676 lines
18 KiB
Groff
676 lines
18 KiB
Groff
.\" Hey Emacs! This file is -*- nroff -*- source.
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.\"
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.\" Copyright (C) 1996 Andries Brouwer <aeb@cwi.nl>
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.\" and Copyright (C) 2006, 2007 Michael Kerrisk <mtk.manpages@gmail.com>
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.\"
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\"
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.\" Modified 1997-01-31 by Eric S. Raymond <esr@thyrsus.com>
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.\" Modified 2000-03-25 by Jim Van Zandt <jrv@vanzandt.mv.com>
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.\" Modified 2001-10-04 by John Levon <moz@compsoc.man.ac.uk>
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.\" Modified 2003-02-02 by Andi Kleen <ak@muc.de>
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.\" Modified 2003-05-21 by Michael Kerrisk <mtk.manpages@gmail.com>
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.\" MAP_LOCKED works from 2.5.37
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.\" Modified 2004-06-17 by Michael Kerrisk <mtk.manpages@gmail.com>
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.\" Modified 2004-09-11 by aeb
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.\" Modified 2004-12-08, from Eric Estievenart <eric.estievenart@free.fr>
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.\" Modified 2004-12-08, mtk, formatting tidy-ups
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.\" Modified 2006-12-04, mtk, various parts rewritten
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.\" 2007-07-10, mtk, Added an example program.
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.\" 2008-11-18, mtk, document MAP_STACK
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.\"
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.TH MMAP 2 2009-09-26 "Linux" "Linux Programmer's Manual"
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.SH NAME
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mmap, mmap64, munmap \- map or unmap files or devices into memory
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.SH SYNOPSIS
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.nf
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.B #include <sys/mman.h>
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.sp
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.BI "void *mmap(void *" addr ", size_t " length \
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", int " prot ", int " flags ,
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.BI " int " fd ", off_t " offset );
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.BI "void *mmap64(void *" addr ", size_t " length \
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", int " prot ", int " flags ,
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.BI " int " fd ", off64_t " offset );
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.BI "int munmap(void *" addr ", size_t " length );
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.fi
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.SH DESCRIPTION
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.BR mmap ()
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creates a new mapping in the virtual address space of
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the calling process.
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The starting address for the new mapping is specified in
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.IR addr .
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The
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.I length
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argument specifies the length of the mapping.
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If
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.I addr
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is NULL,
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then the kernel chooses the address at which to create the mapping;
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this is the most portable method of creating a new mapping.
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If
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.I addr
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is not NULL,
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then the kernel takes it as a hint about where to place the mapping;
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on Linux, the mapping will be created at a nearby page boundary.
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.\" Before Linux 2.6.24, the address was rounded up to the next page
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.\" boundary; since 2.6.24, it is rounded down!
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The address of the new mapping is returned as the result of the call.
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The contents of a file mapping (as opposed to an anonymous mapping; see
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.B MAP_ANONYMOUS
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below), are initialized using
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.I length
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bytes starting at offset
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.I offset
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in the file (or other object) referred to by the file descriptor
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.IR fd .
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.I offset
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must be a multiple of the page size as returned by
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.IR sysconf(_SC_PAGE_SIZE) .
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.LP
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The
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.I prot
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argument describes the desired memory protection of the mapping
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(and must not conflict with the open mode of the file).
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It is either
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.B PROT_NONE
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or the bitwise OR of one or more of the following flags:
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.TP 1.1i
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.B PROT_EXEC
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Pages may be executed.
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.TP
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.B PROT_READ
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Pages may be read.
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.TP
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.B PROT_WRITE
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Pages may be written.
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.TP
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.B PROT_NONE
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Pages may not be accessed.
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.LP
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The
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.I flags
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argument determines whether updates to the mapping
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are visible to other processes mapping the same region,
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and whether updates are carried through to the underlying file.
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This behavior is determined by including exactly one
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of the following values in
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.IR flags :
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.TP 1.1i
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.B MAP_SHARED
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Share this mapping.
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Updates to the mapping are visible to other processes that map this file,
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and are carried through to the underlying file.
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The file may not actually be updated until
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.BR msync (2)
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or
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.BR munmap ()
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is called.
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.TP
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.B MAP_PRIVATE
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Create a private copy-on-write mapping.
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Updates to the mapping are not visible to other processes
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mapping the same file, and are not carried through to
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the underlying file.
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It is unspecified whether changes made to the file after the
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.BR mmap ()
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call are visible in the mapped region.
