358 lines
9.9 KiB
Groff
358 lines
9.9 KiB
Groff
.\" Copyright (c) 1991, 1993
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.\" The Regents of the University of California. 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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.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by the University of
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.\" California, Berkeley and its contributors.
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.\" 4. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS 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 REGENTS 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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.\" @(#)mmap.2 8.4 (Berkeley) 5/11/95
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.\" $FreeBSD$
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.\"
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.Dd May 11, 1995
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.Dt MMAP 2
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.Os
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.Sh NAME
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.Nm mmap
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.Nd allocate memory, or map files or devices into memory
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.Fd #include <sys/types.h>
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.Fd #include <sys/mman.h>
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.Ft void *
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.Fn mmap "void *addr" "size_t len" "int prot" "int flags" "int fd" "off_t offset"
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.Sh DESCRIPTION
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The
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.Fn mmap
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function causes the pages starting at
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.Fa addr
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and continuing for at most
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.Fa len
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bytes to be mapped from the object described by
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.Fa fd ,
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starting at byte offset
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.Fa offset .
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If
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.Fa len
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is not a multiple of the pagesize, the mapped region may extend past the
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specified range.
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Any such extension beyond the end of the mapped object will be zero-filled.
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.Pp
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If
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.Fa addr
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is non-zero, it is used as a hint to the system.
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(As a convenience to the system, the actual address of the region may differ
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from the address supplied.)
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If
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.Fa addr
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is zero, an address will be selected by the system.
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The actual starting address of the region is returned.
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A successful
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.Fa mmap
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deletes any previous mapping in the allocated address range.
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.Pp
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The protections (region accessibility) are specified in the
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.Fa prot
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argument by
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.Em or Ns 'ing
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the following values:
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.Pp
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.Bl -tag -width PROT_WRITE -compact
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.It Dv PROT_NONE
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Pages may not be accessed.
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.It Dv PROT_READ
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Pages may be read.
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.It Dv PROT_WRITE
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Pages may be written.
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.It Dv PROT_EXEC
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Pages may be executed.
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.El
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.Pp
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The
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.Fa flags
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parameter specifies the type of the mapped object, mapping options and
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whether modifications made to the mapped copy of the page are private
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to the process or are to be shared with other references.
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Sharing, mapping type and options are specified in the
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.Fa flags
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argument by
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.Em or Ns 'ing
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the following values:
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.Bl -tag -width MAP_HASSEMAPHORE
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.It Dv MAP_ANON
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Map anonymous memory not associated with any specific file.
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The file descriptor used for creating
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.Dv MAP_ANON
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must be \-1.
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The
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.Fa offset
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parameter is ignored.
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.\".It Dv MAP_FILE
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.\"Mapped from a regular file or character-special device memory.
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.It Dv MAP_FIXED
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Do not permit the system to select a different address than the one
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specified.
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If the specified address cannot be used,
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.Fn mmap
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will fail.
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If
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.Dv MAP_FIXED
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is specified,
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.Fa addr
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must be a multiple of the pagesize.
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Use of this option is discouraged.
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.It Dv MAP_HASSEMAPHORE
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Notify the kernel that the region may contain semaphores and that special
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handling may be necessary.
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.It Dv MAP_INHERIT
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Permit regions to be inherited across
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.Xr execve 2
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system calls.
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.It Dv MAP_NOCORE
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Region is not included in a core file.
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.It Dv MAP_NOSYNC
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Causes data dirtied via this VM map to be flushed to physical media
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only when necessary (usually by the pager) rather then gratuitously.
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Typically this prevents the update daemons from flushing pages dirtied
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through such maps and thus allows efficient sharing of memory across
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unassociated processes using a file-backed shared memory map. Without
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this option any VM pages you dirty may be flushed to disk every so often
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(every 30-60 seconds usually) which can create performance problems if you
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do not need that to occur (such as when you are using shared file-backed
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mmap regions for IPC purposes). Note that VM/filesystem coherency is
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maintained whether you use
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.Dv MAP_NOSYNC
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or not. This option is not portable
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across
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.Ux
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platforms (yet), though some may implement the same behavior
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by default.
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.Pp
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.Em WARNING !
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Extending a file with
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.Xr ftruncate 2 ,
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thus creating a big hole, and then filling the hole by modifying a shared
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.Fn mmap
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can lead to severe file fragmentation.
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In order to avoid such fragmentation you should always pre-allocate the
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file's backing store by
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.Fn write Ns ing
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zero's into the newly extended area prior to modifying the area via your
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.Fn mmap .
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The fragmentation problem is especially sensitive to
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.Dv MAP_NOSYNC
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pages, because pages may be flushed to disk in a totally random order.
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.Pp
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The same applies when using
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.Dv MAP_NOSYNC
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to implement a file-based shared memory store.
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It is recommended that you create the backing store by
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.Fn write Ns ing
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zero's to the backing file rather then
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.Fn ftruncate Ns ing
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it.
