glib/gtimezone.c

   1 /*
   2  * Copyright © 2010 Codethink Limited
   3  *
   4  * This library is free software; you can redistribute it and/or
   5  * modify it under the terms of the GNU Lesser General Public
   6  * License as published by the Free Software Foundation; either
   7  * version 2 of the licence, or (at your option) any later version.
   8  *
   9  * This library is distributed in the hope that it will be useful,
  10  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  11  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  12  * Lesser General Public License for more details.
  13  *
  14  * You should have received a copy of the GNU Lesser General Public
  15  * License along with this library; if not, write to the
  16  * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
  17  * Boston, MA 02111-1307, USA.
  18  *
  19  * Author: Ryan Lortie <desrt@desrt.ca>
  20  */
  21
  22 /* Prologue {{{1 */
  23
  24 #include "gtimezone.h"
  25
  26 #include <string.h>
  27 #include <stdlib.h>
  28 #include <signal.h>
  29
  30 #include "gmappedfile.h"
  31 #include "gtestutils.h"
  32 #include "gfileutils.h"
  33 #include "gstrfuncs.h"
  34 #include "ghash.h"
  35 #include "gthread.h"
  36 #include "gbytes.h"
  37 #include "gslice.h"
  38
  39 /**
  40  * SECTION:timezone
  41  * @title: GTimeZone
  42  * @short_description: a structure representing a time zone
  43  * @see_also: #GDateTime
  44  *
  45  * #GTimeZone is a structure that represents a time zone, at no
  46  * particular point in time.  It is refcounted and immutable.
  47  *
  48  * A time zone contains a number of intervals.  Each interval has
  49  * an abbreviation to describe it, an offet to UTC and a flag indicating
  50  * if the daylight savings time is in effect during that interval.  A
  51  * time zone always has at least one interval -- interval 0.
  52  *
  53  * Every UTC time is contained within exactly one interval, but a given
  54  * local time may be contained within zero, one or two intervals (due to
  55  * incontinuities associated with daylight savings time).
  56  *
  57  * An interval may refer to a specific period of time (eg: the duration
  58  * of daylight savings time during 2010) or it may refer to many periods
  59  * of time that share the same properties (eg: all periods of daylight
  60  * savings time).  It is also possible (usually for political reasons)
  61  * that some properties (like the abbreviation) change between intervals
  62  * without other properties changing.
  63  *
  64  * #GTimeZone is available since GLib 2.26.
  65  */
  66
  67 /**
  68  * GTimeZone:
  69  *
  70  * #GDateTime is an opaque structure whose members cannot be accessed
  71  * directly.
  72  *
  73  * Since: 2.26
  74  **/
  75
  76 /* zoneinfo file format {{{1 */
  77
  78 /* unaligned */
  79 typedef struct { gchar bytes[8]; } gint64_be;
  80 typedef struct { gchar bytes[4]; } gint32_be;
  81 typedef struct { gchar bytes[4]; } guint32_be;
  82
  83 static inline gint64 gint64_from_be (const gint64_be be) {
  84   gint64 tmp; memcpy (&tmp, &be, sizeof tmp); return GINT64_FROM_BE (tmp);
  85 }
  86
  87 static inline gint32 gint32_from_be (const gint32_be be) {
  88   gint32 tmp; memcpy (&tmp, &be, sizeof tmp); return GINT32_FROM_BE (tmp);
  89 }
  90
  91 static inline guint32 guint32_from_be (const guint32_be be) {
  92   guint32 tmp; memcpy (&tmp, &be, sizeof tmp); return GUINT32_FROM_BE (tmp);
  93 }
  94
  95 struct tzhead
  96 {
  97   gchar      tzh_magic[4];
  98   gchar      tzh_version;
  99   guchar     tzh_reserved[15];
 100
 101   guint32_be tzh_ttisgmtcnt;
 102   guint32_be tzh_ttisstdcnt;
 103   guint32_be tzh_leapcnt;
 104   guint32_be tzh_timecnt;
 105   guint32_be tzh_typecnt;
 106   guint32_be tzh_charcnt;
 107 };
 108
 109 struct ttinfo
 110 {
 111   gint32_be tt_gmtoff;
 112   guint8    tt_isdst;
 113   guint8    tt_abbrind;
 114 };
 115
 116 /* GTimeZone structure and lifecycle {{{1 */
 117 struct _GTimeZone
 118 {
 119   gchar   *name;
 120
 121   GBytes *zoneinfo;
 122
 123   const struct tzhead *header;
 124   const struct ttinfo *infos;
 125   const gint64_be     *trans;
 126   const guint8        *indices;
 127   const gchar         *abbrs;
 128   gint                 timecnt;
 129
 130   gint     ref_count;
 131 };
 132
 133 G_LOCK_DEFINE_STATIC (time_zones);
 134 static GHashTable/*<string?, GTimeZone>*/ *time_zones;
 135
 136 /**
 137  * g_time_zone_unref:
 138  * @tz: a #GTimeZone
 139  *
 140  * Decreases the reference count on @tz.
