TCP Concurrent Server

**danielleinad** · 01-10-2017

Hello guys!

I'm having a little bit of struggle understanding how complex Ca /C++ concurrent servers should work. I have to do a project for faculty and I have a list of projects from where I can choose but I have no idea how should I implement any of them.

For example, I can understand how simple concurrent servers work, like when a client connects, I accept that connection on a child process, I deal with that client and I close the connection.

The problems start to appear when my clients won't be disconnected after doing a single action and need to interact between them.

What kind of data structures should I use so that the clients can communicate between them. Should I keep a global array with their descriptors or how can I assure that the server remains concurrent.

For example, a chat application. A client is accepted by a child but obviously they will need to send messages to another clients in different rooms. Or a chess game, a child will accept 1 client but the game won't start until we have 2 of them.What do I do then? And if I wait for a second client, what process will be the server which will assure the communication between them?

I know I sound like a retarded person, but it's getting really frustrating. I passed exams for C#, C++, Java, Php, Html, Css, Sql and others but it's the 3rd year when I'm trying to do something with linux programming and I fail, reason why I can't get my Computer Science license.29 passed exams and this is stopping me. Probably it's a psychological blockage but I really need to understand at least in theory how this should work. Any links or explanation would be very helpful. Thank you.

**Salem** · 01-10-2017

select(2) - Linux manual page
With the select system call, you can observe the state of multiple file descriptors at once.

In the case of a server you describe, you have two classes of descriptor
- a single descriptor which is the socket used to listen and accept new connections
- zero or more descriptors which are your current client set.

A good way to hold all the client descriptors is in a 'fd_set'.

The basic steps are
- create a temporary fd_set from the client descriptor fd_set and the listening socket descriptor.
- call select. Use a timeout if you have other things to do if there is no network activity.
- use the select result to figure out which descriptors need attention

For a new incoming connection, you need to (at a minimum) add the new descriptor to client_fdset.
- You may also need to allocate some instance data, say if it were a chess server, you'd create a new game object.
If the remote disconnects, remove it from client_fdset.

For each client fd, call a handler function, passing in the fd (and your instance data pointer) to read the message and update it's state.

**Elkvis** · 01-10-2017

poll(2) - Linux manual page

I've personally found poll to be a bit easier to work with. It does basically the same thing, but you don't need to deal with the fd_set thing, if you would prefer a simple array. It's also convenient, if you're working in C++, because you can use std::vector<struct pollfd>::data() as the first argument.

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