TZFILE(5)                                             Linux Programmer’s Manual                                            TZFILE(5)



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 infor‐
       mation 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 "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 writ‐
       ten 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
       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 han‐
       dling POSIX-style timezone 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 sup‐
       port the non-glibc-maintained timezone data (which is maintained by some other entity).

SEE ALSO
       time(3), gettimeofday(3), tzset(3), ctime(3)

COLOPHON
       This page is part of release 3.15 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/.



                                                             1996-06-05                                                    TZFILE(5)