Bdtyktl

I have an issue using the clock_gettime() system-call with theCLOCK_MONOTONIC clock. The problem is that the clock returned by this system call seems to behave non-monotonically. First of, some system information:

uname -a

4.4.0-138-generic #164~14.04.1-Ubuntu SMP Fri Oct 5 08:56:16 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux



lsb_release -a

No LSB modules are available.

Distributor ID: Ubuntu

Description:    Ubuntu 14.04.5 LTS

Release:    14.04

Codename:   trusty

I am running the following code which creates a new pthread. This newly created pthread enters an infinite loop and is supposed to do a task every 100ms. In order to measure wall-clock time-intervals, the code uses the clock_gettime(CLOCK_MONOTONIC,...) system call:

#include <pthread.h>

#include <stdio.h>

#include <pthread.h>

#include <time.h>

#include <stdlib.h>



timespec tik;

timespec tok;



void* loop(void* argptr){

    while(true){

        clock_gettime(CLOCK_MONOTONIC,&tik);

        //

        //  do periodic task here

        //

        while(true){

            clock_gettime(CLOCK_MONOTONIC,&tok);

            if(tok.tv_nsec-tik.tv_nsec>100e6){

                printf("10ms have passedn");

                break;

            }

        }

    }

    return NULL;

}



int main()

{

    //create a thread which does a periodic task every 100ms

    pthread_t loopthread;

    if(pthread_create(&loopthread,NULL,loop,NULL)){

        fprintf(stderr,"Error creating thread");

        return EXIT_FAILURE;

    }

    //main thread waits forever

    while(true){



    }

    return 0;

}

Compiling this code with g++ typically prints a couple of lines to the console but then gets stuck in the inner while loop of the function void* loop(void*):

10ms have passed

10ms have passed

10ms have passed

10ms have passed

10ms have passed

10ms have passed

10ms have passed

A bit of debugging shows that the quantity (tok.tv_nsec-tik.tv_nsec) actually becomes negative which doesn't make sense since tok is always set after tik. Does anyone have an idea what the issue could be here?

Thanks!

Addition 1: I just tested the same code on 18.04 and the same issue occurred.

Ok, the solution to this is pretty trivial, one simply needs to replace

tok.tv_nsec-tik.tv_nsec

with

(tok.tv_nsec+tok.tv_sec*1e9)-(tik.tv_nsec+tik.tv_sec*1e9)

since the struct timespec which is set by clock_gettime(...) has two members, one to store the nanoseconds (tv_nsec) and one to store the seconds (tv_sec).