What is the use of __attribute__ (format) in C to define a function?

Question

I came across this line of code in a C project, and I did not understand it.

#define FMT_CHK(fmt, args)  __attribute__ ((format (printf, fmt, args)))

The GNU website does not explain it clearly (https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#Common-Function-Attributes)

What is the purpose of __attribute__ ((format ()) and how should it be used?

Answer 1

So, the fmt and args parameters just tell you which parameter has the format, and which parameter has the arguments.

void myprintf(const char *fmt, ...);
//            ^^ fmt = arg#1
//                             ^^ args = arg#2...

So in this case, the correct attribute is:

__attribute__((format(printf, 1, 2)))

If you have a longer function declaration...

void myprintf(obj *x, const char *fmt, int level, ...)
//                    ^^ format: arg#2
//                                                ^^ args: arg#4...
    __attribute__((format(printf, 2, 4)));