是否应该阻塞，直到足够的数据到达。
阻塞的系统调用，就拿读取来说，直到数据准备好的时候才会返回。
A blocking syscall, like a read will only return once data is available.
不幸的是，read方法也要一直阻塞到数据可用为止，write 方法也一样，。
Unfortunately, the read method also blocks until data is available, as does the write method.
同样，write调用会阻塞，直到可以写数据。
Likewise, a write call blocks until the data can be written.
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.
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.
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