106 lines
3.3 KiB
C
106 lines
3.3 KiB
C
/* Get the system clock resolution.
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Copyright 2021-2025 Free Software Foundation, Inc.
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This file is free software: you can redistribute it and/or modify
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it under the terms of the GNU Lesser General Public License as
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published by the Free Software Foundation, either version 3 of the
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License, or (at your option) any later version.
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This file is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public License
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along with this program. If not, see <https://www.gnu.org/licenses/>. */
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/* Written by Paul Eggert. */
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#include <config.h>
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#include "timespec.h"
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static long int _GL_ATTRIBUTE_CONST
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gcd (long int a, long int b)
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{
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while (b != 0)
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{
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long int r = a % b;
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a = b;
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b = r;
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}
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return a;
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}
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/* Return the system time resolution in nanoseconds. */
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long int
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gettime_res (void)
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{
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#if defined __sun && defined __sparc
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/* On Solaris 11.4/SPARC (with a SPARC-M8 CPU)
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clock_getres (CLOCK_REALTIME, ...) returns a resolution of 1000000 ns,
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and clock_gettime (CLOCK_REALTIME) returns a multiple of 5 ns with a
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probability of ca. 1 - 1/300000. Thus the heuristic below with the
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32 samples guesses a resolution of 5 ns with a probability of ca.
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1 - 1/10000. But occasionally clock_gettime (CLOCK_REALTIME) returns
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only a multiple of 1 ns.
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Simple guesswork is not enough to cope with this irregular behaviour.
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Therefore, here is an override. */
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return 1;
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#else
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struct timespec res;
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# if defined CLOCK_REALTIME && HAVE_CLOCK_GETRES
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clock_getres (CLOCK_REALTIME, &res);
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# elif defined HAVE_TIMESPEC_GETRES
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timespec_getres (&res, TIME_UTC);
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# else
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/* Guess high and let the later code deduce better. */
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res.tv_sec = 1;
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res.tv_nsec = 0;
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# endif
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/* On all Gnulib platforms the following calculations do not overflow. */
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long int hz = TIMESPEC_HZ;
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long int r = res.tv_nsec <= 0 ? hz : res.tv_nsec;
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struct timespec earlier = { .tv_nsec = -1 };
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/* On some platforms, clock_getres (CLOCK_REALTIME, ...) yields a
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too-large resolution, under the mistaken theory that it should
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return the timer interval. For example, on AIX 7.2 POWER8
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clock_getres yields 10 ms even though clock_gettime yields 1 μs
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resolution. Work around the problem with high probability by
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trying clock_gettime several times and observing the resulting
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bounds on resolution. */
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int nsamples = 32;
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for (int i = 0; 1 < r && i < nsamples; i++)
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{
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/* If successive timestamps disagree the clock resolution must
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be small, so exit the inner loop to check this sample.
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Otherwise, arrange for the outer loop to exit but continue
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the inner-loop search for a differing timestamp sample. */
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struct timespec now;
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for (;; i = nsamples)
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{
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now = current_timespec ();
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if (earlier.tv_nsec != now.tv_nsec || earlier.tv_sec != now.tv_sec)
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break;
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}
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earlier = now;
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if (0 < now.tv_nsec)
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r = gcd (r, now.tv_nsec);
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}
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return r;
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#endif
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}
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/*
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* Hey Emacs!
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* Local Variables:
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* coding: utf-8
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* End:
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*/
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