This macro invokes the procedure registered to display
the usage text. Normally, this will be optionUsage from the
AutoOpts library, but you may select another procedure by specifying
usage = "proc_name" program attribute. This procedure must
take two arguments first, a pointer to the option descriptor, and
second the exit code. The macro supplies the option descriptor
automatically. This routine is expected to call exit(3) with
the provided exit code.
The optionUsage routine also behaves differently depending
on the exit code:
EXIT_SUCCESS (the value zero)
It is assumed that full usage help has been requested. Consequently, more
information is provided than when displaying usage and exiting with a
non-zero exit code. Output will be sent to stdout and the program will
exit with a zero status code.
EX_USAGE (64)
The abbreviated usage will be printed to stdout and the program will
exit with a zero status code. EX_USAGE may or may not be 64. If your
system provides /usr/include/sysexits.h that has a different value,
then that value will be used.
any other value
The abbreviated usage will be printed to stderr and the program will
exit with the provided status code.