__CMP_SWAP_QUAD

 Performs a conditional atomic compare and exchange operation on a
 quadword.  The quadword pointed to by source is read and compared
 with the quadword old_value.  If they are equal, the quadword
 new_value is written into the quadword pointed to by source.  The
 read and write is performed atomically, with no intervening access
 to the same memory region.

 The function returns 1 if the write occurs, and 0 otherwise.

 Syntax:

      int __CMP_SWAP_QUAD (volatile void *source, int old_value, int
      new_value);