Red Hat ENTERPRISE LINUX 4 - USING BINUTILS Manual

Red Hat Enterprise Linux 4
Using cpp, the C Preprocessor

Red Hat Enterprise Linux 4: Using cpp, the C Preprocessor
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Table of Contents
1. Overview .......................................................................................................................................... 1
1.1. Character sets ..................................................................................................................... 1
1.2. Initial processing................................................................................................................ 2
1.3. Tokenization....................................................................................................................... 4
1.4. The preprocessing language............................................................................................... 6
2. Header Files..................................................................................................................................... 9
2.1. Include Syntax ................................................................................................................... 9
2.2. Include Operation............................................................................................................... 9
2.3. Search Path....................................................................................................................... 10
2.4. Once-Only Headers.......................................................................................................... 11
2.5. Computed Includes .......................................................................................................... 12
2.6. Wrapper Headers ............................................................................................................. 13
2.7. System Headers................................................................................................................ 13
3. Macros............................................................................................................................................ 15
3.1. Object-like Macros........................................................................................................... 15
3.2. Function-like Macros ....................................................................................................... 16
3.3. Macro Arguments ............................................................................................................ 17
3.4. Stringification................................................................................................................... 19
3.5. Concatenation .................................................................................................................. 20
3.6. Variadic Macros ............................................................................................................... 21
3.7. Predefined Macros ........................................................................................................... 23
3.7.1. Standard Predefined Macros ............................................................................. 23
3.7.2. Common Predefined Macros............................................................................. 25
3.7.3. System-specific Predefined Macros .................................................................. 27
3.7.4. C++ Named Operators ...................................................................................... 28
3.8. Undefining and Redefining Macros ................................................................................. 28
3.9. Directives Within Macro Arguments ............................................................................... 29
3.10. Macro Pitfalls................................................................................................................. 30
3.10.1. Misnesting....................................................................................................... 30
3.10.2. Operator Precedence Problems ....................................................................... 30
3.10.3. Swallowing the Semicolon.............................................................................. 31
3.10.4. Duplication of Side Effects............................................................................. 32
3.10.5. Self-Referential Macros .................................................................................. 33
3.10.6. Argument Prescan ........................................................................................... 34
3.10.7. Newlines in Arguments................................................................................... 35
4. Conditionals................................................................................................................................... 37
4.1. Conditional Uses.............................................................................................................. 37
4.2. Conditional Syntax .......................................................................................................... 37
4.2.1. Ifdef................................................................................................................... 37
4.2.2. If........................................................................................................................ 38
4.2.3. Defined.............................................................................................................. 39
4.2.4. Else.................................................................................................................... 40
4.2.5. Elif..................................................................................................................... 40
4.3. Deleted Code.................................................................................................................... 41

5. Diagnostics..................................................................................................................................... 43
6. Line Control .................................................................................................................................. 45
7. Pragmas ......................................................................................................................................... 47
8. Other Directives ............................................................................................................................ 49
9. Preprocessor Output..................................................................................................................... 51
10. Traditional Mode ........................................................................................................................ 53
10.1. Traditional lexical analysis ............................................................................................ 53
10.2. Traditional macros ......................................................................................................... 54
10.3. Traditional miscellany.................................................................................................... 55
10.4. Traditional warnings ...................................................................................................... 55
11. Implementation Details .............................................................................................................. 57
11.1. Implementation-defined behavior .................................................................................. 57
11.2. Implementation limits .................................................................................................... 58
11.3. Obsolete Features........................................................................................................... 58
11.3.1. Assertions........................................................................................................ 59
11.3.2. Obsolete once-only headers ............................................................................ 60
11.4. Differences from previous versions ............................................................................... 60
12. Invocation .................................................................................................................................... 63
13. Environment Variables ............................................................................................................... 73
14. GNU Free Documentation License............................................................................................ 75
14.1. ADDENDUM: How to use this License for your documents ....................................... 79
Index of Directives ............................................................................................................................ 81
Option Index...................................................................................................................................... 83
Concept Index.................................................................................................................................... 85

