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(autoconf.info)Configuration Headers

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Configuration Header Files
==========================

   When a package tests more than a few C preprocessor symbols, the
command lines to pass `-D' options to the compiler can get quite long.
This causes two problems.  One is that the `make' output is hard to
visually scan for errors.  More seriously, the command lines can exceed
the length limits of some operating systems.  As an alternative to
passing `-D' options to the compiler, `configure' scripts can create a
C header file containing `#define' directives.  The `AC_CONFIG_HEADER'
macro selects this kind of output.  It should be called right after
`AC_INIT'.

   The package should `#include' the configuration header file before
any other header files, to prevent inconsistencies in declarations (for
example, if it redefines `const').  Use `#include <config.h>' instead
of `#include "config.h"', and pass the C compiler a `-I.' option (or
`-I..'; whichever directory contains `config.h').  That way, even if
the source directory is configured itself (perhaps to make a
distribution), other build directories can also be configured without
finding the `config.h' from the source directory.

 - Macro: AC_CONFIG_HEADER (HEADER-TO-CREATE ...)
     Make `AC_OUTPUT' create the file(s) in the whitespace-separated
     list HEADER-TO-CREATE containing C preprocessor `#define'
     statements, and replace `@DEFS@' in generated files with
     `-DHAVE_CONFIG_H' instead of the value of `DEFS'.  The usual name
     for HEADER-TO-CREATE is `config.h'.

     If HEADER-TO-CREATE already exists and its contents are identical
     to what `AC_OUTPUT' would put in it, it is left alone.  Doing this
     allows some changes in configuration without needlessly causing
     object files that depend on the header file to be recompiled.

     Usually the input file is named `HEADER-TO-CREATE.in'; however,
     you can override the input file name by appending to
     HEADER-TO-CREATE, a colon-separated list of input files.  Examples:
          AC_CONFIG_HEADER(defines.h:defines.hin)
          AC_CONFIG_HEADER(defines.h:defs.pre:defines.h.in:defs.post)

     Doing this allows you to keep your file names acceptable to
     MS-DOS, or to prepend and/or append boilerplate to the file.

Header Templates

Input for the configuration headers.

Invoking autoheader

How to create configuration templates.