Get the NTP time in a C++ programm via a simple socket

If you are writing an application it sometimes my be useful to have the exact time. Usually the time of the operating system is not synchronized. However, there is a very old protocol which available which allows servers so synchronize themselves: The Network Timing Protocol (NTP). It uses UDP port 123 for its communication. There is a huge network of free NTP time servers available in the web. The following example program connects to such a timing server and retrieves the NTP time from that server. For that it opens a simple socket connection in C++. The program can be compiled executed as it is here without any changes and also calculates the difference between the NTP time to your current system time. Furthermore you also learn how to use sockets in C++. We create a simple massage here, sent it via a socket to the NTP timing server and then receive the reply which is the desired time.

[sourcecode language=”cpp”]
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <time.h>
#include <string.h>
#include <iostream>
void ntpdate();

int main() {

return 0;

void ntpdate() {
// can be any timing server
// you might have to change the IP if the server is no longer available
char *hostname=(char *)"";
// ntp uses port 123
int portno=123;
int maxlen=1024;
int i;
// buffer for the socket request
unsigned char msg[48]={010,0,0,0,0,0,0,0,0};
// buffer for the reply
unsigned long buf[maxlen];
//struct in_addr ipaddr;
struct protoent *proto; //
struct sockaddr_in server_addr;
int s; // socket
long tmit; // the time — This is a time_t sort of

// open a UDP socket
s=socket(PF_INET, SOCK_DGRAM, proto->p_proto);

//here you can convert hostname to ipaddress if needed
//$ipaddr = inet_aton($HOSTNAME);

memset( &server_addr, 0, sizeof( server_addr ));
server_addr.sin_addr.s_addr = inet_addr(hostname);

* build a message. Our message is all zeros except for a one in the
* protocol version field
* msg[] in binary is 00 001 000 00000000
* it should be a total of 48 bytes long

// send the data to the timing server
i=sendto(s,msg,sizeof(msg),0,(struct sockaddr *)&server_addr,sizeof(server_addr));
// get the data back
struct sockaddr saddr;
socklen_t saddr_l = sizeof (saddr);
// here we wait for the reply and fill it into our buffer

//We get 12 long words back in Network order

* The high word of transmit time is the 4th word we get back
* tmit is the time in seconds not accounting for network delays which
* should be way less than a second if this is a local NTP server

tmit=ntohl((time_t)buf[4]); //# get transmit time

tmit-= 2208988800U;
std::cout << "NTP time is " << ctime(&tmit) << std::endl;
std::cout << "System time is " << (i-tmit) << " seconds off" << std::endl;


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