CODEWITHSUNDEEP
c++linuxwindowstime
Boris
Boris

Reputation: 369

Get seconds since epoch in Linux

Is there cross-platform solution to get seconds since epoch, for windows i use

long long NativesGetTimeInSeconds()
{
    return time (NULL);
}

But how to get on Linux?

Upvotes: 19

Views: 45867

Answers (5)

iurfewijhn
iurfewijhn

Reputation: 1

Here's the ultimate solution to get seconds (as integer or as a double), or to get milliseconds or whatever you want

#include <chrono>

template <typename Duration, typename Clock>
Duration get_duration_since_epoch()
{
    const auto tp = std::chrono::time_point_cast<Duration>(Clock::now());
    return tp.time_since_epoch();
}

Usage:

  • seconds since epoch as integer-type
// std::chrono::seconds since epoch
get_duration_since_epoch<std::chrono::seconds, std::chrono::system_clock>()

// get seconds since epoch as integer-type
get_duration_since_epoch<std::chrono::seconds, std::chrono::system_clock>().count()
  • seconds since epoch as double-type
using double_sec_t = std::chrono::duration<double, std::chrono::seconds::period>;

// double_sec_t since epoch
get_duration_since_epoch<double_sec_t, std::chrono::system_clock>()

// get seconds since epoch as double-type
get_duration_since_epoch<double_sec_t, std::chrono::system_clock>().count()
  • milliseconds since epoch as integer-type
// std::chrono::milliseconds since epoch
get_duration_since_epoch<std::chrono::milliseconds, std::chrono::system_clock>()

// get milliseconds since epoch as integer-type
get_duration_since_epoch<std::chrono::milliseconds, std::chrono::system_clock>().count()

https://wandbox.org/permlink/HULwKXGyc5m0pIVQ

Upvotes: 0

user14803978
user14803978

Reputation:

The Simple, Portable, and Proper Approach

#include <ctime>

long CurrentTimeInSeconds()
{
     return (long)std::time(0); //Returns UTC in Seconds
}

Upvotes: 2

Zeta
Zeta

Reputation: 105876

You're already using it: std::time(0) (don't forget to #include <ctime>). However, whether std::time actually returns the time since epoch isn't specified in the standard (C11, referenced by the C++ standard):

7.27.2.4 The time function

Synopsis

#include <time.h>
time_t time(time_t *timer);

Description

The time function determines the current calendar time. The encoding of the value is unspecified. [emphasis mine]

For C++, C++11 and later provide time_since_epoch. However, before C++20 the epoch of std::chrono::system_clock was unspecified and therefore possibly non-portable in previous standards.

Still, on Linux the std::chrono::system_clock will usually use Unix Time even in C++11, C++14 and C++17, so you can use the following code:

#include <chrono>

// make the decltype slightly easier to the eye
using seconds_t = std::chrono::seconds;

// return the same type as seconds.count() below does.
// note: C++14 makes this a lot easier.
decltype(seconds_t().count()) get_seconds_since_epoch()
{
    // get the current time
    const auto now     = std::chrono::system_clock::now();

    // transform the time into a duration since the epoch
    const auto epoch   = now.time_since_epoch();

    // cast the duration into seconds
    const auto seconds = std::chrono::duration_cast<std::chrono::seconds>(epoch);
    
    // return the number of seconds
    return seconds.count();
}

Upvotes: 31

Quentin Perez
Quentin Perez

Reputation: 2903

In C.

time(NULL);

In C++.

std::time(0);

And the return value of time is : time_t not long long

Upvotes: 17

Mats Petersson
Mats Petersson

Reputation: 129314

The native Linux function for getting time is gettimeofday() [there are some other flavours too], but that gets you the time in seconds and nanoseconds, which is more than you need, so I would suggest that you continue to use time(). [Of course, time() is implemented by calling gettimeofday() somewhere down the line - but I don't see the benefit of having two different pieces of code that does exactly the same thing - and if you wanted that, you'd be using GetSystemTime() or some such on Windows [not sure that's the right name, it's been a while since I programmed on Windows]

Upvotes: 2

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