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Date: Mon, 26 Feb 2007 09:29:46 -0600
From: Brian Ford <Brian DOT Ford AT FlightSafety DOT com>
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Subject: Re: does usleep() sleep more than it's supposed to?
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On Mon, 26 Feb 2007, Carlo Florendo wrote:

> I'm writing an application that requires time precisions up to the
> microsecond level.

Then you shouldn't be using Windows.  Millisecond resolution is all the
scheduler can do (this varies slightly depending on the platform).

> However, I put a hard-coded adjustment of 9000 microseconds since
> usleep() seems to sleep on the average of 9000 microseconds more than
> it's supposed to, at least on my system.

Follow this thread for history and discussion:

http://sourceware.org/ml/cygwin-developers/2005-11/msg00000.html

Put this in its own (separate from all Cygwin code) object file, call
it before any timing calls in your application, and link it with -lwinmm:

#include "windows.h"

void
SetSchedulerMaxRes(void)
{
    TIMECAPS tc;

    /* Set the system scheduler resolution to its maximum.
     * Needed for Cygwin >= 1.5.20, broken in 1.5.19, and unnecessary
     * <= 1.5.18 because it was always done by Cygwin.  Required for */
    if (timeGetDevCaps(&tc, sizeof(tc)) != TIMERR_NOERROR)
    {
        printf("timeGetDevCaps error %d\n", GetLastError());
        tc.wPeriodMin = 1; /* Try 1 ms and hope for the best */
    }

    if (timeBeginPeriod(tc.wPeriodMin) != TIMERR_NOERROR)
        printf("timeBeginPeriod error %d\n", GetLastError());
}

-- 
Brian Ford
Lead Realtime Software Engineer
VITAL - Visual Simulation Systems
FlightSafety International
the best safety device in any aircraft is a well-trained crew...

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