compat/clock_gettime.c

   1 /*****************************************************************************
   2  * clock_gettime.c: POSIX clock_gettime() replacement
   3  *****************************************************************************
   4  * Copyright © 2020 VLC authors and VideoLAN
   5  *
   6  * Author: Marvin Scholz <epirat07 at gmail dot com>
   7  *
   8  * This program is free software; you can redistribute it and/or modify it
   9  * under the terms of the GNU Lesser General Public License as published by
  10  * the Free Software Foundation; either version 2.1 of the License, or
  11  * (at your option) any later version.
  12  *
  13  * This program is distributed in the hope that it will be useful,
  14  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  15  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
  16  * GNU Lesser General Public License for more details.
  17  *
  18  * You should have received a copy of the GNU Lesser General Public License
  19  * along with this program; if not, write to the Free Software Foundation,
  20  * Inc., 51 Franklin Street, Fifth Floor, Boston MA 02110-1301, USA.
  21  *****************************************************************************/
  22
  23 #ifdef __APPLE__
  24
  25 #ifdef HAVE_CONFIG_H
  26 # include <config.h>
  27 #endif
  28
  29 #include <sys/errno.h>
  30 #include <sys/types.h>
  31 #include <sys/time.h>
  32 #include <sys/sysctl.h>
  33
  34 /**
  35  * Get the absolute time at which the system was booted
  36  *
  37  * This time is changed whenever the clock is adjusted to
  38  * correctly reflect the boot time with the adjusted clock,
  39  * so just querying it once and reusing the value is not safe.
  40  *
  41  * \param[out]  tv   Timeval struct to write the boottime to
  42  *
  43  * \note
  44  * The boot time only has microsecond precision
  45  *
  46  * \return 0 on success, else -1 and errno set
  47  */
  48 static int vlc__get_system_boottime(struct timeval *tv)
  49 {
  50     int ret = sysctl((int[]){ CTL_KERN, KERN_BOOTTIME }, 2,
  51                      tv, &(size_t){ sizeof(*tv) }, NULL, 0);
  52
  53     if (ret != 0)
  54         return errno;
  55
  56     return 0;
  57 }
  58
  59 /**
  60  * Get the monotonic time (CLOCK_MONOTONIC)
  61  *
  62  * Calculates a monotically incrasing time since system boot
  63  * that continues to increment when the system is asleep.
  64  *
  65  * Warnings to everyone trying to simplify this:
  66  * - Using mach_continuous_time is not equivalent to this, see
  67  *   the darwin manpage about CLOCK_MONOTONIC_RAW for an explanation.
  68  * - Using mach_absolute_time is not equivalent to this, as it does
  69  *   not continue to increment when the system is asleep.
  70  * - The boot time is queried twice in a loop and only used if both
  71  *   match. This is done to detect if the boot time change since we
  72  *   obtained the current time of day, as the boot time can change
  73  *   when the system wallclock is adjusted, as that will adjust the
  74  *   boot time accordingly.
  75  *
  76  * \param[out]  tv   Timeval struct to write the monotonic time to
  77  *
  78  * \return 0 on success, else -1 and errno set
  79  */
  80 static int vlc__get_monotonic(struct timeval *tv)
  81 {
  82     int ret;
  83     struct timeval currenttime;
  84     struct timeval boottime_begin;
  85     struct timeval boottime_end;
  86
  87     do {
  88         ret = vlc__get_system_boottime(&boottime_begin);
  89         if (ret != 0)
  90             return ret;
  91
  92         ret = gettimeofday(&currenttime, NULL);
  93         if (ret != 0)
  94             return ret;
  95
  96         ret = vlc__get_system_boottime(&boottime_end);
  97         if (ret != 0)
  98             return ret;
  99     } while (timercmp(&boottime_begin, &boottime_end, !=));
 100
 101     timersub(&currenttime, &boottime_begin, tv);
 102     return 0;
 103 }
 104
 105 int clock_gettime(clockid_t clock_id, struct timespec *tp)
 106 {
 107     int ret = 0;
 108     struct timeval tv;
 109
 110     switch (clock_id) {
 111         case CLOCK_MONOTONIC:
 112             ret = vlc__get_monotonic(&tv);
 113             break;
 114         case CLOCK_REALTIME:
 115             ret = gettimeofday(&tv, NULL);
 116             break;
 117         default:
 118             errno = EINVAL;
 119             return -1;
 120     }
 121
 122     if (ret == 0)
 123         TIMEVAL_TO_TIMESPEC(&tv, tp);
 124     return ret;
 125 }
 126
 127 #endif