随笔档案

举一个例子来说明。

父亲叫儿子去请叔叔来下棋，儿子需要开车去叔叔家。

同步：父亲在家什么事也不能做，在家等着。叔叔回来了儿子敲门通知父亲。
异步：父亲有一大堆其它事情要忙，打发儿子完儿子后接着忙一大堆其它的事情，等儿子回来了敲门通知父亲。

上面的同步和异步都使用到了非阻塞操作。
同步和异步的区别就在于：同步必须等到操作的结果，否则其它事都做不了；异步体现在操作发起后不用等待的结果，接着做其它事情，等操作结果出来了会受到通知并作相应处理。


Douglas博士的解释如下：

　AIO is "asynchronous I/O", i.e., the operation is invoked asynchronously and control returns to the client while the OS kernel processes the I/O request.?When the operation completes there is some mechanism for the client to retrieve the results.

　Non-blocking I/O tries an operation (such as a read() or write()) and if it the operation would block (e.g., due to flow control on a TCP connection or due to lack of data in a socket), the call returns -1 and sets errno to EWOULDBLOCK.