Combining select with threads with TCP C Program

Question

I am confused: is it possible/advisable to combine threads and select on fd? For example: I want to make a threaded tcp client-server program where the client can sent a message to the server and the server spreads this to all connected clients (and the client can continue to communicate).

I made a server-client program where all the clients get a separate thread when connecting; but i read in beej guide "What if you're blocking on an accept() call? How are you going to recv() data at the same time? "Use non-blocking sockets!" No way! You don't want to be a CPU hog. What, then?"

I don't see what blocking means in this case; because in my program when I enter a text and i push enter the text is send to the server but why does he mean by receiving data at the same time? While I am typing the text? How is that possible? I think I am confused :p Than I think these are completely different ways of tackling the program but than again i don't see the advantage the select provides? Thx

Greg Inozemtsev · Accepted Answer · 2012-05-24 22:27:34Z

2

It's certainly possible to use multiple threads that call select, but not necessary. A server can serve all clients with just one thread. How? One at a time. The select function (and its replacements such as poll, epoll, and so on) can notify you when an event occurs on one of the sockets from the set you supply. select will also notify you when a listening socket can accept. So the server can have just one event loop processing both listening and connected sockets.

As for what the advantages and disadvantages are, I'll just give you a link to The C10K problem article. This is quite an old debate.

answered May 24, 2012 at 22:27

Greg Inozemtsev

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zoska · Accepted Answer · 2013-08-16 11:17:10Z

I don't see what blocking means in this case; because in my program when I enter a text and i push enter the text is send to the server but why does he mean by receiving data at the same time?

Of course you can. For e.g. you can select() on chosen fd's and handle them in other threads (so duration of this will not block other events that could come on select()).

I don't see what blocking means in this case; because in my program when I enter a text and i push enter the text is send to the server but why does he mean by receiving data at the same time?

He probably meant that when you have one thread for accepting AND receiving data you either block (wait) on accept or receive. Block means you are waiting for incoming connection (accept) or incoming sent data (receive). If you use non-blocking socket if no event comes it will return -1 with either EWOULDBLOCK or EAGAIN errno.

You can wait for both on select() and use both your listen fd and already connected clients fd. Then you can concurrently wait for whatever event that you want on your fd (new connection, received data etc.)

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