 141  *
 142  * Since: 2.26
 143  **/
 144 void
 145 g_time_zone_unref (GTimeZone *tz)
 146 {
 147   int ref_count;
 148
 149 again:
 150   ref_count = g_atomic_int_get (&tz->ref_count);
 151
 152   g_assert (ref_count > 0);
 153
 154   if (ref_count == 1)
 155     {
 156       if (tz->name != NULL)
 157         {
 158           G_LOCK(time_zones);
 159
 160           /* someone else might have grabbed a ref in the meantime */
 161           if G_UNLIKELY (g_atomic_int_get (&tz->ref_count) != 1)
 162             {
 163               G_UNLOCK(time_zones);
 164               goto again;
 165             }
 166
 167           g_hash_table_remove (time_zones, tz->name);
 168           G_UNLOCK(time_zones);
 169         }
 170
 171       if (tz->zoneinfo)
 172         g_bytes_unref (tz->zoneinfo);
 173
 174       g_free (tz->name);
 175
 176       g_slice_free (GTimeZone, tz);
 177     }
 178
 179   else if G_UNLIKELY (!g_atomic_int_compare_and_exchange (&tz->ref_count,
 180                                                           ref_count,
 181                                                           ref_count - 1))
 182     goto again;
 183 }
 184
 185 /**
 186  * g_time_zone_ref:
 187  * @tz: a #GTimeZone
 188  *
 189  * Increases the reference count on @tz.
 190  *
 191  * Returns: a new reference to @tz.
 192  *
 193  * Since: 2.26
 194  **/
 195 GTimeZone *
 196 g_time_zone_ref (GTimeZone *tz)
 197 {
 198   g_assert (tz->ref_count > 0);
 199
 200   g_atomic_int_inc (&tz->ref_count);
 201
 202   return tz;
 203 }
 204
 205 /* fake zoneinfo creation (for RFC3339/ISO 8601 timezones) {{{1 */
 206 /*
 207  * parses strings of the form 'hh' 'hhmm' or 'hh:mm' where:
 208  *  - hh is 00 to 23
 209  *  - mm is 00 to 59
 210  */
 211 static gboolean
 212 parse_time (const gchar *time_,
 213             gint32      *offset)
 214 {
 215   if (*time_ < '0' || '2' < *time_)
 216     return FALSE;
 217
 218   *offset = 10 * 60 * 60 * (*time_++ - '0');
 219
 220   if (*time_ < '0' || '9' < *time_)
 221     return FALSE;
 222
 223   *offset += 60 * 60 * (*time_++ - '0');
 224
 225   if (*offset > 23 * 60 * 60)
 226     return FALSE;
 227
 228   if (*time_ == '\0')
 229     return TRUE;
 230
 231   if (*time_ == ':')
 232     time_++;
 233
 234   if (*time_ < '0' || '5' < *time_)
 235     return FALSE;
 236
 237   *offset += 10 * 60 * (*time_++ - '0');
 238
 239   if (*time_ < '0' || '9' < *time_)
 240     return FALSE;
 241
 242   *offset += 60 * (*time_++ - '0');
 243
 244   return *time_ == '\0';
 245 }
 246
 247 static gboolean
 248 parse_constant_offset (const gchar *name,
 249                        gint32      *offset)
 250 {
 251   switch (*name++)
 252     {
 253     case 'Z':
 254       *offset = 0;
 255       return !*name;
 256
 257     case '+':
 258       return parse_time (name, offset);
 259
 260     case '-':
 261       if (parse_time (name, offset))
 262         {
 263           *offset = -*offset;
 264           return TRUE;
 265         }
 266
 267     default:
 268       return FALSE;
 269     }
 270 }
 271
 272 static GBytes *
 273 zone_for_constant_offset (const gchar *name)
 274 {
 275   const gchar fake_zoneinfo_headers[] =
 276     "TZif" "2..." "...." "...." "...."
