Negative System time with RUsage
 

I have a question and I am unsure if it fits this category since it may be a technical question of either Linux or the exact Distribute but I will ask anyhow:

I am attempting to create a program to measure time for an internal C function call using getrusage. However often the difference between the system time is negative (where the initial system time is larger then the final system time). I was wondering if that is actuate or if I having having problems with my program. I know that time can not go backwards on a computer.


Here is the exact code:
main () {
struct rusage start, end;
double systemtime, usertime;
int i, j;

for (i = 0; i < 10; i++) {

usertime = 0.0;
systemtime = 0.0;

getrusage(RUSAGE_SELF, &start);

for (j = 0; j < 100000000;j++) { printf(""); }


getrusage(RUSAGE_SELF, &end);

usertime = end.ru_utime.tv_sec+end.ru_utime.tv_usec*1e-6 -
(start.ru_utime.tv_sec+start.ru_utime.tv_usec*1e-6);

systemtime = end.ru_stime.tv_sec+end.ru_stime.tv_usec*1e-6 -
(start.ru_stime.tv_sec+start.ru_stime.tv_usec*1e-6);

printf("%f\n", systemtime);
printf("%f\n", usertime);
}
}

It works perfectly fine for me. Here's the cleaned-up code I used, but it's essentially the same as yours:
And my results:

Actually it works fine for short programs and that is why I needed the print statement. If you include the print statement, the process will take longer and that is when the negative numbers show up (like 2 out of 10 times).