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NAME

       tzfile - time zone information

SYNOPSIS

       #include <tzfile.h>

DESCRIPTION

       This page describes the structure of timezone files as commonly found in /usr/lib/zoneinfo
       or /usr/share/zoneinfo.

       The time zone information files used by tzset(3) begin with the magic characters "TZif" to
       identify  then  as  time  zone  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)

COLOPHON

       This  page  is  part of release 3.35 of the Linux man-pages project.  A description of the
       project, and information about reporting bugs, can be found at  http://man7.org/linux/man-
       pages/.

                                            2010-08-31                                  TZFILE(5)