The C Preprocessor

Chapter 1.
Overview
The C preprocessor implements the macro language used to transform C, C++, and Objective-C pro-
grams before they are compiled. It can also be useful on its own.
The C preprocessor, often known as cpp, is a macro processor that is used automatically by the C
compiler to transform your program before compilation. It is called a macro processor because it
allows you to define macros, which are brief abbreviations for longer constructs.
The C preprocessor is intended to be used only with C, C++, and Objective-C source code. In the past,
it has been abused as a general text processor. It will choke on input which does not obey C’s lexical
rules. For example, apostrophes will be interpreted as the beginning of character constants, and cause
errors. Also, you cannot rely on it preserving characteristics of the input which are not significant to
C-family languages. If a Makefile is preprocessed, all the hard tabs will be removed, and the Makefile
will not work.
Having said that, you can often get away with using cpp on things which are not C. Other
Algol-ish programming languages are often safe (Pascal, Ada, etc.) So is assembly, with caution.
-traditional-cpp mode preserves more white space, and is otherwise more permissive. Many of
the problems can be avoided by writing C or C++ style comments instead of native language
comments, and keeping macros simple.
Wherever possible, you should use a preprocessor geared to the language you are writing in. Modern
versions of the GNU assembler have macro facilities. Most high level programming languages have
their own conditional compilation and inclusion mechanism. If all else fails, try a true general text
processor, such as GNU M4.
C preprocessors vary in some details. This manual discusses the GNU C preprocessor, which provides
a small superset of the features of ISO Standard C. In its default mode, the GNU C preprocessor does
not do a few things required by the standard. These are features which are rarely, if ever, used, and
may cause surprising changes to the meaning of a program which does not expect them. To get strict
ISO Standard C, you should use the -std=c89 or -std=c99 options, depending on which version of
the standard you want. To get all the mandatory diagnostics, you must also use -pedantic. Chapter
12 Invocation.
This manual describes the behavior of the ISO preprocessor. To minimize gratuitous differences,
where the ISO preprocessor’s behavior does not conflict with traditional semantics, the traditional
preprocessor should behave the same way. The various differences that do exist are detailed in the
section Chapter 10 Traditional Mode.
For clarity, unless noted otherwise, references to CPP in this manual refer to GNU CPP.

