In this article I will present to you a POSIX function I wrote that can be used to create a server socket to support both IPv4 and IPv6 addresses (IP-Agnostic). Recently, I needed to support this feature in a server application. The implementation uses POSIX system calls and data structures that can be used as generics to support both IPv4 and IPv6.

Below, is the header file introducing the function interface:

/*
 *  Copyright (C) 2014  Efstathios Chatzikyriakidis (stathis.chatzikyriakidis@gmail.com).
 *
 *  This program is free software: you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation, either version 3 of the License, or
 *  (at your option) any later version.
 *
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with this program. If not, see <http://www.gnu.org/licenses/>.
 */

#ifndef _SOCKET_UTILITY_H_
#define _SOCKET_UTILITY_H_

#include <sys/socket.h>

#include "shared-types.h"

/*
 * function prototypes.
 */

int create_server_socket (const string_t const server_address,
                          const string_t const server_port,
                          const int ai_flags,
                          const int ai_family,
                          const int ai_protocol,
                          const int ai_socktype,
                          socklen_t * const server_address_length);

#endif // _SOCKET_UTILITY_H_

Next, follows an example of a client that wants to create an IP-Agnostic server socket:

/*
 *  Copyright (C) 2014  Efstathios Chatzikyriakidis (stathis.chatzikyriakidis@gmail.com).
 *
 *  This program is free software: you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation, either version 3 of the License, or
 *  (at your option) any later version.
 *
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with this program. If not, see <http://www.gnu.org/licenses/>.
 */

#include <netdb.h>

#include "socket-utility.h"

/*
 * functions.
 */

bool_t
server_run (void)
{
  ...

  socklen_t server_address_length;

  const int server_socket_descriptor =
    create_server_socket (options->server_address,
                          options->server_port,
                          AI_PASSIVE | AI_NUMERICSERV,
                          AF_UNSPEC,
                          IPPROTO_TCP,
                          SOCK_STREAM,
                          &server_address_length);
  
  ...
}

Lastly, the implementation of the create_server_socket function:

/*
 *  Copyright (C) 2014  Efstathios Chatzikyriakidis (stathis.chatzikyriakidis@gmail.com).
 *
 *  This program is free software: you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation, either version 3 of the License, or
 *  (at your option) any later version.
 *
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with this program. If not, see <http://www.gnu.org/licenses/>.
 */

#include <netdb.h>
#include <string.h>
#include <unistd.h>

#include "logging-utility.h"
#include "socket-utility.h"
#include "i18n-macros.h"

/*
 * messages.
 */

static const string_t const failed_to_close_server_socket_descriptor_message =
  N_("Failed to close the server socket descriptor.");

/*
 * functions.
 */

int
create_server_socket (const string_t const server_address,
                      const string_t const server_port,
                      const int ai_flags,
                      const int ai_family,
                      const int ai_protocol,
                      const int ai_socktype,
                      socklen_t * const server_address_length)
{
  struct addrinfo hints;

  memset (&hints, 0, sizeof (hints));

  hints.ai_family = ai_family;
  hints.ai_socktype = ai_socktype;
  hints.ai_protocol = ai_protocol;
  hints.ai_flags = ai_flags;

  struct addrinfo * server_addresses = NULL;

  const int return_value = getaddrinfo (server_address,
                                        server_port,
                                        &hints,
                                        &server_addresses);

  if (return_value != 0)
  {
    log_error ("%s", gai_strerror (return_value));

    return -1;
  }

  bool_t server_socket_found = false;

  int server_socket_descriptor = -1;

  const int reuse_address_on = 1;

  for (struct addrinfo * address = server_addresses;
       address;
       address = address->ai_next)
  {
    server_socket_descriptor = socket (address->ai_family,
                                       address->ai_socktype,
                                       address->ai_protocol);

    if (server_socket_descriptor < 0)
    {
      continue;
    }

    if (setsockopt (server_socket_descriptor,
                    SOL_SOCKET,
                    SO_REUSEADDR,
                    &reuse_address_on,
                    sizeof (reuse_address_on)) != -1)
    {
      if (bind (server_socket_descriptor,
                address->ai_addr,
                address->ai_addrlen) != -1)
      {
        *server_address_length = address->ai_addrlen;

        server_socket_found = true;

        break;
      }
    }

    if (close (server_socket_descriptor) < 0)
    {
      log_error (_(failed_to_close_server_socket_descriptor_message));

      break;
    }
  } 

  freeaddrinfo (server_addresses);

  return server_socket_found ? server_socket_descriptor : -1;
}