compare-and-swap

compare-and-swap (比较与交换)

C的实现：

int compare_and_swap(int* reg, int oldval, int newval)
{
  ATOMIC();
  int old_reg_val = *reg;
  if (old_reg_val == oldval)
     *reg = newval;
  END_ATOMIC();
  return old_reg_val;
}

更新reg这块内存地址，如果值是oldval则更新成newval。

思路:如果是期望值，则更新成新值，否则返回老值作为一下次循环的期望值。

java相关：

• java.util.concurrent.atomic implements compareAndSet in various classes
• java.util.concurrent.ConcurrentLinkedQueue

参考：
https://en.wikipedia.org/wiki/Compare-and-swap
http://www.tuicool.com/articles/zuui6z
http://ifeve.com/atomic-operation/