diff --git a/Tools/AP_Bootloader/support.h b/Tools/AP_Bootloader/support.h
index 87450bef2a..817174a69a 100644
--- a/Tools/AP_Bootloader/support.h
+++ b/Tools/AP_Bootloader/support.h
@@ -46,7 +46,7 @@ void led_toggle(unsigned led);
 
 // printf to debug uart (or USB)
 extern "C" {
-void uprintf(const char *fmt, ...);
+void uprintf(const char *fmt, ...) FMT_PRINTF(1,2);
 }
 
 // generate a LED sequence forever
diff --git a/Tools/AP_Periph/AP_Periph.h b/Tools/AP_Periph/AP_Periph.h
index e8e0e6da34..432ea696ea 100644
--- a/Tools/AP_Periph/AP_Periph.h
+++ b/Tools/AP_Periph/AP_Periph.h
@@ -243,6 +243,6 @@ namespace AP
 extern AP_Periph_FW periph;
 
 extern "C" {
-void can_printf(const char *fmt, ...);
+void can_printf(const char *fmt, ...) FMT_PRINTF(1,2);
 }