2 Chapter 1. Overview
1.1. Character sets
Source code character set processing in C and related languages is rather complicated. The C standard
discusses two character sets, but there are really at least four.
The files input to CPP might be in any character set at all. CPP’s very first action, before it even looks
for line boundaries, is to convert the file into the character set it uses for internal processing. That
set is what the C standard calls the source character set. It must be isomorphic with ISO 10646, also
known as Unicode. CPP uses the UTF-8 encoding of Unicode.
At present, GNU CPP does not implement conversion from arbitrary file encodings to the source
character set. Use of any encoding other than plain ASCII or UTF-8, except in comments, will cause
errors. Use of encodings that are not strict supersets of ASCII, such as Shift JIS, may cause errors
even if non-ASCII characters appear only in comments. We plan to fix this in the near future.
All preprocessing work (the subject of the rest of this manual) is carried out in the source character
set. If you request textual output from the preprocessor with the -E option, it will be in UTF-8.
After preprocessing is complete, string and character constants are converted again, into the execution
character set. This character set is under control of the user; the default is UTF-8, matching the source
character set. Wide string and character constants have their own character set, which is not called out
specifically in the standard. Again, it is under control of the user. The default is UTF-16 or UTF-32,
whichever fits in the target’s wchar_t type, in the target machine’s byte order.1Octal and hexadecimal
escape sequences do not undergo conversion; ’\x12’ has the value 0x12 regardless of the currently
selected execution character set. All other escapes are replaced by the character in the source character
set that they represent, then converted to the execution character set, just like unescaped characters.
GCC does not permit the use of characters outside the ASCII range, nor \u and \U escapes, in identi-
fiers. We hope this will change eventually, but there are problems with the standard semantics of such
"extended identifiers" which must be resolved through the ISO C and C++ committees first.
1.2. Initial processing
The preprocessor performs a series of textual transformations on its input. These happen before all
other processing. Conceptually, they happen in a rigid order, and the entire file is run through each
transformation before the next one begins. CPP actually does them all at once, for performance rea-
sons. These transformations correspond roughly to the first three "phases of translation" described in
the C standard.
1. The input file is read into memory and broken into lines.
Different systems use different conventions to indicate the end of a line. GCC accepts the ASCII
control sequences LF,CR LF and CR as end-of-line markers. These are the canonical sequences
used by Unix, DOS and VMS, and the classic Mac OS (before OSX) respectively. You may
therefore safely copy source code written on any of those systems to a different one and use
it without conversion. (GCC may lose track of the current line number if a file doesn’t consis-
tently use one convention, as sometimes happens when it is edited on computers with different
conventions that share a network file system.)
If the last line of any input file lacks an end-of-line marker, the end of the file is considered to
implicitly supply one. The C standard says that this condition provokes undefined behavior, so
GCC will emit a warning message.
2. If trigraphs are enabled, they are replaced by their corresponding single characters. By default
GCC ignores trigraphs, but if you request a strictly conforming mode with the -std option, or
you specify the -trigraphs option, then it converts them.
1. UTF-16 does not meet the requirements of the C standard for a wide character set, but the choice of 16-bit
wchar_t is enshrined in some system ABIs so we cannot fix this.

Chapter 1. Overview 3
These are nine three-character sequences, all starting with ??, that are defined by ISO C to stand
for single characters. They permit obsolete systems that lack some of C’s punctuation to use C.
For example, ??/ stands for \, so ’??/n’ is a character constant for a newline.
Trigraphs are not popular and many compilers implement them incorrectly. Portable code should
not rely on trigraphs being either converted or ignored. With -Wtrigraphs GCC will warn you
when a trigraph may change the meaning of your program if it were converted.
In a string constant, you can prevent a sequence of question marks from being confused with a
trigraph by inserting a backslash between the question marks, or by separating the string literal
at the trigraph and making use of string literal concatenation. "(??\?)" is the string (???), not
(?]. Traditional C compilers do not recognize these idioms.
The nine trigraphs and their replacements are
Trigraph: ??( ??) ??

??

??= ??/ ??’ ??! ??-
Replacement: [ ] { } # \ ^ | ~
3. Continued lines are merged into one long line.
A continued line is a line which ends with a backslash, \. The backslash is removed and the
following line is joined with the current one. No space is inserted, so you may split a line
anywhere, even in the middle of a word. (It is generally more readable to split lines only at
white space.)
The trailing backslash on a continued line is commonly referred to as a backslash-newline.
If there is white space between a backslash and the end of a line, that is still a continued line.
However, as this is usually the result of an editing mistake, and many compilers will not accept
it as a continued line, GCC will warn you about it.
4. All comments are replaced with single spaces.
There are two kinds of comments. Block comments begin with /* and continue until the next
*/. Block comments do not nest:
/* this is /* one comment */ text outside comment
Line comments begin with // and continue to the end of the current line. Line comments do not
nest either, but it does not matter, because they would end in the same place anyway.
// this is // one comment
text outside comment
It is safe to put line comments inside block comments, or vice versa.
/* block comment
// contains line comment
yet more comment
*/ outside comment
// line comment /* contains block comment */
But beware of commenting out one end of a block comment with a line comment.
// l.c. /* block comment begins
oops! this isn’t a comment anymore */

