How to call a function just before returning in C ?

Hi,

I'm trying to execute something at the end of a function just before it returns to the caller. To Do so, I would like to override return in a certain context. The behavior should be the same as __cyg_profile_func_exit, but I would like to activate it only for some functions.

I don't know if it's possible using gcc builtins or this kind of thing.

Thanks.

From stackoverflow

• Nope, not in C per se.

  What you could do is write a #define macro RETURN:

  #define RETURN(func) if(_DEBUG_) func; return ;
  #define RETURNV(func, val) if(_DEBUG_) func; return val ;
  

  (Warning, you probably want to think a little more about guarding special cases than I have.)

  Otherwise, you would need to write something that mangled the code behind the scenes, which is what profilers do.

  LB : Could I just do something with LD_PRELOAD_PATH and over-riding the return when needed ? (pure static approach...)

  Charlie Martin : You can't really override return in C, it's not a function call. It's just a keyword that generates the appropriate instruction to return from a subroutine call. A particular compiler can add frills, like for the performance monitor, but that's not C.

• GCC has an attribute for this, which calls a function when an automatic variable goes out of scope, passing it the address of that variable

  void cleanup_fn(int *p) {
      puts("cleanup called...");
  }
  
  void f(void) {
      int p __attribute__((cleanup(cleanup_fn)));
      puts("in f...");
  }
  
  int main(void) {
      puts("calling f...");
      f();
      puts("out of it...");
      return 0;
  }
  

  Output:

  calling f...
  in f...
  cleanup called...
  out of it...
  

  qrdl : Nice piece of C-Fu. Or GCC-Fu, to be exact.

  Johannes Schaub - litb : I've never heard about GCC-Fu

  qrdl : It is a school inside C-Fu, and RMS is the master :)

  Charlie Martin : Thoroughly nonportable, and not C, but a GCC hack.

  Johannes Schaub - litb : right it is restricted to GCC. i figured that's no problem because he wondered about a GCC builtin :)