lib/time_rz.c

   1 /* Time zone functions such as tzalloc and localtime_rz
   2
   3    Copyright 2015-2016 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 /* Magic cookie timezone_t value, for local time.  It differs from
  51    NULL and from all other timezone_t values.  Only the address
  52    matters; the pointer is never dereferenced.  */
  53 static timezone_t const local_tz = (timezone_t) 1;
  54
  55 #if HAVE_TM_ZONE || HAVE_TZNAME
  56
  57 /* Return true if the values A and B differ according to the rules for
  58    tm_isdst: A and B differ if one is zero and the other positive.  */
  59 static bool
  60 isdst_differ (int a, int b)
  61 {
  62   return !a != !b && 0 <= a && 0 <= b;
  63 }
  64
  65 /* Return true if A and B are equal.  */
  66 static int
  67 equal_tm (const struct tm *a, const struct tm *b)
  68 {
  69   return ! ((a->tm_sec ^ b->tm_sec)
  70             | (a->tm_min ^ b->tm_min)
  71             | (a->tm_hour ^ b->tm_hour)
  72             | (a->tm_mday ^ b->tm_mday)
  73             | (a->tm_mon ^ b->tm_mon)
  74             | (a->tm_year ^ b->tm_year)
  75             | isdst_differ (a->tm_isdst, b->tm_isdst));
  76 }
  77
  78 #endif
  79
  80 /* Copy to ABBRS the abbreviation at ABBR with size ABBR_SIZE (this
  81    includes its trailing null byte).  Append an extra null byte to
  82    mark the end of ABBRS.  */
  83 static void
  84 extend_abbrs (char *abbrs, char const *abbr, size_t abbr_size)
  85 {
  86   memcpy (abbrs, abbr, abbr_size);
  87   abbrs[abbr_size] = '\0';
  88 }
  89
  90 /* Return a newly allocated time zone for NAME, or NULL on failure.
  91    A null NAME stands for wall clock time (which is like unset TZ).  */
  92 timezone_t
  93 tzalloc (char const *name)
  94 {
  95   size_t name_size = name ? strlen (name) + 1 : 0;
  96   size_t abbr_size = name_size < ABBR_SIZE_MIN ? ABBR_SIZE_MIN : name_size + 1;
  97   timezone_t tz = malloc (offsetof (struct tm_zone, abbrs) + abbr_size);
  98   if (tz)
  99     {
 100       tz->next = NULL;
 101 #if HAVE_TZNAME && !HAVE_TM_ZONE
 102       tz->tzname_copy[0] = tz->tzname_copy[1] = NULL;
 103 #endif
 104       tz->tz_is_set = !!name;
 105       tz->abbrs[0] = '\0';
 106       if (name)
 107         extend_abbrs (tz->abbrs, name, name_size);
 108     }
 109   return tz;
 110 }
 111
 112 /* Save into TZ any nontrivial time zone abbreviation used by TM, and
 113    update *TM (if HAVE_TM_ZONE) or *TZ (if !HAVE_TM_ZONE &&
 114    HAVE_TZNAME) if they use the abbreviation.  Return true if
 115    successful, false (setting errno) otherwise.  */
 116 static bool
 117 save_abbr (timezone_t tz, struct tm *tm)
 118 {
 119 #if HAVE_TM_ZONE || HAVE_TZNAME
 120   char const *zone = NULL;
 121   char *zone_copy = (char *) "";
 122
 123 # if HAVE_TZNAME
 124   int tzname_index = -1;
 125 # endif
 126
 127 # if HAVE_TM_ZONE
 128   zone = tm->tm_zone;
 129 # endif
 130
 131 # if HAVE_TZNAME
 132   if (! (zone && *zone) && 0 <= tm->tm_isdst)
 133     {
 134       tzname_index = tm->tm_isdst != 0;
 135       zone = tzname[tzname_index];
 136     }
 137 # endif
 138
 139   /* No need to replace null zones, or zones within the struct tm.  */
 140   if (!zone || ((char *) tm <= zone && zone < (char *) (tm + 1)))
 141     return true;
 142
 143   if (*zone)
 144     {
 145       zone_copy = tz->abbrs;
 146
 147       while (strcmp (zone_copy, zone) != 0)
 148         {
 149           if (! (*zone_copy || (zone_copy == tz->abbrs && tz->tz_is_set)))
 150             {
 151               size_t zone_size = strlen (zone) + 1;
 152               if (zone_size < tz->abbrs + ABBR_SIZE_MIN - zone_copy)
 153                 extend_abbrs (zone_copy, zone, zone_size);
 154               else
 155                 {
 156                   tz = tz->next = tzalloc (zone);
 157                   if (!tz)
 158                     return false;
 159                   tz->tz_is_set = 0;
 160                   zone_copy = tz->abbrs;
 161                 }
 162               break;
 163             }
 164
 165           zone_copy += strlen (zone_copy) + 1;
 166           if (!*zone_copy && tz->next)
