test either an
arithmetic expression or whether a name is defined as a macro.

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

* 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.
* Assertions::		        How and why to use assertions.
* Errors: #error Directive.     Detecting inconsistent compilation parameters.


File: cpp,  Node: Conditional Uses,  Next: Conditional Syntax,  Up: Conditionals

1.5.1 Why Conditionals are Used
-------------------------------

   Generally there are three kinds of reason to use a conditional.

   * 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
     preprocessing conditional, the offending code can be effectively
     excised from the program when it is not valid.

   * 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, or prints the values of those data for
     debugging, while the other does not.

   * 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.

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


File: cpp,  Node: Conditional Syntax,  Next: Deleted Code,  Prev: Conditional Uses,  Up: Conditionals

1.5.2 Syntax of Conditionals
----------------------------

   A conditional in the C preprocessor begins with a "conditional
directive": `#if', `#ifdef' or `#ifndef'.  *Note Conditionals-Macros::,
for information on `#ifdef' and `#ifndef'; only `#if' is explained here.

* Menu:

* If: #if Directive.     Basic conditionals using `#if' and `#endif'.
* Else: #else Directive. Including some text if the condition fails.
* Elif: #elif Directive. Testing several alternative possibilities.


File: cpp,  Node: #if Directive,  Next: #else Directive,  Up: Conditional Syntax

1.5.2.1 The `#if' Directive
...........................

   The `#if' directive in its simplest form consists of

     #if EXPRESSION
     CONTROLLED TEXT
     #endif /* EXPRESSION */

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

   EXPRESSION is a C expression of integer type, subject to stringent
restrictions.  It may contain

   * Integer constants, which are all regarded as `long' or `unsigned
     long'.

   * 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 `char' for these character constants; therefore,
     whether some character codes are negative is determined by the C
     compiler used to compile the preprocessor.  If it treats `char' as
     signed, then character codes large enough to set the sign bit will
     be considered negative; otherwise, no character code is considered
     negative.

   * Arithmetic operators for addition, subtraction, multiplication,
     division, bitwise operations, shifts, comparisons, and logical
     operations (`&&' and `||').

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

   * Macro calls.  All macro calls in the expression are expanded before
     actual computation of the expression's value begins.

   Note that `sizeof' operators and `enum'-type values are not allowed.
`enum'-type values, like all other identifiers that are not taken as
macro calls and expanded, are treated as zero.

   The CONTROLLED TEXT inside of a conditional can include
preprocessing directives.  Then the directives inside the conditional
are obeyed only if that branch of the conditional succeeds.  The text
can also contain other conditional groups.  However, the `#if' and
`#endif' directives must balance.


File: cpp,  Node: #else Directive,  Next: #elif Directive,  Prev: #if Directive,  Up: Conditional Syntax

1.5.2.2 The `#else' Directive
.............................

   The `#else' directive can be added to a conditional to provide
alternative text to be used if the condition is false.  This is what it
looks like:

     #if EXPRESSION
     TEXT-IF-TRUE
     #else /* Not EXPRESSION */
     TEXT-IF-FALSE
     #endif /* Not EXPRESSION */

   If EXPRESSION is nonzero, and thus the TEXT-IF-TRUE is active, then
`#else' acts like a failing conditional and the TEXT-IF-FALSE is
ignored.  Contrariwise, if the `#if' conditional fails, the
TEXT-IF-FALSE is considered included.


File: cpp,  Node: #elif Directive,  Prev: #else Directive,  Up: Conditional Syntax

1.5.2.3 The `#elif' Directive
.............................

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

     #if X == 1
     ...
     #else /* X != 1 */
     #if X == 2
     ...
     #else /* X != 2 */
     ...
     #endif /* X != 2 */
     #endif /* X != 1 */

   Another conditional directive, `#elif', allows this to be abbreviated
as follows:

     #if X == 1
     ...
     #elif X == 2
     ...
     #else /* X != 2 and X != 1*/
     ...
     #endif /* X != 2 and X != 1*/

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

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


File: cpp,  Node: Deleted Code,  Next: Conditionals-Macros,  Prev: Conditional Syntax,  Up: Conditionals

1.5.3 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 `#if 0' before it and `#endif' after it.  This is better
than using comment delimiters `/*' and `*/' since those won't work if
the code already contains comments (C comments do not nest).

