NAME¶
tzfile - timezone information
DESCRIPTION¶
This page describes the structure of the timezone files used by
tzset(3).
These files are typically found under one of the directories
/usr/lib/zoneinfo or
/usr/share/zoneinfo.
Timezone information files begin with the magic characters "TZif" to
identify them as timezone information files, followed by a character
identifying the version of the file's format (as of 2005, either an ASCII NUL
('\0') or a '2') followed by fifteen bytes containing zeros reserved for
future use, followed by six four-byte values of type
long, written in a
"standard" byte order (the high-order byte of the value is written
first). These values are, in order:
- tzh_ttisgmtcnt
- The number of UTC/local indicators stored in the file.
- tzh_ttisstdcnt
- The number of standard/wall indicators stored in the
file.
- tzh_leapcnt
- The number of leap seconds for which data is stored in the
file.
- tzh_timecnt
- The number of "transition times" for which data
is stored in the file.
- tzh_typecnt
- The number of "local time types" for which data
is stored in the file (must not be zero).
- tzh_charcnt
- The number of characters of "timezone abbreviation
strings" stored in the file.
The above header is followed by
tzh_timecnt four-byte values of type
long, sorted in ascending order. These values are written in
"standard" byte order. Each is used as a transition time (as
returned by
time(2)) at which the rules for computing local time
change. Next come
tzh_timecnt one-byte values of type
unsigned
char; each one tells which of the different types of "local
time" types described in the file is associated with the same-indexed
transition time. These values serve as indices into an array of
ttinfo
structures (with
tzh_typecnt entries) that appear next in the file;
these structures are defined as follows:
struct ttinfo {
long tt_gmtoff;
int tt_isdst;
unsigned int tt_abbrind;
};
Each structure is written as a four-byte value for
tt_gmtoff of type
long, in a standard byte order, followed by a one-byte value for
tt_isdst and a one-byte value for
tt_abbrind. In each structure,
tt_gmtoff gives the number of seconds to be added to UTC,
tt_isdst tells whether
tm_isdst should be set by
localtime(3), and
tt_abbrind serves as an index into the array
of timezone abbreviation characters that follow the
ttinfo structure(s)
in the file.
Then there are
tzh_leapcnt pairs of four-byte values, written in standard
byte order; the first value of each pair gives the time (as returned by
time(2)) at which a leap second occurs; the second gives the
total number of leap seconds to be applied after the given time. The
pairs of values are sorted in ascending order by time.
Then there are
tzh_ttisstdcnt standard/wall indicators, each stored as a
one-byte value; they tell whether the transition times associated with local
time types were specified as standard time or wall clock time, and are used
when a timezone file is used in handling POSIX-style timezone environment
variables.
Finally, there are
tzh_ttisgmtcnt UTC/local indicators, each stored as a
one-byte value; they tell whether the transition times associated with local
time types were specified as UTC or local time, and are used when a timezone
file is used in handling POSIX-style timezone environment variables.
localtime(3) uses the first standard-time
ttinfo structure in the
file (or simply the first
ttinfo structure in the absence of a
standard-time structure) if either
tzh_timecnt is zero or the time
argument is less than the first transition time recorded in the file.
NOTES¶
This manual page documents
<tzfile.h> in the glibc source archive,
see
timezone/tzfile.h.
It seems that timezone uses
tzfile internally, but glibc refuses to
expose it to userspace. This is most likely because the standardised functions
are more useful and portable, and actually documented by glibc. It may only be
in glibc just to support the non-glibc-maintained timezone data (which is
maintained by some other entity).
SEE ALSO¶
time(3),
gettimeofday(3),
tzset(3),
ctime(3)
For version-2-format timezone files, the above header and data is followed by a
second header and data, identical in format except that eight bytes are used
for each transition time or leap-second time. After the second header and data
comes a newline-enclosed, POSIX-TZ-environment-variable-style string for use
in handling instants after the last transition time stored in the file (with
nothing between the newlines if there is no POSIX representation for such
instants).
SEE ALSO¶
ctime(3),
tzset(3),
tzselect(8),
timezone/tzfile.h in the glibc source tree
COLOPHON¶
This page is part of release 3.44 of the Linux
man-pages project. A
description of the project, and information about reporting bugs, can be found
at
http://www.kernel.org/doc/man-pages/.