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    A null NAME stands for wall clock time (which is like unset TZ).  */
  94 timezone_t
  95 tzalloc (char const *name)
  96 {
  97   size_t name_size = name ? strlen (name) + 1 : 0;
  98   size_t abbr_size = name_size < ABBR_SIZE_MIN ? ABBR_SIZE_MIN : name_size + 1;
  99   timezone_t tz = malloc (offsetof (struct tm_zone, abbrs) + abbr_size);
 100   if (tz)
 101     {
 102       tz->next = NULL;
 103 #if HAVE_TZNAME && !HAVE_TM_ZONE
 104       tz->tzname_copy[0] = tz->tzname_copy[1] = NULL;
 105 #endif
 106       tz->tz_is_set = !!name;
 107       tz->abbrs[0] = '\0';
 108       if (name)
 109         extend_abbrs (tz->abbrs, name, name_size);
 110     }
 111   return tz;
 112 }
 113
 114 /* Save into TZ any nontrivial time zone abbreviation used by TM, and
 115    update *TM (if HAVE_TM_ZONE) or *TZ (if !HAVE_TM_ZONE &&
 116    HAVE_TZNAME) if they use the abbreviation.  Return true if
 117    successful, false (setting errno) otherwise.  */
 118 static bool
 119 save_abbr (timezone_t tz, struct tm *tm)
 120 {
 121 #if HAVE_TM_ZONE || HAVE_TZNAME
 122   char const *zone = NULL;
 123   char *zone_copy = (char *) "";
 124
 125 # if HAVE_TZNAME
 126   int tzname_index = -1;
 127 # endif
 128
 129 # if HAVE_TM_ZONE
 130   zone = tm->tm_zone;
 131 # endif
 132
 133 # if HAVE_TZNAME
 134   if (! (zone && *zone) && 0 <= tm->tm_isdst)
 135     {
 136       tzname_index = tm->tm_isdst != 0;
 137       zone = tzname[tzname_index];
 138     }
 139 # endif
 140
 141   /* No need to replace null zones, or zones within the struct tm.  */
 142   if (!zone || ((char *) tm <= zone && zone < (char *) (tm + 1)))
 143     return true;
 144
 145   if (*zone)
 146     {
 147       zone_copy = tz->abbrs;
 148
 149       while (strcmp (zone_copy, zone) != 0)
 150         {
 151           if (! (*zone_copy || (zone_copy == tz->abbrs && tz->tz_is_set)))
 152             {
 153               size_t zone_size = strlen (zone) + 1;
 154               if (zone_size < tz->abbrs + ABBR_SIZE_MIN - zone_copy)
 155                 extend_abbrs (zone_copy, zone, zone_size);
 156               else
 157                 {
 158                   tz = tz->next = tzalloc (zone);
 159                   if (!tz)
 160                     return false;
 161                   tz->tz_is_set = 0;
 162                   zone_copy = tz->abbrs;
 163                 }
 164               break;
 165             }
 166
 167           zone_copy += strlen (zone_copy) + 1;
 168           if (!*zone_copy && tz->next)
 169             {
 170               tz = tz->next;
 171               zone_copy = tz->abbrs;
 172             }
 173         }
 174     }
 175
 176   /* Replace the zone name so that its lifetime matches that of TZ.  */
 177 # if HAVE_TM_ZONE
 178   tm->tm_zone = zone_copy;
 179 # else
 180   if (0 <= tzname_index)
 181     tz->tzname_copy[tzname_index] = zone_copy;
 182 # endif
 183 #endif
 184
 185   return true;
 186 }
 187
 188 /* Free a time zone.  */
 189 void
 190 tzfree (timezone_t tz)
 191 {
 192   if (tz != local_tz)
 193     while (tz)
 194       {
 195         timezone_t next = tz->next;
 196         free (tz);
 197         tz = next;
 198       }
 199 }
 200
 201 /* Get and set the TZ environment variable.  These functions can be
 202    overridden by programs like Emacs that manage their own environment.  */
 203
 204 #ifndef getenv_TZ
 205 static char *
 206 getenv_TZ (void)
 207 {
 208   return getenv (TZ);
 209 }
 210 #endif
 211
 212 #ifndef setenv_TZ
 213 static int
 214 setenv_TZ (char const *tz)
 215 {
 216   return tz ? setenv (TZ, tz, 1) : unsetenv (TZ);
 217 }
 218 #endif
 219
 220 /* Change the environment to match the specified timezone_t value.
