1.\" 2.\" Copyright (c) 1991, 1993 3.\" The Regents of the University of California. All rights reserved. 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.\" 3. Neither the name of the University nor the names of its contributors 14.\" may be used to endorse or promote products derived from this software 15.\" without specific prior written permission. 16.\" 17.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 18.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 19.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 20.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 21.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 22.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 23.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 24.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 25.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 26.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 27.\" SUCH DAMAGE. 28.\" 29.Dd March 15, 2017 30.Dt MINHERIT 2 31.Os 32.Sh NAME 33.Nm minherit 34.Nd control the inheritance of pages 35.Sh LIBRARY 36.Lb libc 37.Sh SYNOPSIS 38.In sys/mman.h 39.Ft int 40.Fn minherit "void *addr" "size_t len" "int inherit" 41.Sh DESCRIPTION 42The 43.Fn minherit 44system call 45changes the specified pages to have the inheritance characteristic 46.Fa inherit . 47Not all implementations will guarantee that the inheritance characteristic 48can be set on a page basis; 49the granularity of changes may be as large as an entire region. 50.Fx 51is capable of adjusting inheritance characteristics on a page basis. 52Inheritance only effects children created by 53.Fn fork . 54It has no effect on 55.Fn exec . 56exec'd processes replace their address space entirely. 57This system call also 58has no effect on the parent's address space (other than to potentially 59share the address space with its children). 60.Pp 61Inheritance is a rather esoteric feature largely superseded by the 62.Dv MAP_SHARED 63feature of 64.Fn mmap . 65However, it is possible to use 66.Fn minherit 67to share a block of memory between parent and child that has been mapped 68.Dv MAP_PRIVATE . 69That is, modifications made by parent or child are shared but 70the original underlying file is left untouched. 71.Bl -tag -width ".Dv INHERIT_SHARE" 72.It Dv INHERIT_SHARE 73This option causes the address space in question to be shared between 74parent and child. 75It has no effect on how the original underlying backing 76store was mapped. 77.It Dv INHERIT_NONE 78This option prevents the address space in question from being inherited 79at all. 80The address space will be unmapped in the child. 81.It Dv INHERIT_COPY 82This option causes the child to inherit the address space as copy-on-write. 83This option also has an unfortunate side effect of causing the parent 84address space to become copy-on-write when the parent forks. 85If the original mapping was 86.Dv MAP_SHARED , 87it will no longer be shared in the parent 88after the parent forks and there is no way to get the previous 89shared-backing-store mapping without unmapping and remapping the address 90space in the parent. 91.It Dv INHERIT_ZERO 92This option causes the address space in question to be mapped as new 93anonymous pages, 94which would be initialized to all zero bytes, 95in the child process. 96.El 97.Sh RETURN VALUES 98.Rv -std minherit 99.Sh ERRORS 100The 101.Fn minherit 102system call will fail if: 103.Bl -tag -width Er 104.It Bq Er EINVAL 105The virtual address range specified by the 106.Fa addr 107and 108.Fa len 109arguments is not valid. 110.It Bq Er EACCES 111The flags specified by the 112.Fa inherit 113argument were not valid for the pages specified 114by the 115.Fa addr 116and 117.Fa len 118arguments. 119.El 120.Sh SEE ALSO 121.Xr fork 2 , 122.Xr madvise 2 , 123.Xr mincore 2 , 124.Xr mprotect 2 , 125.Xr msync 2 , 126.Xr munmap 2 , 127.Xr rfork 2 128.Sh HISTORY 129The 130.Fn minherit 131system call first appeared in 132.Ox 133and then in 134.Fx 2.2 . 135.Pp 136The 137.Dv INHERIT_ZERO 138support first appeared in 139.Ox 5.6 140and then in 141.Fx 12.0 . 142.Sh BUGS 143Once you set inheritance to 144.Dv MAP_PRIVATE 145or 146.Dv MAP_SHARED , 147there is no way to recover the original copy-on-write semantics 148short of unmapping and remapping the area. 149