GNU Info

(autoconf.info)Header Templates

---

Next: autoheader Invocation Up: Configuration Headers

---

Enter node , (file) or (file)node

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 Note: autoheader Invocation.