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share/man/man9: document zero_region(9)

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The zero_region() kernel interface was previously undocumented.
Add a new zero_region(9) manual page to document it.

Submitted by:	Ka Ho Ng <khng@freebsdfoundation.org>
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D28914
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Ka Ho Ng 2021-03-02 17:14:05 +08:00 committed by Philip Paeps
parent 95331c228a
commit 43afeee2fb
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@ -442,6 +442,7 @@ MAN= accept_filter.9 \
vrele.9 \
vslock.9 \
watchdog.9 \
zero_region.9 \
zone.9
MLINKS= unr.9 alloc_unr.9 \

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.\" SPDX-License-Identifier: BSD-2-Clause
.\"
.\" Copyright (c) 2021 The FreeBSD Foundation
.\"
.\" 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 AUTHOR 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 AUTHOR 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.
.\"
.\" $FreeBSD$
.\"
.Dd March 2, 2021
.Dt ZERO_REGION 9
.Os
.Sh NAME
.Nm zero_region
.Nd Read-only region prefilled with zeroes
.Sh SYNOPSIS
.In sys/param.h
.In sys/systm.h
.In vm/vm_param.h
.Vt extern const void *zero_region ;
.Sh DESCRIPTION
The global variable
.Va zero_region
points to a read-only region prefilled with zeroes.
The size of the region is specified by the
.Dv ZERO_REGION_SIZE
macro.
.Sh IMPLEMENTATION NOTES
The region
.Va zero_region
points to is mapped to the same page multiple times.
.Sh EXAMPLES
.Bd -literal
/*
* This function writes zeroes to the vnode at offset 0
* with ZERO_REGION_SIZE length.
*/
static int
write_example(struct vnode *vp)
{
struct thread *td;
struct iovec aiov;
struct uio auio;
int error;
td = curthread;
aiov.iov_base = __DECONST(void *, zero_region);
aiov.iov_len = ZERO_REGION_SIZE;
auio.uio_iov = &aiov;
auio.uio_iovcnt = 1;
auio.uio_offset = 0;
auio.uio_resid = ZERO_REGION_SIZE;
auio.uio_segflg = UIO_SYSSPACE;
auio.uio_rw = UIO_WRITE;
auio.uio_td = td;
error = VOP_WRITE(vp, &auio, 0, td->td_ucred);
return (error);
}
.Ed
.Sh SEE ALSO
.Xr pmap 9 ,
.Xr vm_map 9
.Sh AUTHORS
This manual page was written by
.An Ka Ho Ng Aq Mt khng@FreeBSDFoundation.org
under sponsorship from the FreeBSD Foundation.