 277     "\0\0\0\0" "\0\0\0\0" "\0\0\0\0" "\0\0\0\0" "\0\0\0\0" "\0\0\0\0"
 278     "TZif" "2..." "...." "...." "...."
 279     "\0\0\0\0" "\0\0\0\0" "\0\0\0\0" "\0\0\0\0" "\0\0\0\1" "\0\0\0\7";
 280   struct {
 281     struct tzhead headers[2];
 282     struct ttinfo info;
 283     gchar abbr[8];
 284   } *fake;
 285   gint32 offset;
 286
 287   if (name == NULL || !parse_constant_offset (name, &offset))
 288     return NULL;
 289
 290   offset = GINT32_TO_BE (offset);
 291
 292   fake = g_malloc (sizeof *fake);
 293   memcpy (fake, fake_zoneinfo_headers, sizeof fake_zoneinfo_headers);
 294   memcpy (&fake->info.tt_gmtoff, &offset, sizeof offset);
 295   fake->info.tt_isdst = FALSE;
 296   fake->info.tt_abbrind = 0;
 297   strcpy (fake->abbr, name);
 298
 299   return g_bytes_new_take (fake, sizeof *fake);
 300 }
 301
 302 /* Construction {{{1 */
 303 /**
 304  * g_time_zone_new:
 305  * @identifier: (allow-none): a timezone identifier
 306  *
 307  * Creates a #GTimeZone corresponding to @identifier.
 308  *
 309  * @identifier can either be an RFC3339/ISO 8601 time offset or
 310  * something that would pass as a valid value for the
 311  * <varname>TZ</varname> environment variable (including %NULL).
 312  *
 313  * Valid RFC3339 time offsets are <literal>"Z"</literal> (for UTC) or
 314  * <literal>"±hh:mm"</literal>.  ISO 8601 additionally specifies
 315  * <literal>"±hhmm"</literal> and <literal>"±hh"</literal>.
 316  *
 317  * The <varname>TZ</varname> environment variable typically corresponds
 318  * to the name of a file in the zoneinfo database, but there are many
 319  * other possibilities.  Note that those other possibilities are not
 320  * currently implemented, but are planned.
 321  *
 322  * g_time_zone_new_local() calls this function with the value of the
 323  * <varname>TZ</varname> environment variable.  This function itself is
 324  * independent of the value of <varname>TZ</varname>, but if @identifier
 325  * is %NULL then <filename>/etc/localtime</filename> will be consulted
 326  * to discover the correct timezone.
 327  *
 328  * See <ulink
 329  * url='http://tools.ietf.org/html/rfc3339#section-5.6'>RFC3339
 330  * §5.6</ulink> for a precise definition of valid RFC3339 time offsets
 331  * (the <varname>time-offset</varname> expansion) and ISO 8601 for the
 332  * full list of valid time offsets.  See <ulink
 333  * url='http://www.gnu.org/s/libc/manual/html_node/TZ-Variable.html'>The
 334  * GNU C Library manual</ulink> for an explanation of the possible
 335  * values of the <varname>TZ</varname> environment variable.