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

   Conversely, do not use `#if 0' for comments which are not C code.
Use the comment delimiters `/*' and `*/' instead.  The interior of `#if
0' must consist of complete tokens; in particular, singlequote
characters must balance.  But comments often contain unbalanced
singlequote characters (known in English as apostrophes).  These
confuse `#if 0'.  They do not confuse `/*'.


File: cpp,  Node: Conditionals-Macros,  Next: Assertions,  Prev: Deleted Code,  Up: Conditionals

1.5.4 Conditionals and Macros
-----------------------------

   Conditionals are useful in connection with macros or assertions,
because those are the only ways that an expression's value can vary
from one compilation to another.  A `#if' directive whose expression
uses no macros or assertions is equivalent to `#if 1' or `#if 0'; you
might as well determine which one, by computing the value of the
expression yourself, and then simplify the program.

   For example, here is a conditional that tests the expression
`BUFSIZE == 1020', where `BUFSIZE' must be a macro.

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

   (Programmers often wish they could test the size of a variable or
data type in `#if', but this does not work.  The preprocessor does not
understand `sizeof', or typedef names, or even the type keywords such
as `int'.)

   The special operator `defined' is used in `#if' expressions to test
whether a certain name is defined as a macro.  Either `defined NAME' or
`defined (NAME)' is an expression whose value is 1 if NAME is defined
as macro at the current point in the program, and 0 otherwise.  For the
`defined' operator it makes no difference what the definition of the
macro is; all that matters is whether there is a definition.  Thus, for
example,

     #if defined (vax) || defined (ns16000)

would succeed if either of the names `vax' and `ns16000' is defined as
a macro.  You can test the same condition using assertions (*note
Assertions::), like this:

     #if #cpu (vax) || #cpu (ns16000)

   If a macro is defined and later undefined with `#undef', subsequent
use of the `defined' operator returns 0, because the name is no longer
defined.  If the macro is defined again with another `#define',
`defined' will recommence returning 1.

   Conditionals that test whether just one name is defined are very
common, so there are two special short conditional directives for this
case.

`#ifdef NAME'
     is equivalent to `#if defined (NAME)'.

`#ifndef NAME'
     is equivalent to `#if ! defined (NAME)'.

   Macro definitions can vary between compilations for several reasons.

   * Some macros are predefined on each kind of machine.  For example,
     on a Vax, the name `vax' is a predefined macro.  On other
     machines, it would not be defined.

   * 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.

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

   * Macros can be defined or undefined with `-D' and `-U' command
     options when you compile the program.  You can arrange to compile
     the same source file into two different programs by choosing a
     macro name to specify which program you want, writing conditionals
     to test whether or how this macro is defined, and then controlling
     the state of the macro with compiler command options.  *Note
     Invocation::.

   Assertions are usually predefined, but can be defined with
preprocessor directives or command-line options.


File: cpp,  Node: Assertions,  Next: #error Directive,  Prev: Conditionals-Macros,  Up: Conditionals

1.5.5 Assertions
----------------

   "Assertions" are a more systematic alternative to macros in writing
conditionals to test what sort of computer or system the compiled
program will run on.  Assertions are usually predefined, but you can
define them with preprocessing directives or command-line options.

   The macros traditionally used to describe the type of target are not
classified in any way according to which question they answer; they may
indicate a hardware architecture, a particular hardware model, an
operating system, a particular version of an operating system, or
specific configuration options.  These are jumbled together in a single
namespace.  In contrast, each assertion consists of a named question and
an answer.  The question is usually called the "predicate".  An
assertion looks like this:

     #PREDICATE (ANSWER)

You must use a properly formed identifier for PREDICATE.  The value of
ANSWER can be any sequence of words; all characters are significant
except for leading and trailing whitespace, and differences in internal
whitespace sequences are ignored.  Thus, `x + y' is different from
`x+y' but equivalent to `x + y'.  `)' is not allowed in an answer.

