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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 sixteen bytes 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  "time  zone  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
       that appears 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 time zone 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 time zone file is used in handling POSIX-
       style time zone 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  time  zone  file  is  used  in  handling POSIX-style time zone
       environment variables.

       Localtime 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)

                                                                     TZFILE(5)