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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 zeroes 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

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                                  2010-08-31                         TZFILE(5)