Re: usleep

["Lee Chin" <leechin@mail.com>]

Thanks... that helpes (I initially posed my code wrong... my loop I intended to post was a for(i = 0; i < 1000; i++)

Any way... what is setrealtime?  I dont have it on my linux machine

Thanks
Lee
----- Original Message -----
From: Mark Hahn <hahn@physics.mcmaster.ca>
Date: Wed, 11 Jun 2003 19:45:17 -0400 (EDT)
To: Lee Chin <leechin@mail.com>
Subject: Re: usleep

> > I would think the following code would wait for 1 second each itteration
> > before printing hello,
> 
> well, usleep takes microseconds, so as you've written it,
> you should expect it to take 1ms per usleep or 50ms between hello's.
> further, the man page doesn't claim that it'll actually sleep
> for exactly the specified microseconds.
> 
> here:
> #include <unistd.h>
> #include <stdio.h>
> int main() {
>     int c = 20;
>     while (c--) {
>         int i;
>         printf("hello\n");
>         for(i = 0; i < 50; i++) {
>             usleep(1000);
>         }
>     }
>     return 0;
> }
> 
> (not that a correct program needs both the stdio and a return from main.)
> 
> | [hahn@hahn hahn]$ /usr/bin/time ./chin
> | hello
> ...
> | hello
> | 0.00user 0.00system 0:19.99elapsed 0%CPU (0avgtext+0avgdata 0maxresident)k
> | 0inputs+0outputs (71major+10minor)pagefaults 0swaps
> | [hahn@hahn hahn]$ setrealtime time ./chin
> | hello
> ...
> | hello
> | 0.13user 0.87system 0:01.00elapsed 100%CPU (0avgtext+0avgdata 0maxresident)k
> | 0inputs+0outputs (71major+10minor)pagefaults 0swaps
> 
> hmm, so on its own, 20 iterations of your loop take 20 seconds,
> one second per iteration, and the usleep is actually lasting 20ms.
> this is the traditional behavior (and correct, since the man page 
> permits this kind of fuzziness).
> 
> if run in realtime mode, 20*50 usleep(1000)'s take 1.00 seconds, cool.
> 
> also, if I add:
> #include <sys/time.h>
> 
> void myusleep(unsigned us) {
>     struct timeval tv;
>     tv.tv_sec = 0;
>     tv.tv_usec = us;
>     select(0,0,0,0,&tv);
> }
> 
> then it behaves pretty much exactly as expected.  (good code would handle
> the case where us>1000000 and where select is interrupted by a signal.)
> 
> 

-- 
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