   Here is a conditional to test whether the answer ANSWER is asserted
for the predicate PREDICATE:

     #if #PREDICATE (ANSWER)

There may be more than one answer asserted for a given predicate.  If
you omit the answer, you can test whether _any_ answer is asserted for
PREDICATE:

     #if #PREDICATE

   Most of the time, the assertions you test will be predefined
assertions.  GNU C provides three predefined predicates: `system',
`cpu', and `machine'.  `system' is for assertions about the type of
software, `cpu' describes the type of computer architecture, and
`machine' gives more information about the computer.  For example, on a
GNU system, the following assertions would be true:

     #system (gnu)
     #system (mach)
     #system (mach 3)
     #system (mach 3.SUBVERSION)
     #system (hurd)
     #system (hurd VERSION)

and perhaps others.  The alternatives with more or less version
information let you ask more or less detailed questions about the type
of system software.

   On a Unix system, you would find `#system (unix)' and perhaps one of:
`#system (aix)', `#system (bsd)', `#system (hpux)', `#system (lynx)',
`#system (mach)', `#system (posix)', `#system (svr3)', `#system
(svr4)', or `#system (xpg4)' with possible version numbers following.

   Other values for `system' are `#system (mvs)' and `#system (vms)'.

   *Portability note:* Many Unix C compilers provide only one answer
for the `system' assertion: `#system (unix)', if they support
assertions at all.  This is less than useful.

   An assertion with a multi-word answer is completely different from
several assertions with individual single-word answers.  For example,
the presence of `system (mach 3.0)' does not mean that `system (3.0)'
is true.  It also does not directly imply `system (mach)', but in GNU
C, that last will normally be asserted as well.

   The current list of possible assertion values for `cpu' is: `#cpu
(a29k)', `#cpu (alpha)', `#cpu (arm)', `#cpu (clipper)', `#cpu
(convex)', `#cpu (elxsi)', `#cpu (tron)', `#cpu (h8300)', `#cpu
(i370)', `#cpu (i386)', `#cpu (i860)', `#cpu (i960)', `#cpu (m68k)',
`#cpu (m88k)', `#cpu (mips)', `#cpu (ns32k)', `#cpu (hppa)', `#cpu
(pyr)', `#cpu (ibm032)', `#cpu (rs6000)', `#cpu (sh)', `#cpu (sparc)',
`#cpu (spur)', `#cpu (tahoe)', `#cpu (vax)', `#cpu (we32000)'.

   You can create assertions within a C program using `#assert', like
this:

     #assert PREDICATE (ANSWER)

(Note the absence of a `#' before PREDICATE.)

   Each time you do this, you assert a new true answer for PREDICATE.
Asserting one answer does not invalidate previously asserted answers;
they all remain true.  The only way to remove an assertion is with
`#unassert'.  `#unassert' has the same syntax as `#assert'.  You can
also remove all assertions about PREDICATE like this:

     #unassert PREDICATE

   You can also add or cancel assertions using command options when you
run `gcc' or `cpp'.  *Note Invocation::.


File: cpp,  Node: #error Directive,  Prev: Assertions,  Up: Conditionals

1.5.6 The `#error' and `#warning' Directives
--------------------------------------------

   The directive `#error' causes the preprocessor to report a fatal
error.  The rest of the line that follows `#error' is used as the error
message.  The line must consist of complete tokens.

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

     #ifdef __vax__
     #error "Won't work on Vaxen.  See comments at get_last_object."
     #endif

*Note Nonstandard Predefined::, for why this works.

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

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

   The directive `#warning' is like the directive `#error', but causes
the preprocessor to issue a warning and continue preprocessing.  The
rest of the line that follows `#warning' is used as the warning message.

   You might use `#warning' in obsolete header files, with a message
directing the user to the header file which should be used instead.


File: cpp,  Node: Combining Sources,  Next: Other Directives,  Prev: Conditionals,  Up: Top

1.6 Combining Source Files
==========================

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

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

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

   `bison' arranges this by writing `#line' directives into the output
file.  `#line' is a directive that specifies the original line number
and source file name for subsequent input in the current preprocessor
input file.  `#line' has three variants:

`#line LINENUM'
     Here LINENUM is a decimal integer constant.  This specifies that
     the line number of the following line of input, in its original
     source file, was LINENUM.

