Create a free running timer in real time linux

Question

I am studying real time linux and want to create a free running timer. But I am not able to find information regarding the real time linux. Could someone suggests me some document or could someone tell me how to create a free running high precision timer ??

Achievement : After creating a free running timer, So that I can get the time stamp value.

Answer 1

The entire point of using real-time Linux is that it is Linux - all the existing Linux and POSIX services are available, and you should avoid using proprietary non-POSIX OS services if portability is an issue - otherwise you may as well be using a true RTOS.

The standard function clock() has a CLOCKS_PER_SEC of 1000000 (i.e. microseconds) on POSIX. The actual granularity is implementation dependent however. You can check this for your system with:

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

int main()
{
    clock_t t0 = clock() ;
    clock_t t1 = t0 ;
    while( t0 == t1 )
    {
        t1 = clock() ;
    }

    printf( "Clock granularity = %lf seconds\n", (double)(t1 - t0)/(double)(CLOCKS_PER_SEC) ) ;

    return 0 ;
}

On ideone.com, this gave the following result:

Clock granularity = 0.010000 seconds

i.e. 10ms.

If that is not sufficient, a number of clock sources are defined for use with clock_gettime() and the resolution of each can be determined by clock_getres(). The clock ID CLOCK_PROCESS_CPUTIME_ID is likely to be appropriate. Note that resolution and granularity are not the same here either. I performed the following test on ideone.com:

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

int main()
{
    struct timespec t0 ;
    struct timespec t1 ;
    clock_gettime( CLOCK_PROCESS_CPUTIME_ID, &t0) ;
    t1 = t0 ;
    while( t0.tv_nsec == t1.tv_nsec )
    {
        clock_gettime( CLOCK_PROCESS_CPUTIME_ID, &t1) ;
    }

    printf( "Clock granularity = %lf seconds\n", (double)(t1.tv_nsec - t0.tv_nsec)/1e9 ) ;

    struct timespec res ;
    clock_getres( CLOCK_PROCESS_CPUTIME_ID, &res ) ;
    printf( "CPUTIME resolution = %.15lf seconds\n", e, (double)(res.tv_nsec)/1e9) ;


    return 0 ;
}

With results:

Clock granularity = 0.000001 seconds
CPUTIME resolution = 0.000000001000000 seconds

So the granularity is 1 microsecond.