man-pages/man2/mmap.2

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.\" Modified 1997-01-31 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified 2000-03-25 by Jim Van Zandt <jrv@vanzandt.mv.com>
.\" Modified 2001-10-04 by John Levon <moz@compsoc.man.ac.uk>
.\" Modified 2003-02-02 by Andi Kleen <ak@muc.de>
.\" Modified 2003-05-21 by Michael Kerrisk <mtk-manpages@gmx.net>
.\"	MAP_LOCKED works from 2.5.37
.\" Modified 2004-06-17 by Michael Kerrisk <mtk-manpages@gmx.net>
.\" Modified 2004-09-11 by aeb
.\" Modified 2004-12-08, from Eric Estievenart <eric.estievenart@free.fr>
.\" Modified 2004-12-08, mtk, formatting tidy-ups
.\"
.TH MMAP 2 2004-12-08 "Linux 2.6.9" "Linux Programmer's Manual"
.SH NAME
mmap, munmap \- map or unmap files or devices into memory
.SH SYNOPSIS
.nf
.B #include <sys/mman.h>
.sp
.BI "void *mmap(void *" start ", size_t " length ", int " prot ", int " flags ,
.BI "           int " fd ", off_t " offset );
.sp
.BI "int munmap(void *" start ", size_t " length );
.fi
.SH DESCRIPTION
The
.BR mmap ()
function asks to map
.I length
bytes starting at offset
.I offset
from the file (or other object) specified by the file descriptor
.I fd
into memory, preferably at address
.IR start .
This latter address is a hint only, and is usually specified as 0.
The actual place where the object is mapped is returned by
.BR mmap ().
.LP
The
.I prot
argument describes the desired memory protection (and must not
conflict with the open mode of the file). It is either
.B PROT_NONE
or is the bitwise OR of one or more of the other PROT_* flags.
.TP 1.1i
.B PROT_EXEC
Pages may be executed.
.TP
.B PROT_READ
Pages may be read.
.TP
.B PROT_WRITE
Pages may be written.
.TP
.B PROT_NONE
Pages may not be accessed.
.LP
The
.I flags
parameter specifies the type of the mapped object, mapping options and
whether modifications made to the mapped copy of the page are private to
the process or are to be shared with other references.  
It has bits
.TP 1.1i
.B MAP_FIXED
Do not select a different address than the one specified.
If the memory region specified by
.I start
and
.I len
overlaps pages of any existing mapping(s), then the overlapped
part of the existing mapping(s) will be discarded.
If the specified address cannot be used,
.BR mmap ()
will fail.
If
.B MAP_FIXED 
is specified,
.I start
must be a multiple of the page size.  
Use of this option is discouraged.
.TP
.B MAP_SHARED
Share this mapping with all other processes that map this object.
Storing to the region is equivalent to writing to the file.
The file may not actually be updated until
.BR msync (2)
or
.BR munmap (2)
are called.
.TP
.B MAP_PRIVATE
Create a private copy-on-write mapping.
Stores to the region do not affect the original file.
It is unspecified whether changes made to the file after the
.BR mmap ()
call are visible in the mapped region.
.LP
You must specify exactly one of 
.B MAP_SHARED 
and 
.BR MAP_PRIVATE .
.LP
The above three flags are described in POSIX.1-2001.
Linux also knows about the following non-standard flags:
.TP
.B MAP_DENYWRITE
This flag is ignored.
.\" Introduced in 1.1.36, removed in 1.3.24.
(Long ago, it signalled that attempts to write to the underlying file
should fail with 
.BR ETXTBUSY . 
But this was a source of denial-of-service attacks.)
.TP
.B MAP_EXECUTABLE
This flag is ignored.
.\" Introduced in 1.1.38, removed in 1.3.24. Flag tested in proc_follow_link.
.\" (Long ago, it signalled that the underlying file is an executable.
.\" However, that information was not really used anywhere.)
.\" Linus talked about DOS related to MAP_EXECUTABLE, but he was thinking of
.\" MAP_DENYWRITE?
.TP
.B MAP_NORESERVE
Do not reserve swap space for this mapping. 
When swap space is reserved, one has the guarantee
that it is possible to modify the mapping.
When swap space is not reserved one might get SIGSEGV upon a write
if no physical memory is available.
See also the discussion of the file
.I /proc/sys/vm/overcommit_memory
in 
.BR proc (5).
In kernels before 2.6, this flag only had effect for 
private writable mappings.
.TP
.BR MAP_LOCKED " (since Linux 2.5.37)"
Lock the pages of the mapped region into memory in the manner of
.BR mlock () .
This flag is ignored in older kernels.
.\" If set, the mapped pages will not be swapped out.
.TP
.B MAP_GROWSDOWN
Used for stacks. Indicates to the kernel VM system that the mapping
should extend downwards in memory.
.TP
.B MAP_ANONYMOUS
The mapping is not backed by any file; the
.I fd
and
.I offset
arguments are ignored.  
The use of this flag in conjunction with 
.B MAP_SHARED
is only supported on Linux since kernel 2.4.
.TP
.B MAP_ANON
Alias for 
.BR MAP_ANONYMOUS . 
Deprecated.
.TP
.B MAP_FILE
Compatibility flag. Ignored.
.TP
.B MAP_32BIT
Put the mapping into the first 2GB of the process address space.
Ignored when
.B MAP_FIXED
is set. 
This flag is currently only supported on x86-64 for 64bit programs.
.TP
.BR MAP_POPULATE " (since Linux 2.5.46)"
Populate (prefault) page tables for a file mapping, 
by performing read-ahead on the file.
Later accesses to the mapping will not be blocked by page faults.
.TP
.BR MAP_NONBLOCK " (since Linux 2.5.46)"
Only meaningful in conjunction with
.BR MAP_POPULATE .
Don't perform read-ahead: 
only create page tables entries for pages 
that are already present in RAM.
.LP
Some systems document the additional flags MAP_AUTOGROW, MAP_AUTORESRV,
MAP_COPY, and MAP_LOCAL.
.LP
.I fd
should be a valid file descriptor, unless 
.B MAP_ANONYMOUS 
is set.
If 
.B MAP_ANONYMOUS 
is set, then 
.I fd
is ignored on Linux.
However, some implementations require 
.I fd
to be \-1 if 
.B MAP_ANONYMOUS 
(or 
.BR MAP_ANON ) 
is specified,
and portable applications should ensure this.
.LP
.I offset
should be a multiple of the page size as returned by
.BR getpagesize (2).
.LP
Memory mapped by
.BR mmap ()
is preserved across
.BR fork (2),
with the same attributes.
.LP
A file is mapped in multiples of the page size. For a file that is not
a multiple of the page size, the remaining memory is zeroed when mapped,
and writes to that region are not written out to the file. The effect of
changing the size of the underlying file of a mapping on the pages that
correspond to added or removed regions of the file is unspecified.

