c++ select async programming

Question

Is there a way to have 'select' waiting for reads and writes, while also being able to add new file descriptors? Preferrably on one thread?

If you only have one thread, then who is adding the new file descriptors? — Oliver Charlesworth, Apr 11 '11 at 20:06
@Oli: I think OP just wants to cancel the `select` call and add the descriptor before another call to `select` is made. — André Caron, Apr 11 '11 at 20:09
@Andre: But with only one thread, *when* does he want to break the `select`? Is there some external stimulus? — Oliver Charlesworth, Apr 11 '11 at 20:11
@Oli: Well I'm actually using sockets. So more clients may be connecting. — poy, Apr 11 '11 at 20:15
@Andrew: But presumably your listening socket is part of that select, so when a new client connects the select *does* return, allowing you to change the fd set for the next call. — Greg Hewgill, Apr 11 '11 at 20:19
@Greg: I didn't realize that select would return when you get a new client. This DEFINATELY clears things up. Thanks again. — poy, Apr 11 '11 at 20:55

score 3 · Answer 1 · answered Apr 11 '11 at 20:07

3

As far as I think, You can do it in same thread but not at the same time. In a problem like this I normally add my dummy loop-back socket in the descriptor list and whenever I have to add a new socket in FD_LIST, I just send a byte to my dummy socket and it breaks the Select Loop. Then I can update the FD_LIST and resume with the select again.

answered Apr 11 '11 at 20:07

Tayyab

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You can also add a timeout to the `select` call using the last parameter. – André Caron Apr 11 '11 at 20:11
Yes of course. But in that case if you are using long time interval in select then you have to wait until your new socket is added to the FD_LIST. If you are using very short interval in select or you don't need to add socket immediately then using the interval parameter is good option. – Tayyab Apr 11 '11 at 20:17

score 3 · Accepted Answer · answered Apr 11 '11 at 20:18

Now that I know what your scenario is (a socket-based server that may want to accept new incoming connections), did you know that you can append the file-descriptor for your listening socket to the list for select? See e.g. http://www.lowtek.com/sockets/select.html.

(Paraphrased example:)

fd_set socks;

FD_ZERO(&socks);

// Add listener socket
listen(sock, n);
FD_SET(&socks, sock);

// Add existing socket connections
for (i = 0; i < num_existing_connections; i++)
{
    FD_SET(&socks, connection[i]);
}

// Will break if any of the existing connections are active,
// or if a new connection appears.
select(..., &socks, ...);

The website was VERY helpful. Thanks! – poy Apr 11 '11 at 20:54 — poy, Apr 11 '11 at 20:54

c++ select async programming

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