4 Chapter 1. Overview
Comments are not recognized within string literals. "/* blah */" is the string constant /* blah */,
not an empty string.
Line comments are not in the 1989 edition of the C standard, but they are recognized by GCC as an
extension. In C++ and in the 1999 edition of the C standard, they are an official part of the language.
Since these transformations happen before all other processing, you can split a line mechanically
with backslash-newline anywhere. You can comment out the end of a line. You can continue a line
comment onto the next line with backslash-newline. You can even split /*,*/, and // onto multiple
lines with backslash-newline. For example:
/\
*
*/ # /*
*/ defi\
ne FO\
O 10\
20
is equivalent to #define FOO 1020. All these tricks are extremely confusing and should not be used
in code intended to be readable.
There is no way to prevent a backslash at the end of a line from being interpreted as a backslash-
newline. This cannot affect any correct program, however.
1.3. Tokenization
After the textual transformations are finished, the input file is converted into a sequence of prepro-
cessing tokens. These mostly correspond to the syntactic tokens used by the C compiler, but there are
a few differences. White space separates tokens; it is not itself a token of any kind. Tokens do not have
to be separated by white space, but it is often necessary to avoid ambiguities.
When faced with a sequence of characters that has more than one possible tokenization, the prepro-
cessor is greedy. It always makes each token, starting from the left, as big as possible before moving
on to the next token. For instance, a+++++b is interpreted as a ++ ++ + b, not as a ++ + ++ b,
even though the latter tokenization could be part of a valid C program and the former could not.
Once the input file is broken into tokens, the token boundaries never change, except when the ##
preprocessing operator is used to paste tokens together. Section 3.5 Concatenation. For example,
#define foo() bar
foo()baz
==> bar baz
not
==> barbaz
The compiler does not re-tokenize the preprocessor’s output. Each preprocessing token becomes one
compiler token.
Preprocessing tokens fall into five broad classes: identifiers, preprocessing numbers, string literals,
punctuators, and other. An identifier is the same as an identifier in C: any sequence of letters, digits,
or underscores, which begins with a letter or underscore. Keywords of C have no significance to the
preprocessor; they are ordinary identifiers. You can define a macro whose name is a keyword, for

Chapter 1. Overview 5
instance. The only identifier which can be considered a preprocessing keyword is defined. Section
4.2.3 Defined.
This is mostly true of other languages which use the C preprocessor. However, a few of the keywords
of C++ are significant even in the preprocessor. Section 3.7.4 C++ Named Operators.
In the 1999 C standard, identifiers may contain letters which are not part of the "basic source charac-
ter set," at the implementation’s discretion (such as accented Latin letters, Greek letters, or Chinese
ideograms). This may be done with an extended character set, or the \u and \U escape sequences.
GCC does not presently implement either feature in the preprocessor or the compiler.
As an extension, GCC treats $as a letter. This is for compatibility with some systems, such as VMS,
where $is commonly used in system-defined function and object names. $is not a letter in strictly
conforming mode, or if you specify the -$ option. Chapter 12 Invocation.
Apreprocessing number has a rather bizarre definition. The category includes all the normal integer
and floating point constants one expects of C, but also a number of other things one might not initially
recognize as a number. Formally, preprocessing numbers begin with an optional period, a required
decimal digit, and then continue with any sequence of letters, digits, underscores, periods, and expo-
nents. Exponents are the two-character sequences e+,e-,E+,E-,p+,p-,P+, and P-. (The exponents
that begin with por Pare new to C99. They are used for hexadecimal floating-point constants.)
The purpose of this unusual definition is to isolate the preprocessor from the full complexity of nu-
meric constants. It does not have to distinguish between lexically valid and invalid floating-point
numbers, which is complicated. The definition also permits you to split an identifier at any position
and get exactly two tokens, which can then be pasted back together with the ## operator.
It’s possible for preprocessing numbers to cause programs to be misinterpreted. For example, 0xE+12
is a preprocessing number which does not translate to any valid numeric constant, therefore a syntax
error. It does not mean 0xE + 12, which is what you might have intended.
String literals are string constants, character constants, and header file names (the argument of
#include).2String constants and character constants are straightforward: ". . . " or ’. . . ’. In either
case embedded quotes should be escaped with a backslash: ’\” is the character constant for ’. There
is no limit on the length of a character constant, but the value of a character constant that contains
more than one character is implementation-defined. Chapter 11 Implementation Details.
Header file names either look like string constants, ". . . ", or are written with angle brackets instead,

...