`#line LINENUM FILENAME'
     Here LINENUM is a decimal integer constant and FILENAME is a
     string constant.  This specifies that the following line of input
     came originally from source file FILENAME and its line number there
     was LINENUM.  Keep in mind that FILENAME is not just a file name;
     it is surrounded by doublequote characters so that it looks like a
     string constant.

`#line ANYTHING ELSE'
     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.

   `#line' directives alter the results of the `__FILE__' and
`__LINE__' predefined macros from that point on.  *Note Standard
Predefined::.

   The output of the preprocessor (which is the input for the rest of
the compiler) contains directives that look much like `#line'
directives.  They start with just `#' instead of `#line', but this is
followed by a line number and file name as in `#line'.  *Note Output::.


File: cpp,  Node: Other Directives,  Next: Output,  Prev: Combining Sources,  Up: Top

1.7 Miscellaneous Preprocessing Directives
==========================================

   This section describes three additional preprocessing directives.
They are not very useful, but are mentioned for completeness.

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

   The ANSI standard specifies that the effect of the `#pragma'
directive is implementation-defined.  In the GNU C preprocessor,
`#pragma' directives are not used, except for `#pragma once' (*note
Once-Only::).  However, they are left in the preprocessor output, so
they are available to the compilation pass.

   The `#ident' directive is supported for compatibility with certain
other systems.  It is followed by a line of text.  On some systems, the
text is copied into a special place in the object file; on most systems,
the text is ignored and this directive has no effect.  Typically
`#ident' is only used in header files supplied with those systems where
it is meaningful.


File: cpp,  Node: Output,  Next: Invocation,  Prev: Other Directives,  Up: Top

1.8 C Preprocessor Output
=========================

   The output from the C preprocessor looks much like the input, except
that all preprocessing directive lines have been replaced with blank
lines and all comments with spaces.  Whitespace within a line is not
altered; however, unless `-traditional' is used, spaces may be inserted
into the expansions of macro calls to prevent tokens from being
concatenated.

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

     # LINENUM FILENAME FLAGS

which are inserted as needed into the middle of the input (but never
within a string or character constant).  Such a line means that the
following line originated in file FILENAME at line LINENUM.

   After the file name comes zero or more flags, which are `1', `2',
`3', or `4'.  If there are multiple flags, spaces separate them.  Here
is what the flags mean:

`1'
     This indicates the start of a new file.

`2'
     This indicates returning to a file (after having included another
     file).

`3'
     This indicates that the following text comes from a system header
     file, so certain warnings should be suppressed.

`4'
     This indicates that the following text should be treated as C.


File: cpp,  Node: Invocation,  Next: Concept Index,  Prev: Output,  Up: Top

1.9 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 on its own.

   The C preprocessor expects two file names as arguments, INFILE and
OUTFILE.  The preprocessor reads INFILE together with any other files
it specifies with `#include'.  All the output generated by the combined
input files is written in OUTFILE.

   Either INFILE or OUTFILE may be `-', which as INFILE means to read
from standard input and as OUTFILE means to write to standard output.
Also, if OUTFILE or both file names are omitted, the standard output
and standard input are used for the omitted file names.

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

`-P'
     Inhibit generation of `#'-lines with line-number information in
     the output from the preprocessor (*note Output::).  This might be
     useful when running the preprocessor on something that is not C
     code and will be sent to a program which might be confused by the
     `#'-lines.

`-C'
     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.

`-traditional'
     Try to imitate the behavior of old-fashioned C, as opposed to ANSI
     C.

        * Traditional macro expansion pays no attention to singlequote
          or doublequote characters; macro argument symbols are
          replaced by the argument values even when they appear within
          apparent string or character constants.

        * 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.

        * However, traditionally the end of the line terminates a
          string or character constant, with no error.

        * In traditional C, a comment is equivalent to no text at all.
          (In ANSI C, a comment counts as whitespace.)

        * Traditional C does not have the concept of a "preprocessing
          number".  It considers `1.0e+4' to be three tokens: `1.0e',
          `+', and `4'.

        * A macro is not suppressed within its own definition, in
          traditional C.  Thus, any macro that is used recursively
          inevitably causes an error.

        * The character `#' has no special meaning within a macro
          definition in traditional C.

        * 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 ANSI C.)

        * Traditionally, `\' inside a macro argument suppresses the
          syntactic significance of the following character.

