Function definition(?) without {}
av_printf_format is a macro, which can optionally add a GCC attribute to the function declaration. It's defined in attributes.h:
#ifdef __GNUC__# define av_builtin_constant_p __builtin_constant_p# define av_printf_format(fmtpos, attrpos) __attribute__((__format__(__printf__, fmtpos, attrpos)))#else# define av_builtin_constant_p(x) 0# define av_printf_format(fmtpos, attrpos)#endifSo this is actually a function declaration, which may have a specific attribute if compiled on GCC.
The format attribute tells GCC that the function takes its arguments like printf, which helps diagnose some errors.