First UDP socket example: NTP client

Author: alexander-naumov

sockets/UDP/ntp_client.c

@@ -0,0 +1,86 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <time.h>
+#include <sys/types.h>
+#include <sys/socket.h>
+#include <netinet/in.h>
+#include <netdb.h>
+
+#define oops(msg,x) {perror(msg);exit(x);}
+#define VERSION "0.1"
+#define NTP_TIMESTAMP_DELTA 2208988800ull
+
+void usage(char** argv)
+{
+	printf("Yet another NTP client v%s\n", VERSION);
+	printf("Usage:   %s <hostname OR ip_address>\n", *argv); //example: 64.113.32.5
+	exit(1);
+}
+
+int main(int argc, char* argv[])
+{
+	if (argc != 2)
+		usage(argv);
+
+	typedef struct 
+	{
+		unsigned leap	: 2;
+		unsigned ver	: 3;
+		unsigned mode	: 3;
+		
+		uint8_t stratum;
+		uint8_t poll;
+		uint8_t precision;
+		
+		uint32_t root_delay;
+		uint32_t root_dis;
+		uint32_t ref_id;
+		
+		uint32_t ref_Tm_s;
+		uint32_t ref_Tm_f;
+		
+		uint32_t orig_Tm_s;
+		uint32_t orig_Tm_f;
+		
+		uint32_t rcv_Tm_s;
+		uint32_t rcv_Tm_f;
+		
+		uint32_t tsm_Tm_s;
+		uint32_t tsm_xTm_f;
+	} ntp_packet;
+	
+	ntp_packet packet = {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0};
+	*((char *) &packet + 0) = 0x1b;
+
+	struct sockaddr_in serv_addr;
+	struct hostent *server;
+	int sockfd;
+
+	if ((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) < 0)
+		oops("can't create socket", 2);
+	
+	if ((server = gethostbyname(*++argv)) == NULL)
+		oops("can't find this host", 2);
+	
+	bzero((void *)&serv_addr, sizeof(serv_addr));
+	bcopy((void *)server->h_addr, (void *)&serv_addr.sin_addr, server->h_length);
+	
+	serv_addr.sin_port = htons(123);
+  serv_addr.sin_family = AF_INET;
+	
+	if (sendto(sockfd, &packet, sizeof(packet), 0,
+						(struct sockaddr *) &serv_addr, sizeof(serv_addr)) == -1)
+		oops("sendto failed", 3);
+
+	socklen_t saddrlen = sizeof(serv_addr);	
+	if (recvfrom(sockfd, (char*) &packet, sizeof(packet), 0,
+              (struct sockaddr *) &serv_addr, &saddrlen) == -1)
+		oops("recvfrom failed", 3)
+
+	packet.tsm_Tm_s = ntohl(packet.tsm_Tm_s);
+	time_t tTm = (time_t) (packet.tsm_Tm_s - NTP_TIMESTAMP_DELTA); 
+	
+	printf("Time: %s", ctime((const time_t*) &tTm));
+	return 0;
+}