Question

I am writing a thread library, and when scheduling the threads I need to know how long they’ve been ready. Every Thread instance has a timeval _timeInReady field, and when I push an instance to the ready queue I call this function:

void Thread::startTiming() {
    gettimeofday(&_timeInReady, NULL);
}

When I want to check the current _timeInReady value I call:

double Thread::getTimeInReady() const {
    return TIME(_timeInReady.tv_sec,_timeInReady.tv_usec);
}

Where TIME is #define TIME(a,b) ((a*1000000) + b) So that I get the total time in microseconds.

My problem is that for some reason, I get crazy negative values (such as -10293843) when I check that field after a while.

Any ideas? Thanks!

Answer 1

You should return the result as a 64-bit unsigned integer, as you want an integer microsecond result, not a fractional second (which would imply returning double):

unsigned long long Thread::getTimeInReady() const
{
    return (unsigned long long)_timeInReady.tv_sec * 1000000ULL +
        (unsigned long long)_timeInReady.tv_usec;
}

Your system might have uint64_t which is a more concise than unsigned long long, or you could typedef it.