TZSET(3) | Library Functions Manual | TZSET(3) |
tzset
, tzsetwall
— initialize time conversion information
Standard C Library (libc, -lc)
#include
<time.h>
void
tzset
(void);
void
tzsetwall
(void);
The
tzset
()
function initializes time conversion information used by the library routine
localtime(3). The environment
variable TZ
specifies how this is done.
If TZ
does not appear in the environment,
the best available approximation to local wall clock time, as specified by
the tzfile(5)-format file #ifdef
UNIFDEF_MOVE_LOCALTIME /var/db/timezone/localtime,
#else /* !UNIFDEF_MOVE_LOCALTIME */ /etc/localtime,
#endif /* UNIFDEF_MOVE_LOCALTIME */ is used.
If TZ
appears in the environment but its
value is a null string, Coordinated Universal Time (UTC) is used (without
leap second correction).
If TZ
appears in the environment and its
value begins with a colon (‘:
’), the
rest of its value is used as a pathname of a
tzfile(5)-format file from which to
read the time conversion information. If the first character of the pathname
is a slash (‘/
’), it is used as an
absolute pathname; otherwise, it is used as a pathname relative to the
system time conversion information directory.
If its value does not begin with a colon, it is first used as the pathname of a file (as described above) from which to read the time conversion information. If that file cannot be read, the value is then interpreted as a direct specification (the format is described below) of the time conversion information.
If the TZ
environment variable does not
specify a tzfile(5)-format file and
cannot be interpreted as a direct specification, UTC is used.
The
tzsetwall
()
function sets things up so that
localtime returns the best available
approximation of local wall clock time.
When TZ
is used directly as a
specification of the time conversion information, it must have the following
syntax (spaces inserted for clarity):
Where:
:
’), digits, comma
(‘,
’), minus
(‘-
’), plus
(‘+
’), and ASCII
NUL
are allowed.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
(‘+
’)).
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.
The format of date is one of the following:
The time has the same
format as offset except that no leading sign
(‘-
’) or
(‘+
’) is allowed. The default,
if time is not given, is
02:00:00.
If no rule is present in the
TZ
specification, 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.
#ifdef UNIFDEF_TZDIR_SYMLINK #else /* !UNIFDEF_TZDIR_SYMLINK */ #endif /* UNIFDEF_TZDIR_SYMLINK */
If the file /usr/share/zoneinfo/GMT does not exist, UTC leap seconds are loaded from /usr/share/zoneinfo/posixrules.
date(1), gettimeofday(2), ctime(3), getenv(3), time(3), tzfile(5)
The tzset
() and
tzsetwall
() functions first appeared in
4.4BSD.
November 17, 1993 | macOS 14.6 |