 336  *
 337  * You should release the return value by calling g_time_zone_unref()
 338  * when you are done with it.
 339  *
 340  * Returns: the requested timezone
 341  *
 342  * Since: 2.26
 343  **/
 344 GTimeZone *
 345 g_time_zone_new (const gchar *identifier)
 346 {
 347   GTimeZone *tz;
 348   GMappedFile *file;
 349
 350   G_LOCK (time_zones);
 351   if (time_zones == NULL)
 352     time_zones = g_hash_table_new (g_str_hash, g_str_equal);
 353
 354   if (identifier)
 355     tz = g_hash_table_lookup (time_zones, identifier);
 356   else
 357     tz = NULL;
 358
 359   if (tz == NULL)
 360     {
 361       tz = g_slice_new0 (GTimeZone);
 362       tz->name = g_strdup (identifier);
 363       tz->ref_count = 0;
 364
 365       tz->zoneinfo = zone_for_constant_offset (identifier);
 366
 367       if (tz->zoneinfo == NULL)
 368         {
 369           gchar *filename;
 370
 371           if (identifier != NULL)
 372             {
 373               const gchar *tzdir;
 374
 375               tzdir = getenv ("TZDIR");
 376               if (tzdir == NULL)
 377                 tzdir = "/usr/share/zoneinfo";
 378
 379               filename = g_build_filename (tzdir, identifier, NULL);
 380             }
 381           else
 382             filename = g_strdup ("/etc/localtime");
 383
 384           file = g_mapped_file_new (filename, FALSE, NULL);
 385           if (file != NULL)
 386             {
 387               tz->zoneinfo = g_bytes_new_with_free_func (g_mapped_file_get_contents (file),
 388                                                          g_mapped_file_get_length (file),
 389                                                          (GDestroyNotify)g_mapped_file_unref,
 390                                                          g_mapped_file_ref (file));
 391               g_mapped_file_unref (file);
 392             }
 393           g_free (filename);
 394         }
 395
 396       if (tz->zoneinfo != NULL)
 397         {
 398           gsize size;
 399           const struct tzhead *header = g_bytes_get_data (tz->zoneinfo, &size);
 400
 401           /* we only bother to support version 2 */
 402           if (size < sizeof (struct tzhead) || memcmp (header, "TZif2", 5))
 403             {
 404               g_bytes_unref (tz->zoneinfo);
 405               tz->zoneinfo = NULL;
 406             }
 407           else
 408             {
 409               gint typecnt;
 410
 411               /* we trust the file completely. */
 412               tz->header = (const struct tzhead *)
 413                 (((const gchar *) (header + 1)) +
 414                   guint32_from_be(header->tzh_ttisgmtcnt) +
 415                   guint32_from_be(header->tzh_ttisstdcnt) +
 416                   8 * guint32_from_be(header->tzh_leapcnt) +
 417                   5 * guint32_from_be(header->tzh_timecnt) +
 418                   6 * guint32_from_be(header->tzh_typecnt) +
 419                   guint32_from_be(header->tzh_charcnt));
 420
 421               typecnt     = guint32_from_be (tz->header->tzh_typecnt);
 422               tz->timecnt = guint32_from_be (tz->header->tzh_timecnt);
 423               tz->trans   = (gconstpointer) (tz->header + 1);
 424               tz->indices = (gconstpointer) (tz->trans + tz->timecnt);
 425               tz->infos   = (gconstpointer) (tz->indices + tz->timecnt);
 426               tz->abbrs   = (gconstpointer) (tz->infos + typecnt);
 427             }
 428         }
 429
 430       if (identifier)
 431         g_hash_table_insert (time_zones, tz->name, tz);
 432     }
 433   g_atomic_int_inc (&tz->ref_count);
 434   G_UNLOCK (time_zones);
 435
 436   return tz;
 437 }
 438
 439 /**
 440  * g_time_zone_new_utc:
 441  *
 442  * Creates a #GTimeZone corresponding to UTC.
