lib/tzset.c

   1 /* Provide tzset for systems that don't have it or for which it's broken.
   2
   3    Copyright (C) 2001-2003, 2005-2007, 2009-2024 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 Jim Meyering */
  19
  20 #include <config.h>
  21
  22 /* Specification.  */
  23 #include <time.h>
  24
  25 #include <stdlib.h>
  26 #include <string.h>
  27 #if defined _WIN32 && ! defined __CYGWIN__
  28 # include <wchar.h>
  29 #endif
  30
  31 void
  32 rpl_tzset (void)
  33 #undef tzset
  34 {
  35 #if defined _WIN32 && ! defined __CYGWIN__
  36   /* Rectify the value of the environment variable TZ.
  37      There are four possible kinds of such values:
  38        - Traditional US time zone names, e.g. "PST8PDT".  Syntax: see
  39          <https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/tzset>
  40        - Time zone names based on geography, that contain one or more
  41          slashes, e.g. "Europe/Moscow".
  42        - Time zone names based on geography, without slashes, e.g.
  43          "Singapore".
  44        - Time zone names that contain explicit DST rules.  Syntax: see
  45          <https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08_03>
  46      The Microsoft CRT understands only the first kind.  It produces incorrect
  47      results if the value of TZ is of the other kinds.
  48      But in a Cygwin environment, /etc/profile.d/tzset.sh sets TZ to a value
  49      of the second kind for most geographies, or of the first kind in a few
  50      other geographies.  If it is of the second kind, neutralize it.  For the
  51      Microsoft CRT, an absent or empty TZ means the time zone that the user
  52      has set in the Windows Control Panel.
  53      If the value of TZ is of the third or fourth kind -- Cygwin programs
  54      understand these syntaxes as well --, it does not matter whether we
  55      neutralize it or not, since these values occur only when a Cygwin user
  56      has set TZ explicitly; this case is 1. rare and 2. under the user's
  57      responsibility.  */
  58   const char *tz = getenv ("TZ");
  59   if (tz != NULL && strchr (tz, '/') != NULL)
  60     {
  61       /* Neutralize it, in a way that is multithread-safe.
  62          (If we were to use _putenv ("TZ="), it would free the memory allocated
  63          for the environment variable "TZ", and thus other threads that are
  64          using the previously fetched value of getenv ("TZ") could crash.)  */
  65       char **env = _environ;
  66       wchar_t **wenv = _wenviron;
  67       if (env != NULL)
  68         for (char **ep = env; *ep != NULL; ep++)
  69           {
  70             char *s = *ep;
  71             if (s[0] == 'T' && s[1] == 'Z' && s[2] == '=')
  72               s[0] = '$';
  73           }
  74       if (wenv != NULL)
  75         for (wchar_t **wep = wenv; *wep != NULL; wep++)
  76           {
  77             wchar_t *ws = *wep;
  78             if (ws[0] == L'T' && ws[1] == L'Z' && ws[2] == L'=')
  79               ws[0] = L'$';
  80           }
  81     }
  82
  83   /* On native Windows, tzset() is deprecated.  Use _tzset() instead.  See
  84      <https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/posix-tzset>
  85      <https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/tzset>  */
  86   _tzset ();
  87 #else
  88   tzset ();
  89 #endif
  90 }