Next: autoheader Invocation, Up: Configuration Headers
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 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. */ #undef 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 <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. 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.