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.LP
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Both of these flags are described in POSIX.1-2001.
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In addition, zero or more of the following values can be ORed in
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.IR flags :
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.TP
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.BR MAP_32BIT " (since Linux 2.4.20, 2.6)"
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Put the mapping into the first 2 Gigabytes of the process address space.
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This flag is only supported on x86-64, for 64-bit programs.
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It was added to allow thread stacks to be allocated somewhere
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in the first 2GB of memory,
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so as to improve context-switch performance on some early
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64-bit processors.
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.\" See http://lwn.net/Articles/294642 "Tangled up in threads", 19 Aug 08
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Modern x86-64 processors no longer have this performance problem,
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so use of this flag is not required on those systems.
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The
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.B MAP_32BIT
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flag is ignored when
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.B MAP_FIXED
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is set.
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.TP
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.B MAP_ANON
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Synonym for
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.BR MAP_ANONYMOUS .
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Deprecated.
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.TP
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.B MAP_ANONYMOUS
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The mapping is not backed by any file;
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its contents are initialized to zero.
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The
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.I fd
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and
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.I offset
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arguments are ignored;
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however, some implementations require
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.I fd
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to be \-1 if
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.B MAP_ANONYMOUS
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(or
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.BR MAP_ANON )
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is specified,
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and portable applications should ensure this.
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The use of
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.B MAP_ANONYMOUS
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in conjunction with
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.B MAP_SHARED
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is only supported on Linux since kernel 2.4.
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.TP
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.B MAP_DENYWRITE
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This flag is ignored.
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.\" Introduced in 1.1.36, removed in 1.3.24.
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(Long ago, it signaled that attempts to write to the underlying file
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should fail with
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.BR ETXTBUSY .
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But this was a source of denial-of-service attacks.)
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.TP
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.B MAP_EXECUTABLE
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This flag is ignored.
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.\" Introduced in 1.1.38, removed in 1.3.24. Flag tested in proc_follow_link.
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.\" (Long ago, it signaled that the underlying file is an executable.
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.\" However, that information was not really used anywhere.)
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.\" Linus talked about DOS related to MAP_EXECUTABLE, but he was thinking of
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.\" MAP_DENYWRITE?
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.TP
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.B MAP_FILE
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Compatibility flag.
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Ignored.
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.\" On some systems, this was required as the opposite of
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.\" MAP_ANONYMOUS -- mtk, 1 May 2007
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.TP
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.B MAP_FIXED
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Don't interpret
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.I addr
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as a hint: place the mapping at exactly that address.
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.I addr
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must be a multiple of the page size.
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If the memory region specified by
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.I addr
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and
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.I len
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overlaps pages of any existing mapping(s), then the overlapped
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part of the existing mapping(s) will be discarded.
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If the specified address cannot be used,
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.BR mmap ()
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will fail.
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Because requiring a fixed address for a mapping is less portable,
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the use of this option is discouraged.
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.TP
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.B MAP_GROWSDOWN
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Used for stacks.
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Indicates to the kernel virtual memory system that the mapping
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should extend downwards in memory.
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.TP
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.BR MAP_HUGETLB " (since Linux 2.6.32)"
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Allocate the mapping using "huge pages."
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See the kernel source file
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.I Documentation/vm/hugetlbpage.txt
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for further information.
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.TP
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.BR MAP_LOCKED " (since Linux 2.5.37)"
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Lock the pages of the mapped region into memory in the manner of
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.BR mlock (2).
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This flag is ignored in older kernels.
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.\" If set, the mapped pages will not be swapped out.
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.TP
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.BR MAP_NONBLOCK " (since Linux 2.5.46)"
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Only meaningful in conjunction with
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.BR MAP_POPULATE .
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Don't perform read-ahead:
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only create page tables entries for pages
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that are already present in RAM.
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Since Linux 2.6.23, this flag causes
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.BR MAP_POPULATE
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to do nothing.
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One day the combination of
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.BR MAP_POPULATE
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and
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.BR MAP_NONBLOCK
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may be reimplemented.
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.TP
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.B MAP_NORESERVE
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Do not reserve swap space for this mapping.
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When swap space is reserved, one has the guarantee
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that it is possible to modify the mapping.
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When swap space is not reserved one might get
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.B SIGSEGV
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upon a write
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if no physical memory is available.
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See also the discussion of the file
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.I /proc/sys/vm/overcommit_memory
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in
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.BR proc (5).
