diff --git a/src/mm.c b/src/mm.c
index ff62148..dc337b7 100644
--- a/src/mm.c
+++ b/src/mm.c
@@ -106,6 +106,15 @@ oserr(char *s, char *r){
 	} \
 	return retval
 
+/* Prints a usage text, in which s is the program being run (i.e. argv[0]), and
+ * returns an exit status appropriate for a usage error. */
+int usage(char *s){
+
+	fprintf(stderr, "Usage: %s (-aenu) (-i [input])... (-o [output])...\n", s);
+
+	return EX_USAGE;
+}
+
 int main(int argc, char *argv[]){
 	int c;
 	struct Files files[2]; /* {read, write} */
@@ -178,12 +187,15 @@ int main(int argc, char *argv[]){
 				k = 1;
 				break;
 			default:
-				fprintf(stderr, "Usage: %s (-aenu) (-i [input])..."
-					" (-o [output])...\n", argv[0]);
-				retval = EX_USAGE;
+				retval = usage(argv[0]);
 				terminate;
 			}
 
+	if(optind != argc){
+		retval = usage(argv[0]);
+		terminate;
+	}
+
 	files[0].s += files[0].s == 0;
 	files[1].s += files[1].s == 0;