std :: gmtime does not return UTC time

Hello!

The following code should print the current UTC and local times, respectively:

#pragma (disable : 4996)

#include <ctime>
using namespace std;

int main()
{
 tm* p_current_tm_0 = gmtime(&current_time);
 tm* p_current_tm_ = localtime(&current_time);

 char* current_tm_0 = asctime(p_current_tm_0);
 cout << "Current UTC date and time: " << current_tm_0;

 char* current_tm_ = asctime(p_current_tm_);
 cout << "Current local date and time: " << current_tm_;
}

However, when I run the program, the output is the local time repeated twice.

I do not understand why that is the case.

>Solution :

The problem here is that gmtime and localtime share a single global struct tm and return a pointer to that. Thus, the localtime call overwrites the structure.

Try this instead, which grabs a copy of the structure:

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

int main()
{
    time_t current_time = time(NULL);
    tm p_current_tm_0 = * gmtime(&current_time);
    tm p_current_tm_ = * localtime(&current_time);

    char* current_tm_0 = asctime(&p_current_tm_0);
    cout << "Current UTC date and time: " << current_tm_0;

    char* current_tm_ = asctime(&p_current_tm_);
    cout << "Current local date and time: " << current_tm_;
}

(Yes, the variable names are no longer accurate, since they aren’t pointers.)

Alternatively, change the order:

    tm * p_current_tm_0 = gmtime(&current_time);
    char* current_tm_0 = asctime(p_current_tm_0);
    cout << "Current UTC date and time: " << current_tm_0;

    tm * p_current_tm_ = localtime(&current_time);
    char* current_tm_ = asctime(p_current_tm_);
    cout << "Current local date and time: " << current_tm_;