newtzset.3

.\" This file is in the public domain, so clarified as of
.\" 2009-05-17 by Arthur David Olson.
.TH newtzset 3 "" "Time Zone Database"
.SH NAME
tzset \- initialize time conversion information
.SH SYNOPSIS
.nf
.ie \n(.g .ds - \f(CR-\fP
.el .ds - \-
.B #include <time.h>
.PP
.BI "timezone_t tzalloc(char const *" TZ );
.PP
.BI "void tzfree(timezone_t " tz );
.PP
.B void tzset(void);
.PP
/\(** Optional and obsolescent:  \(**/
.br
.B extern char *tzname[];
.br
.B extern long timezone;
.br
.B extern int daylight;
.PP
.B cc ... \*-ltz
.fi
.SH DESCRIPTION
.ie '\(en'' .ds en \-
.el .ds en \(en
.ie '\(lq'' .ds lq \&"\"
.el .ds lq \(lq\"
.ie '\(rq'' .ds rq \&"\"
.el .ds rq \(rq\"
.de q
\\$3\*(lq\\$1\*(rq\\$2
..
The
.B tzalloc
function
allocates and returns a timezone object described by
.IR TZ .
.PP
If
.I TZ
is a null pointer,
.B tzalloc
uses the best available approximation to local (wall
clock) time, as specified by the
.BR tzfile (5)-format
file
.B localtime
in the system time conversion information directory.
.PP
If
.I TZ
is the empty string,
.B tzalloc
uses Universal Time (UT), with the abbreviation "UTC"
and without leap second correction; please see
.BR newctime (3)
for more about UT, UTC, and leap seconds.
.PP
If
.I TZ
is nonnull and nonempty:
.IP
if the value begins with a colon, it is used as a pathname of a file
from which to read the time conversion information;
.IP
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.
.PP
When
.I TZ
contents are used as a pathname, a pathname beginning with
.q "/"
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
.BR tzfile (5).
.PP
When
.I TZ
is used directly as a specification of the time conversion information,
it must have the following syntax:
.IP
\fIstd\|offset\fR[\fIdst\fR[\fIoffset\fR][\fB,\fIrule\fR]]
.PP
Where:
.RS
.TP
.IR std " and " dst
Three or more bytes that are the designation for the standard
.RI ( std )
or the alternative
.RI ( dst ,
such as daylight saving time)
time zone.  Only
.I std
is required; if
.I 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
.RB ( : ),
digits, comma
.RB ( , ),
ASCII minus
.RB ( \*- ),
ASCII plus
.RB ( + ),
and NUL bytes are allowed.
Alternatively, a designation can be surrounded by angle brackets
.B <
and
.BR > ;
in this case, the designation can contain any characters other than
.B >
and NUL.
.TP
.I offset
Indicates the value one must add to the local time to arrive at
Coordinated Universal Time.  The
.I offset
has the form:
.RS
.IP
\fIhh\fR[\fB:\fImm\fR[\fB:\fIss\fR]]
.RE
.IP
The minutes
.RI ( mm )
and seconds
.RI ( ss )
are optional.  The hour
.RI ( hh )
is required and may be a single digit.  The
.I offset
following
.I std
is required.  If no
.I offset
follows
.IR 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) \*(en if present \*(en between zero and 59.  If preceded by a
.q "\*-" ,
the time zone shall be east of the Prime Meridian; otherwise it shall be
west (which may be indicated by an optional preceding
.q "+" .
.TP
.I rule
Indicates when to change to and back from daylight saving time.  The
.I rule
has the form:
.RS
.IP
\fIdate\fB/\fItime\fB,\fIdate\fB/\fItime\fR
.RE
.IP
where the first
.I date
describes when the change from standard to daylight saving time occurs and the
second
.I date
describes when the change back happens.  Each
.I 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.
.IP
The format of
.I date
is one of the following:
.RS
.TP
.BI J n
The Julian day
.I n
.RI "(1\ \(<=" "\ n\ " "\(<=\ 365).
Leap days are not counted; that is, in all years \*(en including leap
years \*(en February 28 is day 59 and March 1 is day 60.  It is
impossible to explicitly refer to the occasional February 29.
.TP
.I n
The zero-based Julian day
.RI "(0\ \(<=" "\ n\ " "\(<=\ 365).
Leap days are counted, and it is possible to refer to February 29.
