Implementing createServerSocket

• The code is dense, but totally accessible to us.
  • Provided a server is prepared to listen to a specified port on any of its IP addresses, the following function returns a properly configured server socket:

  static const int kServerSocketFailure = -1; // sentinel for no valid socket
  static const int kReuseAddresses = 1;   // 1 means true here
  static const int kDefaultBacklog = 128; // allow 128 clients to queue up before they are "accept"ed, drop/ignore 129th
  int createServerSocket(unsigned short port) {
    int serverSocket = socket(AF_INET, SOCK_STREAM, 0);
    if (serverSocket < 0) return kServerSocketFailure;
    if (setsockopt(serverSocket, SOL_SOCKET, SO_REUSEADDR,
                   &kReuseAddresses, sizeof(int)) < 0) {
      close(serverSocket);
      return kServerSocketFailure;
    } // setsockopt used here so port becomes available even is server crashes and reboots
  
    struct sockaddr_in serverAddress; // IPv4-style socket address
    memset(&serverAddress, 0, sizeof(serverAddress));
    serverAddress.sin_family = AF_INET; // sin_family field used to self-identify sockaddr type
    serverAddress.sin_addr.s_addr = htonl(INADDR_ANY);
    serverAddress.sin_port = htons(port);
  
    if (bind(serverSocket, (struct sockaddr *) &serverAddress, sizeof(struct sockaddr_in)) == 0 &&
        listen(serverSocket, kDefaultBacklog) == 0) return serverSocket;
  
    close(serverSocket);
    return kServerSocketFailure;
  }

  • Note that createServerSocket returns a socket we listen to for incoming connections.
    • It's categorized as a descriptor, so it needs to be closed when we're done with it.
    • It also gets cloned across fork boundaries as any descriptor would.
    • Server sockets, however, are not compatible with read and write system calls. The only "read"-oriented system call it's compatible with is accept.