c - 选择()在C : the Rosetta stone?

Question

主要是select()就设置哪些标志而言很简单:

• r 如果数据可读，
• 如果它是可写的，
• e 用于套接字紧急数据

但是这条规则有很多异常(exception)(或者，也许是重要的扩展):'w' 设置为已完成的非阻塞连接，'r' 设置为 listen()带有待处理的 accept()，等等。

有没有人找到在什么情况下设置哪些标志的简单列表？

Answer 1

来自 this reference page :

A descriptor shall be considered ready for reading when a call to an input function with O_NONBLOCK clear would not block, whether or not the function would transfer data successfully. (The function might return data, an end-of-file indication, or an error other than one indicating that it is blocked, and in each of these cases the descriptor shall be considered ready for reading.)

A descriptor shall be considered ready for writing when a call to an output function with O_NONBLOCK clear would not block, whether or not the function would transfer data successfully.

If a socket has a pending error, it shall be considered to have an exceptional condition pending. Otherwise, what constitutes an exceptional condition is file type-specific.

除了异常集中的描述符外，其他都非常清楚。如果阻塞描述符(无论它是文件、套接字还是其他描述符)不会阻塞，那么它在各自的集合中被标记为就绪。接受和接收是“读”操作，而连接和写是“写”操作。

唯一有问题的是异常状态，这取决于您在集合中传递的描述符类型。