 167             {
 168               tz = tz->next;
 169               zone_copy = tz->abbrs;
 170             }
 171         }
 172     }
 173
 174   /* Replace the zone name so that its lifetime matches that of TZ.  */
 175 # if HAVE_TM_ZONE
 176   tm->tm_zone = zone_copy;
 177 # else
 178   if (0 <= tzname_index)
 179     tz->tzname_copy[tzname_index] = zone_copy;
 180 # endif
 181 #endif
 182
 183   return true;
 184 }
 185
 186 /* Free a time zone.  */
 187 void
 188 tzfree (timezone_t tz)
 189 {
 190   if (tz != local_tz)
 191     while (tz)
 192       {
 193         timezone_t next = tz->next;
 194         free (tz);
 195         tz = next;
 196       }
 197 }
 198
 199 /* Get and set the TZ environment variable.  These functions can be
 200    overridden by programs like Emacs that manage their own environment.  */
 201
 202 #ifndef getenv_TZ
 203 static char *
 204 getenv_TZ (void)
 205 {
 206   return getenv ("TZ");
 207 }
 208 #endif
 209
 210 #ifndef setenv_TZ
 211 static int
 212 setenv_TZ (char const *tz)
 213 {
 214   return tz ? setenv ("TZ", tz, 1) : unsetenv ("TZ");
 215 }
 216 #endif
 217
 218 /* Change the environment to match the specified timezone_t value.
 219    Return true if successful, false (setting errno) otherwise.  */
 220 static bool
 221 change_env (timezone_t tz)
 222 {
 223   if (setenv_TZ (tz->tz_is_set ? tz->abbrs : NULL) != 0)
 224     return false;
 225   tzset ();
 226   return true;
 227 }
 228
 229 /* Temporarily set the time zone to TZ, which must not be null.
 230    Return LOCAL_TZ if the time zone setting is already correct.
 231    Otherwise return a newly allocated time zone representing the old
 232    setting, or NULL (setting errno) on failure.  */
 233 static timezone_t
 234 set_tz (timezone_t tz)
 235 {
 236   char *env_tz = getenv_TZ ();
 237   if (env_tz
 238       ? tz->tz_is_set && strcmp (tz->abbrs, env_tz) == 0
 239       : !tz->tz_is_set)
 240     return local_tz;
 241   else
 242     {
 243       timezone_t old_tz = tzalloc (env_tz);
 244       if (!old_tz)
 245         return old_tz;
 246       if (! change_env (tz))
 247         {
 248           int saved_errno = errno;
 249           tzfree (old_tz);
 250           errno = saved_errno;
 251           return NULL;
 252         }
 253       return old_tz;
 254     }
 255 }
 256
 257 /* Restore an old setting returned by set_tz.  It must not be null.
 258    Return true (preserving errno) if successful, false (setting errno)
 259    otherwise.  */
 260 static bool
 261 revert_tz (timezone_t tz)
 262 {
 263   if (tz == local_tz)
 264     return true;
 265   else
 266     {
 267       int saved_errno = errno;
 268       bool ok = change_env (tz);
 269       if (!ok)
 270         saved_errno = errno;
 271       tzfree (tz);
 272       errno = saved_errno;
 273       return ok;
 274     }
 275 }
 276
 277 /* Use time zone TZ to compute localtime_r (T, TM).  */
 278 struct tm *
 279 localtime_rz (timezone_t tz, time_t const *t, struct tm *tm)
 280 {
 281   if (!tz)
 282     return gmtime_r (t, tm);
 283   else
 284     {
 285       timezone_t old_tz = set_tz (tz);
 286       if (old_tz)
 287         {
 288           bool abbr_saved = localtime_r (t, tm) && save_abbr (tz, tm);
 289           if (revert_tz (old_tz) && abbr_saved)
 290             return tm;
 291         }
 292       return NULL;
 293     }
 294 }
 295
 296 /* Use time zone TZ to compute mktime (TM).  */
 297 time_t
 298 mktime_z (timezone_t tz, struct tm *tm)
 299 {
 300   if (!tz)
 301     return timegm (tm);
 302   else
 303     {
 304       timezone_t old_tz = set_tz (tz);
 305       if (old_tz)
 306         {
 307           time_t t = mktime (tm);
 308 #if HAVE_TM_ZONE || HAVE_TZNAME
 309           time_t badtime = -1;
 310           struct tm tm_1;
 311           if ((t != badtime
 312                || (localtime_r (&t, &tm_1) && equal_tm (tm, &tm_1)))
 313               && !save_abbr (tz, tm))
 314             t = badtime;
 315 #endif
 316           if (revert_tz (old_tz))
 317             return t;
 318         }
 319       return -1;
 320     }
 321 }