Function definition(?) without {}

Question

I was reading avio.h (part of ffmpeg) and there is definition(?) like this: `int avio_printf(AVIOContext *s, const char *fmt, ...) av_printf_format(2, 3);`. I don't get it. Could someone explain me what this do? Thanks.

Accepted Answer

`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)
#endif
```

So 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.

Function definition(?) without {}

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