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, 2009-2020 Free Software Foundation, Inc.
   4
   5    This program is free software; you can redistribute it and/or modify
   6    it under the terms of the GNU General Public License as published by
   7    the Free Software Foundation; either version 2, or (at your option)
   8    any later version.
   9
  10    This program 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 General Public License for more details.
  14
  15    You should have received a copy of the GNU 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 "localtime-buffer.h"
  26
  27 /* This is a wrapper for tzset, for systems on which tzset may clobber
  28    the static buffer used for localtime's result.
  29    Work around the bug in some systems whereby tzset clobbers the
  30    static buffer that localtime uses for its return value.  The
  31    tzset function from Solaris 2.5, 2.5.1, and 2.6 has this problem.  */
  32
  33 void
  34 tzset (void)
  35 #undef tzset
  36 {
  37 #if TZSET_CLOBBERS_LOCALTIME
  38   /* Save and restore the contents of the buffer used for localtime's
  39      result around the call to tzset.  */
  40   struct tm save = *localtime_buffer_addr;
  41 #endif
  42
  43 #if defined _WIN32 && ! defined __CYGWIN__
  44   /* Rectify the value of the environment variable TZ.
  45      There are four possible kinds of such values:
  46        - Traditional US time zone names, e.g. "PST8PDT".  Syntax: see
  47          <https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/tzset>
  48        - Time zone names based on geography, that contain one or more
  49          slashes, e.g. "Europe/Moscow".
  50        - Time zone names based on geography, without slashes, e.g.
  51          "Singapore".
  52        - Time zone names that contain explicit DST rules.  Syntax: see
  53          <https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08_03>
  54      The Microsoft CRT understands only the first kind.  It produces incorrect
  55      results if the value of TZ is of the other kinds.
  56      But in a Cygwin environment, /etc/profile.d/tzset.sh sets TZ to a value
  57      of the second kind for most geographies, or of the first kind in a few
  58      other geographies.  If it is of the second kind, neutralize it.  For the
  59      Microsoft CRT, an absent or empty TZ means the time zone that the user
  60      has set in the Windows Control Panel.
  61      If the value of TZ is of the third or fourth kind -- Cygwin programs
  62      understand these syntaxes as well --, it does not matter whether we
  63      neutralize it or not, since these values occur only when a Cygwin user
  64      has set TZ explicitly; this case is 1. rare and 2. under the user's
  65      responsibility.  */
  66   const char *tz = getenv ("TZ");
  67   if (tz != NULL && strchr (tz, '/') != NULL)
  68     _putenv ("TZ=");
  69
  70   /* On native Windows, tzset() is deprecated.  Use _tzset() instead.  See
  71      <https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/posix-tzset>
  72      <https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/tzset>  */
  73   _tzset ();
  74 #elif HAVE_TZSET
  75   tzset ();
  76 #else
  77   /* Do nothing.  Avoid infinite recursion.  */
  78 #endif
  79
  80 #if TZSET_CLOBBERS_LOCALTIME
  81   *localtime_buffer_addr = save;
  82 #endif
  83 }