How do I measure time in C?

user_110028 0
Viewed 81932

I want to find out for how long (approximately) some block of code executes. Something like this:

startStopwatch();
// do some calculations
stopStopwatch();
printf("%lf", timeMesuredInSeconds);

How?

winapi time c windows
8 Answers

The standard C library provides the time function and it is useful if you only need to compare seconds. If you need millisecond precision, though, the most portable way is to call timespec_get. It can tell time up to nanosecond precision, if the system supports. Calling it, however, takes a bit more effort because it involves a struct. Here's a function that just converts the struct to a simple 64-bit integer.

#include <stdio.h>
#include <inttypes.h>
#include <time.h>

int64_t millis()
{
    struct timespec now;
    timespec_get(&now, TIME_UTC);
    return ((int64_t) now.tv_sec) * 1000 + ((int64_t) now.tv_nsec) / 1000000;
}

int main(void)
{
    printf("Unix timestamp with millisecond precision: %" PRId64 "\n", millis());
}

Unlike clock, this function returns a Unix timestamp so it will correctly account for the time spent in blocking functions, such as sleep. This is a useful property for benchmarking and implementing delays that take running time into account.

user_134707540

For sake of completeness, there is more precise clock counter than GetTickCount() or clock() which gives you only 32-bit result that can overflow relatively quickly. It's QueryPerformanceCounter(). QueryPerformanceFrequency() gets clock frequency which is a divisor for two counters difference. Something like CLOCKS_PER_SEC in <time.h>.

#include <stdio.h>
#include <windows.h>

int main()
    {
    LARGE_INTEGER tu_freq, tu_start, tu_end;
    __int64 t_ns;

    QueryPerformanceFrequency(&tu_freq);
    QueryPerformanceCounter(&tu_start);
    /* do your stuff */
    QueryPerformanceCounter(&tu_end);

    t_ns = 1000000000ULL * (tu_end.QuadPart - tu_start.QuadPart) / tu_freq.QuadPart;
    printf("dt = %g[s]; (%llu)[ns]\n", t_ns/(double)1e+9, t_ns);

    return 0;
    }
user_84061720
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