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The C Preprocessor

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

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

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

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

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

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

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

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


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