Problems creating and binding port in unix. (New to network programming)

Question

Hi, I'm having trouble creating a port in Unix. This code keeps returning "Error creating socket (3)", which makes no sense whatsoever, since sockfd must be smaller than 0 to print the error line.

server.c:

int main (int argc, char **argv)
{
  int sockfd;
  struct sockaddr_in server_sockaddr;
  char buff[TRANSFER_BUFFER_LEN];

  /* check number of command line arguments */
  if (argc < 2)
  {
    fprintf (stderr, "Usage: %s <port>\n");
    exit (EXIT_FAILURE);
  }

  /* create server socket */
  if ((sockfd = createUdpSocket (&sockfd,
                                 &server_sockaddr,
                                 atoi (*(argv + 1)))
      ) < 0);
  {
    fprintf (stderr, "Error creating socket (%d).\n", sockfd);
    exit (EXIT_FAILURE);
  }

  ...

return 0;
}

socket_utils.h:

int createUdpSocket (int *sockfd, struct sockaddr_in *client_sockaddr, int port)
{
  /* create socket */
  if ((*sockfd = socket (AF_INET, SOCK_DGRAM, 0)) == -1)
    return -1;

  memset (client_sockaddr, 0, sizeof (struct sockaddr_in));
  client_sockaddr->sin_family = AF_INET;
  client_sockaddr->sin_addr.s_addr = htonl (INADDR_ANY);
  client_sockaddr->sin_port = htons (port);

  /* bind socket */
  if (!bind (*sockfd, (struct sockaddr*) &client_sockaddr, sizeof (sockfd)))
  {
    close (*sockfd);
    return -2;
  }

  return *sockfd;
}

Any ideas? This same function works fine in my client program that doesn't take a port value (instead takes 0).

Cheers,

Rhys

Answer 1

if ((sockfd = createUdpSocket (&sockfd,
                                 &server_sockaddr,
                                 atoi (*(argv + 1)))
      ) < 0);

Whoops, where'd that semi-colon come from? You've got a stowaway!

---

Also, this line won't do what you expect:

if (!bind (*sockfd, (struct sockaddr*) &client_sockaddr, sizeof (sockfd)))

The bind() function returns 0 on success, -1 on failure. Your test is backwards and will error out when bind succeeds rather than fails. It's also got an incorrect & where you say &client_sockaddr. And the third argument needs to be the size of the sockaddr_in struct, not sizeof(sockfd). Try:

if (bind (*sockfd, (struct sockaddr*) client_sockaddr, sizeof (struct sockaddr_in)) != 0)

---

And heck while we're here, your usage printout is missing its argv[0] argument:

fprintf (stderr, "Usage: %s <port>\n");

Answer 2

Looks like you have too many parentheses which is confusing the code and yourself....Also, here is a useful guide to understanding network programming with sockets courtesy of beej...