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| <h4 class="subsection">16.10.4 Example of Reading Datagrams</h4> |
| |
| <p>Here is the client program corresponding to the server above. |
| |
| <p>It sends a datagram to the server and then waits for a reply. Notice |
| that the socket for the client (as well as for the server) in this |
| example has to be given a name. This is so that the server can direct |
| a message back to the client. Since the socket has no associated |
| connection state, the only way the server can do this is by |
| referencing the name of the client. |
| |
| <pre class="smallexample"> #include <stdio.h> |
| #include <errno.h> |
| #include <unistd.h> |
| #include <stdlib.h> |
| #include <sys/socket.h> |
| #include <sys/un.h> |
| |
| #define SERVER "/tmp/serversocket" |
| #define CLIENT "/tmp/mysocket" |
| #define MAXMSG 512 |
| #define MESSAGE "Yow!!! Are we having fun yet?!?" |
| |
| int |
| main (void) |
| { |
| extern int make_named_socket (const char *name); |
| int sock; |
| char message[MAXMSG]; |
| struct sockaddr_un name; |
| size_t size; |
| int nbytes; |
| |
| /* <span class="roman">Make the socket.</span> */ |
| sock = make_named_socket (CLIENT); |
| |
| /* <span class="roman">Initialize the server socket address.</span> */ |
| name.sun_family = AF_LOCAL; |
| strcpy (name.sun_path, SERVER); |
| size = strlen (name.sun_path) + sizeof (name.sun_family); |
| |
| /* <span class="roman">Send the datagram.</span> */ |
| nbytes = sendto (sock, MESSAGE, strlen (MESSAGE) + 1, 0, |
| (struct sockaddr *) & name, size); |
| if (nbytes < 0) |
| { |
| perror ("sendto (client)"); |
| exit (EXIT_FAILURE); |
| } |
| |
| /* <span class="roman">Wait for a reply.</span> */ |
| nbytes = recvfrom (sock, message, MAXMSG, 0, NULL, 0); |
| if (nbytes < 0) |
| { |
| perror ("recfrom (client)"); |
| exit (EXIT_FAILURE); |
| } |
| |
| /* <span class="roman">Print a diagnostic message.</span> */ |
| fprintf (stderr, "Client: got message: %s\n", message); |
| |
| /* <span class="roman">Clean up.</span> */ |
| remove (CLIENT); |
| close (sock); |
| } |
| </pre> |
| <p>Keep in mind that datagram socket communications are unreliable. In |
| this example, the client program waits indefinitely if the message |
| never reaches the server or if the server's response never comes |
| back. It's up to the user running the program to kill and restart |
| it if desired. A more automatic solution could be to use |
| <code>select</code> (see <a href="Waiting-for-I_002fO.html#Waiting-for-I_002fO">Waiting for I/O</a>) to establish a timeout period |
| for the reply, and in case of timeout either re-send the message or |
| shut down the socket and exit. |
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