CODEWITHSUNDEEP
c++linuxtime
Dylan
Dylan

Reputation: 580

clock_gettime returning a nonsense value

I am trying to simulate a point falling at a constant rate in linux. For this to work, I need to get the time to millisecond resolution. Now this part is fine, but I am having a problem with clock_gettime.

When the 'tv_nsec' field wraps around to about 100000000, it starts back near zero, and the time retrieved by clock_gettime is is before the time retrieved the previous iteration. Note that this doesn't happen every time the field wraps, but it does happen.

To debug, I made it write the values returned from clock_gettime and the delta value retrived:

Iteration:

gettime.seconds: 1362720808 , gettime.us: 993649771, total: 1362721801649 us
delta: 0.014

Another iteration:

gettime.seconds: 1362720808 , gettime.us: 993667981, total: 1362721801667 us
delta: 0.015

Another iteration:

gettime.seconds: 1362720808 , gettime.us: 993686119, total: 1362721801686 us
delta: 0.015

Iteration in question:

gettime.seconds: 1362720809 , gettime.us: 20032630, total: 1362720829032 us
delta: -972.661

Note that the delta is in seconds, which is calculated by dividing the milliseconds by 1000, which combined with subtracting a time from the future from a time from the past which equals a negative, and then dividing that by 1000, it makes the delta a positive.

The code to reproduce the problem is here:

#include <iostream>
#include <sys/time.h>

using namespace std

double prevMillis = 0.0;

double getMillis()
{
    timespec ts;

    clock_gettime(CLOCK_REALTIME, &ts);

    cout << "gettime.seconds: " << ts.tv_sec << " , gettime.us: " << ts.tv_nsec << ", total: " << ((ts.tv_sec * 1000) + (ts.tv_nsec / 1000)) << " ms" << endl;

    return ((ts.tv_sec * 1000) + (ts.tv_nsec / 1000)) + 0.5;
}

int main()
{
    double delta = 0.0;

    prevMillis = getMillis();

    while(delta >= 0)
    {
        delta = (getMillis() - prevMillis) / 1000;

        prevMillis = getMillis();

        cout << "Delta: " << delta << endl << endl;
    }

    return 0;
}

Note that it must be compiled with '-lrt' for the the clock functions.

This will loop until the problem occurs, i.e. the delta is negative because of the time. It only takes a few seconds on my PC.

Sorry about the verbose question, but thanks for any help I may get in advance :)

Upvotes: 1

Views: 3156

Answers (1)

congusbongus
congusbongus

Reputation: 14622

tv_nsec is nanoseconds, i.e. 1 billionth (1 / 1,000,000,000) of one second. Your calcuation however is treating it as if it's microseconds.

Here's the fix:

return ((ts.tv_sec * 1000) + (ts.tv_nsec / 1000000)) + 0.5;
                                               ^^^

Upvotes: 4

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