     Use the `-traditional' option when preprocessing Fortran code, so
     that singlequotes and doublequotes within Fortran comment lines
     (which are generally not recognized as such by the preprocessor)
     do not cause diagnostics about unterminated character or string
     constants.

     However, this option does not prevent diagnostics about
     unterminated comments when a C-style comment appears to start, but
     not end, within Fortran-style commentary.

     So, the following Fortran comment lines are accepted with
     `-traditional':

          C This isn't an unterminated character constant
          C Neither is "20000000000, an octal constant
          C in some dialects of Fortran

     However, this type of comment line will likely produce a
     diagnostic, or at least unexpected output from the preprocessor,
     due to the unterminated comment:

          C Some Fortran compilers accept /* as starting
          C an inline comment.

     Note that `g77' automatically supplies the `-traditional' option
     when it invokes the preprocessor.  However, a future version of
     `g77' might use a different, more-Fortran-aware preprocessor in
     place of `cpp'.

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

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

`-pedantic'
     Issue warnings required by the ANSI C standard in certain cases
     such as when text other than a comment follows `#else' or `#endif'.

`-pedantic-errors'
     Like `-pedantic', except that errors are produced rather than
     warnings.

`-Wtrigraphs'
     Warn if any trigraphs are encountered.  Currently this only works
     if you have turned trigraphs on with `-trigraphs' or `-ansi'; in
     the future this restriction will be removed.

`-Wcomment'
     Warn whenever a comment-start sequence `/*' appears in a `/*'
     comment, or whenever a Backslash-Newline appears in a `//' comment.

`-Wall'
     Requests both `-Wtrigraphs' and `-Wcomment' (but not
     `-Wtraditional' or `-Wundef').

`-Wtraditional'
     Warn about certain constructs that behave differently in
     traditional and ANSI C.

`-Wundef'
     Warn if an undefined identifier is evaluated in an `#if' directive.

`-I DIRECTORY'
     Add the directory DIRECTORY to the head of the list of directories
     to be searched for header files (*note Include Syntax::).  This
     can be used to override a system header file, substituting your
     own version, since these directories are searched before the system
     header file directories.  If you use more than one `-I' option,
     the directories are scanned in left-to-right order; the standard
     system directories come after.

`-I-'
     Any directories specified with `-I' options before the `-I-'
     option are searched only for the case of `#include "FILE"'; they
     are not searched for `#include <FILE>'.

     If additional directories are specified with `-I' options after
     the `-I-', these directories are searched for all `#include'
     directives.

     In addition, the `-I-' option inhibits the use of the current
     directory as the first search directory for `#include "FILE"'.
     Therefore, the current directory is searched only if it is
     requested explicitly with `-I.'.  Specifying both `-I-' and `-I.'
     allows you to control precisely which directories are searched
     before the current one and which are searched after.

`-nostdinc'
     Do not search the standard system directories for header files.
     Only the directories you have specified with `-I' options (and the
     current directory, if appropriate) are searched.

`-nostdinc++'
     Do not search for header files in the C++-specific standard
     directories, but do still search the other standard directories.
     (This option is used when building the C++ library.)

`-remap'
     When searching for a header file in a directory, remap file names
     if a file named `header.gcc' exists in that directory.  This can
     be used to work around limitations of file systems with file name
     restrictions.  The `header.gcc' file should contain a series of
     lines with two tokens on each line: the first token is the name to
     map, and the second token is the actual name to use.

`-D NAME'
     Predefine NAME as a macro, with definition `1'.

`-D NAME=DEFINITION'
     Predefine NAME as a macro, with definition DEFINITION.  There are
     no restrictions on the contents of 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.  If you
     use more than one `-D' for the same NAME, the rightmost definition
     takes effect.

`-U NAME'
     Do not predefine NAME.  If both `-U' and `-D' are specified for
     one name, the `-U' beats the `-D' and the name is not predefined.

`-undef'
     Do not predefine any nonstandard macros.

`-gcc'
     Define the macros __GNUC__ and __GNUC_MINOR__.  These are defined
     automatically when you use `gcc -E'; you can turn them off in that
     case with `-no-gcc'.

`-A PREDICATE(ANSWER)'
     Make an assertion with the predicate PREDICATE and answer ANSWER.
     *Note Assertions::.

