X That's just the special symbol in C that says don't pass X, that is don't pass a copy of X. Rather figure out where x is in memory, where he is in that frame and provide swap the numeric address in RAM of that value so that swap can go do anything it wants at that address.

这在C中是一个特殊的符号，指明不传送，也不是传递X的拷贝，而是指出x在内存中哪个地方,它在那个框架中的哪个地方,提供给swap它的地址值,这样swap就可以使用那个地址中的数据。

哈佛公开课 - 计算机科学课程节选