 443  *
 444  * This is equivalent to calling g_time_zone_new() with a value like
 445  * "Z", "UTC", "+00", etc.
 446  *
 447  * You should release the return value by calling g_time_zone_unref()
 448  * when you are done with it.
 449  *
 450  * Returns: the universal timezone
 451  *
 452  * Since: 2.26
 453  **/
 454 GTimeZone *
 455 g_time_zone_new_utc (void)
 456 {
 457   return g_time_zone_new ("UTC");
 458 }
 459
 460 /**
 461  * g_time_zone_new_local:
 462  *
 463  * Creates a #GTimeZone corresponding to local time.  The local time
 464  * zone may change between invocations to this function; for example,
 465  * if the system administrator changes it.
 466  *
 467  * This is equivalent to calling g_time_zone_new() with the value of the
 468  * <varname>TZ</varname> environment variable (including the possibility
 469  * of %NULL).
 470  *
 471  * You should release the return value by calling g_time_zone_unref()
 472  * when you are done with it.
 473  *
 474  * Returns: the local timezone
 475  *
 476  * Since: 2.26
 477  **/
 478 GTimeZone *
 479 g_time_zone_new_local (void)
 480 {
 481   return g_time_zone_new (getenv ("TZ"));
 482 }
 483
 484 /* Internal helpers {{{1 */
 485 inline static const struct ttinfo *
 486 interval_info (GTimeZone *tz,
 487                gint       interval)
 488 {
 489   if (interval)
 490     return tz->infos + tz->indices[interval - 1];
 491
 492   return tz->infos;
 493 }
 494
 495 inline static gint64
 496 interval_start (GTimeZone *tz,
 497                 gint       interval)
 498 {
 499   if (interval)
 500     return gint64_from_be (tz->trans[interval - 1]);
 501
 502   return G_MININT64;
 503 }
 504
 505 inline static gint64
 506 interval_end (GTimeZone *tz,
 507               gint       interval)
 508 {
 509   if (interval < tz->timecnt)
 510     return gint64_from_be (tz->trans[interval]) - 1;
 511
 512   return G_MAXINT64;
 513 }
 514
 515 inline static gint32
 516 interval_offset (GTimeZone *tz,
 517                  gint       interval)
 518 {
 519   return gint32_from_be (interval_info (tz, interval)->tt_gmtoff);
 520 }
 521
 522 inline static gboolean
 523 interval_isdst (GTimeZone *tz,
 524                 gint       interval)
 525 {
 526   return interval_info (tz, interval)->tt_isdst;
 527 }
 528
 529 inline static guint8
 530 interval_abbrind (GTimeZone *tz,
 531                   gint       interval)
 532 {
 533   return interval_info (tz, interval)->tt_abbrind;
 534 }
 535
 536 inline static gint64
 537 interval_local_start (GTimeZone *tz,
 538                       gint       interval)
 539 {
 540   if (interval)
 541     return interval_start (tz, interval) + interval_offset (tz, interval);
 542
 543   return G_MININT64;
 544 }
 545
 546 inline static gint64
 547 interval_local_end (GTimeZone *tz,
 548                     gint       interval)
 549 {
 550   if (interval < tz->timecnt)
 551     return interval_end (tz, interval) + interval_offset (tz, interval);
 552
 553   return G_MAXINT64;
 554 }
 555
 556 static gboolean
 557 interval_valid (GTimeZone *tz,
 558                 gint       interval)
 559 {
 560   return interval <= tz->timecnt;
 561 }
 562
 563 /* g_time_zone_find_interval() {{{1 */
 564
 565 /**
 566  * g_time_zone_adjust_time:
 567  * @tz: a #GTimeZone
 568  * @type: the #GTimeType of @time_
 569  * @time_: a pointer to a number of seconds since January 1, 1970
 570  *
 571  * Finds an interval within @tz that corresponds to the given @time_,
 572  * possibly adjusting @time_ if required to fit into an interval.
 573  * The meaning of @time_ depends on @type.
