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Copyright (C) 2022 Frank Eskesen.

This file is free content, distributed under the MIT license. (See accompanying file LICENSE.MIT or the original contained within https://opensource.org/licenses/MIT)

This journal describes socket programming and usage techniques used to avoid practical problems.

If your code works, but not across machines

  • First, check your firewalls. Most likely it's the server's firewall disallowing an inbound socket, but it's also possible that outbound (client) restrictions exist.
  • Most routers handle UDP (packets) without any required setup. Check whether yours is an exception, especially if using multi-cast.

2022/05/15 Re-use of a socket connection's port pairs.

Problem: A client repeatedly makes short connections to the same server. After about 30K connections, the client is unable to make further connections getting the error "Address already in use."

If an client application connects to the same server application many times in a short interval, it's possible to run into a situation where the client fails to connect because the address port pair is already in use.

This occurs because the kernel does not immediately release the connection information when a socket is closed. Instead, the socket goes into the TIME_WAIT state. This logic is usually useful, since it prevents packets from old connections interfering with new connections. However, if a server can accept a large number of separate connections from the same client in a short interval, it may want reset a connection rather than just closing it when processing a client close. This is accomplished using SO_LINGER with linger l_onoff= 1 and l_linger=0 before the close is invoked. This immediately terminates the connection, avoiding the TIME_WAIT before the socket address pair can be reused.

Sample code:

    struct linger option;
    option.l_onoff= 1;
    option.l_linger= 0;
    int rc= setsockopt(handle, SOL_SOCKET, SO_LINGER, &option, sizeof(option));
    if( rc != 0 ) { /* Replace comment with your error recovery procedure */ }

2022/05/15 Server accept handling

Problem: It's possible for an accept operation to block. There are multiple possible causes for this. (One of them is as a result of a client being unable to connect because of the "Address already in use" problem described above.) This blocked accept condition leaves a server in an unusable and unstoppable state when a client and server reside on the same machine. The server is unable to immediately complete a connect operation to complete its blocked accept, because that connection's refused too.

A server can prevent the blocked condition from occurring. Before the accept operation, a server should either use polling code such as

     pollfd pfd= {};
     pfd.fd= handle;
     pfd.events= POLLIN;
     int rc= poll(&pfd, 1, 3000);   // Replace this ms timeout as needed
     if( rc <= 0 ) {                // If polling error or timeout
       // Handle timeout or error condition ...
     }
     client= ::accept(handle, ...); // (The accept won't block)

to insure that the accept won't block or non-blocking listener sockets.