Clock_gettime nanoseconds calculation

Question

ref: linux clock_gettime

I found a formula which works well to get the processing time, but there's something I don't understand. See the result below.

The first 2 rows is just to show the forumla in their respective columns.

I'm only showing 3 results from a quick run.
The interesting part is in the last row, why is 5551 - 999896062 nanoseconds = 18446744072709661105?
Why is 18446744072709661105+1/1E9 = 0.000109?

I think there's some data conversion going on that affects the results?

xx:      | t1.tv_sec |   | t1.tv_nsec |          | t2.tv_sec |   | t2.tv_nsec 
xx:      t2-t1(sec)      t2-t1(nsec)         (t2-t1(sec))+(t2-t1(nsec))/1E9

52291:   | 30437 |   | 999649886 |       | 30437 |   | 999759331 
52291:   0   109445          0.000109

52292:   | 30437 |   | 999772970 |       | 30437 |   | 999882416 
52292:   0   109446          0.000109

52293:   | 30437 |   | 999896062 |       | 30438 |   | 5551 
52293:   1   18446744072709661105        0.000109

source code:

int main() {
    struct timespec t1, t2;

    int i = 0;
    while(1) {
        clock_gettime(CLOCK_MONOTONIC, &t1);
            for(int j=0;j<25000;j++) { };
        clock_gettime(CLOCK_MONOTONIC, &t2);
        printf("%d: \t | %llu | \t | %lu | \t\t | %llu | \t | %lu \n", i, (unsigned long long) t1.tv_sec, t1.tv_nsec, (unsigned long long) t2.tv_sec, t2.tv_nsec);
        printf("%d: \t %llu \t %lu \t\t %lf\n", i, (unsigned long long) t2.tv_sec - t1.tv_sec, t2.tv_nsec - t1.tv_nsec, (t2.tv_sec - t1.tv_sec)+(t2.tv_nsec - t1.tv_nsec)/1E9);
        if ((t2.tv_sec - t1.tv_sec) == 1) break;
        i++;
    }
    return 0;
}

Solution

Because 5551 - 999896062 is some negative value, stored in a temp variable of type long, but interpreted by printf as "unsigned long" due to the %lu conversion specifier.

Note that the tv_nsec field in struct timespec is of type long, not unsigned long. Similarly, on Linux and other Unix systems time_t is a typedef for a signed integer type. So get rid of all the unsigned stuff in your code.

Btw, a way to to substract two timespec instances is

timespec diff(timespec start, timespec end)
{
        timespec temp;
        if ((end.tv_nsec - start.tv_nsec) < 0) 
        {
                temp.tv_sec = end.tv_sec - start.tv_sec - 1;
                temp.tv_nsec = 1000000000 + end.tv_nsec - start.tv_nsec;
        } 
        else 
        {
                temp.tv_sec = end.tv_sec - start.tv_sec;
                temp.tv_nsec = end.tv_nsec - start.tv_nsec;
        }
        return temp;
}