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/* Get the system clock resolution.
Copyright 2021-2023 Free Software Foundation, Inc.
This file is free software: you can redistribute it and/or modify
it under the terms of the GNU Lesser General Public License as
published by the Free Software Foundation, either version 3 of the
License, or (at your option) any later version.
This file is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public License
along with this program. If not, see <https://www.gnu.org/licenses/>. */
/* Written by Paul Eggert. */
#include <config.h>
#include "timespec.h"
static long int _GL_ATTRIBUTE_CONST
gcd (long int a, long int b)
{
while (b != 0)
{
long int r = a % b;
a = b;
b = r;
}
return a;
}
/* Return the system time resolution in nanoseconds. */
long int
gettime_res (void)
{
struct timespec res;
#if defined CLOCK_REALTIME && HAVE_CLOCK_GETRES
clock_getres (CLOCK_REALTIME, &res);
#elif defined HAVE_TIMESPEC_GETRES
timespec_getres (&res, TIME_UTC);
#else
/* Guess high and let the later code deduce better. */
res.tv_sec = 1;
res.tv_nsec = 0;
#endif
/* On all Gnulib platforms the following calculations do not overflow. */
long int hz = TIMESPEC_HZ;
long int r = res.tv_nsec <= 0 ? hz : res.tv_nsec;
struct timespec earlier = { .tv_nsec = -1 };
/* On some platforms, clock_getres (CLOCK_REALTIME, ...) yields a
too-large resolution, under the mistaken theory that it should
return the timer interval. For example, on AIX 7.2 POWER8
clock_getres yields 10 ms even though clock_gettime yields 1 μs
resolution. Work around the problem with high probability by
trying clock_gettime several times and observing the resulting
bounds on resolution. */
int nsamples = 32;
for (int i = 0; 1 < r && i < nsamples; i++)
{
/* If successive timestamps disagree the clock resolution must
be small, so exit the inner loop to check this sample.
Otherwise, arrange for the outer loop to exit but continue
the inner-loop search for a differing timestamp sample. */
struct timespec now;
for (;; i = nsamples)
{
now = current_timespec ();
if (earlier.tv_nsec != now.tv_nsec || earlier.tv_sec != now.tv_sec)
break;
}
earlier = now;
if (0 < now.tv_nsec)
r = gcd (r, now.tv_nsec);
}
return r;
}
/*
* Hey Emacs!
* Local Variables:
* coding: utf-8
* End:
*/
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