NAME | DESCRIPTION | SEE ALSO | COLOPHON

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

NAME         top

       tzfile - timezone information

DESCRIPTION         top

       This page describes the structure of the timezone files used by
       tzset(3).  These files are typically found under one of the
       directories /usr/lib/zoneinfo or /usr/share/zoneinfo.

       Timezone information files begin with the magic characters "TZif" to
       identify them as timezone 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.

       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         top

       ctime(3), tzset(3), tzselect(8),

       timezone/tzfile.h in the glibc source tree

COLOPHON         top

       This page is part of release 3.70 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       http://www.kernel.org/doc/man-pages/.

                                 2012-05-04                        TZFILE(5)