.Dd 27 April 2024
.Dt USAGE 3
.Os
.Sh NAME
.Nm usage
.Nd print a program usage diagnostic
.Sh LIBRARY
.Lb mlib
.Sh SYNOPSIS
.In errors.h
.Ft void
.Fn usage "const char *s" "..."
.Sh DESCRIPTION
The
.Fn usage
macro prints a program usage diagnostic to the standard error.
The one-or-more program usages are provided as null-terminated strings to
.Fn usage ,
and the program name must be set via a prior invocation of
.Fn mlib_setprogname .
None of the provided arguments should be
.Dv nullptr .
.Sh EXAMPLES
The following call to
.Fn usage
prints a diagnostic showing the different command-line arguments that can
be provided to the example executable.
.Bd -literal -offset indent
#include <stdlib.h>

#include <errors.h>
#include <macros.h>
#include <optparse.h>
 
static const struct op_option opts[] = {
	{'a', nullptr, OPT_NONE},
	{'b', nullptr, OPT_NONE},
	{'h', nullptr, OPT_NONE},
};

int
main(int argc, char **argv)
{
	mlib_setprogname("my-prog");

	rune opt;
	struct optparse op = mkoptparser(argv);
	while (opt = optparse(&op, opts, lengthof(opts))) {
		switch (opt) {
		/* … */
		case 'h':
			[[fallthrough]];
		default:
			/* Prints the following diagnostic:

			   Usage: my-prog [-ab]
			          my-prog -h
			 */
			usage("[-ab]", "-h");
			exit(opt == 'h' ? EXIT_SUCCESS : EXIT_FAILURE);
		}
	}

	/* … */

	return EXIT_SUCCESS;
}
.Ed
.Sh SEE ALSO
.Xr mlib_setprogname 3 ,
.Xr optparse.h 3
.Sh AUTHORS
.An Thomas Voss Aq Mt mail@thomasvoss.com