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\input texinfo

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:

o+ Inclusion of header files. These are files of de-
clarations that can be substituted into your pro-

o+ Macro expansion. You can define macros, which are
abbreviations for arbitrary fragments of C code,
and then the C preprocessor will replace the mac-
ros with their definitions throughout the program.

o+ Conditional compilation. Using special preproces-
sor commands, you can include or exclude parts of
the program according to various conditions.

o+ 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 incom-
patibility 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 Stan-
dard C make it undesirable to do this. See section Invoca-

1.1. Transformations Made Globally

Most C preprocessor features are inactive unless you
give specific commands to request their use. (Preprocessor
commands are lines starting with `#'; see section

2 The C Preprocessor

Commands). But there are three transformations that the
preprocessor always makes on all the input it receives, even
in the absence of commands.

o+ All C comments are replaced with single spaces.

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

o+ Predefined macro names are replaced with their ex-
pansions (see section 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 cosmet-
ically with Backslash-Newline anywhere (except when tri-
graphs are in use; see below).

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

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


This behavior is unclean: in all other contexts, a Backslash
can be inserted in a string constant as an ordinary charac-
ter 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 transforma-

o+ C comments and predefined macro names are not
recognized inside a `#include' command in which

The C Preprocessor 3

the file name is delimited with `<' and `>'.

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

o+ 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 in-
side, 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.
See section Invocation.

1.2. 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 can-
not define new preprocessor commands.

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

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 signifi-
cant 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

4 The C Preprocessor

expanding to `define', that does not make `#foo' a valid
preprocessor command.

1.3. Header Files

A header file is a file containing C declarations and
macro definitions (see section 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'.

1.3.1. Uses of Header Files

Header files serve two kinds of purposes.

o+ System header files declare the interfaces to
parts of the operating system. You include them
in your program to supply the definitions and de-
clarations you need to invoke system calls and li-

o+ Your own header files contain declarations for in-
terfaces 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'.

1.3.2. The `#include' Command

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

This variant is used for system header files. It
searches for a file named file in a list of direc-

The C Preprocessor 5

tories specified by you, then in a standard list
of system directories. You specify directories to
search for header files with the command option
`-I' (see section 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 ' 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 un-
likely 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 direc-
tories 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 in-
put file refers to. (If the `-I-' option is used,
the special treatment of the current directory is

The argument file may not contain `"' characters.
If backslashes occur within file, they are con-
sidered ordinary text characters, not escape char-
acters. None of the character escape sequences
appropriate to string constants in C are pro-
cessed. 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 (see section 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

6 The C Preprocessor

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 al-
low 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.

1.3.3. How `#include' Works

The `#include' command works by directing the C prepro-
cessor 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; those are merely the typical uses. Any
fragment of a C program can be included from another file.

The C Preprocessor 7

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 con-
stant 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.

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 waste-
ful. 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 `__'
to avoid conflicts with user programs, and it should contain
the name of the file and some additional text, to avoid con-
flicts 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 sub-
sequent `#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 command to tell the prepro-
cessor 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

8 The C Preprocessor

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.

`#include' 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.

1.4. 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 straightfor-

For example, suppose an application program uses the
system header file `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 would be con-
venient to define a ``local'' version
`/usr/local/include/sys/signal.h' to override or add to the
one supplied by the system.

You can do this by using the option `-I.' for compila-
tion, 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 ' 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 ' would find the
proper file, but that is not clean, since it makes an
assumption about where the system header file is found.

The C Preprocessor 9

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 the
directive `#include_next', which means, ``Include the next
file with this name.'' This command 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.

Thus, suppose the list of directories to search con-
tains `/usr/local/include' and `/usr/include', and both
directories contain a file named `sys/signal.h'. Ordinary
`#include ' finds the file under
`/usr/local/include'. If that file contains `#include_next
', it starts searching after that directory,
and finds the file in `/usr/include'.

1.5. 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.

1.5.1. 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 expli-
citly 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

10 The C Preprocessor

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 charac-
ter 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 charac-
ter 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

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;

The C Preprocessor 11

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

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

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 expan-
sion is checked for more macro names. See section Cascaded

1.5.2. 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.

