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