Why are gettimeofday() intervals occasionally negative?

c++ c linux timing gettimeofday
Andres Jaan Tack picture Andres Jaan Tack · May 6, 2010 · Viewed 8.1k times · Source

I have an experimental library whose performance I'm trying to measure. To do this, I've written the following:

struct timeval begin;
gettimeofday(&begin, NULL);
{
    // Experiment!
}
struct timeval end;
gettimeofday(&end, NULL);

// Print the time it took!
std::cout << "Time: " << 100000 * (end.tv_sec - begin.tv_sec) + (end.tv_usec - begin.tv_usec) << std::endl;

Occasionally, my results include negative timings, some of which are nonsensical. For instance:

Time: 226762
Time: 220222
Time: 210883
Time: -688976

What's going on?

Answer

Michael Ulm picture Michael Ulm · May 6, 2010

You've got a typo. Corrected last line (note the number of 0s):

std::cout << "Time: " << 1000000 * (end.tv_sec - begin.tv_sec) + (end.tv_usec - begin.tv_usec) << std::endl;

BTW, timersub is a built in method to get the difference between two timevals.

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