     You can use `-A-' to disable all predefined assertions; it also
     undefines all predefined macros and all macros that preceded it on
     the command line.

`-dM'
     Instead of outputting the result of preprocessing, output a list of
     `#define' directives for all the macros defined during the
     execution of the preprocessor, including predefined macros.  This
     gives you a way of finding out what is predefined in your version
     of the preprocessor; assuming you have no file `foo.h', the command

          touch foo.h; cpp -dM foo.h

     will show the values of any predefined macros.

`-dD'
     Like `-dM' except in two respects: it does _not_ include the
     predefined macros, and it outputs _both_ the `#define' directives
     and the result of preprocessing.  Both kinds of output go to the
     standard output file.

`-dI'
     Output `#include' directives in addition to the result of
     preprocessing.

`-M [-MG]'
     Instead of outputting the result of preprocessing, output a rule
     suitable for `make' describing the dependencies of the main source
     file.  The preprocessor outputs one `make' rule containing the
     object file name for that source file, a colon, and the names of
     all the included files.  If there are many included files then the
     rule is split into several lines using `\'-newline.

     `-MG' says to treat missing header files as generated files and
     assume they live in the same directory as the source file.  It
     must be specified in addition to `-M'.

     This feature is used in automatic updating of makefiles.

`-MM [-MG]'
     Like `-M' but mention only the files included with `#include
     "FILE"'.  System header files included with `#include <FILE>' are
     omitted.

`-MD FILE'
     Like `-M' but the dependency information is written to FILE.  This
     is in addition to compiling the file as specified--`-MD' does not
     inhibit ordinary compilation the way `-M' does.

     When invoking `gcc', do not specify the FILE argument.  `gcc' will
     create file names made by replacing ".c" with ".d" at the end of
     the input file names.

     In Mach, you can use the utility `md' to merge multiple dependency
     files into a single dependency file suitable for using with the
     `make' command.

`-MMD FILE'
     Like `-MD' except mention only user header files, not system
     header files.

`-H'
     Print the name of each header file used, in addition to other
     normal activities.

`-imacros FILE'
     Process FILE as input, discarding the resulting output, before
     processing the regular input file.  Because the output generated
     from FILE is discarded, the only effect of `-imacros FILE' is to
     make the macros defined in FILE available for use in the main
     input.

`-include FILE'
     Process FILE as input, and include all the resulting output,
     before processing the regular input file.

`-idirafter DIR'
     Add the directory DIR to the second include path.  The directories
     on the second include path are searched when a header file is not
     found in any of the directories in the main include path (the one
     that `-I' adds to).

`-iprefix PREFIX'
     Specify PREFIX as the prefix for subsequent `-iwithprefix' options.

`-iwithprefix DIR'
     Add a directory to the second include path.  The directory's name
     is made by concatenating PREFIX and DIR, where PREFIX was
     specified previously with `-iprefix'.

`-isystem DIR'
     Add a directory to the beginning of the second include path,
     marking it as a system directory, so that it gets the same special
     treatment as is applied to the standard system directories.

`-x c'
`-x c++'
`-x objective-c'
`-x assembler-with-cpp'
     Specify the source language: C, C++, Objective-C, or assembly.
     This has nothing to do with standards conformance or extensions;
     it merely selects which base syntax to expect.  If you give none
     of these options, cpp will deduce the language from the extension
     of the source file: `.c', `.cc', `.m', or `.S'.  Some other common
     extensions for C++ and assembly are also recognized.  If cpp does
     not recognize the extension, it will treat the file as C; this is
     the most generic mode.

     *Note:* Previous versions of cpp accepted a `-lang' option which
     selected both the language and the standards conformance level.
     This option has been removed, because it conflicts with the `-l'
     option.

`-std=STANDARD'
`-ansi'
     Specify the standard to which the code should conform.  Currently
     cpp only knows about the standards for C; other language standards
     will be added in the future.

     STANDARD may be one of:
    `iso9899:1990'
          The ISO C standard from 1990.

    `iso9899:199409'
    `c89'
          The 1990 C standard, as amended in 1994.  `c89' is the
          customary shorthand for this version of the standard.

          The `-ansi' option is equivalent to `-std=c89'.

