C++: how to get the actual time with time and localtime?

Question

I'm looking for a way to save the time in a HH::MM::SS fashion in C++. I saw here that they are many solutions and after a little research I opted for time and localtime. However, it seems like the localtime function is a little tricky, since it says:

All calls to localtime and gmtime use the same static structure, so each call overwrites the results of the previous call.

The problem that this causes is shown in the next snippet of code:

#include <ctime>
#include <iostream>
using namespace std;

int main() {
time_t t1 = time(0);   // get time now
struct tm * now = localtime( & t1 );

std::cout << t1 << std::endl;
sleep(2);
time_t t2 = time(0);   // get time now
struct tm * now2 = localtime( & t2 );
std::cout << t2 << std::endl;

cout << (now->tm_year + 1900) << '-'
     << (now->tm_mon + 1) << '-'
     <<  now->tm_mday << ", "
     << now->tm_hour << ":" << now->tm_min << ":" << now->tm_sec
     << endl;

cout << (now2->tm_year + 1900) << '-'
     << (now2->tm_mon + 1) << '-'
     <<  now2->tm_mday << ", "
     << now2->tm_hour << ":" << now2->tm_min << ":" << now2->tm_sec
     << endl;
}

A typical output for this is:

1320655946
1320655948
2011-11-7, 9:52:28
2011-11-7, 9:52:28

So as you can see, the time_t timestamps are correct, but the localtime messes everything up.

My question is: how do I convert a timestamp ot type time_t into a human-readable time?

Answer 1

If you are worried about reentrancy in localtime and gmtime, there is localtime_r and gmtime_r which can handle multiple calls.

When it comes to formatting the time to your liking, check the function strftime.