C++ obtaining milliseconds time on Linux — clock()

Question

On Windows, clock() returns the time in milliseconds, but on this Linux box I'm working on, it rounds it to the nearest 1000 so the precision is only to the "second" level and not to the milliseconds level.

I found a solution with Qt using the QTime class, instantiating an object and calling start() on it then calling elapsed() to get the number of milliseconds elapsed.

I got kind of lucky because I'm working with Qt to begin with, but I'd like a solution that doesn't rely on third party libraries,

Is there no standard way to do this?

UPDATE

Please don't recommend Boost ..

If Boost and Qt can do it, surely it's not magic, there must be something standard that they're using!

Answer 1

clock() doesn't return milliseconds or seconds on linux. Usually clock() returns microseconds on a linux system. The proper way to interpret the value returned by clock() is to divide it by CLOCKS_PER_SEC to figure out how much time has passed.

Answer 2

gettimeofday - the problem is that will can have lower values if you change you hardware clock (with NTP for example) Boost - not available for this project clock() - usually returns a 4 bytes integer, wich means that its a low capacity, and after some time it returns negative numbers.

I prefer to create my own class and update each 10 miliseconds, so this way is more flexible, and I can even improve it to have subscribers.

class MyAlarm {
static int64_t tiempo;
static bool running;
public:
static int64_t getTime() {return tiempo;};
static void callback( int sig){
    if(running){
        tiempo+=10L;
    }
}
static void run(){ running = true;}
};

int64_t MyAlarm::tiempo = 0L;
bool MyAlarm::running = false;

to refresh it I use setitimer:

int main(){
struct sigaction sa; 
struct itimerval timer; 

MyAlarm::run();
memset (&sa, 0, sizeof (sa)); 
sa.sa_handler = &MyAlarm::callback; 

sigaction (SIGALRM, &sa, NULL); 


timer.it_value.tv_sec = 0; 
timer.it_value.tv_usec = 10000; 



timer.it_interval.tv_sec = 0; 
timer.it_interval.tv_usec = 10000; 


setitimer (ITIMER_REAL, &timer, NULL); 
.....

Look at setitimer and the ITIMER_VIRTUAL and ITIMER_REAL.

Don't use the alarm or ualarm functions, you will have low precision when your process get a hard work.

Answer 3

With C++11 and std::chrono::high_resolution_clock you can do this:

#include <iostream>
#include <chrono>
#include <thread>
typedef std::chrono::high_resolution_clock Clock;

int main()
{
    std::chrono::milliseconds three_milliseconds{3};

    auto t1 = Clock::now();
    std::this_thread::sleep_for(three_milliseconds);
    auto t2 = Clock::now();

    std::cout << "Delta t2-t1: " 
              << std::chrono::duration_cast<std::chrono::milliseconds>(t2 - t1).count()
              << " milliseconds" << std::endl;
}

Output:

Delta t2-t1: 3 milliseconds

Link to demo: http://cpp.sh/2zdtu

Answer 4

In the POSIX standard clock has its return value defined in terms of the CLOCKS_PER_SEC symbol and an implementation is free to define this in any convenient fashion. Under Linux, I have had good luck with the times() function.

Answer 5

I prefer the Boost Timer library for its simplicity, but if you don't want to use third-parrty libraries, using clock() seems reasonable.

Answer 6

This should work...tested on a mac...

#include <stdio.h>
#include <sys/time.h>

int main() {
        struct timeval tv;
        struct timezone tz;
        struct tm *tm;
        gettimeofday(&tv,&tz);
        tm=localtime(&tv.tv_sec);
        printf("StartTime: %d:%02d:%02d %d \n", tm->tm_hour, tm->tm_min, tm->tm_sec, tv.tv_usec);
}

Yeah...run it twice and subtract...