smallest resolution using gettimeofday() in C

Applications Programming
1

Hello ...
Can somebody help telling me how smallest clock resolution on ones sytem can be in obtained in C using gettimeofday() .

Thankz in advance...

2

The smallest resolution for gettimeofday would be microseconds according to the manpage.

3

1 microsecond is the granularity of the system call, not the resolution. To obtain the resolution you should use clock_getres() if you have it. Unix systems without clock_getres() tend to have a resolution of 10 milliseconds. There is no reliable way to obtain the resolution with gettimeofday().

4

gettimeofday will give you the current date and time specified in seconds or microseconds from the UNIX Epoch but not the resolution of the system-wide clock so be specific as to what you want?

5

/*

  • Print the gettimeofday() clock resolution for this machine.
    *
  • Code taken from Iozone. Iozone Filesystem Benchmark
  • Author: Don Capps
    *
    /
    #include <sys/time.h>
    #include <stdio.h>
    #include <stdlib.h>
    #include <sys/types.h>
    #define THISVERSION " Version $Revision: 1.2 $"
    double time_res,delay;
    void get_resolution(); /     Works with most compilers /
    static double time_so_far1(); /     Works with most compilers /
    /
  • Measure and print the gettimeofday() resolution.
    /
    main()
    {
    printf("\n\nMeasuring the gettimeofday() resolution\n\n");
    get_resolution();
    printf("Time resolution of gettimeofday() = %f seconds\n",time_res);
    printf("Time resolution of gettimeofday() = %f milli seconds\n",
    time_res    1000);
    printf("Time resolution of gettimeofday() = %f micro seconds\n\n",
    time_res*(10001000));
    printf("Provided, courtesy of Iozone. http://www.iozone.org\n\n");
    }
    /
  • Lifted code from Iozone.
    /
    #ifdef HAVE_ANSIC_C
    void
    get_resolution(void)
    #else
    void
    get_resolution()
    #endif
    {
    double starttime, finishtime;
    long j;
    again:
    finishtime=time_so_far1(); /     Warm up the instruction cache /
    starttime=time_so_far1(); /     Warm up the instruction cache /
    delay=j=0; /     Warm up the data cache /
    while(1)
    {
    starttime=time_so_far1();
    for(j=0;j< delay;j++)
    ;
    finishtime=time_so_far1();
    if(starttime==finishtime)
    delay++;
    else
    break;
    }
    time_res = (finishtime-starttime)/1000000.0;
    }
    /
  • Lifted code from Iozone.
    /
    /    /
    /     Time measurement routines. /
    /     Return time in microseconds /
    /    /
    #ifdef HAVE_ANSIC_C
    static double
    time_so_far1(void)
    #else
    static double
    time_so_far1()
    #endif
    {
    /     For Windows the time_of_day() is useless. It increments in 55 milli second /
    /     increments. By using the Win32api one can get access to the high performance /
    /     measurement interfaces. With this one can get back into the 8 to 9 /
    /     microsecond resolution. */
    #ifdef Windows
    LARGE_INTEGER freq,counter;
    double wintime;
    double bigcounter;
    QueryPerformanceFrequency(&freq);
    QueryPerformanceCounter(&counter);
    bigcounter=(double)counter.HighPart (double)0xffffffff +
    (double)counter.LowPart;
    wintime = (double)(bigcounter/(double)freq.LowPart);
    return((double)wintime    1000000.0);
    #else
    #if defined (OSFV4) || defined(OSFV3) || defined(OSFV5)
    struct timespec gp;
    if (getclock(TIMEOFDAY, (struct timespec *) &gp) == -1)
    perror("getclock");
    return (( (double) (gp.tv_sec)*1000000.0) +
    ( ((float)(gp.tv_nsec)) * 0.001 ));
    #else
    struct timeval tp;
    if (gettimeofday(&tp, (struct timezone *) NULL) == -1)
    perror("gettimeofday");
    return ((double) (tp.tv_sec)*1000000.0) +
    (((double) tp.tv_usec) );
    #endif
    #endif
    }