Question

I’m implementing a TCP socket server in C and Linux. It will be a chat server handling many requests simultaneously. Here is the pseudo code for how it is setup

create a socket
bind to a well-known port
use listen to place in passive mode
while (1)
{
    accept a client connection
    fork
    if (child)
    {
        communicate with new socket
        close new socket
        exit
     }
 else
 {close new socket}
}

From what I’ve seen, the “communicate with new socket” part consists of reading the buffer and then possibly writing to the buffer, then the socket closes. I thought sockets were suppose to be a persistent connection? If the client has to reconnect every time it wants to send more data to the server, isn’t this inefficient and defeating the purpose of sockets?

Answer 1

1. Don’t close the socket. Put the code which receives and responds to the messages in a loop and using blocking I/O ( the default ) to block on reading the socket until a request arrives. This way you’ll have a forked process dealing with each client and using minimal resources. You only close the socket and exit the child process when the client explicitly closes the connection because it’s done.

2. Use UDP instead of TCP. This way there is no “connection” between the server and the client. You multiplex multiple clients to a single socket without forking. If you care about reliability you’d need to add a sequence number to each message to keep them straight and allow retransmission on failures.

3. Keep a single process using TCP, but store a socket descriptor set and use select to check for readable sockets. You can then retransmit that data on each or some of the other client sockets.

I’d point out that forking a child for each client in a chat server seems like a poor implementation. How do two people who want to chat communicate? Each one will be served by a separate process – how do the processes communicate with each other so that chat data can pass between clients?