.TP
.BI M m . n . d
The
.IR d' th
day
.RI "(0\ \(<=" "\ d\ " "\(<=\ 6)
of week
.I n
of month
.I m
of the year
.RI "(1\ \(<=" "\ n\ " "\(<=\ 5,
.RI "1\ \(<=" "\ m\ " "\(<=\ 12,
where week 5 means
.q "the last \fId\fP day in month \fIm\fP"
which may occur in either the fourth or the fifth week).  Week 1 is the
first week in which the
.IR d' th
day occurs.  Day zero is Sunday.
.RE
.IP
The
.I time
has the same format as
.I offset
except that the hours part of
.I time
can range from \-167 through 167; this allows for unusual rules such
as
.q "the Saturday before the first Sunday of March" .
The default, if
.I time
is not given, is
.BR 02:00:00 .
.RE
.LP
Here are some examples of
.I TZ
values that directly specify the timezone.
.TP
.B EST5
stands for US Eastern Standard
Time (EST), 5 hours behind UT, without daylight saving.
.TP
.B <+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
.q "+12"
and
.q "+13".
.TP
.B 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.
.TP
.B <\*-04>4<\*-03>,J1/0,J365/25
stands for permanent daylight saving time, 3 hours behind UT with
abbreviation
.q "\*-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
.B <\*-04>
is a placeholder.
.TP
.B <\*-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
.q "\*-03"
and
.q "\*-02".
.PP
If
.I TZ
specifies daylight saving time but does not specify a
.IR rule ,
and the optional
.BR tzfile (5)-format
file
.B posixrules
is present in the system time conversion information directory, the
rules in
.B posixrules
are used, with the
.B posixrules
standard and daylight saving time offsets from UT
replaced by those specified by the
.I offset
values in
.IR TZ .
However, the
.B posixrules
file is obsolete: if it is present it is only for backward compatibility,
and it does not work reliably.
Therefore, if a
.I TZ
string directly specifies a timezone with daylight saving time,
it should specify the daylight saving rules explicitly.
.PP
For compatibility with System V Release 3.1, a semicolon
.RB ( ; )
may be used to separate the
.I rule
from the rest of the specification;
this is an extension to POSIX.
.PP
The
.B tzfree
function
frees a timezone object
.IR tz ,
which should have been successfully allocated by
.BR tzalloc .
This invalidates any
.B tm_zone
pointers that
.I tz
was used to set.
.PP
The
.B tzset
function
acts like
.BR tzalloc(getenv("TZ")) ,
except it saves any resulting timezone object into internal
storage that is accessed by
.BR localtime ,
.BR localtime_r ,
and
.BR mktime .
The anonymous shared timezone object is freed by the next call to
.BR tzset .
If the implied call to
.B getenv
fails,
.B tzset
acts like
.BR tzalloc(nullptr) ;
if the implied call to
.B tzalloc
fails,
.B tzset
falls back on UT.
.PP
As a side effect, the
.B tzset
function sets some external variables if the platform defines them.
It sets
.BR tzname [0]
and
.BR tzname [1]
to pointers to strings that are time zone abbreviations to be used with
standard and daylight saving time, respectively.
It also sets
.B timezone
to be the number of seconds that standard time is west of the Prime Meridian,
and
.B daylight
to be zero if daylight saving time is never in effect, non-zero otherwise.
.SH "RETURN VALUE"
If successful, the
.B tzalloc
function returns a nonnull pointer to the newly allocated object.
Otherwise, it returns a null pointer and sets
.IR errno .
.SH ERRORS
.TP
.B EOVERFLOW
.I 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.
.PP
The
.B tzalloc
function may also fail and set
.I errno
for any of the errors specified for the routines
.BR access (2),
.BR close (2),
.BR malloc (3),
.BR open (2),
and
.BR read (2).
.SH FILES
.ta \w'/usr/share/zoneinfo/posixrules\0\0'u
/etc/localtime	local timezone file
.br
/usr/share/zoneinfo	timezone directory
.br
/usr/share/zoneinfo/posixrules	default DST rules (obsolete)
.br
/usr/share/zoneinfo/GMT	for UTC leap seconds
.PP
If /usr/share/zoneinfo/GMT is absent,
UTC leap seconds are loaded from /usr/share/zoneinfo/GMT0 if present.
.SH SEE ALSO
.BR getenv (3),
.BR newctime (3),
.BR newstrftime (3),
.BR time (2),
.BR tzfile (5).
.SH NOTES
Portable code should not rely on the contents of the external variables
.BR tzname ,
.B timezone
and
.B 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
.BR tzset .
A future version of POSIX is planned to remove these variables;
callers can instead use the
.I tm_gmtoff
and
.I tm_zone
members of
.B struct tm,
or use
.B strftime
with "%z" or "%Z".