why C clock() returns 0

Question

I\'ve got something like this:

clock_t start, end;
start=clock();

something_else();

end=clock();
printf(\"\\nClock cycles are: %d - %d\\n\",start,end);
         


        

Answer 1

I have used the little program below to investigate wall clock time and CPU time.

On my test sytem this prints

CLOCKS_PER_SEC 1000000

CPU time usage resolution looks to be 0.010000 seconds

gettimeofday changed by 9634 uSwhen CPU time changed by 0.010000

gettimeofday resolution looks to be 1 us

#include 
#include 
#include 
#include 


int main(int argc, char** argv) {

    struct  timeval now; // wall clock times
    struct  timeval later;

    clock_t tNow = clock(); // clock measures CPU time of this Linux thread
    gettimeofday(&now, NULL); // wall clock time when CPU time first read

    clock_t tLater = tNow;
    while (tNow == tLater)
           tLater = clock(); // consume CPU time

    gettimeofday(&later, NULL); // wall clock time when CPU time has ticked

    printf("CLOCKS_PER_SEC %ld\n",CLOCKS_PER_SEC);

    double cpuRes = (double)(tLater - tNow)/CLOCKS_PER_SEC;

    printf("CPU time usage resolution looks to be %f seconds\n", cpuRes);

    unsigned long long nowUs = ((unsigned long long)now.tv_sec) * 1000000ULL;
    nowUs += (unsigned long long)now.tv_usec;

    unsigned long long laterUs = ((unsigned long long)later.tv_sec) * 1000000ULL;
    laterUs += (unsigned long long)later.tv_usec;

    printf("gettimeofday changed by %d uS when CPU time changed by %f seconds\n", (int)(laterUs - nowUs), cpuRes);

    // now measure resolution of gettimeofday

    gettimeofday(&now, NULL);
    later = now;

    while ((now.tv_sec  == later.tv_sec) && (now.tv_usec == later.tv_usec))
            gettimeofday(&later, NULL);

    nowUs = ((unsigned long long)now.tv_sec) * 1000000ULL;
    nowUs += (unsigned long long)now.tv_usec;

    laterUs = ((unsigned long long)later.tv_sec) * 1000000ULL;
    laterUs += (unsigned long long)later.tv_usec;

    printf("gettimeofday resolution looks to be %d us\n", (int)(laterUs - nowUs));

}