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You can test file fragmentation by observing the KB/t (kilobytes per
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transfer) results from an
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.Dq Li iostat 1
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while reading a large file sequentially, e.g. using
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.Dq Li dd if=filename of=/dev/null bs=32k .
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.Pp
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The
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.Xr fsync 2
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function will flush all dirty data and metadata associated with a file,
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including dirty NOSYNC VM data, to physical media. The
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.Xr sync 8
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command and
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.Xr sync 2
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system call generally do not flush dirty NOSYNC VM data.
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The
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.Xr msync 2
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system call is obsolete since
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.Bx
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implements a coherent filesystem buffer cache. However, it may be
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used to associate dirty VM pages with filesystem buffers and thus cause
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them to be flushed to physical media sooner rather then later.
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.It Dv MAP_PRIVATE
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Modifications are private.
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.It Dv MAP_SHARED
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Modifications are shared.
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.It Dv MAP_STACK
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This option is only available if your system has been compiled with
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.Dv VM_STACK
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defined when compiling the kernel.
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This is the default for
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i386 only.
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Consider adding
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.Li -DVM_STACK
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to
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.Va COPTFLAGS
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in your
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.Pa /etc/make.conf
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to enable this option for other architechures.
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.Dv MAP_STACK
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implies
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.Dv MAP_ANON ,
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and
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.Fa offset
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of 0.
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.Fa fd
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must be -1 and
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.Fa prot
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must include at least
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.Dv PROT_READ
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and
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.Dv PROT_WRITE .
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This option creates
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a memory region that grows to at most
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.Fa len
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bytes in size, starting from the stack top and growing down. The
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stack top is the starting address returned by the call, plus
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.Fa len
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bytes. The bottom of the stack at maximum growth is the starting
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address returned by the call.
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.El
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.Pp
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The
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.Xr close 2
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function does not unmap pages, see
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.Xr munmap 2
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for further information.
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.Pp
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The current design does not allow a process to specify the location of
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swap space.
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In the future we may define an additional mapping type,
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.Dv MAP_SWAP ,
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in which
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the file descriptor argument specifies a file or device to which swapping
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should be done.
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.Sh RETURN VALUES
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Upon successful completion,
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.Fn mmap
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returns a pointer to the mapped region.
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Otherwise, a value of
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.Dv MAP_FAILED
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is returned and
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.Va errno
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is set to indicate the error.
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.Sh ERRORS
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.Fn Mmap
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will fail if:
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.Bl -tag -width Er
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.It Bq Er EACCES
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The flag
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.Dv PROT_READ
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was specified as part of the
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.Fa prot
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parameter and
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.Fa fd
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was not open for reading.
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The flags
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.Dv MAP_SHARED
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and
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.Dv PROT_WRITE
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were specified as part of the
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.Fa flags
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and
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.Fa prot
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parameters and
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.Fa fd
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was not open for writing.
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.It Bq Er EBADF
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.Fa fd
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is not a valid open file descriptor.
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.It Bq Er EINVAL
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.Dv MAP_FIXED
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was specified and the
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.Fa addr
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parameter was not page aligned, or part of the desired address space
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resides out of the valid address space for a user process.
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.It Bq Er EINVAL
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.Fa Len
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was negative.
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.It Bq Er EINVAL
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.Dv MAP_ANON
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was specified and the
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.Fa fd
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parameter was not -1.
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.It Bq Er EINVAL
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.Dv MAP_ANON
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has not been specified and
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.Fa fd
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did not reference a regular or character special file.
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.It Bq Er EINVAL
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.Fa Offset
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was not page-aligned.
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(See
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.Sx BUGS
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below.)
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.It Bq Er ENOMEM
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.Dv MAP_FIXED
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was specified and the
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.Fa addr
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parameter wasn't available, or the system has reached the per-process mmap
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limit specified in the
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.Va vm.max_proc_mmap
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sysctl.
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.Dv MAP_ANON
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was specified and insufficient memory was available.
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.El
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.Sh SEE ALSO
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.Xr madvise 2 ,
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.Xr mincore 2 ,
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.Xr mlock 2 ,
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.Xr mprotect 2 ,
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.Xr msync 2 ,
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.Xr munlock 2 ,
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.Xr munmap 2 ,
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.Xr getpagesize 3
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.Sh BUGS
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.Fa len
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is limited to 2GB. Mmapping slightly more than 2GB doesn't work, but
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it is possible to map a window of size (filesize % 2GB) for file sizes
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of slightly less than 2G, 4GB, 6GB and 8GB.
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.Pp
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The limit is imposed for a variety of reasons.
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Most of them have to do
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with
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.Fx
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not wanting to use 64 bit offsets in the VM system due to
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the extreme performance penalty.
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So
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.Fx
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uses 32bit page indexes and
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this gives
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.Fx
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a maximum of 8TB filesizes.
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It's actually bugs in
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the filesystem code that causes the limit to be further restricted to
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1TB (loss of precision when doing blockno calculations).
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.Pp
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Another reason for the 2GB limit is that filesystem metadata can
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reside at negative offsets.
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.Pp
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We currently can only deal with page aligned file offsets.
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