Can select() be used for clients, not just servers?

Question

I'd like to make a TCP client that makes multiple connections while a select() loop that receives data from them is running in a separate thread. I'm not sure this is possible, though, because the select() loop is already running and thus I don't see how it would "notice" a new socket was added even if the thread-safety issues are dealt with.

Is there a way to do this, or must I spawn a new thread and use recv() every time I make a new connection?

(Edited for clarity.)

Answer 1

Of course it is possible. The select() function accepts file handles in three sets, one for read, one for write and one for errors. Just add your socket to the read set, and you'll be noticed when the server has sent you something.

This page has code showing how this is done.

Answer 2

Another good reason to select() on client sockets is to track outgoing TCP connections progress. This allows, for example, to setup connection timeout.

• Set client socket to be non-blocking.
• Call connect(). Probably it would return with EINPROGRESS error set (connection is in progress, you are not blocked because socket is non-blocking).
• Now select() with FD_SET configured to track client-socket as 'write-ready'. Also you can set timeout.
• Analyze select() result.
• Analyze if last client socket operation was failed or succeed.

The most useful thing is you can use this on several sockets in different state. So you get truly non-blocking handling of number of sockets (client, server, outgoing, listening, accepted ...). And all of this with only one thread.