xapian-applications/omega/timegm.cc

   1 /** @file
   2  * @brief Portable implementation of timegm().
   3  */
   4 /* Copyright (c) 2013,2024 Olly Betts
   5  *
   6  * Permission is hereby granted, free of charge, to any person obtaining a copy
   7  * of this software and associated documentation files (the "Software"), to
   8  * deal in the Software without restriction, including without limitation the
   9  * rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
  10  * sell copies of the Software, and to permit persons to whom the Software is
  11  * furnished to do so, subject to the following conditions:
  12  *
  13  * The above copyright notice and this permission notice shall be included in
  14  * all copies or substantial portions of the Software.
  15  *
  16  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  17  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  18  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
  19  * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  20  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
  21  * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
  22  * IN THE SOFTWARE.
  23  */
  24
  25 #include <config.h>
  26 #include "timegm.h"
  27
  28 #include "setenv.h"
  29 #include <time.h>
  30
  31 time_t
  32 safe_mktime(struct tm* tm)
  33 {
  34     time_t r = mktime(tm);
  35     if (r == time_t(-1) && tm->tm_year < 70 &&
  36         (sizeof(time_t) > 4 || tm->tm_year >= 1)) {
  37         /* Microsoft's mktime() treats years before 1970 as an error, unlike
  38          * most other implementations.
  39          *
  40          * We workaround this to support older years by calling mktime() for a
  41          * date offset such that it's after 1970 (but before 3000 which is the
  42          * highest year Microsoft's mktime() handles, and also before 2038 for
  43          * 32-bit time_t), and such that the leap year pattern matches.  For
  44          * 32-bit time_t we just need to add a multiple of 4, but for 64-bit
  45          * time_t we need to add a multiple of 400.
  46          *
  47          * We require the year to be >= 1901 for 32-bit time_t since the
  48          * oldest representable date in signed 32-bit time_t is in 1901
  49          * so there's no point retrying anything older:
  50          *
  51          *   Fri 13 Dec 1901 20:45:52 UTC
  52          *
  53          * For larger time_t we support any tm_year value which fits in an int.
  54          * This is somewhat dubious before the adoption of the Gregorian
  55          * calendar but it matches what most mktime() implementations seem to
  56          * do.
  57          */
  58         int y = tm->tm_year;
  59         int y_offset = sizeof(time_t) > 4 ?
  60             ((-1 - y) / 400 + 1) * 400 :
  61             (76 - y) & ~3;
  62         tm->tm_year = y + y_offset;
  63         r = mktime(tm);
  64         tm->tm_year = y;
  65         if (r != time_t(-1)) {
  66             // The two magic numbers are the average number of seconds in a
  67             // year for a 400 year cycle and for a 4 year cycle (one which
  68             // includes a leap year).
  69             r -= y_offset * time_t(sizeof(time_t) > 4 ? 31556952 : 31557600);
  70         }
  71     }
  72     return r;
  73 }
  74
  75 #ifndef HAVE_TIMEGM
  76
  77 time_t
  78 timegm(struct tm* tm)
  79 {
  80     static bool set_tz = false;
  81     if (!set_tz) {
  82         setenv("TZ", "", 1);
  83         tzset();
  84         set_tz = true;
  85     }
  86     return safe_mktime(tm);
  87 }
  88
  89 #endif