    `iso9899:199x'
    `c9x'
          The revised ISO C standard, which is expected to be
          promulgated some time in 1999.  It has not been approved yet,
          hence the `x'.

    `gnu89'
          The 1990 C standard plus GNU extensions.  This is the default.

    `gnu9x'
          The 199x C standard plus GNU extensions.

`-Wp,-lint'
     Look for commands to the program checker `lint' embedded in
     comments, and emit them preceded by `#pragma lint'.  For example,
     the comment `/* NOTREACHED */' becomes `#pragma lint NOTREACHED'.

     Because of the clash with `-l', you must use the awkward syntax
     above.  In a future release, this option will be replaced by
     `-flint' or `-Wlint'; we are not sure which yet.

`-$'
     Forbid the use of `$' in identifiers.  The C standard does not
     permit this, but it is a common extension.


File: cpp,  Node: Concept Index,  Next: Index,  Prev: Invocation,  Up: Top

Concept Index
*************

* Menu:

* ##:                                    Concatenation.
* arguments in macro definitions:        Argument Macros.
* assertions:                            Assertions.
* assertions, undoing:                   Assertions.
* blank macro arguments:                 Argument Macros.
* cascaded macros:                       Cascaded Macros.
* commenting out code:                   Deleted Code.
* computed #include:                     Include Syntax.
* concatenation:                         Concatenation.
* conditionals:                          Conditionals.
* directives:                            Directives.
* expansion of arguments:                Argument Prescan.
* Fortran:                               Invocation.
* function-like macro:                   Argument Macros.
* g77:                                   Invocation.
* header file:                           Header Files.
* including just once:                   Once-Only.
* inheritance:                           Inheritance.
* invocation of the preprocessor:        Invocation.
* line control:                          Combining Sources.
* macro argument expansion:              Argument Prescan.
* macro body uses macro:                 Cascaded Macros.
* macros with argument:                  Argument Macros.
* manifest constant:                     Simple Macros.
* newlines in macro arguments:           Newlines in Args.
* null directive:                        Other Directives.
* options:                               Invocation.
* output format:                         Output.
* overriding a header file:              Inheritance.
* parentheses in macro bodies:           Macro Parentheses.
* pitfalls of macros:                    Macro Pitfalls.
* predefined macros:                     Predefined.
* predicates:                            Assertions.
* preprocessing directives:              Directives.
* prescan of macro arguments:            Argument Prescan.
* problems with macros:                  Macro Pitfalls.
* redefining macros:                     Redefining.
* repeated inclusion:                    Once-Only.
* retracting assertions:                 Assertions.
* second include path:                   Invocation.
* self-reference:                        Self-Reference.
* semicolons (after macro calls):        Swallow Semicolon.
* side effects (in macro arguments):     Side Effects.
* simple macro:                          Simple Macros.
* space as macro argument:               Argument Macros.
* standard predefined macros:            Standard Predefined.
* stringification:                       Stringification.
* testing predicates:                    Assertions.
* unassert:                              Assertions.
* undefining macros:                     Undefining.
* unsafe macros:                         Side Effects.
* unterminated:                          Invocation.


File: cpp,  Node: Index,  Prev: Concept Index,  Up: Top

Index of Directives, Macros and Options
***************************************

* Menu:

