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Measuring Short Time Slots in C on Ubuntu / VMware

I am trying to measure short time intervals within a few milliseconds. This turns out to be some non-trivial task, since the ubiquitous function time()works with precision of whole seconds. After some research, I developed four methods discussed in the code example below:

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

int main(int argc, char *argv[])
{
    clock_t t0, t1;
    double dt0;

    struct timespec t2, t3;
    double dt1;

    struct timespec t4, t5;
    double dt2;

    struct timeval t6, t7;
    double dt3;

    volatile long long i;

    t2.tv_sec = 0;
    t2.tv_nsec = 0;
    clock_settime(CLOCK_MONOTONIC, &t2);
    clock_settime(CLOCK_REALTIME,  &t2);

    t0 = clock();
    clock_gettime(CLOCK_REALTIME,  &t2);
    clock_gettime(CLOCK_MONOTONIC, &t4);
    gettimeofday(&t6, NULL);

    getchar();
    for (i=0; i<1e9; i++) {};

    gettimeofday(&t7, NULL);
    clock_gettime(CLOCK_MONOTONIC, &t5);
    clock_gettime(CLOCK_REALTIME,  &t3);
    t1 = clock();

    dt0 = (double) (t1 - t0) / CLOCKS_PER_SEC;
    dt1 = (double) (t3.tv_nsec - t2.tv_nsec) / 1e9;
    dt2 = (double) (t5.tv_nsec - t4.tv_nsec) / 1e9;
    dt3 = (double) (t7.tv_usec - t6.tv_usec) / 1e6;

    printf("1. clock():          [0]=%10.0f, [1]=%10.0f, [1-0]=%10.6f sec\n", (double) t0, (double) t1, dt0);
    printf("2. clock_gettime(R): [2]=%10ld, [3]=%10ld, [3-2]=%10f sec\n", (long) t2.tv_nsec, (long) t3.tv_nsec, dt1);
    printf("3. clock_gettime(M): [2]=%10ld, [3]=%10ld, [3-2]=%10f sec\n", (long) t4.tv_nsec, (long) t5.tv_nsec, dt2);
    printf("4. gettimeofday():   [4]=%10ld, [5]=%10ld, [5-4]=%10f sec\n", (long) t6.tv_usec, (long) t7.tv_usec, dt3);

    return 0;
}

Then I compiled and ran it with:

gcc -lrt -o timer.e timer.c; time ./timer.e

I used the call getchar()to enter a couple of seconds of delay to see the difference between the time realand and reports user, and the result:

1. clock():          [0]=         0, [1]=   3280000, [1-0]=  3.280000 sec
2. clock_gettime(R): [2]= 823922476, [3]= 478650549, [3-2]= -0.345272 sec
3. clock_gettime(M): [2]= 671137949, [3]= 325864897, [3-2]= -0.345273 sec
4. gettimeofday():   [4]=    823924, [5]=    478648, [5-4]= -0.345276 sec

real    0m6.659s
user    0m3.280s
sys     0m0.010s

As you can see, the only significant result is the correlation between method 1 and the time userreported by the command time.

: 1. 2-4 ? 2. , , real number time?

- Ubuntu 10.04 LTS 64- VMware Windows 7 HP AMD.

+5
3

clock_gettime().

clock_settime(): clock_gettime() .

, tv_sec, tv_nsec; .

Ubuntu , :

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

int main(int argc, char *argv[])
{
    struct timespec t2, t3;
    double dt1;

    volatile long long i;

    clock_gettime(CLOCK_MONOTONIC,  &t2);
    for (i=0; i<1e9; i++) {};
    clock_gettime(CLOCK_MONOTONIC,  &t3);
    //time in seconds
    dt1 = (t3.tv_sec - t2.tv_sec) + (double) (t3.tv_nsec - t2.tv_nsec) * 1e-9;
    printf("%f\n", dt1);

    return 0;
}
+12

Your challenges clock_settimedo not fit your needs. If you had permission, they just set the system clock to 0maybe not what you want. Almost all POSIX functions have a return value that tells you whether the call was successful. It is usually bad to ignore this value. For your case, in particular, I think that it should have given this value, since it can be found on the page man:

EPERM clock_settime () is not allowed to set the clock indicated.

Fortunately, you do not have permission to set the clock as easy as that :)

+1
source

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