.TH TZFILE 5
.SH NAME
tzfile \- time zone information
.SH SYNOPSIS
.B
#include "tzfile.h"
.SH DESCRIPTION
The time zone information files used by
.IR settz (3)
and
.IR newlocaltime (3)
begin with a
.I tzinfo
structure (as defined in the include file
.B 
"tzfile.h"\c
):
.sp
.nf
.in +.5i
.ta .5i +\w'unsigned short  'u
struct tzhead {
	char	tzh_reserved[14];
	unsigned short	tzh_timecnt;
	unsigned short	tzh_typecnt;
	unsigned short	tzh_charcnt;
};
.in -.5i
.fi
.PP
The
.B tzh_reserved
element is currently unused.
The
.B tzh_timecnt
element gives the number of "transition times" for which data is stored
in the file;
the
.B tzh_typecnt
(which must not be zero)
element gives the number of "local time types" for which data is stored
in the file;
and the
.B tzh_charcnt
element gives the number of characters of "time zone abbreviation strings"
stored in the file.
.PP
The above header is followed by
.B tzh_timecnt
values of type
.BR long ,
sorted in ascending order;
each is used as a transition time (as returned by
.IR time (2))
at which the rules for computing local time change.
Next come
.B tzh_timecnt
values of type
.BR "unsigned char" ;
each one tells which of the different types of "local time" described in the
file is associated with the same-indexed transition time.
These values serve as indices into an array of
.B ttinfo
structures that appears next in the file; 
these structures are defined as follows:
.in +.5i
.sp
.nf
.ta .5i +\w'unsigned short  'u
struct ttinfo {	
	long	tt_gmtoff;
	int	tt_isdst;
	unsigned int	tt_abbrind;
};
.in -.5i
.fi
.sp
In each structure,
.B tt_gmtoff
gives the number of seconds to be added to GMT,
.B
tt_isdst
tells whether
.B
tm_isdst
should be set by
.IR newlocaltime (3),
and
.B tt_abbrind
serves as an index into the array of time zone abbreviation chaaracters
that follow the
.B ttinfo
structure(s) in the file.
.PP
.I Newlocaltime
uses the first
.B ttinfo
structure in the file
if either
.B tzh_timecnt
is zero or
.IR newlocaltime 's
argument is less than
the first transition time recorded in the file.
.SH SEE ALSO
settz(3)
.. %W%