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. See section 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

12 The C Preprocessor

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 argu-
ments 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.

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 contin-
ues. 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)))

The C Preprocessor 13

: (c))

(Line breaks shown here for clarity would not actually be

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 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:

14 The C Preprocessor

#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.

1.5.3. 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. 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 exten-

This macro expands to the name of the current in-
put file, in the form of a C string constant. The
precise name returned is the one that was speci-
fied in `#include' or as the input file name argu-

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.

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

The C Preprocessor 15

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 con-
tained 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 pro-
cessing moves to the line after the `#in-

The expansions of both `__FILE__' and
`__LINE__' are altered if a `#line' command is
used. See section Combining Sources.

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"'

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"'.

This macro expands to the constant 1, to sig-
nify that this is ANSI Standard C. (Whether
that is actually true depends on what C com-
piler will operate on the output from the

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 unde-

This macro is defined if and only if the `-
ansi' switch was specified when GNU C was in-

16 The C Preprocessor

voked. Its definition is the null string.
This macro exists primarily to direct certain
GNU header files not to define certain tradi-
tional Unix constructs which are incompatible
with ANSI C.

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 incor-
porate it into a string constant.

This macro is defined in optimizing compila-
tions. 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.

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 prac-
tice to refer to this macro yourself; instead,
refer to the standard macros defined in
`limit.h'. The preprocessor uses this macro
to determine whether or not to sign-extend
large character constants written in octal;
see `#if Command,,The `#if' Command'. Nonstandard Predefined Macros

The C preprocessor normally has several predefined mac-
ros 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 section Invocation.

Some nonstandard predefined macros describe the operat-
ing system in use, with more or less specificity. For exam-

`unix' is normally predefined on all Unix systems.

The C Preprocessor 17

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

`m68k' is also predefined on most computers whose
CPU is a 68000, 68010 or 68020; however, some mak-
ers 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' has been observed to be predefined on
some systems that use 68020 CPUs---in addition to
`mc68000' and `m68k', which are less specific.


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

`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 comput-

pyr `pyr' is predefined on all models of Pyramid com-

`sequent' is predefined on all models of Sequent

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'

18 The C Preprocessor

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 sup-
plied 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 com-
pile 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 pur-
pose, 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 con-
taining `-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'.

1.5.4. 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 argu-
ment 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 `#'.

The C Preprocessor 19

Here is an example of a macro definition that uses

#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 section 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 con-
stants 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 double-
quote 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

20 The C Preprocessor

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 sin-
gle space in the stringified result.

1.5.5. 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. Specifi-
cally, 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 func-
tion, 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

The C Preprocessor 21

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 concaten-
ate 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 lexi-
cal 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 charac-
ter. Also, you can freely use comments next to a `##' in a
macro definition, or in actual arguments that will be con-
catenated, because the comments will be converted to spaces
at first sight, and concatenation will later discard the

1.5.6. Undefining Macros

To undefine a macro means to cancel its definition.
This is done with the `#undef' command. `#undef' is fol-
lowed 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.

22 The C Preprocessor

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 defini-
tions with arguments or definitions that don't expect argu-
ments. The `#undef' command has no effect when used on a
name not currently defined as a macro.

1.5.7. 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 (see section 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 preproces-
sor. However, sometimes it is useful to change the defini-
tion 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:

o+ Whitespace may be added or deleted at the begin-
ning or the end.

The C Preprocessor 23

o+ Whitespace may be changed in the middle (but not
inside strings). However, it may not be eliminat-
ed entirely, and it may not be added where there
was no whitespace at all.

Recall that a comment counts as whitespace.

