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What is the most accurate timer on a unix system? (c language)

Posted on 2004-04-09
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Last Modified: 2013-12-26
I am simulating the datalink layer between two  entities in a network.

I need to get down to microseconds if possible.

I'm compiling c code on a the gcc compiler.

I basically need to do this.

StartTime = getTime();


loop(   )
{
........
  CurrentTime = getTime();
  if(CurrentTime - StartTime > 50 microseconds)
  {
     do this;
  }
  ......
}


Any ideas on the best timer function to use?

Thank you.


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Comment
Question by:oxygen_728
3 Comments
 
LVL 23

Assisted Solution

by:brettmjohnson
brettmjohnson earned 200 total points
ID: 10795453
Although gettimeofday() supports microsecond resolution, the clocks on most
machines do not tick more than a few hundred times per second, causing the
returned microsecond values from gettimeofday() to jump by large quanta.
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LVL 3

Accepted Solution

by:
idt earned 300 total points
ID: 10795654
Like Brett.. said, gettimeofday supports usec resolution, but diiferent systems may jump by large increments, run the following test app to test your system.

#include <stdio.h>
#include <sys/time.h>
#include <time.h>
#include <string.h>

/*
const char* TimeToString(const struct tm* t) {
  static char buf[30];
  strftime(buf,sizeof(buf),"%Y-%m-%d %H:%M:%S",t);
  return buf;
}

const char* FormatTime(const struct timeval* v) {
  static char buf[128];
  struct tm t;
  localtime_r(&v->tv_sec, &t);
  sprintf(buf,"%s.%ld", TimeToString(&t),v->tv_usec);
  return buf;
}
*/

void difftimeval(const struct timeval* v, const struct timeval* w, struct timeval *r) {
  r->tv_sec=(v->tv_sec - w->tv_sec);
  r->tv_usec=(v->tv_usec - w->tv_usec);
  return;
}

int main(void) {
  int i;
  struct timeval v,w,r;
  gettimeofday(&w,0);
  for(i=0;i<10;i++) {
    gettimeofday(&v,0);
    difftimeval(&v,&w,&r);
    //printf("time is now:%s  elapsed:%ld seconds, %ld usecs\n",FormatTime(&v),r.tv_sec,r.tv_usec);
    printf("elapsed:%ld seconds, %ld usecs\n",r.tv_sec,r.tv_usec);
    memcpy(&w,&v,sizeof(v));
  }
  return 0;
}

My output is:

elapsed:0 seconds, 1 usecs
elapsed:0 seconds, 56 usecs
elapsed:0 seconds, 4 usecs
elapsed:0 seconds, 4 usecs
elapsed:0 seconds, 3 usecs
elapsed:0 seconds, 4 usecs
elapsed:0 seconds, 3 usecs
elapsed:0 seconds, 4 usecs
elapsed:0 seconds, 3 usecs
elapsed:0 seconds, 3 usecs

Unremark for more verbose date/time output
time is now:2004-04-10 00:17:53.583650  elapsed:0 seconds, 1 usecs
time is now:2004-04-10 00:17:53.583863  elapsed:0 seconds, 213 usecs
time is now:2004-04-10 00:17:53.583872  elapsed:0 seconds, 9 usecs
time is now:2004-04-10 00:17:53.583880  elapsed:0 seconds, 8 usecs
time is now:2004-04-10 00:17:53.583888  elapsed:0 seconds, 8 usecs
time is now:2004-04-10 00:17:53.583895  elapsed:0 seconds, 7 usecs
time is now:2004-04-10 00:17:53.583902  elapsed:0 seconds, 7 usecs
time is now:2004-04-10 00:17:53.583909  elapsed:0 seconds, 7 usecs
time is now:2004-04-10 00:17:53.583917  elapsed:0 seconds, 8 usecs
time is now:2004-04-10 00:17:53.583924  elapsed:0 seconds, 7 usecs

compiled with gcc version 3.2.2 (Mandrake Linux 9.1 3.2.2-3mdk)

Daniel
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Author Comment

by:oxygen_728
ID: 10796023
Thank you both for your time. Have a great day both of you!
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