[SOLVED] Time conversions not deterministic

Issue

I have written a program which should convert iso 8601 strings into std::chrono::system_clock::time_point. But it has sometimes an indeterministic behavior. If you execute the program multiple times, the parsed timestamps differ by 1 hour.

As far as i know, when you want to convert date-strings(iso 8601) to std::chrono::system_clock types, you currently need some functions from ctime (this means, that you need to use c-functions), if you don't want to use external libraries.

So i used strptime(), mktime() and std::tm_t.

Here is the MWE:

#include <chrono>
#include <ctime>
#include <iostream>
#include <sstream>

int main(void) {
    std::tm t;
    strptime("2016-01-01T00:00:00", "%Y-%m-%dT%H:%M:%S", &t);
    auto time = mktime(&t);
    std::cout << "UNIX-seconds: " << time << std::endl;
    //std::cout << "tm-hours: " << t.tm_hour << ", UNIX-seconds: " << time << std::endl;
    auto tp = std::chrono::system_clock::from_time_t(time);
    {
        std::stringstream ss;
        std::chrono::system_clock::to_time_t(tp);
    }
}

If you compile it with  g++ --std=c++14 main.cpp -o out

and run it multiple times, I get one of the following 2 outputs:

UNIX-seconds: 1451599200

or

UNIX-seconds: 1451602800

But I currently don't know why I get sometimes the first and sometimes the second one.

If you uncomment the commented line, the indeterministic behavior goes away and I get the following line:

tm-hours: 0, UNIX-seconds: 1451602800

What can I do, to get deterministic behavior?

I have debian 8, g++v4.9 and my pc is in timezone Europe/Vienna.

Solution

mktime respects tm_isdst member of std::tm. strptime does not change that flag and it retais its previous value. As std::tm is POD, tm_isdst is not initialized by default construction, so you need to initialize it manually, or you will get indeteminate value.

Answered By - Revolver_Ocelot