C中没有复合语句的函数定义

Question

I am confused by the following function definition without a compound statement in C:

void
__tu_finishme(const char *file, int line, const char *format, ...)
   tu_printflike(3, 4);

It seems to not result in a function in generated object files, while the linker still expects __tu_finishme to have been written. Especially odd to me since

void
__tu_finishme(const char *file, int line, const char *format, ...) {
   tu_printflike(3, 4);
}

seems to have different (AKA "normal") linkage than the former.

Can someone please explain which concept and niche of the C language I encounter here and how it works?

Bonus points for explaining things like:

void
foo(const char* c)
   bar()
{
    ha = hoo();
    boo(ha);
}

Answer 1

tu_printflike is very likely a macro that expands to an attribute like :

__attribute__ ((format (printf, 3, 4)))

The above is GCC specific, so the use of a macro is there to enable portability across compilers, it can be defined as something akin to

#ifdef __GNUC__
#  define tu_printflike(i, j) __attribute__ ((format (printf, i, j)))
#else
#  define tu_printflike(i, j) 
#endif

Your bonus point can be explained just the same with

#define bar()

Where the definition is just an empty token sequence, the function like macro expands to nothing.