. In either case, backslash is an ordinary character. There is no way to escape the closing quote
or angle bracket. The preprocessor looks for the header file in different places depending on which
form you use. Section 2.2 Include Operation.
No string literal may extend past the end of a line. Older versions of GCC accepted multi-line string
constants. You may use continued lines instead, or string constant concatenation. Section 11.4 Differ-
ences from previous versions.
Punctuators are all the usual bits of punctuation which are meaningful to C and C++. All but three of
the punctuation characters in ASCII are C punctuators. The exceptions are @,$, and ‘. In addition,
all the two- and three-character operators are punctuators. There are also six digraphs, which the C++
standard calls alternative tokens, which are merely alternate ways to spell other punctuators. This is
a second attempt to work around missing punctuation in obsolete systems. It has no negative side
effects, unlike trigraphs, but does not cover as much ground. The digraphs and their corresponding
normal punctuators are:
Digraph:

% %

: :

%: %:%:
Punctuator: { } [ ] # ##
2. The C standard uses the term string literal to refer only to what we are calling string constants.

6 Chapter 1. Overview
Any other single character is considered "other." It is passed on to the preprocessor’s output unmo-
lested. The C compiler will almost certainly reject source code containing "other" tokens. In ASCII,
the only other characters are @,$,‘, and control characters other than NUL (all bits zero). (Note that
$is normally considered a letter.) All characters with the high bit set (numeric range 0x7F-0xFF) are
also "other" in the present implementation. This will change when proper support for international
character sets is added to GCC.
NUL is a special case because of the high probability that its appearance is accidental, and because it
may be invisible to the user (many terminals do not display NUL at all). Within comments, NULs are
silently ignored, just as any other character would be. In running text, NUL is considered white space.
For example, these two directives have the same meaning.
#define X^@1
#define X 1
(where ^@ is ASCII NUL). Within string or character constants, NULs are preserved. In the latter two
cases the preprocessor emits a warning message.
1.4. The preprocessing language
After tokenization, the stream of tokens may simply be passed straight to the compiler’s parser. How-
ever, if it contains any operations in the preprocessing language, it will be transformed first. This stage
corresponds roughly to the standard’s "translation phase 4" and is what most people think of as the
preprocessor’s job.
The preprocessing language consists of directives to be executed and macros to be expanded. Its
primary capabilities are:
•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. The preprocessor will replace the macros with their definitions throughout the program. Some
macros are automatically defined for you.
•Conditional compilation. 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 where each source line
originally came from.
•Diagnostics. You can detect problems at compile time and issue errors or warnings.
There are a few more, less useful, features.
Except for expansion of predefined macros, all these operations are triggered with preprocessing di-
rectives. Preprocessing directives are lines in your program that start with #. Whitespace is allowed
before and after the #. The #is followed by an identifier, the directive name. It specifies the opera-
tion to perform. Directives are commonly referred to as #name where name is the directive name. For
example, #define is the directive that defines a macro.
The #which begins a directive cannot come from a macro expansion. Also, the directive name is not
macro expanded. Thus, if foo is defined as a macro expanding to define, that does not make #foo
a valid preprocessing directive.
The set of valid directive names is fixed. Programs cannot define new preprocessing directives.

Chapter 1. Overview 7
Some directives require arguments; these make up the rest of the directive line and must be separated
from the directive name by whitespace. For example, #define must be followed by a macro name
and the intended expansion of the macro.
A preprocessing directive cannot cover more than one line. The line may, however, be continued with
backslash-newline, or by a block comment which extends past the end of the line. In either case, when
the directive is processed, the continuations have already been merged with the first line to make one
long line.

