-extern void guestfs_error (guestfs_h *g, const char *fs, ...);
-extern void guestfs_perrorf (guestfs_h *g, const char *fs, ...);
+extern void guestfs_error (guestfs_h *g, const char *fs, ...)
+ __attribute__((format (printf,2,3)));
+extern void guestfs_perrorf (guestfs_h *g, const char *fs, ...)
+ __attribute__((format (printf,2,3)));