* #assert:                               Assertions.
* #cpu:                                  Assertions.
* #define:                               Argument Macros.
* #elif:                                 #elif Directive.
* #else:                                 #else Directive.
* #error:                                #error Directive.
* #ident:                                Other Directives.
* #if:                                   Conditional Syntax.
* #ifdef:                                Conditionals-Macros.
* #ifndef:                               Conditionals-Macros.
* #import:                               Once-Only.
* #include:                              Include Syntax.
* #include_next:                         Inheritance.
* #line:                                 Combining Sources.
* #machine:                              Assertions.
* #pragma:                               Other Directives.
* #pragma once:                          Once-Only.
* #system:                               Assertions.
* #unassert:                             Assertions.
* #warning:                              #error Directive.
* -$:                                    Invocation.
* -A:                                    Invocation.
* -ansi:                                 Invocation.
* -C:                                    Invocation.
* -D:                                    Invocation.
* -dD:                                   Invocation.
* -dI:                                   Invocation.
* -dM:                                   Invocation.
* -gcc:                                  Invocation.
* -H:                                    Invocation.
* -I:                                    Invocation.
* -idirafter:                            Invocation.
* -imacros:                              Invocation.
* -include:                              Invocation.
* -iprefix:                              Invocation.
* -isystem:                              Invocation.
* -iwithprefix:                          Invocation.
* -lint:                                 Invocation.
* -M:                                    Invocation.
* -MD:                                   Invocation.
* -MM:                                   Invocation.
* -MMD:                                  Invocation.
* -nostdinc:                             Invocation.
* -nostdinc++:                           Invocation.
* -P:                                    Invocation.
* -pedantic:                             Invocation.
* -pedantic-errors:                      Invocation.
* -remap:                                Invocation.
* -std:                                  Invocation.
* -traditional:                          Invocation.
* -trigraphs:                            Invocation.
* -U:                                    Invocation.
* -undef:                                Invocation.
* -Wall:                                 Invocation.
* -Wcomment:                             Invocation.
* -Wtraditional:                         Invocation.
* -Wtrigraphs:                           Invocation.
* -Wundef:                               Invocation.
* -x assembler-with-cpp:                 Invocation.
* -x c:                                  Invocation.
* -x objective-c:                        Invocation.
* __BASE_FILE__:                         Standard Predefined.
* __CHAR_UNSIGNED__:                     Standard Predefined.
* __cplusplus:                           Standard Predefined.
* __DATE__:                              Standard Predefined.
* __FILE__:                              Standard Predefined.
* __GNUC__:                              Standard Predefined.
* __GNUC_MINOR__:                        Standard Predefined.
* __GNUG__:                              Standard Predefined.
* __INCLUDE_LEVEL_:                      Standard Predefined.
* __LINE__:                              Standard Predefined.
* __OPTIMIZE__:                          Standard Predefined.
* __REGISTER_PREFIX__:                   Standard Predefined.
* __STDC__:                              Standard Predefined.
* __STDC_VERSION__:                      Standard Predefined.
* __STRICT_ANSI__:                       Standard Predefined.
* __TIME__:                              Standard Predefined.
* __USER_LABEL_PREFIX__:                 Standard Predefined.
* __VERSION__:                           Standard Predefined.
* _AM29000:                              Nonstandard Predefined.
* _AM29K:                                Nonstandard Predefined.
* BSD:                                   Nonstandard Predefined.
* defined:                               Conditionals-Macros.
* M68020:                                Nonstandard Predefined.
* m68k:                                  Nonstandard Predefined.
* mc68000:                               Nonstandard Predefined.
* ns32000:                               Nonstandard Predefined.
* pyr:                                   Nonstandard Predefined.
* sequent:                               Nonstandard Predefined.
* sun:                                   Nonstandard Predefined.
* system header files:                   Header Uses.
* unix:                                  Nonstandard Predefined.
* vax:                                   Nonstandard Predefined.



Tag Table:
Node: Top920
Node: Global Actions3828
Node: Directives6349
Node: Header Files8039
Node: Header Uses8699
Node: Include Syntax10198
Node: Include Operation13345
Node: Once-Only15214
Node: Inheritance17646
Node: Macros20186
Node: Simple Macros21103
Node: Argument Macros24098
Node: Predefined29903
Node: Standard Predefined30340
Node: Nonstandard Predefined38009
Node: Stringification41596
Node: Concatenation44529
Node: Undefining47809
Node: Redefining48855
Node: Macro Pitfalls50161
Node: Misnesting51272
Node: Macro Parentheses52297
Node: Swallow Semicolon54176
Node: Side Effects56085
Node: Self-Reference57794
Node: Argument Prescan60081
Node: Cascaded Macros65093
Node: Newlines in Args66249
Node: Conditionals67601
Node: Conditional Uses68956
Node: Conditional Syntax70386
Node: #if Directive70979
Node: #else Directive73279
Node: #elif Directive73957
Node: Deleted Code75346
Node: Conditionals-Macros76414
Node: Assertions80105
Node: #error Directive84347
Node: Combining Sources85839
Node: Other Directives88753
Node: Output90209
Node: Invocation91521
Node: Concept Index106306
Node: Index109313

End Tag Table
