4.8.1 Configuration Header Templates
Your distribution should contain a template file that looks as you want
the final header file to look, including comments, with
statements which are used as hooks. For example, suppose your
configure.ac makes these calls:
Then you could have code like the following in conf.h.in. On
systems that have unistd.h, configure defines
‘HAVE_UNISTD_H’ to 1. On other systems, the whole line is
commented out (in case the system predefines that symbol).
/* Define as 1 if you have unistd.h. */
Pay attention that ‘#undef’ is in the first column, and there is
nothing after ‘HAVE_UNISTD_H’, not even white space. You can
then decode the configuration header using the preprocessor directives:
# include <unistd.h>
/* We are in trouble. */
The use of old form templates, with ‘#define’ instead of
‘#undef’ is strongly discouraged. Similarly with old templates
with comments on the same line as the ‘#undef’. Anyway, putting
comments in preprocessor macros has never been a good idea.
Since it is a tedious task to keep a template header up to date, you may
use autoheader to generate it, see autoheader Invocation.