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NAMEtzset, tzname, timezone, daylight - initialize time conversion information
#include <time.h> void tzset (void); extern char *tzname; extern long timezone; extern int daylight;
DESCRIPTIONThe fBtzset()fP function initializes the fItznamefP variable from the TZ environment variable. This function is automatically called by the other time conversion functions that depend on the time zone. In a SysV-like environment it will also set the variables fItimezonefP (seconds West of GMT) and fIdaylightfP (0 if this time zone does not have any daylight savings time rules, nonzero if there is a time during the year when daylight savings time applies).
If the TZ variable does not appear in the environment, the fItznamefP variable is initialized with the best approximation of local wall clock time, as specified by the tzfile(5)-format file fIlocaltimefP found in the system timezone directory (see below). (One also often sees /etc/localtime used here, a symlink to the right file in the system timezone directory.)
If the TZ variable does appear in the environment but its value is NULL or its value cannot be interpreted using any of the formats specified below, Coordinated Universal Time (UTC) is used.
The value of TZ can be one of three formats. The first format is used when there is no daylight saving time in the local time zone:
The fIstdfP string specifies the name of the time zone and must be three or more alphabetic characters. The fIoffsetfP string immediately follows fIstdfP and specifies the time value to be added to the local time to get Coordinated Universal Time (UTC). The fIoffsetfP is positive if the local time zone is west of the Prime Meridian and negative if it is east. The hour must be between 0 and 24, and the minutes and seconds 0 and 59.
The second format is used when there is daylight saving time:
There are no spaces in the specification. The initial fIstdfP and fIoffsetfP specify the standard time zone, as described above. The fIdstfP string and fIoffsetfP specify the name and offset for the corresponding daylight savings time zone. If the offset is omitted, it defaults to one hour ahead of standard time.
The fIstartfP field specifies when daylight savings time goes into effect and the fIendfP field specifies when the change is made back to standard time. These fields may have the following formats:
The fItimefP fields specify when, in the local time currently in effect, the change to the other time occurs. If omitted, the default is 02:00:00.
The third format specifies that the time zone information should be read from a file:
If the file specification fIfilespecfP is omitted, the time zone information is read from the file localtime in the system timezone directory, which nowadays usually is /usr/share/zoneinfo. This file is in tzfile(5) format. If fIfilespecfP is given, it specifies another tzfile(5)-format file to read the time zone information from. If fIfilespecfP does not begin with a `/', the file specification is relative to the system timezone directory.
FILESThe system time zone directory used depends on the (g)libc version. Libc4 and libc5 use /usr/lib/zoneinfo, and, since libc-5.4.6, when this doesn't work, will try /usr/share/zoneinfo. Glibc2 will use the environment variable TZDIR, when that exists. Its default depends on how it was installed, but normally is /usr/share/zoneinfo.
This timezone directory contains the files
localtime local time zone file posixrules rules for POSIX-style TZ's
CONFORMING TOSVID 3, POSIX, BSD 4.3
NOTESNote that the variable fIdaylightfP does not indicate that daylight savings time applies right now. It used to give the number of some algorithm (see the variable fItz_dsttimefP in gettimeofday(2)). It has been obsolete for many years but is required by SUSv2.
BSD4.3 had a routine fIchar *timezone(zone,dst)fP that returned the name of the time zone corresponding to its first argument (minutes West of GMT). If the second argument was 0, the standard name was used, otherwise the daylight savings time version.
SEE ALSOdate(1), gettimeofday(2), time(2), ctime(3), getenv(3), tzfile(5)