Difference of two gettimeofday() calls gives negative number

Question

I'm trying to calculate the average overhead of a system call, so I am repeatedly doing a 0 byte read system call and calculating the average overhead as the difference in time divided by the number of iterations. However sometimes when I do this I get a negative number. Here is my code:

#include <unistd.h>
#include <stdio.h>
#include <sys/time.h>
#define NUM_ITER 1000000
#define NUM_EPOCHS 10

int main(){
    char buf[1];
    struct timeval tv1, tv2;
    for(int i = 0; i<NUM_EPOCHS; i++){
        gettimeofday(&tv1, NULL);
        for(int j = 0; j < NUM_ITER; j++)
            read(0, buf, 0);
        gettimeofday(&tv2, NULL);
        float time_of_sys_call = (float)(tv2.tv_usec - tv1.tv_usec) / NUM_ITER;
        printf("Avg cost of system call: %fms\n", time_of_sys_call);
    }
}

Here is sample output:

Avg cost of system call: 0.199954ms
Avg cost of system call: 0.213105ms
Avg cost of system call: 0.203455ms
Avg cost of system call: 0.200443ms
Avg cost of system call: -0.793516ms
Avg cost of system call: 0.203922ms
Avg cost of system call: 0.209279ms
Avg cost of system call: 0.201137ms
Avg cost of system call: 0.204261ms
Avg cost of system call: -0.800930ms

Any idea what is going on here?

Answer 1

tv_usec gives the number of microseconds within the current second. When the time accumulates to a full second, tv_sec increases, and tv_usec restarts from zero.

When you subtract a number shortly after the restart from a number shortly before the restart, the result is negative.