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In kernels before 2.6, this flag only had effect for
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private writable mappings.
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.TP
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.BR MAP_POPULATE " (since Linux 2.5.46)"
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Populate (prefault) page tables for a mapping.
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For a file mapping, this causes read-ahead on the file.
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Later accesses to the mapping will not be blocked by page faults.
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.BR MAP_POPULATE
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is only supported for private mappings since Linux 2.6.23.
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.LP
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Of the above flags, only
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.B MAP_FIXED
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is specified in POSIX.1-2001.
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However, most systems also support
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.B MAP_ANONYMOUS
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(or its synonym
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.BR MAP_ANON ).
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.TP
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.BR MAP_STACK " (since Linux 2.6.27)"
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Allocate the mapping at an address suitable for a process
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or thread stack.
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This flag is currently a no-op,
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but is used in the glibc threading implementation so that
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if some architectures require special treatment for stack allocations,
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support can later be transparently implemented for glibc.
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.\" See http://lwn.net/Articles/294642 "Tangled up in threads", 19 Aug 08
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.\" commit cd98a04a59e2f94fa64d5bf1e26498d27427d5e7
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.\" http://thread.gmane.org/gmane.linux.kernel/720412
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.\" "pthread_create() slow for many threads; also time to revisit 64b
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.\" context switch optimization?"
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.LP
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Some systems document the additional flags
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.BR MAP_AUTOGROW ,
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.BR MAP_AUTORESRV ,
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.BR MAP_COPY ,
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and
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.BR MAP_LOCAL .
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.LP
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Memory mapped by
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.BR mmap ()
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is preserved across
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.BR fork (2),
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with the same attributes.
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.LP
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A file is mapped in multiples of the page size.
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For a file that is not
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a multiple of the page size, the remaining memory is zeroed when mapped,
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and writes to that region are not written out to the file.
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The effect of
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changing the size of the underlying file of a mapping on the pages that
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correspond to added or removed regions of the file is unspecified.
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.SS mmap64()
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The
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.BR mmap64 ()
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system call operates in exactly the same way as
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.BR mmap (),
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except that the final argument specifies the offset as
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a 64-bit off64_t. This enables applications to aceess the large
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files.
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.SS munmap()
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The
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.BR munmap ()
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system call deletes the mappings for the specified address range, and
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causes further references to addresses within the range to generate
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invalid memory references.
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The region is also automatically unmapped
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when the process is terminated.
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On the other hand, closing the file
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descriptor does not unmap the region.
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.LP
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The address
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.I addr
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must be a multiple of the page size.
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All pages containing a part
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of the indicated range are unmapped, and subsequent references
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to these pages will generate
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.BR SIGSEGV .
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It is not an error if the
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indicated range does not contain any mapped pages.
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.SS Timestamps changes for file-backed mappings
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For file-backed mappings, the
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.I st_atime
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field for the mapped file may be updated at any time between the
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.BR mmap ()
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and the corresponding unmapping; the first reference to a mapped
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page will update the field if it has not been already.
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.LP
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The
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.I st_ctime
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and
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.I st_mtime
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field for a file mapped with
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.B PROT_WRITE
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and
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.B MAP_SHARED
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will be updated after
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a write to the mapped region, and before a subsequent
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.BR msync (2)
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with the
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.B MS_SYNC
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or
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.B MS_ASYNC
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flag, if one occurs.
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.SH "RETURN VALUE"
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On success,
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.BR mmap ()
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returns a pointer to the mapped area.
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On error, the value
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.B MAP_FAILED
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(that is,
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.IR "(void\ *)\ \-1" )
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is returned, and
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.I errno
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is set appropriately.
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On success,
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.BR munmap ()
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returns 0, on failure \-1, and
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.I errno
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is set (probably to
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.BR EINVAL ).
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.SH ERRORS
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.TP
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.B EACCES
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A file descriptor refers to a non-regular file.
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Or
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.B MAP_PRIVATE
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was requested, but
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.I fd
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is not open for reading.
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Or
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.B MAP_SHARED
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was requested and
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.B PROT_WRITE
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is set, but
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.I fd
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is not open in read/write
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.RB ( O_RDWR )
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mode.
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Or
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.B PROT_WRITE
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is set, but the file is append-only.
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.TP
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.B EAGAIN
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The file has been locked, or too much memory has been locked (see
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.BR setrlimit (2)).
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.TP
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.B EBADF
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.I fd
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is not a valid file descriptor (and
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.B MAP_ANONYMOUS
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was not set).
