diff --git a/monitor.c b/monitor.c
index 64de10a..6766e49 100644
--- a/monitor.c
+++ b/monitor.c
@@ -258,29 +258,36 @@ static void monitor_puts(Monitor *mon, const char *str)
     }
 }
 
-void monitor_vprintf(Monitor *mon, const char *fmt, va_list ap)
+int monitor_vprintf(Monitor *mon, const char *fmt, va_list ap)
 {
     char buf[4096];
+    int ret;
 
-    if (!mon)
-        return;
-
+    if (!mon) {
+        return -1;
+    }
     mon_print_count_inc(mon);
 
     if (monitor_ctrl_mode(mon)) {
-        return;
+        return -1;
     }
 
-    vsnprintf(buf, sizeof(buf), fmt, ap);
+    ret = vsnprintf(buf, sizeof(buf), fmt, ap);
     monitor_puts(mon, buf);
+
+    return ret;
 }
 
-void monitor_printf(Monitor *mon, const char *fmt, ...)
+int monitor_printf(Monitor *mon, const char *fmt, ...)
 {
     va_list ap;
+    int ret;
+
     va_start(ap, fmt);
-    monitor_vprintf(mon, fmt, ap);
+    ret = monitor_vprintf(mon, fmt, ap);
     va_end(ap);
+
+    return ret;
 }
 
 void monitor_print_filename(Monitor *mon, const char *filename)
diff --git a/monitor.h b/monitor.h
index ea15469..32c0170 100644
--- a/monitor.h
+++ b/monitor.h
@@ -45,8 +45,8 @@ int monitor_read_bdrv_key_start(Monitor *mon, BlockDriverState *bs,
 
 int monitor_get_fd(Monitor *mon, const char *fdname);
 
-void monitor_vprintf(Monitor *mon, const char *fmt, va_list ap);
-void monitor_printf(Monitor *mon, const char *fmt, ...)
+int monitor_vprintf(Monitor *mon, const char *fmt, va_list ap);
+int monitor_printf(Monitor *mon, const char *fmt, ...)
     __attribute__ ((__format__ (__printf__, 2, 3)));
 void monitor_print_filename(Monitor *mon, const char *filename);
 void monitor_flush(Monitor *mon);
diff --git a/qemu-tool.c b/qemu-tool.c
index b39af86..f6ce6cd 100644
--- a/qemu-tool.c
+++ b/qemu-tool.c
@@ -43,12 +43,14 @@ void monitor_set_error(Monitor *mon, QError *qerror)
 {
 }
 
-void monitor_vprintf(Monitor *mon, const char *fmt, va_list ap)
+int monitor_vprintf(Monitor *mon, const char *fmt, va_list ap)
 {
+    return -1;
 }
 
-void monitor_printf(Monitor *mon, const char *fmt, ...)
+int monitor_printf(Monitor *mon, const char *fmt, ...)
 {
+    return -1;
 }
 
 void monitor_print_filename(Monitor *mon, const char *filename)