Wrong time from clock

Question

I have a program that counts the time to send or receive a amount of data. When I receive the data the clocks says it only takes about half of the time it actually takes.

Output from the terminal that recives the data:

Amount of data recived: 60296112/300000000
Time: 4
Start time: 3269
End time: 4849790
Clocks per sec: 1000000

And the output from the terminal that sends the data:

Sent 300000000 bytes of data
Time to send was 10.793425

The terminal that sends the data will send a stop signal after it has sent all other data. When I watch the terminal that receives the data I can see that it starts counting when the other terminal starts sending data and I can see it print out output from clock() way longer that the output says it does.

My code for the receive part:

static void recive_sock(int socket_fd,char *buffert, size_t buffert_size, struct sockaddr *other, socklen_t *other_size){
    clock_t start_t, end_t;
    long int total_time = 0;
    printf("Listning for data\n" );
    fflush(stdout);
    int run = 1;
    char start = 1;
    int amount = 0;
    int recive_length;

    while(run){
        recive_length = recvfrom(socket_fd, buffert, buffert_size, 0, other, other_size );
        if(recive_length < 0){
            die("Recvfrom failed");
        }
        if(strncmp(buffert, "HELLO!", 6) == 0){
            amount += recive_length;
            if(start == 1){
                start = 0;
                start_t = clock();
                printf("Start: %ld\n",start_t );
            }
            printf("%ld\n",clock() );
        }
        else if (strncmp(buffert, "die", 3) == 0) {
            run = 0;
            end_t = clock();
            printf("End %ld\n",end_t );
            total_time = (end_t - start_t) / CLOCKS_PER_SEC;
            printf("Amount of data recived: %d/%d\nTime: %ld\nStart time: %ld\nEnd time: %ld\n,Clocks per sec: %ld", amount, AMOUNT, total_time, start_t, end_t, CLOCKS_PER_SEC);
        }
    }
} 

Answer 1

The function clock will return the CPU time which is probably not what you are looking for, instead you want to use something like gettimeofday or clock_gettime for system that support it. Then you can compare the time before and after to get the time elapsed. For future readers, here is how to do it with clock_gettime if your system supports it:

#include <stdio.h>
#include <time.h>                // for clock_gettime()

int main(void) {
    int i;
    int j;
    int sum = 1;
    struct timespec t1, t2;
    double elapsedTime;

    // start timer
    clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &t1);

    // do something here
    for (i = 0; i < 10000; i++) {
        for (j = 0; j < 10000; j++) {
            sum *= i+j;
        }
    }

    // stop timer
    clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &t2);

    // compute and print the elapsed time in millisec
    elapsedTime = (t2.tv_sec - t1.tv_sec) * 1000.0;
    elapsedTime += (t2.tv_nsec - t1.tv_nsec) / 1000000.0;
    printf("%.3f ms elapsed\n", elapsedTime);

    return 0;
}

Here is a simple example on how to measure time with gettimeofday (less accurate for high-precision timing):

#include <stdio.h>
#include <time.h>                // for gettimeofday()

int main(void) {
    int i;
    int j;
    int sum = 1;
    struct timeval t1, t2;
    double elapsedTime;

    // start timer
    gettimeofday(&t1, NULL);

    // do something here
    for (i = 0; i < 10000; i++) {
        for (j = 0; j < 10000; j++) {
            sum *= i+j;
        }
    }

    // stop timer
    gettimeofday(&t2, NULL);

    // compute and print the elapsed time in millisec
    elapsedTime = (t2.tv_sec - t1.tv_sec) * 1000.0;
    elapsedTime += (t2.tv_usec - t1.tv_usec) / 1000.0;
    printf("%.3f ms elapsed\n", elapsedTime);

    return 0;
}