Go to the first, previous, next, last section, table of contents.
When a package tests more than a few C preprocessor symbols, the command
lines to pass @option{-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 @option{-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. 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
@option{-I.} option (or @option{-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.
AC_OUTPUT create the file(s) in the
whitespace-separated list header containing C preprocessor
#define statements, and replace `@DEFS@' in generated
files with @option{-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
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.in'; however, you can override the input file name by appending to header, a colon-separated list of input files. Examples:
AC_CONFIG_HEADERS(config.h:config.hin) AC_CONFIG_HEADERS(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.
See section Taking Configuration Actions, for more details on header.
Go to the first, previous, next, last section, table of contents.