Originální popis anglicky:
tzset, tzname, timezone, daylight - initialize time conversion information
Návod, kniha: Linux Programmer's Manual
#include <time.h>
void tzset (void);
extern char *tzname[2];
extern long timezone;
extern int daylight;
The
tzset() function initializes the
tzname 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
timezone (seconds West of GMT) and
daylight (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
tzname
variable is initialized with the best approximation of local wall clock time,
as specified by the
tzfile(5)-format file
localtime 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:
std offset
The
std string specifies the name of the time zone and must be three or
more alphabetic characters. The
offset string immediately follows
std and specifies the time value to be added to the local time to get
Coordinated Universal Time (UTC). The
offset 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:
std offset dst
[offset],start[/time],end[/time]
There are no spaces in the specification. The initial
std and
offset specify the standard time zone, as described above. The
dst string and
offset 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
start field specifies when daylight savings time goes into effect and
the
end field specifies when the change is made back to standard time.
These fields may have the following formats:
- Jn
- This specifies the Julian day with n between 1 and
365. February 29 is never counted even in leap years.
- n
- This specifies the Julian day with n between 1 and
365. February 29 is counted in leap years.
- Mm.w.d
- This specifies day d (0 <= d <= 6) of
week w (1 <= w <= 5) of month m (1 <=
m <= 12). Week 1 is the first week in which day d occurs
and week 5 is the last week in which day d occurs. Day 0 is a
Sunday.
The
time 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:
:[filespec]
If the file specification
filespec 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
filespec is given, it specifies another
tzfile(5)-format file to read the time zone information from. If
filespec does not begin with a `/', the file specification is relative
to the system timezone directory.
The 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
Often
/etc/localtime is a symlink to the file
localtime or to the
correct time zone file in the system time zone directory.
SVID 3, POSIX, BSD 4.3
Note that the variable
daylight does not indicate that daylight savings
time applies right now. It used to give the number of some algorithm (see the
variable
tz_dsttime in
gettimeofday(2)). It has been obsolete
for many years but is required by SUSv2.
BSD4.3 had a routine
char *timezone(zone,dst) 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.
date(1),
gettimeofday(2),
time(2),
ctime(3),
getenv(3),
tzfile(5)