1.5.8. 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. 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

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

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

24 The C Preprocessor Unintended Grouping of Arithmetic

You may have noticed that in most of the macro defini-
tion examples shown above, each occurrence of a macro argu-
ment name had parentheses around it. In addition, another
pair of parentheses usually surround the entire macro defin-
ition. 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

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)

The C Preprocessor 25

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 dif-
ferent. 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)) Swallowing the Semicolon

Often it is desirable to define a macro that expands
into a compound statement. Consider, for example, the fol-
lowing 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 defini-
tion, 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

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 minim-
izes confusion if you can use it like a function call, writ-
ing a semicolon afterward, as in `SKIP_SPACES (p, lim);'

26 The C Preprocessor

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' state-
ment. 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. 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));

The C Preprocessor 27

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

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'). Self-Referential Macros

A self-referential macro is one whose name appears in
its definition. A special feature of ANSI Standard C is

28 The C Preprocessor

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)'. There-
fore, this macro definition has the possibly useful effect
of causing the program to add 4 to the value of `foo' wher-
ever `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 vari-
able `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 `y''s expansion 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 defini-
tion 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))'.

The C Preprocessor 29

It is not clear that this behavior would ever be use-
ful, but it is specified by the ANSI C standard, so you may
need to understand it. 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 (see section 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

30 The C Preprocessor


#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:

o+ Nested calls to a macro.

o+ Macros that call other macros that stringify or

o+ 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 can-
cels an undesirable side effect (in the medical, not compu-
tational, 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))


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

The C Preprocessor 31

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 state-
ments. 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 `({...})' con-
struct which turns a compound statement into an expression:

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

Or you can rewrite the macro definition to avoid such

#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

32 The C Preprocessor

forms the actual argument for `str'. `str' uses its argu-
ment without prescan because it performs stringification;
but it cannot prevent or undo the prescanning already done
by `xstr'. 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

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
#undef BUFSIZE
#define BUFSIZE 37

Now `TABLESIZE' expands (in two stages) to `37'.

1.6. Conditionals

In a macro processor, a conditional is a command that
allows a part of the program to be ignored during compila-
tion, on some conditions. In the C preprocessor, a condi-
tional 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 under-
stand 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

The C Preprocessor 33

from run to run, depending on the data it is operating on.
The condition in a preprocessor conditional command is
tested when your program is compiled. Its purpose is to
allow different code to be included in the program depending
on the situation at the time of compilation.

1.6.1. Why Conditionals are Used

Generally there are three kinds of reason to use a con-

o+ 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 sys-
tem may be erroneous on another operating system;
for example, it might refer to library routines
that do not exist on the other system. When this
happens, it is not enough to avoid executing the
invalid code: merely having it in the program
makes it impossible to link the program and run
it. With a preprocessor conditional, the offend-
ing code can be effectively excised from the pro-
gram when it is not valid.

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

o+ 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 preprocessor conditionals.

1.6.2. Syntax of Conditionals

A conditional in the C preprocessor begins with a con-
ditional command: `#if', `#ifdef' or `#ifndef'. See section
Conditionals-Macros, for information on `#ifdef' and
`#ifndef'; only `#if' is explained here. The `#if' Command

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

#if expression

34 The C Preprocessor

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

o+ Integer constants, which are all regarded as long
or unsigned long.

o+ Character constants, which are interpreted accord-
ing to the character set and conventions of the
machine and operating system on which the prepro-
cessor is running. The GNU C preprocessor uses
the C data type `char' for these character con-
stants; 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; oth-
erwise, no character code is considered negative.

o+ Arithmetic operators for addition, subtraction,
multiplication, division, bitwise operations,
shifts, comparisons, and `&&' and `||'.

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

o+ 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
preprocessor commands. Then the commands inside the condi-
tional are obeyed only if that branch of the conditional
succeeds. The text can also contain other conditional
groups. However, the `#if''s and `#endif''s must balance.

The C Preprocessor 35 The `#else' Command

The `#else' command 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
#else /* Not expression */
#endif /* Not expression */

If expression is nonzero, and the text-if-true is con-
sidered included, then `#else' acts like a failing condi-
tional and the text-if-false is ignored. Contrariwise, if
the `#if' conditional fails, the text-if-false is considered
included. The `#elif' Command

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 command, `#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*/

36 The C Preprocessor

`#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' command includes an expression to be

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

1.6.3. 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 refer-
ence, the easy way to do this is to put `#if 0' before it
and `#endif' after it.

