4.9.1 Configuration Header Templates

Your distribution should contain a template file that looks as you want the final header file to look, including comments, with #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’. The ‘conf.h’ created by configure defines ‘HAVE_UNISTD_H’ to 1, if and only if the system has ‘unistd.h’.

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

The format of the template file is stricter than what the C preprocessor is required to accept. A directive line should contain only whitespace, ‘#undef’, and ‘HAVE_UNISTD_H’. The use of ‘#define’ instead of ‘#undef’, or of comments on the same line as ‘#undef’, is strongly discouraged. Each hook should only be listed once. Other preprocessor lines, such as ‘#ifdef’ or ‘#include’, are copied verbatim from the template into the generated header.

Since it is a tedious task to keep a template header up to date, you may use autoheader to generate it, see Using autoheader to Create ‘config.h.in’.

During the instantiation of the header, each ‘#undef’ line in the template file for each symbol defined by ‘AC_DEFINE’ is changed to an appropriate ‘#define’. If the corresponding ‘AC_DEFINE’ has not been executed during the configure run, the ‘#undef’ line is commented out. (This is important, e.g., for ‘_POSIX_SOURCE’: on many systems, it can be implicitly defined by the compiler, and undefining it in the header would then break compilation of subsequent headers.)

Currently, all remaining ‘#undef’ lines in the header template are commented out, whether or not there was a corresponding ‘AC_DEFINE’ for the macro name; but this behavior is not guaranteed for future releases of Autoconf.

Generally speaking, since you should not use ‘#define’, and you cannot guarantee whether a ‘#undef’ directive in the header template will be converted to a ‘#define’ or commented out in the generated header file, the template file cannot be used for conditional definition effects. Consequently, if you need to use the construct

#ifdef THIS
# define THAT
#endif

you must place it outside of the template. If you absolutely need to hook it to the config header itself, please put the directives to a separate file, and ‘#include’ that file from the config header template. If you are using autoheader, you would probably use ‘AH_BOTTOM’ to append the ‘#include’ directive.