Can I undo or remove an atexit command?

Question

If I place atexit( fn ); on the exit stack, it will get executed when the program exits: returns from main() or via exit().

Can I remove it from the stack?

Why do I want to do this, you ask?

I was experimenting with a simple try-catch mechanism using atexit, setjmp and longjmp. It would be just perfect if I could undo-atexit(fn); - even if it would only work for the last registered function.

Edit:

Following monoceres' suggestion to make my own stack...

The stack only works with one exception catcher for now.

void (*_catchFn[10])()  = {0,0,0,0,0,0,0,0,0,0};

void _catch(){
  if ( _catchFn[0] != 0 ){
    (_catchFn[0])();
  }
}

void _addCatch( void (*fn)() ){
  _catchFn[0]=fn;
}

void _remCatch( void (*fn)() ){
  _catchFn[0]=0;
}

void test(){
  jmp_buf env;

  void catch(){                  // we get here after an exit with a registered catch
    longjmp(env,1);              // return to the line marked except...
                               //   that first will get the value 1
  }
  int first = setjmp( env);      // ** return here **
  fprintf( stderr , "test: After setjmp. first=%d\n" , first );
  if( first == 0 ){              // try this code
    _addCatch(catch);            // register the catch function to 'catch' the exit
    fprintf( stderr , "test: Before CHECK\n" );
    // CHECK something and something bad happens and it exits
    exit(1);                     // like this
    fprintf( stderr , "test: After CHECK - THIS SHOULD NEVER BE SEEN AFTER AN EXCEPTION.\n" );
  }else{
    fprintf( stderr , "test: After longjmp return. first=%d\n" , first );
  }
  _remCatch( catch);
  fprintf( stderr , "test: IT WORKED!\n");
  exit(1);  // exit again to see if we are safe
}

int main(){
  atexit( _catch );              // register my global exception stack
  test();
}

Answer 1

No, you cannot do it, but you can use global flag so your exit handler will be doing nothing if the flag is set.

Alternatively you can call _Exit() (C99) - it will perform normal exit procedure (close all open descriptors, send all needed signals and parent/children) but will not call exit handler.

Answer 2

Why not build your own stack that you call from a single atexit() function? That way you could manipulate the stack all you want.