20000917-1.c

this is a gcc file, you can download it and learn its usetility, for further detail please look at t

/* This bug exists in gcc-2.95, egcs-1.1.2, gcc-2.7.2 and probably
   every other version as well.  */

typedef struct int3 { int a, b, c; } int3;

int3
one (void)
{
  return (int3) { 1, 1, 1 };
}

int3
zero (void)
{
  return (int3) { 0, 0, 0 };
}

int
main (void)
{
  int3 a;

  /* gcc allocates a temporary for the inner expression statement
     to store the return value of `one'.

     gcc frees the temporaries for the inner expression statement.

     gcc realloates the same temporary slot to store the return
     value of `zero'.

     gcc expands the call to zero ahead of the expansion of the
     statement expressions.  The temporary gets the value of `zero'.

     gcc expands statement expressions and the stale temporary is
     clobbered with the value of `one'.  The bad value is copied from
     the temporary into *&a.  */

  *({ ({ one (); &a; }); }) = zero ();
  if (a.a && a.b && a.c)
    abort ();
  exit (0);
}