问Clock_Gettime()抖动？

EN

#include <stdio.h>
#include <time.h>

long elapsed_milli( struct timespec * t1, struct timespec *t2 )
{
    return (long)(t2->tv_sec - t1->tv_sec) * 1000L
         + (t2->tv_nsec - t1->tv_nsec) / 1000000L;
}

int main()
{
    const long period_milli = 500;
    struct timespec ts_last;
    struct timespec ts_next;
    const struct timespec ts_sleep = { 0, 1000000L };

    clock_gettime( CLOCK_REALTIME, &ts_last );

    while( 1 )
    {
        nanosleep( &ts_sleep, NULL );
        clock_gettime( CLOCK_REALTIME, &ts_next );
        long elapsed = elapsed_milli( &ts_last, &ts_next );

        if( elapsed >= period_milli )
        {
            printf( "Elapsed : %ld\n", elapsed );

            ts_last.tv_nsec += period_milli * 1000000L;
            if( ts_last.tv_nsec >= 1000000000L )
            {
                ts_last.tv_nsec -= 1000000000L;
                ts_last.tv_sec++;
            }
        }
    }
    return 0;
}