cgram: provide usage on incorrect invocation

diff --git a/cgram/cgram.c b/cgram/cgram.c
index 77c56a545cdc2307a8a22673f4bcc906b0343c82..00701ab77ca6b45255e71a104fbcfb0b586067a2 100644 (file)
--- a/cgram/cgram.c
+++ b/cgram/cgram.c
@@ -1,4 +1,4 @@
-/* $NetBSD: cgram.c,v 1.19 2021/04/25 20:14:29 rillig Exp $ */
+/* $NetBSD: cgram.c,v 1.20 2021/04/25 20:19:19 rillig Exp $ */
 
 /*-
  * Copyright (c) 2013, 2021 The NetBSD Foundation, Inc.
@@ -31,7 +31,7 @@
 
 #include <sys/cdefs.h>
 #if defined(__RCSID) && !defined(lint)
-__RCSID("$NetBSD: cgram.c,v 1.19 2021/04/25 20:14:29 rillig Exp $");
+__RCSID("$NetBSD: cgram.c,v 1.20 2021/04/25 20:19:19 rillig Exp $");
 #endif
 
 #include <assert.h>
@@ -594,11 +594,23 @@ clean_up(void)
        stringarray_cleanup(&lines);
 }
 
-////////////////////////////////////////////////////////////
+
+static void __dead
+usage(void)
+{
+
+       fprintf(stderr, "usage: %s [file]\n", getprogname());
+       exit(1);
+}
 
 int
 main(int argc, char *argv[])
 {
+
+       setprogname(argv[0]);
+       if (argc != 1 && argc != 2)
+               usage();
+
        init(argc > 1 ? argv[1] : NULL);
        loop();
        clean_up();