lib/gettime-res.c

   1 /* Get the system clock resolution.
   2
   3    Copyright 2021-2024 Free Software Foundation, Inc.
   4
   5    This file is free software: you can redistribute it and/or modify
   6    it under the terms of the GNU Lesser General Public License as
   7    published by the Free Software Foundation, either version 3 of the
   8    License, or (at your option) any later version.
   9
  10    This file is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  13    GNU Lesser General Public License for more details.
  14
  15    You should have received a copy of the GNU Lesser General Public License
  16    along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
  17
  18 /* Written by Paul Eggert.  */
  19
  20 #include <config.h>
  21
  22 #include "timespec.h"
  23
  24 static long int _GL_ATTRIBUTE_CONST
  25 gcd (long int a, long int b)
  26 {
  27   while (b != 0)
  28     {
  29       long int r = a % b;
  30       a = b;
  31       b = r;
  32     }
  33   return a;
  34 }
  35
  36 /* Return the system time resolution in nanoseconds.  */
  37
  38 long int
  39 gettime_res (void)
  40 {
  41 #if defined __sun && defined __sparc
  42   /* On Solaris 11.4/SPARC (with a SPARC-M8 CPU)
  43      clock_getres (CLOCK_REALTIME, ...) returns a resolution of 1000000 ns,
  44      and clock_gettime (CLOCK_REALTIME) returns a multiple of 5 ns with a
  45      probability of ca. 1 - 1/300000.  Thus the heuristic below with the
  46      32 samples guesses a resolution of 5 ns with a probability of ca.
  47      1 - 1/10000.  But occasionally clock_gettime (CLOCK_REALTIME) returns
  48      only a multiple of 1 ns.
  49      Simple guesswork is not enough to cope with this irregular behaviour.
  50      Therefore, here is an override.  */
  51   return 1;
  52 #else
  53   struct timespec res;
  54 # if defined CLOCK_REALTIME && HAVE_CLOCK_GETRES
  55   clock_getres (CLOCK_REALTIME, &res);
  56 # elif defined HAVE_TIMESPEC_GETRES
  57   timespec_getres (&res, TIME_UTC);
  58 # else
  59   /* Guess high and let the later code deduce better.  */
  60   res.tv_sec = 1;
  61   res.tv_nsec = 0;
  62 # endif
  63
  64   /* On all Gnulib platforms the following calculations do not overflow.  */
  65
  66   long int hz = TIMESPEC_HZ;
  67   long int r = res.tv_nsec <= 0 ? hz : res.tv_nsec;
  68   struct timespec earlier = { .tv_nsec = -1 };
  69
  70   /* On some platforms, clock_getres (CLOCK_REALTIME, ...) yields a
  71      too-large resolution, under the mistaken theory that it should
  72      return the timer interval.  For example, on AIX 7.2 POWER8
  73      clock_getres yields 10 ms even though clock_gettime yields 1 μs
  74      resolution.  Work around the problem with high probability by
  75      trying clock_gettime several times and observing the resulting
  76      bounds on resolution.  */
  77   int nsamples = 32;
  78   for (int i = 0; 1 < r && i < nsamples; i++)
  79     {
  80       /* If successive timestamps disagree the clock resolution must
  81          be small, so exit the inner loop to check this sample.
  82          Otherwise, arrange for the outer loop to exit but continue
  83          the inner-loop search for a differing timestamp sample.  */
  84       struct timespec now;
  85       for (;; i = nsamples)
  86         {
  87           now = current_timespec ();
  88           if (earlier.tv_nsec != now.tv_nsec || earlier.tv_sec != now.tv_sec)
  89             break;
  90         }
  91       earlier = now;
  92
  93       if (0 < now.tv_nsec)
  94         r = gcd (r, now.tv_nsec);
  95     }
  96
  97   return r;
  98 #endif
  99 }
 100
 101 /*
 102  * Hey Emacs!
 103  * Local Variables:
 104  * coding: utf-8
 105  * End:
 106  */