Node:Header Templates, Next:autoheader Invocation, Up:Configuration Headers

Configuration Header Templates

Your distribution should contain a template file that looks as you want the final header file to look, including comments, with #undef statements which are used as hooks. For example, suppose your configure.ac makes these calls:

AC_CONFIG_HEADERS([conf.h])
AC_CHECK_HEADERS([unistd.h])

Then you could have code like the following in conf.h.in. On systems that have unistd.h, configure will #define HAVE_UNISTD_H to 1. On other systems, the whole line will be commented out (in case the system predefines that symbol).

/* Define as 1 if you have unistd.h.  */
#undef HAVE_UNISTD_H

You can then decode the configuration header using the preprocessor directives:

#include <conf.h>

#if HAVE_UNISTD_H
# include <unistd.h>
#else
/* We are in trouble. */
#endif

The use of old form templates, with #define instead of #undef is strongly discouraged.

Since it is a tedious task to keep a template header up to date, you may use autoheader to generate it, see autoheader Invocation.