tzfile


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, fol-
       lowed 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  "stan-
       dard"  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;
           };

       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 environ-
       ment 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-main-
       tained 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  fol-
       lowed  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)
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