1  #include <stdio.h>
       2  
       3  extern int __attribute__ ((weak)) fun (void);
       4  
       5  int
       6  main (void)
       7  {
       8    if (&fun != 0)
       9      fun ();
      10    else
      11      printf ("Weak undefined\n");
      12    return 0;
      13  }