 574  *
 575  * This function is similar to g_time_zone_find_interval(), with the
 576  * difference that it always succeeds (by making the adjustments
 577  * described below).
 578  *
 579  * In any of the cases where g_time_zone_find_interval() succeeds then
 580  * this function returns the same value, without modifying @time_.
 581  *
 582  * This function may, however, modify @time_ in order to deal with
 583  * non-existent times.  If the non-existent local @time_ of 02:30 were
 584  * requested on March 13th 2010 in Toronto then this function would
 585  * adjust @time_ to be 03:00 and return the interval containing the
 586  * adjusted time.
 587  *
 588  * Returns: the interval containing @time_, never -1
 589  *
 590  * Since: 2.26
 591  **/
 592 gint
 593 g_time_zone_adjust_time (GTimeZone *tz,
 594                          GTimeType  type,
 595                          gint64    *time_)
 596 {
 597   gint i;
 598
 599   if (tz->zoneinfo == NULL)
 600     return 0;
 601
 602   /* find the interval containing *time UTC
 603    * TODO: this could be binary searched (or better) */
 604   for (i = 0; i < tz->timecnt; i++)
 605     if (*time_ <= interval_end (tz, i))
 606       break;
 607
 608   g_assert (interval_start (tz, i) <= *time_ && *time_ <= interval_end (tz, i));
 609
 610   if (type != G_TIME_TYPE_UNIVERSAL)
 611     {
 612       if (*time_ < interval_local_start (tz, i))
 613         /* if time came before the start of this interval... */
 614         {
 615           i--;
 616
 617           /* if it's not in the previous interval... */
 618           if (*time_ > interval_local_end (tz, i))
 619             {
 620               /* it doesn't exist.  fast-forward it. */
 621               i++;
 622               *time_ = interval_local_start (tz, i);
 623             }
 624         }
 625
 626       else if (*time_ > interval_local_end (tz, i))
 627         /* if time came after the end of this interval... */
 628         {
 629           i++;
 630
 631           /* if it's not in the next interval... */
 632           if (*time_ < interval_local_start (tz, i))
 633             /* it doesn't exist.  fast-forward it. */
 634             *time_ = interval_local_start (tz, i);
 635         }
 636
 637       else if (interval_isdst (tz, i) != type)
 638         /* it's in this interval, but dst flag doesn't match.
 639          * check neighbours for a better fit. */
 640         {
 641           if (i && *time_ <= interval_local_end (tz, i - 1))
 642             i--;
 643
 644           else if (i < tz->timecnt &&
 645                    *time_ >= interval_local_start (tz, i + 1))
 646             i++;
 647         }
 648     }
 649
 650   return i;
 651 }
 652
 653 /**
 654  * g_time_zone_find_interval:
 655  * @tz: a #GTimeZone
 656  * @type: the #GTimeType of @time_
 657  * @time_: a number of seconds since January 1, 1970
 658  *
 659  * Finds an the interval within @tz that corresponds to the given @time_.
 660  * The meaning of @time_ depends on @type.
 661  *
 662  * If @type is %G_TIME_TYPE_UNIVERSAL then this function will always
 663  * succeed (since universal time is monotonic and continuous).
 664  *
 665  * Otherwise @time_ is treated is local time.  The distinction between
 666  * %G_TIME_TYPE_STANDARD and %G_TIME_TYPE_DAYLIGHT is ignored except in
 667  * the case that the given @time_ is ambiguous.  In Toronto, for example,
 668  * 01:30 on November 7th 2010 occurred twice (once inside of daylight
 669  * savings time and the next, an hour later, outside of daylight savings
 670  * time).  In this case, the different value of @type would result in a
 671  * different interval being returned.
 672  *
 673  * It is still possible for this function to fail.  In Toronto, for
 674  * example, 02:00 on March 14th 2010 does not exist (due to the leap
 675  * forward to begin daylight savings time).  -1 is returned in that
 676  * case.