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.TP
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.B EINVAL
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We don't like
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.IR addr ,
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.IR length ,
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or
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.I offset
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(e.g., they are too large, or not aligned on a page boundary).
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.TP
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.B EINVAL
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(since Linux 2.6.12)
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.I length
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was 0.
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.TP
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.B EINVAL
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.I flags
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contained neither
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.B MAP_PRIVATE
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or
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.BR MAP_SHARED ,
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or contained both of these values.
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.TP
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.B ENFILE
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.\" This is for shared anonymous segments
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.\" [2.6.7] shmem_zero_setup()-->shmem_file_setup()-->get_empty_filp()
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The system limit on the total number of open files has been reached.
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.\" .TP
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.\" .B ENOEXEC
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.\" A file could not be mapped for reading.
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.TP
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.B ENODEV
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The underlying file system of the specified file does not support
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memory mapping.
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.TP
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.B ENOMEM
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No memory is available, or the process's maximum number of mappings would
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have been exceeded.
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.TP
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.B EPERM
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The
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.I prot
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argument asks for
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.B PROT_EXEC
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|
but the mapped area belongs to a file on a file system that
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was mounted no-exec.
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.\" (Since 2.4.25 / 2.6.0.)
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.TP
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.B ETXTBSY
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.B MAP_DENYWRITE
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was set but the object specified by
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.I fd
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is open for writing.
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.LP
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Use of a mapped region can result in these signals:
|
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.TP
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.B SIGSEGV
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Attempted write into a region mapped as read-only.
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.TP
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.B SIGBUS
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|
Attempted access to a portion of the buffer that does not correspond
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|
to the file (for example, beyond the end of the file, including the
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case where another process has truncated the file).
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.SH "CONFORMING TO"
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|
SVr4, 4.4BSD, POSIX.1-2001.
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.\" SVr4 documents additional error codes ENXIO and ENODEV.
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.\" SUSv2 documents additional error codes EMFILE and EOVERFLOW.
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.SH AVAILABILITY
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|
On POSIX systems on which
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.BR mmap (),
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.BR msync (2)
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and
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.BR munmap ()
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are available,
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.B _POSIX_MAPPED_FILES
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|
is defined in \fI<unistd.h>\fP to a value greater than 0.
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(See also
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.BR sysconf (3).)
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|
.\" POSIX.1-2001: It shall be defined to -1 or 0 or 200112L.
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.\" -1: unavailable, 0: ask using sysconf().
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.\" glibc defines it to 1.
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.SH NOTES
|
|
Since kernel 2.4, this system call has been superseded by
|
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.BR mmap2 (2).
|
|
Nowadays,
|
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.\" Since around glibc 2.1/2.2, depending on the platform.
|
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the glibc
|
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.BR mmap ()
|
|
wrapper function invokes
|
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.BR mmap2 (2)
|
|
with a suitably adjusted value for
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.IR offset .
|
|
|
|
On some hardware architectures (e.g., i386),
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.B PROT_WRITE
|
|
implies
|
|
.BR PROT_READ .
|
|
It is architecture dependent whether
|
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.B PROT_READ
|
|
implies
|
|
.B PROT_EXEC
|
|
or not.
|
|
Portable programs should always set
|
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.B PROT_EXEC
|
|
if they intend to execute code in the new mapping.
|
|
|
|
The portable way to create a mapping is to specify
|
|
.I addr
|
|
as 0 (NULL), and omit
|
|
.B MAP_FIXED
|
|
from
|
|
.IR flags .
|
|
In this case, the system chooses the address for the mapping;
|
|
the address is chosen so as not to conflict with any existing mapping,
|
|
and will not be 0.
|
|
If the
|
|
.B MAP_FIXED
|
|
flag is specified, and
|
|
.I addr
|
|
is 0 (NULL), then the mapped address will be 0 (NULL).
|
|
.SH BUGS
|
|
On Linux there are no guarantees like those suggested above under
|
|
.BR MAP_NORESERVE .
|
|
By default, any process can be killed
|
|
at any moment when the system runs out of memory.
|
|
|
|
In kernels before 2.6.7, the
|
|
.B MAP_POPULATE
|
|
flag only has effect if
|
|
.I prot
|
|
is specified as
|
|
.BR PROT_NONE .
|
|
|
|
SUSv3 specifies that
|
|
.BR mmap ()
|
|
should fail if
|
|
.I length
|
|
is 0.
