11.15 udp_connect Function

Our udp_connect function creates a connected UDP socket.

#include "unp.h"
int udp_connect (const char *hostname, const char *service);
Returns: connected socket descriptor if OK, no return on error

With a connected UDP socket, the final two arguments required by udp_client are no longer needed. The caller can call write instead of sendto, so our function need not return a socket address structure and its length.

Figure 11.17 shows the source code.

Figure 11.17 udp_connect function: creates a connected UDP socket.

lib/udp_connect.c

 1 #include     "unp.h"

 2 int
 3 udp_connect (const char *host, const char *serv)
 4 {
 5     int     sockfd, n;
 6     struct addrinfo hints, *res, *ressave;

 7     bzero (&hints, sizeof (struct addrinfo));
 8     hints.ai_family = AF_UNSPEC;
 9     hints.ai_socktype = SOCK_DGRAM;

10     if ( (n = getaddrinfo (host, serv, &hints, &res)) != 0)
11         err_quit ("udp_connect error for %s, %s: %s",
12                  host, serv, gai_strerror (n));
13     ressave = res;

14     do {
15         sockfd = socket (res->ai_family, res->ai_socktype, res->ai_protocol);
16         if (sockfd < 0)
17             continue;         /* ignore this one */

18         if (connect (sockfd, res->ai_addr, res->ai_addrlen) == 0)
19             break;            /* success */

20         Close (sockfd);       /* ignore this one */
21     } while ( (res = res->ai_next) ! = NULL);

22     if (res == NULL)          /* errno set from final connect () */
23         err_sys ("udp_connect error for %s, %s", host, serv);

24     freeaddrinfo (ressave);

25     return (sockfd);
26 }

This function is nearly identical to tcp_connect. One difference, however, is that the call to connect with a UDP socket does not send anything to the peer. If something is wrong (the peer is unreachable or there is no server at the specified port), the caller does not discover that until it sends a datagram to the peer.