1.\" SPDX-License-Identifier: BSD-2-Clause 2.\" 3.\" Copyright (c) 2021 The FreeBSD Foundation 4.\" 5.\" Redistribution and use in source and binary forms, with or without 6.\" modification, are permitted provided that the following conditions 7.\" are met: 8.\" 1. Redistributions of source code must retain the above copyright 9.\" notice, this list of conditions and the following disclaimer. 10.\" 2. Redistributions in binary form must reproduce the above copyright 11.\" notice, this list of conditions and the following disclaimer in the 12.\" documentation and/or other materials provided with the distribution. 13.\" 14.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 15.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 16.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 17.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE 18.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 19.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 20.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 21.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 22.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 23.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 24.\" SUCH DAMAGE. 25.\" 26.Dd March 2, 2021 27.Dt ZERO_REGION 9 28.Os 29.Sh NAME 30.Nm zero_region 31.Nd Read-only region prefilled with zeroes 32.Sh SYNOPSIS 33.In sys/param.h 34.In sys/systm.h 35.In vm/vm_param.h 36.Vt extern const void *zero_region ; 37.Sh DESCRIPTION 38The global variable 39.Va zero_region 40points to a read-only region prefilled with zeroes. 41The size of the region is specified by the 42.Dv ZERO_REGION_SIZE 43macro. 44.Sh IMPLEMENTATION NOTES 45The region 46.Va zero_region 47points to is mapped to the same page multiple times. 48.Sh EXAMPLES 49.Bd -literal 50/* 51 * This function writes zeroes to the vnode at offset 0 52 * with ZERO_REGION_SIZE length. 53 */ 54static int 55write_example(struct vnode *vp) 56{ 57 struct thread *td; 58 struct iovec aiov; 59 struct uio auio; 60 int error; 61 62 td = curthread; 63 64 aiov.iov_base = __DECONST(void *, zero_region); 65 aiov.iov_len = ZERO_REGION_SIZE; 66 auio.uio_iov = &aiov; 67 auio.uio_iovcnt = 1; 68 auio.uio_offset = 0; 69 auio.uio_resid = ZERO_REGION_SIZE; 70 auio.uio_segflg = UIO_SYSSPACE; 71 auio.uio_rw = UIO_WRITE; 72 auio.uio_td = td; 73 74 error = VOP_WRITE(vp, &auio, 0, td->td_ucred); 75 return (error); 76} 77.Ed 78.Sh SEE ALSO 79.Xr pmap 9 , 80.Xr vm_map 9 81.Sh AUTHORS 82This manual page was written by 83.An Ka Ho Ng Aq Mt khng@FreeBSDFoundation.org 84under sponsorship from the FreeBSD Foundation. 85