Net2/usr/src/usr.bin/gcc/doc/cpp.info-1

Info file cpp.info, produced by Makeinfo, -*- Text -*- from input
file cpp.texinfo.

This file documents the GNU C Preprocessor.

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

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

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

File: cpp.info, Node: Top, Next: Global Actions, Up: (DIR)

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 preprocessor commands,
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::.

* Menu:

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

File: cpp.info, Node: Global Actions, Next: Commands, Prev: Top, Up: Top

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

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

* 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 preprocessor commands 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' command 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.info, Node: Commands, Next: Header Files, Prev: Global Actions, Up: Top

Preprocessor Commands
=====================

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

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

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

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

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

The `#' and the command 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 preprocessor command.

File: cpp.info, Node: Header Files, Next: Macros, Prev: Commands, Up: Top

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 preprocessor command `#include'.

* Menu:

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

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

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 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'.

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

The `#include' Command
----------------------

Both user and system header files are included using the preprocessor
command `#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
command 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'
command 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.

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

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

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

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

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

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

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

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

File: cpp.info, Node: Once-Only, Prev: Include Operation, Up: Header Files

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; its name should begin with `__', and should
contain the name of the file to avoid accidental conflicts.

One drawback of this method is that the preprocessor must scan the
input file completely in order to determine that all of it is to be
ignored. This makes compilation slower. You can avoid the delay by
inserting the following command near the beginning of file *in
addition to the conditionals described above*:

#pragma once

This command tells the GNU C preprocessor to ignore any future
commands to include the same file (whichever file the `#pragma'
appears in).

You should not *rely* on `#pragma once' to prevent multiple inclusion
of the file. It is just a hint, and a nonstandard one at that. Most
C compilers will ignore it entirely. For this reason, you still need
the conditionals if you want to make certain that the file's contents
are not included twice.

Note that `#pragma once' works by file name; if a file has more than
one name, it can be included once under each name, even in GNU CC,
despite `#pragma once'.

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

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.info, Node: Simple Macros, Next: Argument Macros, Prev: Macros, Up: Macros

Simple Macros
-------------

A "simple macro" is a kind of abbreviation. It is a name which
stands for a fragment of code.

Before you can use a macro, you must "define" it explicitly with the
`#define' command. `#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'. Therefore, if somewhere after this `#define' command 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 definition must be a single line; however, it may not end in the
middle of a multi-line string constant or character constant.

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

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

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

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

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.info, Node: Argument Macros, Next: Predefined, Prev: Simple Macros, Up: Macros

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.

To define a macro that uses arguments, you write a `#define' command
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; thus, 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

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

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

If you use the macro name followed by something other than an
open-parenthesis (after ignoring any spaces, tabs and comments that
follow), it is not a call to the macro, and the preprocessor leaves
the name unaltered. 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'.

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

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

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.info, Node: Standard Predefined, Next: Nonstandard Predefined, Prev: Predefined, Up: Predefined

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.

`__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.

`__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' command 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' command, 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' command 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 `"Jan 29 1987"' or
`"Apr 1 1905"'.

`__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.)

`__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.

`__STRICT_ANSI__'
This macro is defined 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.

`__VERSION__'
This macro expands to a string which describes the version
number of GNU C. The string is normally a sequence of decimal
numbers separated by periods, such as `"1.18"'. The only
reasonable use of this macro is to incorporate it into a string
constant.

`__OPTIMIZE__'
This macro is defined in optimizing compilations. It causes
certain GNU header files to define alternative macro definitions
for some system library functions. It is unwise to 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__'
This macro is defined if and only if the data type `char' is
unsigned on the target machine. It exists to cause the standard
header file `limit.h' to work correctly. It is bad practice to
refer to this macro yourself; instead, refer to the standard
macros defined in `limit.h'.

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

Nonstandard Predefined Macros
.............................

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

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

`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' that are
less specific.

`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 is to 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 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"

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

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; *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
`EXP''s actual argument 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.info, Node: Concatenation, Next: Undefining, Prev: Stringification, Up: Macros

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.info, Node: Undefining, Next: Redefining, Prev: Concatenation, Up: Macros

Undefining Macros
-----------------

To "undefine" a macro means to cancel its definition. This is done
with the `#undef' command. `#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' command will cancel definitions with
arguments or definitions that don't expect arguments. The `#undef'
command has no effect when used on a name not currently defined as a
macro.

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

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.info, Node: Macro Pitfalls, Prev: Redefining, Up: Macros

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.

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

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.info, Node: Macro Parentheses, Next: Swallow Semicolon, Prev: Misnesting, Up: Macro Pitfalls

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''s are needed to hold a certain number of
`char''s.) 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.

However, 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.info, Node: Swallow Semicolon, Next: Side Effects, Prev: Macro Parentheses, Up: Macro Pitfalls

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.info, Node: Side Effects, Next: Self-Reference, Prev: Swallow Semicolon, Up: Macro Pitfalls

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 I assume that `foo' returns type `int').