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I'm working on a network project (sockets). I have read some article like link>> and link>> but I can't understand them very well. I want to handle operations server-side and I should understand them. Now I think the readfds is handles the incoming data operation (Am I right? I don't know) and the writefds is handles the outgoing data (with send() method) operation (Am I right? I don't know). And the last one; the exceptfds is handles the socket exceptions like disconnection/close operation (Am I right? I don't know). These are only my quesses, I don't know anything about them (without a article description) and I want to ask them in here to understand. Can anybody help me about this? I'll implement a I/O system server-side (high-performance) and I need to understand them very much! Thanks...

Mehmet Fatih Marabaoğlu
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    You got *readfds* and *writefds* right. Link to [an explanation of *exceptfds*](http://stackoverflow.com/a/1343995/315052). Also, `select()` is probably not your best choice for a high-performance system. Consider `epoll` (on Linux) or `kqueue` (on bsd) instead. Even `poll()` would probably do a little better than `select()`. – jxh Jun 26 '13 at 18:08

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In the most general sense, select() is a way of having your application wait until something interesting happens. This interesting event can be:

  • Data is available on one of the sockets you listed in readfds, so attempting to read() from that socket will not block.

  • Write room becomes available on one of the sockets you listed in writefds, so attempting to write() to that socket will not block.

  • Something unusual (like out-of-band data being received) happens on one of the sockets you listed in exceptfds.

  • The amount of time you specified in timeout has passed, but nothing else has happened.

  • What do you think about high-performance sockets? Is select() method is suitable for it? What else? – Mehmet Fatih Marabaoğlu Jun 26 '13 at 18:19
  • There are some techniques (e.g, AIO, `epoll()`, and `kqueue()`) which can end up performing slightly better under very high concurrency, but you are unlikely to ever need them. If you're really interested, though, you may want to check out [libev](http://software.schmorp.de/pkg/libev.html), which abstracts away some of the platform-specific nastiness. –  Jun 26 '13 at 18:34