How do you keep a socket connection open indefinitely in C?

I'm trying to implement a C socket server in Linux using the code from Beej's sockets guide, which is here:

http://beej.us/guide/bgnet/examples/server.c

This works, and I've written a Windows client in C# to communicate with it. Once the client connects, I have it send a byte array to the server, the server reads it, then sends back a byte array. This works.

However, after this, if I have the client try to send another byte array, I get a Windows popup saying "An established connection was aborted by the software in your host machine." Then I have to re-connect with the client again. I want to keep the connection open indefinitely, until the client sends a disconnect command, but despite reading through Beej's guide, I just don't seem to get it. I'm not even trying to implement the disconnect command at present, I'm just trying to keep the connection open until I close the server.

I've tried removing the close() calls in Beej's code:

while(1) {  // main accept() loop
    sin_size = sizeof their_addr;
    new_fd = accept(sockfd, (struct sockaddr *)&their_addr, &sin_size);
    if (new_fd == -1) {
        perror("accept");
        continue;
    }

    inet_ntop(their_addr.ss_family,
        get_in_addr((struct sockaddr *)&their_addr),
        s, sizeof s);
    printf("server: got connection from %s\n", s);

    if (!fork()) { // this is the child process
        close(sockfd); // child doesn't need the listener
        ProcessRequest(new_fd); // this is not Beej's code, I've replaced his code here (which was a simple string send()) with a function call that does a read() call, processes some data, then sends back a byte array to the client using send().
        close(new_fd);
        exit(0);
    }
    close(new_fd);  // parent doesn't need this
}

But that just gets me an infinite loop of "socket accept: bad file descriptor" (I tried removing both the close(new_fd) lines, together and apart, and the close(sockfd) as well. Can anyone more versed with C socket programming give me a hint where I should be looking? Thank you.

The reason for the accept() problem is that sockfd isn't valid. You must have closed it somewhere. NB if you get such an error you shouldn't just keep retrying as though it hadn't happened.

The reason for the client problem is that you're only processing one request in ProcessRequest(), as its name suggests, and as you describe in your comment. Use a loop, reading requests until recv() returns zero or an error occurs.