udptime.c

comer s TCPIP-vol3源代码...当然是经典咯

/* UDPtime.c - main */

#include <sys/types.h>

#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <stdio.h>

#define	BUFSIZE 64

#define	UNIXEPOCH	2208988800UL	/* UNIX epoch, in UCT secs	*/
#define	MSG		"what time is it?\n"

extern int	errno;

int	connectUDP(const char *host, const char *service);
int	errexit(const char *format, ...);

/*------------------------------------------------------------------------
 * main - UDP client for TIME service that prints the resulting time
 *------------------------------------------------------------------------
 */
int
main(int argc, char *argv[])
{
	char	*host = "localhost";	/* host to use if none supplied	*/
	char	*service = "time";	/* default service name		*/
	time_t	now;			/* 32-bit integer to hold time	*/ 
	int	s, n;			/* socket descriptor, read count*/

	switch (argc) {
	case 1:
		host = "localhost";
		break;
	case 3:
		service = argv[2];
		/* FALL THROUGH */
	case 2:
		host = argv[1];
		break;
	default:
		fprintf(stderr, "usage: UDPtime [host [port]]\n");
		exit(1);
	}

	s = connectUDP(host, service);

	(void) write(s, MSG, strlen(MSG));

	/* Read the time */

	n = read(s, (char *)&now, sizeof(now));
	if (n < 0)
		errexit("read failed: %s\n", strerror(errno));
	now = ntohl((unsigned long)now);	/* put in host order	*/
	now -= UNIXEPOCH;		/* convert UCT to UNIX epoch	*/
	printf("%s", ctime(&now));
	exit(0);
}