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

1.6.4. Conditionals and Macros

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

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

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

The special operator `defined' may be 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

The C Preprocessor 37

`defined' operator it makes no difference what the defini-
tion of the macro is; all that matters is whether there is a
definition. Thus, for example,

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

would include the following code if either of the names
`vax' and `ns16000' is defined as a macro.

If a macro is defined and later undefined with
`#undef', subsequent use of the `defined' operator will
return 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 just the definedness of one name
are very common, so there are two special short conditional
commands for this case. They are

#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.

o+ 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.

o+ Many more macros are defined by system header
files. Different systems and machines define dif-
ferent 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.

o+ Macros are a common way of allowing users to cus-
tomize a program for different machines or appli-
cations. For example, the macro `BUFSIZE' might
be defined in a configuration file for your pro-
gram that is included as a header file in each
source file. You would use `BUFSIZE' in a prepro-
cessor conditional in order to generate different
code depending on the chosen configuration.

38 The C Preprocessor

o+ 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. See section

1.6.5. The `#error' and `#warning' Commands

The command `#error' causes the preprocessor to report
a fatal error. The rest of the line that follows `#error'
is used as the error message.

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

#ifdef vax
#error Won't work on Vaxen. See comments at get_last_object.

See section 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

The command `#warning' is like the command `#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.

The C Preprocessor 39

1.7. 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 preproces-
sor output to be different from the line's number in the
original source file. You will appreciate the value of mak-
ing both the C compiler (in error messages) and symbolic
debuggers such as GDB use the line numbers in your source

The C preprocessor builds on this feature by offering a
command by which you can control the feature explicitly.
This is useful when a file for input to the C preprocessor
is the output from another program such as the 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 output.

bison arranges this by writing `#line' commands into
the output file. `#line' is a command 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

#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 double-
quote characters so that it looks like a string

#line anything else
anything else is checked for macro calls, which

40 The C Preprocessor

are expanded. The result should be a decimal in-
teger constant followed optionally by a string
constant, as described above.

`#line' commands alter the results of the `__FILE__'
and `__LINE__' predefined macros from that point on. See
section Standard Predefined.

1.8. Miscellaneous Preprocessor Commands

This section describes three additional preprocessor
commands. They are not very useful, but are mentioned for

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

The ANSI standard specifies that the `#pragma' command
has an arbitrary, implementation-defined effect. In the GNU
C preprocessor, `#pragma' commands are ignored, except for
`#pragma once' (see section Once-Only).

The `#ident' command 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.

1.9. C Preprocessor Output

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

Source file name and line number information is con-
veyed by lines of the form

# linenum filename flag

The C Preprocessor 41

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.

The third field, flag, may be a number, or may be
absent. It is `1' for the beginning of a new source file,
and `2' for return to an old source file at the end of an
included file. It is absent otherwise.

1.10. 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 use-
ful individually.

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

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 compil-
ing a C program; they are passed along automatically to the
preprocessor when it is invoked by the compiler.

Inhibit generation of `#'-lines with line-number
information in the output from the preprocessor
(see section 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.

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.

Process ANSI standard trigraph sequences. These
are three-character sequences, all starting with
`??', that are defined by ANSI C to stand for sin-
gle characters. For example, `??/' stands for

42 The C Preprocessor

`\', so `'??/n'' is a character constant for a
newline. Strictly speaking, the GNU C preproces-
sor does not support all programs in ANSI Standard
C unless `-trigraphs' is used, but if you ever no-
tice the difference it will be with relief.

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

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

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

Warn if any trigraphs are encountered (assuming
they are enabled).

Warn whenever a comment-start sequence `/*' ap-
pears in a comment.

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

Warn about certain constructs that behave dif-
ferently in traditional and ANSI C.

`-I directory'
Add the directory directory to the end of the list
of directories to be searched for header files
(see section 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 direc-
tories. If you use more than one `-I' option, the
directories are scanned in left-to-right order;
the standard system directories come after.

