Advanced Programming in the UNIX® Environment

ptg10805159

Section 16.5 Data Transfer 621

int sockfd, err;

struct sigaction sa;

if (argc != 2)

err_quit("usage: ruptime hostname");

sa.sa_handler = sigalrm;

sa.sa_flags = 0;

sigemptyset(&sa.sa_mask);

if (sigaction(SIGALRM, &sa, NULL) < 0)

err_sys("sigaction error");

memset(&hint, 0, sizeof(hint));

hint.ai_socktype = SOCK_DGRAM;

hint.ai_canonname = NULL;

hint.ai_addr = NULL;

hint.ai_next = NULL;

if ((err = getaddrinfo(argv[1], "ruptime", &hint, &ailist)) != 0)

err_quit("getaddrinfo error: %s", gai_strerror(err));

for (aip = ailist; aip != NULL; aip = aip->ai_next) {

if ((sockfd = socket(aip->ai_family, SOCK_DGRAM, 0)) < 0) {

err = errno;

}else {

print_uptime(sockfd, aip);

exit(0);

}

}

fprintf(stderr, "can’t contact %s: %s\n", argv[1], strerror(err));

exit(1);

}

Figure 16.19 Client command using datagram service

The main function for the datagram-based client is similar to the one for the

connection-oriented client, with the addition of installing a signal handler forSIGALRM.

We use thealarmfunction to avoid blocking indefinitely in the call torecvfrom.

With the connection-oriented protocol, we needed to connect to the server before

exchanging data. The arrival of the connect request was enough for the server to

determine that it needed to provide service to a client. But with the datagram-based

protocol, we need a way to notify the server that we want it to perform its service on

our behalf. In this example, we simply send the server a 1-byte message. The server

will receive it, get our address from the packet, and use this address to transmit its

response. If the server offered multiple services, we could use this request message to

indicate the service we want, but since the server does only one thing, the content of the

1 - byte message doesn’t matter.

If the server isn’t running, the client will block indefinitely in the call torecvfrom.

With the connection-oriented example, theconnectcall will fail if the server isn’t

running. Toavoid blocking indefinitely, we set an alarm clock beforecallingrecvfrom.