 221    Return true if successful, false (setting errno) otherwise.  */
 222 static bool
 223 change_env (timezone_t tz)
 224 {
 225   if (setenv_TZ (tz->tz_is_set ? tz->abbrs : NULL) != 0)
 226     return false;
 227   tzset ();
 228   return true;
 229 }
 230
 231 /* Temporarily set the time zone to TZ, which must not be null.
 232    Return LOCAL_TZ if the time zone setting is already correct.
 233    Otherwise return a newly allocated time zone representing the old
 234    setting, or NULL (setting errno) on failure.  */
 235 static timezone_t
 236 set_tz (timezone_t tz)
 237 {
 238   char *env_tz = getenv_TZ ();
 239   if (env_tz
 240       ? tz->tz_is_set && strcmp (tz->abbrs, env_tz) == 0
 241       : !tz->tz_is_set)
 242     return local_tz;
 243   else
 244     {
 245       timezone_t old_tz = tzalloc (env_tz);
 246       if (!old_tz)
 247         return old_tz;
 248       if (! change_env (tz))
 249         {
 250           int saved_errno = errno;
 251           tzfree (old_tz);
 252           errno = saved_errno;
 253           return NULL;
 254         }
 255       return old_tz;
 256     }
 257 }
 258
 259 /* Restore an old setting returned by set_tz.  It must not be null.
 260    Return true (preserving errno) if successful, false (setting errno)
 261    otherwise.  */
 262 static bool
 263 revert_tz (timezone_t tz)
 264 {
 265   if (tz == local_tz)
 266     return true;
 267   else
 268     {
 269       int saved_errno = errno;
 270       bool ok = change_env (tz);
 271       if (!ok)
 272         saved_errno = errno;
 273       tzfree (tz);
 274       errno = saved_errno;
 275       return ok;
 276     }
 277 }
 278
 279 /* Use time zone TZ to compute localtime_r (T, TM).  */
 280 struct tm *
 281 localtime_rz (timezone_t tz, time_t const *t, struct tm *tm)
 282 {
 283   if (!tz)
 284     return gmtime_r (t, tm);
 285   else
 286     {
 287       timezone_t old_tz = set_tz (tz);
 288       if (old_tz)
 289         {
 290           bool abbr_saved = localtime_r (t, tm) && save_abbr (tz, tm);
 291           if (revert_tz (old_tz) && abbr_saved)
 292             return tm;
 293         }
 294       return NULL;
 295     }
 296 }
 297
 298 /* Use time zone TZ to compute mktime (TM).  */
 299 time_t
 300 mktime_z (timezone_t tz, struct tm *tm)
 301 {
 302   if (!tz)
 303     return timegm (tm);
 304   else
 305     {
 306       timezone_t old_tz = set_tz (tz);
 307       if (old_tz)
 308         {
 309           time_t t = mktime (tm);
 310 #if HAVE_TM_ZONE || HAVE_TZNAME
 311           time_t badtime = -1;
 312           struct tm tm_1;
 313           if ((t != badtime
 314                || (localtime_r (&t, &tm_1) && equal_tm (tm, &tm_1)))
 315               && !save_abbr (tz, tm))
 316             t = badtime;
 317 #endif
 318           if (revert_tz (old_tz))
 319             return t;
 320         }
 321       return -1;
 322     }
 323 }