991118-1.c

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

struct tmp
{
  long long int pad : 12;
  long long int field : 52;
};

struct tmp2
{
  long long int field : 52;
  long long int pad : 12;
};

struct tmp3
{
  long long int pad : 11;
  long long int field : 53;
};

struct tmp4
{
  long long int field : 53;
  long long int pad : 11;
};

struct tmp
sub (struct tmp tmp)
{
  tmp.field ^= 0x0008765412345678LL;
  return tmp;
}

struct tmp2
sub2 (struct tmp2 tmp2)
{
  tmp2.field ^= 0x0008765412345678LL;
  return tmp2;
}

struct tmp3
sub3 (struct tmp3 tmp3)
{
  tmp3.field ^= 0x0018765412345678LL;
  return tmp3;
}

struct tmp4
sub4 (struct tmp4 tmp4)
{
  tmp4.field ^= 0x0018765412345678LL;
  return tmp4;
}

struct tmp tmp = {0x123, 0x123456789ABCDLL};
struct tmp2 tmp2 = {0x123456789ABCDLL, 0x123};
struct tmp3 tmp3 = {0x123, 0x1FFFF00000000LL};
struct tmp4 tmp4 = {0x1FFFF00000000LL, 0x123};

main()
{

  if (sizeof (long long) != 8)
    exit (0);

  tmp = sub (tmp);
  tmp2 = sub2 (tmp2);

  if (tmp.pad != 0x123 || tmp.field != 0xFFF9551175BDFDB5LL)
    abort ();
  if (tmp2.pad != 0x123 || tmp2.field != 0xFFF9551175BDFDB5LL)
    abort ();

  tmp3 = sub3 (tmp3);
  tmp4 = sub4 (tmp4);
  if (tmp3.pad != 0x123 || tmp3.field != 0xFFF989AB12345678LL)
    abort ();
  if (tmp4.pad != 0x123 || tmp4.field != 0xFFF989AB12345678LL)
    abort ();
  exit (0);
}