[thedigitalengel]

For multiple functions taking different arguments you could exploit atexit's specified calling order

    data_t *global;
    
    int main() {
      atexit(callback);
      atexit(set_global_to_x);
      atexit(callback);
      atexit(set_global_to_y);
    }

Since functions are called in reverse order of their installation using atexit, you end up with two calls to callback; one with global set to y and one with global set to x.