8 Chapter 1. Overview

Chapter 2.
Header Files
A header file is a file containing C declarations and macro definitions (Chapter 3 Macros) to be shared
between several source files. You request the use of a header file in your program by including it, with
the C preprocessing directive #include.
Header files serve two purposes.
•System header files declare the interfaces to parts of the operating system. You include them in your
program to supply the definitions and declarations you need to invoke system calls and libraries.
•Your own header files contain declarations for interfaces between the source files of your program.
Each time you have a group of related declarations and macro definitions all or most of which are
needed in several different source files, it is a good idea to create a header file for them.
Including a header file produces the same results as copying the header file into each source file that
needs it. Such copying would be time-consuming and error-prone. With a header file, the related dec-
larations 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.
In C, the usual convention is to give header files names that end with .h. It is most portable to use
only letters, digits, dashes, and underscores in header file names, and at most one dot.
2.1. Include Syntax
Both user and system header files are included using the preprocessing directive #include. It has
two variants:
#include
 file 
This variant is used for system header files. It searches for a file named file in a standard list
of system directories. You can prepend directories to this list with the -I option (Chapter 12
Invocation).
#include "file"
This variant is used for header files of your own program. It searches for a file named file first
in the directory containing the current file, then in the same directories used for

file

.
The argument of #include, whether delimited with quote marks or angle brackets, behaves like
a string constant in that comments are not recognized, and macro names are not expanded. Thus,
#include

x/*y

specifies inclusion of a system header file named x/*y.
However, 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. (Some systems inter-
pret \as a pathname separator. All of these also interpret /the same way. It is most portable to use
only /.)
It is an error if there is anything (other than comments) on the line after the file name.

10 Chapter 2. Header Files
2.2. Include Operation
The #include directive works by directing the C preprocessor to scan the specified file as input
before continuing with the rest of the current file. The output from the preprocessor contains the
output already generated, followed by the output resulting from the included file, followed by the
output that comes from the text after the #include directive. For example, if you have a header file
header.h as follows,
char *test (void);
and a main program called program.c that uses the header file, like this,
int x;
#include "header.h"
int
main (void)
{
puts (test ());
}
the compiler will see the same token stream as it would if program.c read
int x;
char *test (void);
int
main (void)
{
puts (test ());
}
Included files are not limited to declarations and macro definitions; those are merely the typical uses.
Any fragment of a C program can be included from another file. The include file could even contain
the beginning of a statement that is concluded in the containing file, or the end of a statement that was
started in the including file. However, an included file must consist of complete tokens. Comments
and string literals which have not been closed by the end of an included file are invalid. For error
recovery, they are considered to end at the end of the file.
To avoid confusion, it is best if header files contain only complete syntactic units--function declara-
tions or definitions, type declarations, etc.
The line following the #include directive is always treated as a separate line by the C preprocessor,
even if the included file lacks a final newline.
2.3. Search Path
GCC looks in several different places for headers. On a normal Unix system, if you do not instruct it
otherwise, it will look for headers requested with #include

