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+.\"	$NetBSD: mstohz.9,v 1.12 2019/09/28 15:11:19 christos Exp $
+.\"
+.\" Copyright (c) 2002 Manuel Bouyer.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
+.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
+.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+.\" SUCH DAMAGE.
+.\"
+.\"
+.Dd September 28, 2019
+.Dt MSTOHZ 9
+.Os
+.Sh NAME
+.Nm mstohz ,
+.Nm hztoms
+.Nd convert between milliseconds and system clock ticks
+.Sh SYNOPSIS
+.In sys/param.h
+.Ft unsigned int
+.Fn mstohz "unsigned int ms"
+.Ft unsigned int
+.Fn hztoms "unsigned int hz"
+.Sh DESCRIPTION
+The
+.Fn mstohz
+macro can be used to convert time
+in milliseconds to system clock ticks, as used
+by the
+.Xr callout 9
+facility, in an integer-overflow safe way.
+Conversely,
+.Fn hztoms
+converts system clock ticks to milliseconds.
+.Pp
+These macros are defined in the
+.Aq Em sys/param.h
+header.
+Individual ports can have a processor-specific, more efficient
+version implemented in their
+.Aq Em machine/param.h
+header as a define.
+.Sh RETURN VALUES
+The return value is either the number of clock ticks or milliseconds
+for the specified value.
+.Sh SEE ALSO
+.Xr param 3 ,
+.Xr callout 9 ,
+.Xr tvtohz 9
+.Sh BUGS
+Neither
+.Fn mstohz
+nor
+.Fn hztoms
+make use of expensive 64-bit integer arithmetic.
+The result from
+.Fn mstohz
+will be rounded down to one second if the parameter is larger
+than 131072 milliseconds.