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Section: C Library Functions (3)
Index Return to Main Contents


tzset - initialize time conversion information  


ds - -#include <time.h>

timezone_t tzalloc(char const *TZ);

void tzfree(timezone_t tz);

void tzset(void);

cc ... ltz


Tzalloc allocates and returns a time zone object described by TZ. If TZ is not a valid time zone description, or if the object cannot be allocated, tzalloc returns a null pointer and sets errno.

Tzfree frees a time zone object tz, which should have been successfully allocated by tzalloc. This invalidates any tm_zone pointers that tz was used to set.

Tzset acts like tzalloc(getenv(TZ)), except it saves any resulting time zone object into internal storage that is accessed by localtime, localtime_r, and mktime. The anonymous shared time zone object is freed by the next call to tzset. If the implied call to tzalloc fails, tzset falls back on UTC.

If TZ is null, the best available approximation to local wall clock time, as specified by the tzfile(5)-format file localtime in the system time conversion information directory, is used. If TZ is the empty string, Universal Time (UT) is used, with the abbreviation "UTC" and without leap second correction; please see newctime(3) for more about UT, UTC, and leap seconds. If TZ is nonnull and nonempty:

if the value begins with a colon, it is used as a pathname of a file from which to read the time conversion information;
if the value does not begin with a colon, it is first used as the pathname of a file from which to read the time conversion information, and, if that file cannot be read, is used directly as a specification of the time conversion information.

When TZ is used as a pathname, if it begins with a slash, it is used as an absolute pathname; otherwise, it is used as a pathname relative to a system time conversion information directory. The file must be in the format specified in tzfile(5).

When TZ is used directly as a specification of the time conversion information, it must have the following syntax (spaces inserted for clarity):



std and dst
Three or more bytes that are the designation for the standard (std) or summer (dst) time zone. Only std is required; if dst is missing, then summer time does not apply in this locale. Upper- and lowercase letters are explicitly allowed. Any characters except a leading colon (:), digits, comma (,), ASCII minus (), ASCII plus (+), and NUL bytes are allowed. Alternatively, a designation can be surrounded by angle brackets < and >; in this case, the designation can contain any characters other than > and NUL.
Indicates the value one must add to the local time to arrive at Coordinated Universal Time. The offset has the form:
The minutes (mm) and seconds (ss) are optional. The hour (hh) is required and may be a single digit. The offset following std is required. If no offset follows dst, summer time is assumed to be one hour ahead of standard time. One or more digits may be used; the value is always interpreted as a decimal number. The hour must be between zero and 24, and the minutes (and seconds) - if present - between zero and 59. If preceded by a the time zone shall be east of the Prime Meridian; otherwise it shall be west (which may be indicated by an optional preceding
Indicates when to change to and back from summer time. The rule has the form:
where the first date describes when the change from standard to summer time occurs and the second date describes when the change back happens. Each time field describes when, in current local time, the change to the other time is made. As an extension to POSIX, daylight saving is assumed to be in effect all year if it begins January 1 at 00:00 and ends December 31 at 24:00 plus the difference between daylight saving and standard time, leaving no room for standard time in the calendar.
The format of date is one of the following:
The Julian day n (1 < n < 365). Leap days are not counted; that is, in all years - including leap years - February 28 is day 59 and March 1 is day 60. It is impossible to explicitly refer to the occasional February 29.
The zero-based Julian day (0 < n < 365). Leap days are counted, and it is possible to refer to February 29.
The d'th day (0 < d < 6) of week n of month m of the year (1 < n < 5, 1 < m < 12, where week 5 means which may occur in either the fourth or the fifth week). Week 1 is the first week in which the d'th day occurs. Day zero is Sunday.
The time has the same format as offset except that POSIX does not allow a leading sign ( or As an extension to POSIX, the hours part of time can range from -167 through 167; this allows for unusual rules such as The default, if time is not given, is 02:00:00.

Here are some examples of TZ values that directly specify the time zone rules; they use some of the extensions to POSIX.

stands for US Eastern Standard Time (EST), 5 hours behind UTC, without daylight saving.
stands for Fiji Time (FJT) and Fiji Summer Time (FJST), 12 hours ahead of UTC, springing forward on November's first Sunday at 02:00, and falling back on January's third Thursday at 75:00 (i.e., 03:00 on the first Sunday on or after January 18).
stands for Israel Standard Time (IST) and Israel Daylight Time (IDT), 2 hours ahead of UTC, springing forward on March's fourth Thursday at 26:00 (i.e., 02:00 on the first Friday on or after March 23), and falling back on October's last Sunday at 02:00.
stands for Western Argentina Summer Time (WARST), 3 hours behind UTC. There is a dummy fall-back transition on December 31 at 25:00 daylight saving time (i.e., 24:00 standard time, equivalent to January 1 at 00:00 standard time), and a simultaneous spring-forward transition on January 1 at 00:00 standard time, so daylight saving time is in effect all year and the initial WART is a placeholder.
stands for Western Greenland Time (WGT) and Western Greenland Summer Time (WGST), 3 hours behind UTC, where clocks follow the EU rules of springing forward on March's last Sunday at 01:00 UTC (-02:00 local time) and falling back on October's last Sunday at 01:00 UTC (-01:00 local time).

If no rule is present in TZ, the rules specified by the tzfile(5)-format file posixrules in the system time conversion information directory are used, with the standard and summer time offsets from UTC replaced by those specified by the offset values in TZ.

For compatibility with System V Release 3.1, a semicolon (;) may be used to separate the rule from the rest of the specification.  


/usr/local/etc/zoneinfo             time zone information directory

/usr/local/etc/zoneinfo/localtime  local time zone file

/usr/local/etc/zoneinfo/posixrules used with POSIX-style TZ's

/usr/local/etc/zoneinfo/GMT        for UTC leap seconds

If /usr/local/etc/zoneinfo/GMT is absent, UTC leap seconds are loaded from /usr/local/etc/zoneinfo/posixrules.  


getenv(3), newctime(3), newstrftime(3), time(2), tzfile(5)




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