file

in:
/usr/local/include
libdir/gcc/target/version/include
/usr/target/include

Chapter 2. Header Files 11
/usr/include
For C++ programs, it will also look in /usr/include/g++-v3, first. In the above, target is the
canonical name of the system GCC was configured to compile code for; often but not always the same
as the canonical name of the system it runs on. version is the version of GCC in use.
You can add to this list with the -Idir command line option. All the directories named by -I are
searched, in left-to-right order, before the default directories. The only exception is when dir is al-
ready searched by default. In this case, the option is ignored and the search order for system directories
remains unchanged.
Duplicate directories are removed from the quote and bracket search chains before the two chains are
merged to make the final search chain. Thus, it is possible for a directory to occur twice in the final
search chain if it was specified in both the quote and bracket chains.
You can prevent GCC from searching any of the default directories with the -nostdinc option. This
is useful when you are compiling an operating system kernel or some other program that does not use
the standard C library facilities, or the standard C library itself. -I options are not ignored as described
above when -nostdinc is in effect.
GCC looks for headers requested with #include "file"first in the directory containing the current
file, then in the same places it would have looked for a header requested with angle brackets. For ex-
ample, if /usr/include/sys/stat.h contains #include "types.h", GCC looks for types.h
first in /usr/include/sys, then in its usual search path.
#line (Chapter 6 Line Control) does not change GCC’s idea of the directory containing the current
file.
You may put -I- at any point in your list of -I options. This has two effects. First, directories appear-
ing before the -I- in the list are searched only for headers requested with quote marks. Directories
after -I- are searched for all headers. Second, the directory containing the current file is not searched
for anything, unless it happens to be one of the directories named by an -I switch.
-I. -I- is not the same as no -I options at all, and does not cause the same behavior for
 includes
that "" includes get with no special options. -I. searches the compiler’s current working directory
for header files. That may or may not be the same as the directory containing the current file.
If you need to look for headers in a directory named -, write -I./-.
There are several more ways to adjust the header search path. They are generally less useful. Chapter
12 Invocation.
2.4. Once-Only Headers
If a header file happens to be included twice, the compiler will process its contents twice. This is very
likely to cause an error, e.g. when the compiler sees the same structure definition twice. Even if it does
not, it will certainly waste time.
The standard way to prevent this is to enclose the entire real contents of the file in a conditional, like
this:
/* File foo. */
#ifndef FILE_FOO_SEEN
#define FILE_FOO_SEEN
the entire file
#endif /* !FILE_FOO_SEEN */

12 Chapter 2. Header Files
This construct is commonly known as a wrapper #ifndef . When the header is included again, the
conditional will be false, because FILE_FOO_SEEN is defined. The preprocessor will skip over the
entire contents of the file, and the compiler will not see it twice.
CPP optimizes even further. It remembers when a header file has a wrapper #ifndef. If a subsequent
#include specifies that header, and the macro in the #ifndef is still defined, it does not bother to
rescan the file at all.
You can put comments outside the wrapper. They will not interfere with this optimization.
The macro FILE_FOO_SEEN is called the controlling macro or guard macro. In a user header file, the
macro name should not begin with _. In a system header file, it should begin with __ to avoid conflicts
with user programs. In any kind of header file, the macro name should contain the name of the file
and some additional text, to avoid conflicts with other header files.
2.5. Computed Includes
Sometimes it is necessary to select one of several different header files to be included into your pro-
gram. They might specify configuration parameters to be used on different sorts of operating systems,
for instance. You could do this with a series of conditionals,
#if SYSTEM_1
# include "system_1.h"
#elif SYSTEM_2
# include "system_2.h"
#elif SYSTEM_3
...
#endif
That rapidly becomes tedious. Instead, the preprocessor offers the ability to use a macro for the header
name. This is called a computed include. Instead of writing a header name as the direct argument of
#include, you simply put a macro name there instead:
#define SYSTEM_H "system_1.h"
...
#include SYSTEM_H
SYSTEM_H will be expanded, and the preprocessor will look for system_1.h as if the #include had
been written that way originally. SYSTEM_H could be defined by your Makefile with a -D option.
You must be careful when you define the macro. #define saves tokens, not text. The preprocessor has
no way of knowing that the macro will be used as the argument of #include, so it generates ordinary
tokens, not a header name. This is unlikely to cause problems if you use double-quote includes, which
are close enough to string constants. If you use angle brackets, however, you may have trouble.
The syntax of a computed include is actually a bit more general than the above. If the first non-
whitespace character after #include is not "or

, then the entire line is macro-expanded like running
text would be.
If the line expands to a single string constant, the contents of that string constant are the file to be in-
cluded. CPP does not re-examine the string for embedded quotes, but neither does it process backslash
escapes in the string. Therefore
#define HEADER "a\"b"

