(autoconf.info)Configuration Headers

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4.9 Configuration Header Files

When a package contains more than a few tests that define 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_HEADERS' macro selects this kind of output.  Though it
can be called anywhere between `AC_INIT' and `AC_OUTPUT', it is
customary to call it 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').

   To provide for VPATH builds, remember to pass the C compiler a `-I.'
option (or `-I..'; whichever directory contains `config.h').  Even if
you use `#include "config.h"', the preprocessor searches only the
directory of the currently read file, i.e., the source directory, not
the build directory.

   With the appropriate `-I' option, you can use `#include <config.h>'.
Actually, it's a good habit to use it, because in the rare case when
the source directory contains another `config.h', the build directory
should be searched first.

     This macro is one of the instantiating macros; see *Note
     Configuration Actions::.  Make `AC_OUTPUT' create the file(s) in
     the blank-or-newline-separated list HEADER 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 is `config.h'.

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

     Usually the input file is named `HEADER.in'; however, you can
     override the input file name by appending to HEADER a
     colon-separated list of input files.  For example, you might need
     to make the input file name acceptable to DOS variants:


 -- Macro: AH_HEADER
     This macro is defined as the name of the first declared config
     header and undefined if no config headers have been declared up to
     this point.  A third-party macro may, for example, require use of
     a config header without invoking AC_CONFIG_HEADERS twice, like

                  [m4_ifndef([AH_HEADER], [AC_CONFIG_HEADERS([config.h])])])

   Note: Configuration Actions, for more details on HEADER.

* Header Templates
Input for the configuration headers
* autoheader Invocation
How to create configuration templates
* Autoheader Macros
How to specify CPP templates

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