Implementing time delay function in C

Question

I want to implement a delay function using null loops. But the amount of time needed to complete a loop once is compiler and machine dependant. I want my program to determine the time on its own and delay the program for the specified amount of time. Can anyone give me any idea how to do this? N. B. There is a function named delay() which suspends the system for the specified milliseconds. Is it possible to suspend the system without using this function?

Answer 1

First of all, you should never sit in a loop doing nothing. Not only does it waste energy (as it keeps your CPU 100% busy counting your loop counter) -- in a multitasking system it also decreases the whole system performance, because your process is getting time slices all the time as it appears to be doing something.

Next point is ... I don't know of any delay() function. This is not standard C. In fact, until C11, there was no standard at all for things like this.

POSIX to the rescue, there is usleep(3) (deprecated) and nanosleep(2). If you're on a POSIX-compliant system, you'll be fine with those. They block (means, the scheduler of your OS knows they have nothing to do and schedules them only after the end of the call), so you don't waste CPU power.

If you're on windows, for a direct delay in code, you only have Sleep(). Note that THIS function takes milliseconds, but has normally only a precision around 15ms. Often good enough, but not always. If you need better precision on windows, you can request more timer interrupts using timeBeginPeriod() ... timeBeginPeriod(1); will request a timer interrupt each millisecond. Don't forget calling timeEndPeriod() with the same value as soon as you don't need the precision any more, because more timer interrupts come with a cost: they keep the system busy, thus wasting more energy.

I had a somewhat similar problem developing a little game recently, I needed constant ticks in 10ms intervals, this is what I came up with for POSIX-compliant systems and for windows. The ticker_wait() function in this code just suspends until the next tick, maybe this is helpful if your original intent was some timing issue.

Answer 2

Unless you're on a real-time operating system, anything you program yourself directly is not going to be accurate. You need to use a system function to sleep for some amount of time like usleep in Linux or Sleep in Windows.

Because the operating system could interrupt the process sooner or later than the exact time expected, you should get the system time before and after you sleep to determine how long you actually slept for.

Edit:

On Linux, you can get the current system time with gettimeofday, which has microsecond resolution (whether the actual clock is that accurate is a different story). On Windows, you can do something similar with GetSystemTimeAsFileTime:

int gettimeofday(struct timeval *tv, struct timezone *tz)
{
    const unsigned __int64 epoch_diff = 11644473600000000;
    unsigned __int64 tmp;
    FILETIME t;

    if (tv) {
        GetSystemTimeAsFileTime(&t);

        tmp = 0;
        tmp |= t.dwHighDateTime;
        tmp <<= 32;
        tmp |= t.dwLowDateTime;

        tmp /= 10;
        tmp -= epoch_diff;
        tv->tv_sec = (long)(tmp / 1000000);
        tv->tv_usec = (long)(tmp % 1000000);
    }

    return 0;
}