Chapter 2. Header Files 13
#include HEADER
looks for a file named a\"b. CPP searches for the file according to the rules for double-quoted in-
cludes.
If the line expands to a token stream beginning with a

token and including a

token, then the tokens
between the

and the first

are combined to form the filename to be included. Any whitespace
between tokens is reduced to a single space; then any space after the initial

is retained, but a trailing
space before the closing

is ignored. CPP searches for the file according to the rules for angle-bracket
includes.
In either case, if there are any tokens on the line after the file name, an error occurs and the directive
is not processed. It is also an error if the result of expansion does not match either of the two expected
forms.
These rules are implementation-defined behavior according to the C standard. To minimize the risk
of different compilers interpreting your computed includes differently, we recommend you use only
a single object-like macro which expands to a string constant. This will also minimize confusion for
people reading your program.
2.6. Wrapper Headers
Sometimes it is necessary to adjust the contents of a system-provided header file without editing it
directly. GCC’s fixincludes operation does this, for example. One way to do that would be to create
a new header file with the same name and insert it in the search path before the original header. That
works fine as long as you’re willing to replace the old header entirely. But what if you want to refer to
the old header from the new one?
You cannot simply include the old header with #include. That will start from the beginning, and find
your new header again. If your header is not protected from multiple inclusion (Section 2.4 Once-Only
Headers), it will recurse infinitely and cause a fatal error.
You could include the old header with an absolute pathname:
#include "/usr/include/old-header.h"
This works, but is not clean; should the system headers ever move, you would have to edit the new
headers to match.
There is no way to solve this problem within the C standard, but you can use the GNU exten-
sion #include_next. It means, "Include the next file with this name." This directive works like
#include except in searching for the specified file: it starts searching the list of header file directo-
ries after the directory in which the current file was found.
Suppose you specify -I /usr/local/include, and the list of directories to search also
includes /usr/include; and suppose both directories contain signal.h. Ordinary #include

signal.h

finds the file under /usr/local/include. If that file contains #include_next

signal.h

, it starts searching after that directory, and finds the file in /usr/include.
#include_next does not distinguish between

file

and "file"inclusion, nor does it check that
the file you specify has the same name as the current file. It simply looks for the file named, starting
with the directory in the search path after the one where the current file was found.
The use of #include_next can lead to great confusion. We recommend it be used only when there is
no other alternative. In particular, it should not be used in the headers belonging to a specific program;
it should be used only to make global corrections along the lines of fixincludes.

14 Chapter 2. Header Files
2.7. System Headers
The header files declaring interfaces to the operating system and runtime libraries often cannot be
written in strictly conforming C. Therefore, GCC gives code found in system headers special treat-
ment. All warnings, other than those generated by #warning (Chapter 5 Diagnostics), are suppressed
while GCC is processing a system header. Macros defined in a system header are immune to a few
warnings wherever they are expanded. This immunity is granted on an ad-hoc basis, when we find
that a warning generates lots of false positives because of code in macros defined in system headers.
Normally, only the headers found in specific directories are considered system headers. These direc-
tories are determined when GCC is compiled. There are, however, two ways to make normal headers
into system headers.
The -isystem command line option adds its argument to the list of directories to search for headers,
just like -I. Any headers found in that directory will be considered system headers.
All directories named by -isystem are searched after all directories named by -I, no matter what
their order was on the command line. If the same directory is named by both -I and -isystem, the
-I option is ignored. GCC provides an informative message when this occurs if -v is used.
There is also a directive, #pragma GCC system_header, which tells GCC to consider the rest of
the current include file a system header, no matter where it was found. Code that comes before the
#pragma in the file will not be affected. #pragma GCC system_header has no effect in the primary
source file.
On very old systems, some of the pre-defined system header directories get even more special treat-
ment. GNU C++ considers code in headers found in those directories to be surrounded by an extern
"C" block. There is no way to request this behavior with a #pragma, or from the command line.