198 lines
5.6 KiB
Groff
198 lines
5.6 KiB
Groff
.\" Copyright (c) 1997 Wolfgang Helbig
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd November 29, 1997
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.Dt CALENDAR 3
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.Os
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.Sh NAME
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.Nm easterg ,
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.Nm easterog ,
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.Nm easteroj ,
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.Nm gdate ,
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.Nm jdate ,
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.Nm ndaysg ,
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.Nm ndaysj ,
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.Nm week ,
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.Nm weekday
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.Nd Calendar arithmetic for the Christian era.
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.Sh SYNOPSIS
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.Fd #include <calendar.h>
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.Ft struct date *
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.Fn easterg "int year" "struct date *dt"
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.Ft struct date *
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.Fn easterog "int year" "struct date *dt"
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.Ft struct date *
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.Fn easteroj "int year" "struct date *dt"
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.Ft struct date *
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.Fn gdate "int nd" "struct date *dt"
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.Ft struct date *
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.Fn jdate "int nd" "struct date *dt"
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.Ft int
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.Fn ndaysg "struct date *dt"
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.Ft int
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.Fn ndaysj "struct date *dt"
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.Ft int
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.Fn week "int nd" "int *year"
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.Ft int
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.Fn weekday "int nd"
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.Sh DESCRIPTION
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These functions provide calendar arithmetic for a large range of years,
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starting at March 1st, year zero (i. e. 1 B.C.) and ending way beyond
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year 100000.
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.Pp
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Programs should be linked with
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.Fl lcalendar .
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.Pp
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The functions
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.Fn easterg ,
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.Fn easterog
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and
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.Fn easteroj
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store the date of Easter Sunday into the structure pointed at by
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.Fa dt
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and return a pointer to this structure.
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The function
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.Fn easterg
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assumes Gregorian Calendar (adopted by most western churches after 1582) and
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the functions
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.Fn easterog
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and
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.Fn easteroj
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compute the date of Easter Sunday according to the orthodox rules
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(Western churches before 1582, Greek and Russian Orthodox Church
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until today).
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The result returned by
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.Fn easterog
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is the date in Gregorian Calendar, whereas
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.Fn easteroj
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returns the date in Julian Calendar.
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.Pp
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The functions
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.Fn gdate ,
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.Fn jdate ,
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.Fn ndaysg
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and
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.Fn ndaysj
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provide conversions between the common "year, month, day" notation
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of a date and the "number of days" representation, which is better suited
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for calculations. The days are numbered from March 1st year 1 B.C., starting
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with zero, so the number of a day gives the number of days since March 1st,
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year 1 B.C. The conversions work for nonnegative day numbers only.
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.Pp
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The
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.Fn gdate
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and
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.Fn jdate
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functions
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store the date corresponding to the day number
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.Fa nd
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into the structure pointed at by
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.Fa dt
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and return a pointer to this structure.
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.Pp
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The
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.Fn ndaysg
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and
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.Fn ndaysj
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functions
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return the day number of the date pointed at by
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.Fa dt .
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.Pp
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The
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.Fn gdate
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and
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.Fn ndaysg
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functions
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assume Gregorian Calendar after October 4, 1582 and Julian Calendar before,
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whereas
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.Fn jdate
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and
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.Fn ndaysj
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assume Julian Calendar throughout.
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.Pp
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The two calendars differ by the definition of the leap year. The
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Julian Calendar says every year that is a multiple of four is a
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leap year. The Gregorian Calendar excludes years that are multiples of
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100 and not multiples of 400.
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This means the years 1700, 1800, 1900, 2100 are not leap years
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and the year 2000 is
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a leap year.
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The new rules were inaugurated on October 4, 1582 by deleting ten
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days following this date. Most catholic countries adopted the new
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calendar by the end of the 16th century, whereas others stayed with
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the Julian Calendar until the 20th century. The United Kingdom and
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their colonies switched on September 2, 1752. They already had to
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delete 11 days.
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.Pp
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The function
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.Fn week
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returns the number of the week which contains the day numbered
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.Fa nd .
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The argument
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.Fa *year
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is set with the year that contains (the greater part of) the week.
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The weeks are numbered per year starting with week 1, which is the
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first week in a year that includes more than three days of the year.
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Weeks start on Monday.
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This function is defined for Gregorian Calendar only.
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.Pp
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The function
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.Fn weekday
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returns the weekday (Mo = 0 .. Su = 6) of the day numbered
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.Fa nd .
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.Pp
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The structure
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.Fa date
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is defined in
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.Aq Pa calendar.h .
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It contains these fields:
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.Bd -literal -offset indent
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int y; /\(** year (0000 - ????) \(**/
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int m; /\(** month (1 - 12) \(**/
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int d; /\(** day of month (1 - 31) \(**/
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.Ed
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.Pp
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The year zero is written as "1 B.C." by historians and "0" by astronomers
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and in this library.
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.Sh SEE ALSO
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.Xr ncal 1 ,
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.Xr strftime 3
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.Rs
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.%A A. B. Author
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.%D November 1997
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.Sh STANDARDS
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The week number conforms to ISO 8601: 1988.
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.Sh HISTORY
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The
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.Nm calendar
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library first appeared in
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.Fx 3.0 .
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.Sh AUTHORS
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This manual page and the library was written by
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.An Wolfgang Helbig Aq helbig@FreeBSD.org .
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.Sh BUGS
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The library was coded with great care so there are no bugs left.
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