lib/time_rz.c

   1 /* Time zone functions such as tzalloc and localtime_rz
   2
   3    Copyright 2015 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 3, 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 along
  16    with this program; if not, see <http://www.gnu.org/licenses/>.  */
  17
  18 /* Written by Paul Eggert.  */
  19
  20 /* Although this module is not thread-safe, any races should be fairly
  21    rare and reasonably benign.  For complete thread-safety, use a C
  22    library with a working timezone_t type, so that this module is not
  23    needed.  */
  24
  25 #include <config.h>
  26
  27 #include <time.h>
  28
  29 #include <errno.h>
  30 #include <stdbool.h>
  31 #include <stddef.h>
  32 #include <stdlib.h>
  33 #include <string.h>
  34
  35 #include "time-internal.h"
  36
  37 #if !HAVE_TZSET
  38 static void tzset (void) { }
  39 #endif
  40
  41 /* The approximate size to use for small allocation requests.  This is
  42    the largest "small" request for the GNU C library malloc.  */
  43 enum { DEFAULT_MXFAST = 64 * sizeof (size_t) / 4 };
  44
  45 /* Minimum size of the ABBRS member of struct abbr.  ABBRS is larger
  46    only in the unlikely case where an abbreviation longer than this is
  47    used.  */
  48 enum { ABBR_SIZE_MIN = DEFAULT_MXFAST - offsetof (struct tm_zone, abbrs) };
  49
  50 static char const TZ[] = "TZ";
  51
  52 /* Magic cookie timezone_t value, for local time.  It differs from
  53    NULL and from all other timezone_t values.  Only the address
  54    matters; the pointer is never dereferenced.  */
  55 static timezone_t const local_tz = (timezone_t) 1;
  56
  57 #if HAVE_TM_ZONE || HAVE_TZNAME
  58
  59 /* Return true if the values A and B differ according to the rules for
  60    tm_isdst: A and B differ if one is zero and the other positive.  */
  61 static bool
  62 isdst_differ (int a, int b)
  63 {
  64   return !a != !b && 0 <= a && 0 <= b;
  65 }
  66
  67 /* Return true if A and B are equal.  */
  68 static int
  69 equal_tm (const struct tm *a, const struct tm *b)
  70 {
  71   return ! ((a->tm_sec ^ b->tm_sec)
  72             | (a->tm_min ^ b->tm_min)
  73             | (a->tm_hour ^ b->tm_hour)
  74             | (a->tm_mday ^ b->tm_mday)
  75             | (a->tm_mon ^ b->tm_mon)
  76             | (a->tm_year ^ b->tm_year)
  77             | isdst_differ (a->tm_isdst, b->tm_isdst));
  78 }
  79
  80 #endif
  81
  82 /* Copy to ABBRS the abbreviation at ABBR with size ABBR_SIZE (this
  83    includes its trailing null byte).  Append an extra null byte to
  84    mark the end of ABBRS.  */
  85 static void
  86 extend_abbrs (char *abbrs, char const *abbr, size_t abbr_size)
  87 {
  88   memcpy (abbrs, abbr, abbr_size);
  89   abbrs[abbr_size] = '\0';
  90 }
  91
  92 /* Return a newly allocated time zone for NAME, or NULL on failure.
  93    As a special case, return a nonzero constant for wall clock time, a
  94    constant that survives freeing.  */
  95 timezone_t
  96 tzalloc (char const *name)
  97 {
  98   size_t name_size = name ? strlen (name) + 1 : 0;
  99   size_t abbr_size = name_size < ABBR_SIZE_MIN ? ABBR_SIZE_MIN : name_size + 1;
 100   timezone_t tz = malloc (offsetof (struct tm_zone, abbrs) + abbr_size);
 101   if (tz)
 102     {
 103       tz->next = NULL;
 104 #if HAVE_TZNAME && !HAVE_TM_ZONE
 105       tz->tzname_copy[0] = tz->tzname_copy[1] = NULL;
 106 #endif
 107       tz->tz_is_set = !!name;
 108       extend_abbrs (tz->abbrs, name, name_size);
 109     }
 110   return tz;
 111 }
 112
 113 /* Save into TZ any nontrivial time zone abbreviation used by TM, and
 114    update *TM (if HAVE_TM_ZONE) or *TZ (if !HAVE_TM_ZONE &&
 115    HAVE_TZNAME) if they use the abbreviation.  Return true if
 116    successful, false (setting errno) otherwise.  */
 117 static bool
 118 save_abbr (timezone_t tz, struct tm *tm)
 119 {
 120 #if HAVE_TM_ZONE || HAVE_TZNAME
 121   char const *zone = NULL;
 122   char *zone_copy = (char *) "";
 123
 124 # if HAVE_TZNAME
 125   int tzname_index = -1;
 126 # endif
 127
 128 # if HAVE_TM_ZONE
 129   zone = tm->tm_zone;
 130 # endif
 131
 132 # if HAVE_TZNAME
 133   if (! (zone && *zone) && 0 <= tm->tm_isdst)
 134     {
 135       tzname_index = tm->tm_isdst != 0;
 136       zone = tzname[tzname_index];
 137     }
 138 # endif
 139
 140   /* No need to replace null zones, or zones within the struct tm.  */
 141   if (!zone || ((char *) tm <= zone && zone < (char *) (tm + 1)))
 142     return true;
 143
 144   if (*zone)
 145     {
 146       zone_copy = tz->abbrs;
 147
 148       while (strcmp (zone_copy, zone) != 0)
 149         {
 150           if (! (*zone_copy || (zone_copy == tz->abbrs && tz->tz_is_set)))
 151             {
