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newtzset(3) Library Functions Manual newtzset(3)
NAME
tzset - initialize time conversion information
SYNOPSIS
#include <time.h>
timezone_t tzalloc(char const *TZ);
void tzfree(timezone_t tz);
void tzset(void);
/* Optional and obsolescent: */
extern char *tzname[];
extern long timezone;
extern int daylight;
cc ... -ltz
DESCRIPTION
The tzalloc function allocates and returns a timezone object described
by TZ.
If TZ is a null pointer, tzalloc uses 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.
If TZ is the empty string, tzalloc uses Universal Time (UT), 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 contents are used as a pathname, a pathname beginning with "/"
is used as-is; otherwise the pathname is 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:
stdoffset[dst[offset][,rule]]
Where:
std and dst
Three or more bytes that are the designation for the
standard (std) or the alternative (dst, such as daylight
saving time) time zone. Only std is required; if dst is
missing, then daylight saving 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.
offset Indicates the value one must add to the local time to
arrive at Coordinated Universal Time. The offset has the
form:
hh[:mm[:ss]]
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,
daylight saving 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 "+".
rule Indicates when to change to and back from daylight saving
time. The rule has the form:
date/time,date/time
where the first date describes when the change from
standard to daylight saving 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. 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:
Jn 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.
n The zero-based Julian day (0 <= n <= 365). Leap
days are counted, and it is possible to refer to
February 29.
Mm.n.d 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 "the last d day in month m" 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 the
hours part of time can range from -167 through 167; this
allows for unusual rules such as "the Saturday before the
first Sunday of March". The default, if time is not
given, is 02:00:00.
Here are some examples of TZ values that directly specify the timezone.
EST5 stands for US Eastern Standard Time (EST), 5 hours behind UT,
without daylight saving.
<+12>-12<+13>,M11.1.0,M1.2.1/147
stands for Fiji time, 12 hours ahead of UT, springing forward on
November's first Sunday at 02:00, and falling back on January's
second Monday at 147:00 (i.e., 03:00 on the first Sunday on or
after January 14). The abbreviations for standard and daylight
saving time are "+12" and "+13".
IST-2IDT,M3.4.4/26,M10.5.0
stands for Israel Standard Time (IST) and Israel Daylight Time
(IDT), 2 hours ahead of UT, 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.
<-04>4<-03>,J1/0,J365/25
stands for permanent daylight saving time, 3 hours behind UT
with abbreviation "-03". 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 <-04> is a placeholder.
<-03>3<-02>,M3.5.0/-2,M10.5.0/-1
stands for time in western Greenland, 3 hours behind UT, where
clocks follow the EU rules of springing forward on March's last
Sunday at 01:00 UT (-02:00 local time, i.e., 22:00 the previous
day) and falling back on October's last Sunday at 01:00 UT
(-01:00 local time, i.e., 23:00 the previous day). The
abbreviations for standard and daylight saving time are "-03"
and "-02".
If TZ specifies daylight saving time but does not specify a rule, and
the optional tzfile(5)-format file posixrules is present in the system
time conversion information directory, the rules in posixrules are
used, with the posixrules standard and daylight saving time offsets
from UT replaced by those specified by the offset values in TZ.
However, the posixrules file is obsolete: if it is present it is only
for backward compatibility, and it does not work reliably. Therefore,
if a TZ string directly specifies a timezone with daylight saving time,
it should specify the daylight saving rules explicitly.
For compatibility with System V Release 3.1, a semicolon (;) may be
used to separate the rule from the rest of the specification; this is
an extension to POSIX.
The tzfree function frees a timezone object tz, which should have been
successfully allocated by tzalloc. This invalidates any tm_zone
pointers that tz was used to set.
The tzset function acts like tzalloc(getenv("TZ")), except it saves any
resulting timezone object into internal storage that is accessed by
localtime, localtime_r, and mktime. The anonymous shared timezone
object is freed by the next call to tzset. If the implied call to
getenv fails, tzset acts like tzalloc(nullptr); if the implied call to
tzalloc fails, tzset falls back on UT.
As a side effect, the tzset function sets some external variables if
the platform defines them. It sets tzname[0] and tzname[1] to pointers
to strings that are time zone abbreviations to be used with standard
and daylight saving time, respectively. It also sets timezone to be
the number of seconds that standard time is west of the Prime Meridian,
and daylight to be zero if daylight saving time is never in effect,
non-zero otherwise.
RETURN VALUE
If successful, the tzalloc function returns a nonnull pointer to the
newly allocated object. Otherwise, it returns a null pointer and sets
errno.
ERRORS
EOVERFLOW
TZ directly specifies time conversion information, and contains
an integer out of machine range or a time zone abbreviation that
is too long for this platform.
The tzalloc function may also fail and set errno for any of the errors
specified for the routines access(2), close(2), malloc(3), open(2), and
read(2).
FILES
/etc/localtime local timezone file
/usr/share/zoneinfo timezone directory
/usr/share/zoneinfo/posixrules default DST rules (obsolete)
/usr/share/zoneinfo/GMT for UTC leap seconds
If /usr/share/zoneinfo/GMT is absent, UTC leap seconds are loaded from
/usr/share/zoneinfo/GMT0 if present.
SEE ALSO
getenv(3), newctime(3), newstrftime(3), time(2), tzfile(5).
NOTES
Portable code should not rely on the contents of the external variables
tzname, timezone and daylight as their contents are unspecified (and do
not make sense in general) when a geographical TZ is used. In
multithreaded applications behavior is undefined if one thread accesses
one of these variables while another thread invokes tzset. A future
version of POSIX is planned to remove these variables; callers can
instead use the tm_gmtoff and tm_zone members of struct tm, or use
strftime with "%z" or "%Z".
Time Zone Database newtzset(3)
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