File: autoconf.info,  Node: Header Templates,  Next: autoheader Invocation,  Up: Configuration Headers

4.9.1 Configuration Header Templates
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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 *note autoheader Invocation::.

   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.