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

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

The C Preprocessor 43

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

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.

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

`-D name=definition'
Predefine name as a macro, with definition defini-
tion. There are no restrictions on the contents
of definition, but if you are invoking the prepro-
cessor 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

`-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.

Do not predefine any nonstandard macros.

Instead of outputting the result of preprocessing,
output a list of `#define' commands for all the
macros defined during the execution of the prepro-
cessor, 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.

44 The C Preprocessor

Like `-dM' except in two respects: it does not
include the predefined macros, and it outputs
both the `#define' commands and the result of
preprocessing. Both kinds of output go to the
standard output file.

Instead of outputting the result of prepro-
cessing, 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.

This feature is used in automatic updating of

Like `-M' but mention only the files included
with `#include "file"'. System header files
included with `#include ' are omitted.

Like `-M' but the dependency information is
written to files with names made by replacing
`.c' with `.d' at the end of the input file
names. This is in addition to compiling the
file as specified---`-MD' does not inhibit or-
dinary compilation the way `-M' does.

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

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

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

`-imacros file'
Process file as input, discarding the result-
ing output, before processing the regular in-
put 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.

The C Preprocessor 45

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




Specify the source language. `-lang-c++'
makes the preprocessor handle C++ comment syn-
tax, and includes extra default include direc-
tories for C++, and `-lang-objc' enables the
Objective C `#import' directive. `-lang-c'
explicitly turns off both of these extensions,
and `-lang-objc++' enables both.

These options are generated by the compiler
driver gcc, but not passed from the `gcc' com-
mand line.

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 NO-

This option is available only when you call
cpp directly; gcc will not pass it from its
command line.

Forbid the use of `$' in identifiers. This is
required for ANSI conformance. gcc automati-
cally supplies this option to the preprocessor
if you specify `-ansi', but gcc doesn't recog-
nize the `-$' option itself---to use it
without the other effects of `-ansi', you must
call the preprocessor directly.

Concept Index

Index of Commands, Macros and Options

The C Preprocessor i

Table of Contents

1 The C Preprocessor ............................ 1
1.1 Transformations Made Globally ................ 1
1.2 Preprocessor Commands ........................ 3
1.3 Header Files ................................. 4
1.3.1 Uses of Header Files ......................... 4
1.3.2 The `#include' Command ....................... 4
1.3.3 How `#include' Works ......................... 6
1.3.4 Once-Only Include Files ...................... 7
1.4 Inheritance and Header Files ................. 8
1.5 Macros ....................................... 9
1.5.1 Simple Macros ................................ 9
1.5.2 Macros with Arguments ........................ 11
1.5.3 Predefined Macros ............................ 14 Standard Predefined Macros ................... 14 Nonstandard Predefined Macros ................ 16
1.5.4 Stringification .............................. 18
1.5.5 Concatenation ................................ 20
1.5.6 Undefining Macros ............................ 21
1.5.7 Redefining Macros ............................ 22
1.5.8 Pitfalls and Subtleties of Macros ............ 23 Improperly Nested Constructs ................. 23 Unintended Grouping of Arithmetic ............ 24 Swallowing the Semicolon ..................... 25 Duplication of Side Effects .................. 26 Self-Referential Macros ...................... 27 Separate Expansion of Macro Arguments ........ 29 Cascaded Use of Macros ....................... 32
1.6 Conditionals ................................. 32
1.6.1 Why Conditionals are Used .................... 33
1.6.2 Syntax of Conditionals ....................... 33 The `#if' Command ............................ 33 The `#else' Command .......................... 35 The `#elif' Command .......................... 35
1.6.3 Keeping Deleted Code for Future Reference
.................................................. 36
1.6.4 Conditionals and Macros ...................... 36
1.6.5 The `#error' and `#warning' Commands ......... 38
1.7 Combining Source Files ....................... 39
1.8 Miscellaneous Preprocessor Commands .......... 40
1.9 C Preprocessor Output ........................ 40
1.10 Invoking the C Preprocessor ................. 41
Concept Index ......................................... 45
Index of Commands, Macros and Options ................. 45

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