异步 I/O 让应用程序能够在不阻塞的情况下读写数据。
The asynchronous I/O allows applications to read and write data without blocking.
EWOULDBLOCK在不阻塞的情况下返回当没有数据可用。
EWOULDBLOCK is returned when no data is available without blocking.
一个明显的例子是，操作可以不阻塞地完成时。
One obvious example is when the requested operation can be completed without blocking.
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