// define the "before main" function
int x = 0;
__attribute__((constructor))
static void run_before_main_func(void){
  x = 100;
}


// main
#include <stdio.h>
int main(){
  printf("x = %d\n", x);
  return(0);
}


#if 0
// wrapped in a macro:

#define BEFORE_MAIN(n) \
__attribute__((constructor)) static void n(void)

BEFORE_MAIN(before_main_rule){
  // do work here
}

#endif