Index: generic/nsf.h
===================================================================
diff -u -r92ab630ebd3c1b907e3d0fdf97cc07914245c028 -rc24b68646f2b5bc897704b212a10fdd35fa5c33a
--- generic/nsf.h	(.../nsf.h)	(revision 92ab630ebd3c1b907e3d0fdf97cc07914245c028)
+++ generic/nsf.h	(.../nsf.h)	(revision c24b68646f2b5bc897704b212a10fdd35fa5c33a)
@@ -351,12 +351,20 @@
 #else
 # define __GNUC_PREREQ(maj, min) (0)
 #endif
+
 #if __GNUC_PREREQ(3, 3)
 # define NSF_nonnull(ARGS) __attribute__((__nonnull__(ARGS)))
 #else
 # define NSF_nonnull(ARGS)
 #endif
 
+/* unforunately, we can't combine NSF_attribute_format() with functions called via stubs */
+#if __GNUC_PREREQ(3, 4)
+# define NSF_attribute_format(ARGS) __attribute__((format ARGS))
+#else
+# define NSF_attribute_format(ARGS)
+#endif
+
 EXTERN int
 Nsf_ArgumentParse(Tcl_Interp *interp, int objc, Tcl_Obj *CONST objv[],
 		  Nsf_Object *object, Tcl_Obj *procNameObj,
@@ -402,7 +410,7 @@
 
 EXTERN void
 NsfLog(Tcl_Interp *interp, int requiredLevel, CONST char *fmt, ...)
-  NSF_nonnull(1) NSF_nonnull(3);
+  NSF_nonnull(1) NSF_nonnull(3) NSF_attribute_format((printf,3,4));
 
 /*
  * Nsf Pointer converter interface