passing time info from NTP server to strftime function

Question

I am using the code ntp client code at the bottom by referring the webpage on here. The code receives the time info then I would like to store the time info as YYYYMMDDHHMM like 201304211405. The code receives the time info from the NTP server, but I feel trouble to find out how to pass that info to the strftime, how should I pass the received time info to the strftime?

Here is the relevant part of the code

i=recv(s,buf,sizeof(buf),0);

tmit=ntohl((time_t)buf[10]);    //# get transmit time
tmit-= 2208988800U;
printf("tmit=%d\n",tmit);

//#compare to system time
printf("Time is time: %s",ctime(&tmit));
char buffer[13];
struct tm * timeinfo;
timeinfo = ctime(&tmit);

strftime (buffer,13,"%04Y%02m%02d%02k%02M",timeinfo);
printf("new buffer:%s\n" ,buffer);

Here is the full code that I am using

#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>

void ntpdate();

int main() {
    ntpdate();
    return 0;
}

void ntpdate() {
char    *hostname="79.99.6.190 2";
int portno=123;     //NTP is port 123
int maxlen=1024;        //check our buffers
int i;          // misc var i
unsigned char msg[48]={010,0,0,0,0,0,0,0,0};    // the packet we send
unsigned long  buf[maxlen]; // the buffer we get back
//struct in_addr ipaddr;        //
struct protoent *proto;     //
struct sockaddr_in server_addr;
int s;  // socket
int tmit;   // the time -- This is a time_t sort of

//use Socket;
proto=getprotobyname("udp");
s=socket(PF_INET, SOCK_DGRAM, proto->p_proto);

memset( &server_addr, 0, sizeof(server_addr));
server_addr.sin_family=AF_INET;
server_addr.sin_addr.s_addr = inet_addr(hostname);
server_addr.sin_port=htons(portno);
// send the data
i=sendto(s,msg,sizeof(msg),0,(struct sockaddr *)&server_addr,sizeof(server_addr));


/***************HERE WE START**************/
// get the data back
i=recv(s,buf,sizeof(buf),0);

tmit=ntohl((time_t)buf[10]);    //# get transmit time
tmit-= 2208988800U;
printf("tmit=%d\n",tmit);

//#compare to system time
printf("Time is time: %s",ctime(&tmit));
char buffer[13];
struct tm * timeinfo;
timeinfo = ctime(&tmit);

strftime (buffer,13,"%04Y%02m%02d%02k%02M",timeinfo);
printf("new buffer:%s\n" ,buffer);
}

Answer 1

The problem is with the line...

timeinfo = ctime(&tmit);

If timeinfo is of type struct tm *, you can't just point it to the char * human-readable string returned by ctime().

If you're converting to struct tm *, you'll need to use either gmtime() or localtime(), depending on whether you want the struct tm * to be a UTC time, or expressed relative to your local timezone.

Since ctime() uses the local timezone, I'll assume you want it that way, so replace that line with...

timeinfo = localtime(&tmit);