 677  *
 678  * Returns: the interval containing @time_, or -1 in case of failure
 679  *
 680  * Since: 2.26
 681  */
 682 gint
 683 g_time_zone_find_interval (GTimeZone *tz,
 684                            GTimeType  type,
 685                            gint64     time_)
 686 {
 687   gint i;
 688
 689   if (tz->zoneinfo == NULL)
 690     return 0;
 691
 692   for (i = 0; i < tz->timecnt; i++)
 693     if (time_ <= interval_end (tz, i))
 694       break;
 695
 696   if (type == G_TIME_TYPE_UNIVERSAL)
 697     return i;
 698
 699   if (time_ < interval_local_start (tz, i))
 700     {
 701       if (time_ > interval_local_end (tz, --i))
 702         return -1;
 703     }
 704
 705   else if (time_ > interval_local_end (tz, i))
 706     {
 707       if (time_ < interval_local_start (tz, ++i))
 708         return -1;
 709     }
 710
 711   else if (interval_isdst (tz, i) != type)
 712     {
 713       if (i && time_ <= interval_local_end (tz, i - 1))
 714         i--;
 715
 716       else if (i < tz->timecnt && time_ >= interval_local_start (tz, i + 1))
 717         i++;
 718     }
 719
 720   return i;
 721 }
 722
 723 /* Public API accessors {{{1 */
 724
 725 /**
 726  * g_time_zone_get_abbreviation:
 727  * @tz: a #GTimeZone
 728  * @interval: an interval within the timezone
 729  *
 730  * Determines the time zone abbreviation to be used during a particular
 731  * @interval of time in the time zone @tz.
 732  *
 733  * For example, in Toronto this is currently "EST" during the winter
 734  * months and "EDT" during the summer months when daylight savings time
 735  * is in effect.
 736  *
 737  * Returns: the time zone abbreviation, which belongs to @tz
 738  *
 739  * Since: 2.26
 740  **/
 741 const gchar *
 742 g_time_zone_get_abbreviation (GTimeZone *tz,
 743                               gint       interval)
 744 {
 745   g_return_val_if_fail (interval_valid (tz, interval), NULL);
 746
 747   if (tz->header == NULL)
 748     return "UTC";
 749
 750   return tz->abbrs + interval_abbrind (tz, interval);
 751 }
 752
 753 /**
 754  * g_time_zone_get_offset:
 755  * @tz: a #GTimeZone
 756  * @interval: an interval within the timezone
 757  *
 758  * Determines the offset to UTC in effect during a particular @interval
 759  * of time in the time zone @tz.
 760  *
 761  * The offset is the number of seconds that you add to UTC time to
 762  * arrive at local time for @tz (ie: negative numbers for time zones
 763  * west of GMT, positive numbers for east).
 764  *
 765  * Returns: the number of seconds that should be added to UTC to get the
 766  *          local time in @tz
 767  *
 768  * Since: 2.26
 769  **/
 770 gint32
 771 g_time_zone_get_offset (GTimeZone *tz,
 772                         gint       interval)
 773 {
 774   g_return_val_if_fail (interval_valid (tz, interval), 0);
 775
 776   if (tz->header == NULL)
 777     return 0;
 778
 779   return interval_offset (tz, interval);
 780 }
 781
 782 /**
 783  * g_time_zone_is_dst:
 784  * @tz: a #GTimeZone
 785  * @interval: an interval within the timezone
 786  *
 787  * Determines if daylight savings time is in effect during a particular
 788  * @interval of time in the time zone @tz.
 789  *
 790  * Returns: %TRUE if daylight savings time is in effect
 791  *
 792  * Since: 2.26
 793  **/
 794 gboolean
 795 g_time_zone_is_dst (GTimeZone *tz,
 796                     gint       interval)
 797 {
 798   g_return_val_if_fail (interval_valid (tz, interval), FALSE);
 799
 800   if (tz->header == NULL)
 801     return FALSE;
 802
 803   return interval_isdst (tz, interval);
 804 }
 805
 806 /* Epilogue {{{1 */
 807 /* vim:set foldmethod=marker: */