diff --git a/common/rfb/util.h b/common/rfb/util.h
index 98ce642..9ad1772 100644
--- a/common/rfb/util.h
+++ b/common/rfb/util.h
@@ -32,6 +32,12 @@
 
 struct timeval;
 
+#ifdef __GNUC__
+#  define __printf_attr(a, b) __attribute__((__format__ (__printf__, a, b)))
+#else
+#  define __printf_attr(a, b)
+#endif // __GNUC__
+
 namespace rfb {
 
   // -=- Class to handle cleanup of arrays of characters
@@ -45,6 +51,7 @@
     ~CharArray() {
       delete [] buf;
     }
+    void format(const char *fmt, ...) __printf_attr(2, 3);
     // Get the buffer pointer & clear it (i.e. caller takes ownership)
     char* takeBuf() {char* tmp = buf; buf = 0; return tmp;}
     void replaceBuf(char* b) {delete [] buf; buf = b;}