 152               size_t zone_size = strlen (zone) + 1;
 153               if (zone_size < tz->abbrs + ABBR_SIZE_MIN - zone_copy)
 154                 extend_abbrs (zone_copy, zone, zone_size);
 155               else
 156                 {
 157                   tz = tz->next = tzalloc (zone);
 158                   if (!tz)
 159                     return false;
 160                   tz->tz_is_set = 0;
 161                   zone_copy = tz->abbrs;
 162                 }
 163               break;
 164             }
 165
 166           zone_copy += strlen (zone_copy) + 1;
 167           if (!*zone_copy && tz->next)
 168             {
 169               tz = tz->next;
 170               zone_copy = tz->abbrs;
 171             }
 172         }
 173     }
 174
 175   /* Replace the zone name so that its lifetime matches that of TZ.  */
 176 # if HAVE_TM_ZONE
 177   tm->tm_zone = zone_copy;
 178 # else
 179   if (0 <= tzname_index)
 180     tz->tzname_copy[tzname_index] = zone_copy;
 181 # endif
 182 #endif
 183
 184   return true;
 185 }
 186
 187 /* Free a time zone.  */
 188 void
 189 tzfree (timezone_t tz)
 190 {
 191   if (tz != local_tz)
 192     while (tz)
 193       {
 194         timezone_t next = tz->next;
 195         free (tz);
 196         tz = next;
 197       }
 198 }
 199
 200 /* Get and set the TZ environment variable.  These functions can be
 201    overridden by programs like Emacs that manage their own environment.  */
 202
 203 #ifndef getenv_TZ
 204 static char *
 205 getenv_TZ (void)
 206 {
 207   return getenv (TZ);
 208 }
 209 #endif
 210
 211 #ifndef setenv_TZ
 212 static int
 213 setenv_TZ (char const *tz)
 214 {
 215   return tz ? setenv (TZ, tz, 1) : unsetenv (TZ);
 216 }
 217 #endif
 218
 219 /* Change the environment to match the specified timezone_t value.
 220    Return true if successful, false (setting errno) otherwise.  */
 221 static bool
 222 change_env (timezone_t tz)
 223 {
 224   if (setenv_TZ (tz->tz_is_set ? tz->abbrs : NULL) != 0)
 225     return false;
 226   tzset ();
 227   return true;
 228 }
 229
 230 /* Temporarily set the time zone to TZ, which must not be null.
 231    Return LOCAL_TZ if the time zone setting is already correct.
 232    Otherwise return a newly allocated time zone representing the old
 233    setting, or NULL (setting errno) on failure.  */
 234 static timezone_t
 235 set_tz (timezone_t tz)
 236 {
 237   char *env_tz = getenv_TZ ();
 238   if (env_tz
 239       ? tz->tz_is_set && strcmp (tz->abbrs, env_tz) == 0
 240       : !tz->tz_is_set)
 241     return local_tz;
 242   else
 243     {
 244       timezone_t old_tz = tzalloc (env_tz);
 245       if (!old_tz)
 246         return old_tz;
 247       if (! change_env (tz))
 248         {
 249           int saved_errno = errno;
 250           tzfree (old_tz);
 251           errno = saved_errno;
 252           return NULL;
 253         }
 254       return old_tz;
 255     }
 256 }
 257
 258 /* Restore an old setting returned by set_tz.  It must not be null.
 259    Return true (preserving errno) if successful, false (setting errno)
 260    otherwise.  */
 261 static bool
 262 revert_tz (timezone_t tz)
 263 {
 264   if (tz == local_tz)
 265     return true;
 266   else
 267     {
 268       int saved_errno = errno;
 269       bool ok = change_env (tz);
 270       if (!ok)
 271         saved_errno = errno;
 272       tzfree (tz);
 273       errno = saved_errno;
 274       return ok;
 275     }
 276 }
 277
 278 /* Use time zone TZ to compute localtime_r (T, TM).  */
 279 struct tm *
 280 localtime_rz (timezone_t tz, time_t const *t, struct tm *tm)
 281 {
 282   if (!tz)
 283     return gmtime_r (t, tm);
 284   else
 285     {
 286       timezone_t old_tz = set_tz (tz);
 287       if (old_tz)
 288         {
 289           tm = localtime_r (t, tm);
 290           if (tm && !save_abbr (tz, tm))
 291             tm = NULL;
 292           if (revert_tz (old_tz))
 293             return tm;
 294         }
 295       return NULL;
 296     }
 297 }
 298
 299 /* Use time zone TZ to compute mktime (TM).  */
 300 time_t
 301 mktime_z (timezone_t tz, struct tm *tm)
 302 {
 303   if (!tz)
 304     return timegm (tm);
 305   else
 306     {
 307       timezone_t old_tz = set_tz (tz);
 308       if (old_tz)
 309         {
 310           time_t t = mktime (tm);
 311 #if HAVE_TM_ZONE || HAVE_TZNAME
 312           time_t badtime = -1;
 313           struct tm tm_1;
 314           if ((t != badtime
 315                || (localtime_r (&t, &tm_1) && equal_tm (tm, &tm_1)))
 316               && !save_abbr (tz, tm))
 317             t = badtime;
 318 #endif
 319           if (revert_tz (old_tz))
 320             return t;
 321         }
 322       return -1;
 323     }
 324 }