diff --git a/c/debug.h b/c/debug.h
index e16081f..4a51166 100644
--- a/c/debug.h
+++ b/c/debug.h
@@ -19,22 +19,26 @@
 
 #include "bits.h"
 
+#define _unused __attribute__((__unused__))
+#define _printf __attribute__ ((format (printf, 6, 7)))
 #ifdef DEBUG_DEBUG
+#define _printf __attribute__ ((format (printf, 6, 7)))
 void debug_init(u32 level);
 void debug_level_set(u32 level);
 void debug_devel_set(u32 level);
-void debug(u32 level, bool timestamp, u32 indent,
-           const char *src, u32 line, const char *, ...);
+void _printf debug(u32 level, bool timestamp,
+                   u32 indent, const char *src,
+                   u32 line, const char *, ...);
 #else  /* DEBUG_DEBUG */
-#define _unused __attribute__((__unused__))
 static inline void debug_init(_unused u32 level) {}
 static inline void debug_level_set(_unused u32 level) {}
 static inline void debug_devel_set(_unused u32 level) {}
-static inline void debug(_unused u32 level, _unused bool timestamp,
-                         _unused u32 indent, _unused const char *src,
-                         _unused u32 line, const char *, ...) {}
-#undef _unused
+static inline void _printf debug(_unused u32 level, _unused bool timestamp,
+                                 _unused u32 indent, _unused const char *src,
+                                 _unused u32 line, const char *, ...) {}
 #endif  /* DEBUG_DEBUG */
+#undef _unused
+#undef _printf
 
 /* format: only printf
  */