|
|
However, in kernels before 2.6.12,
|
|
.BR mmap ()
|
|
succeeded in this case: no mapping was created and the call returned
|
|
.IR addr .
|
|
Since kernel 2.6.12,
|
|
.BR mmap ()
|
|
fails with the error
|
|
.B EINVAL
|
|
for this case.
|
|
.SH EXAMPLE
|
|
.\" FIXME . Add an example here that uses an anonymous shared region for
|
|
.\" IPC between parent and child.
|
|
.PP
|
|
The following program prints part of the file specified in
|
|
its first command-line argument to standard output.
|
|
The range of bytes to be printed is specified via offset and length
|
|
values in the second and third command-line arguments.
|
|
The program creates a memory mapping of the required
|
|
pages of the file and then uses
|
|
.BR write (2)
|
|
to output the desired bytes.
|
|
.nf
|
|
|
|
#include <sys/mman.h>
|
|
#include <sys/stat.h>
|
|
#include <fcntl.h>
|
|
#include <stdio.h>
|
|
#include <stdlib.h>
|
|
#include <unistd.h>
|
|
|
|
#define handle_error(msg) \\
|
|
do { perror(msg); exit(EXIT_FAILURE); } while (0)
|
|
|
|
int
|
|
main(int argc, char *argv[])
|
|
{
|
|
char *addr;
|
|
int fd;
|
|
struct stat sb;
|
|
off_t offset, pa_offset;
|
|
size_t length;
|
|
ssize_t s;
|
|
|
|
if (argc < 3 || argc > 4) {
|
|
fprintf(stderr, "%s file offset [length]\\n", argv[0]);
|
|
exit(EXIT_FAILURE);
|
|
}
|
|
|
|
fd = open(argv[1], O_RDONLY);
|
|
if (fd == \-1)
|
|
handle_error("open");
|
|
|
|
if (fstat(fd, &sb) == \-1) /* To obtain file size */
|
|
handle_error("fstat");
|
|
|
|
offset = atoi(argv[2]);
|
|
pa_offset = offset & ~(sysconf(_SC_PAGE_SIZE) \- 1);
|
|
/* offset for mmap() must be page aligned */
|
|
|
|
if (offset >= sb.st_size) {
|
|
fprintf(stderr, "offset is past end of file\\n");
|
|
exit(EXIT_FAILURE);
|
|
}
|
|
|
|
if (argc == 4) {
|
|
length = atoi(argv[3]);
|
|
if (offset + length > sb.st_size)
|
|
length = sb.st_size \- offset;
|
|
/* Can\(aqt display bytes past end of file */
|
|
|
|
} else { /* No length arg ==> display to end of file */
|
|
length = sb.st_size \- offset;
|
|
}
|
|
|
|
addr = mmap(NULL, length + offset \- pa_offset, PROT_READ,
|
|
MAP_PRIVATE, fd, pa_offset);
|
|
if (addr == MAP_FAILED)
|
|
handle_error("mmap");
|
|
|
|
s = write(STDOUT_FILENO, addr + offset \- pa_offset, length);
|
|
if (s != length) {
|
|
if (s == \-1)
|
|
handle_error("write");
|
|
|
|
fprintf(stderr, "partial write");
|
|
exit(EXIT_FAILURE);
|
|
}
|
|
|
|
exit(EXIT_SUCCESS);
|
|
} /* main */
|
|
.fi
|
|
.SH "SEE ALSO"
|
|
.BR getpagesize (2),
|
|
.BR mincore (2),
|
|
.BR mlock (2),
|
|
.BR mmap2 (2),
|
|
.BR mprotect (2),
|
|
.BR mremap (2),
|
|
.BR msync (2),
|
|
.BR remap_file_pages (2),
|
|
.BR setrlimit (2),
|
|
.BR shmat (2),
|
|
.BR shm_open (3),
|
|
.BR shm_overview (7)
|
|
.br
|
|
B.O. Gallmeister, POSIX.4, O'Reilly, pp. 128-129 and 389-391.
|
|
.\"
|
|
.\" Repeat after me: private read-only mappings are 100% equivalent to
|
|
.\" shared read-only mappings. No ifs, buts, or maybes. -- Linus
|
|
.SH COLOPHON
|
|
This page is part of release 3.24 of the Linux
|
|
.I man-pages
|
|
project.
|
|
A description of the project,
|
|
and information about reporting bugs,
|
|
can be found at
|
|
http://www.kernel.org/doc/man-pages/.
|