diff --git a/lupan-clock/stdout-clock.c b/lupan-clock/stdout-clock.c
index dd34ae4..13f36e2 100644
--- a/lupan-clock/stdout-clock.c
+++ b/lupan-clock/stdout-clock.c
@@ -1,27 +1,40 @@
 #include <X11/Xlib.h>
 #include <stdio.h>
+#include <stdlib.h>
 #include <time.h>
 #include <unistd.h>
 #include <string.h>
 
-void show(struct tm *tm)
+void show(char *prefix, struct tm *tm)
 {
 	char s[6];
 	strftime(s, sizeof(s), "%H:%M", tm);
-	printf("%s\n", s);
+	printf("%s%s\n", prefix, s);
 	fflush(stdout);
 }
 
-int main() {
-	int h, m, init = 60;
+int main(int argc, char **argv) {
+	int h, m, init = 60, opt;
 	time_t t;
 	struct tm *tm;
+	char *prefix = "";
+
+	while ((opt = getopt(argc, argv, "p:")) != -1) {
+		switch (opt) {
+		case 'p':
+			prefix = optarg;
+		break;
+		default:
+			fprintf(stderr, "Usage: %s [-p PREFIX]\n", argv[0]);
+			exit(EXIT_FAILURE);
+		}
+	}
 
 	while (1) {
 		t = time(NULL);
 		tm = localtime(&t);
 		if (tm->tm_hour != h || tm->tm_min != m || init > 0) {
-			show(tm);
+			show(prefix, tm);
 			h = tm->tm_hour;
 			m = tm->tm_min;
 			init = (init > 0) ? init - 1 : 0;