The format attribute marks a variadic function as taking a format string and corresponding arguments which should be type-checked against the format string. Marking printf_, sprintf_, etc. this way allows compilers to issue warnings when the supplied arguments do not have the correct type, as they would for the standard library functions.

The code change is very small, and currently implemented in svkampen@ba025a5 svkampen@d8aeaa9 (see comments below). I've guarded the __attribute__ declarations with #ifdef __GNUC__ ... #endif, so that they don't interfere with compilers that don't support them.