The
.BR munmap ()
system call deletes the mappings for the specified address range, and
causes further references to addresses within the range to generate
invalid memory references.  The region is also automatically unmapped
when the process is terminated.  On the other hand, closing the file
descriptor does not unmap the region.
.LP
The address
.I start
must be a multiple of the page size. All pages containing a part
of the indicated range are unmapped, and subsequent references
to these pages will generate SIGSEGV. It is not an error if the
indicated range does not contain any mapped pages.

For file-backed mappings, the
.I st_atime
field for the mapped file may be updated at any time between the
.BR mmap ()
and the corresponding unmapping; the first reference to a mapped
page will update the field if it has not been already.
.LP
The
.I st_ctime
and
.I st_mtime
field for a file mapped with 
.B PROT_WRITE 
and 
.B MAP_SHARED 
will be updated after
a write to the mapped region, and before a subsequent
.BR msync ()
with the 
.B MS_SYNC 
or 
.BR MS_ASYNC 
flag, if one occurs.
.SH "RETURN VALUE"
On success,
.BR mmap ()
returns a pointer to the mapped area.
On error, the value
.B MAP_FAILED
(that is, (void *) \-1) is returned, and
.I errno
is set appropriately.
On success,
.BR munmap ()
returns 0, on failure \-1, and
.I errno
is set (probably to 
.BR EINVAL ).
.SH NOTES
It is architecture dependent whether
.B PROT_READ
includes
.B PROT_EXEC
or not. Portable programs should always set
.B PROT_EXEC
if they intend to execute code in the new mapping.
.SH ERRORS
.TP
.B EACCES
A file descriptor refers to a non-regular file.
Or 
.B MAP_PRIVATE 
was requested, but
.I fd
is not open for reading.
Or 
.B MAP_SHARED 
was requested and 
.B PROT_WRITE 
is set, but
.I fd
is not open in read/write (O_RDWR) mode.
Or 
.B PROT_WRITE 
is set, but the file is append-only.
.TP
.B EAGAIN
The file has been locked, or too much memory has been locked (see
.BR setrlimit (2)).
.TP
.B EBADF
.I fd
is not a valid file descriptor (and 
.B MAP_ANONYMOUS
was not set).
.TP
.B EINVAL
We don't like
.IR start ,
.IR length ,
or
.IR offset
(e.g., they are too large, or not aligned on a page boundary).
.TP
.B EINVAL
(since Linux 2.6.12),
.I length
was 0.
.TP
.B EINVAL
.I flags
contained neither
.B MAP_PRIVATE
or
.BR MAP_SHARED ,
or contained both of these values.
.TP
.B ENFILE
.\" This is for shared anonymous segments
.\" [2.6.7] shmem_zero_setup()-->shmem_file_setup()-->get_empty_filp()
The system limit on the total number of open files has been reached.
.\" .TP
.\" .B ENOEXEC
.\" A file could not be mapped for reading.
.TP
.B ENODEV
The underlying filesystem of the specified file does not support
memory mapping.
.TP
.B ENOMEM
No memory is available, or the process's maximum number of mappings would
have been exceeded.
.TP
.B EPERM
The
.I prot
argument asks for
.B PROT_EXEC
but the mapped area belongs to a file on a filesystem that
was mounted no-exec.
.\" (Since 2.4.25 / 2.6.0.)
.TP
.B ETXTBSY
.B MAP_DENYWRITE 
was set but the object specified by
.I fd
is open for writing.
.LP
Use of a mapped region can result in these signals:
.TP
.B SIGSEGV
Attempted write into a region mapped as read-only.
.TP
.B SIGBUS
Attempted access to a portion of the buffer that does not correspond
to the file (for example, beyond the end of the file, including the
case where another process has truncated the file).
.SH AVAILABILITY
On POSIX systems on which
.BR mmap (),
.BR msync ()
and
.BR munmap ()
are available,
.B _POSIX_MAPPED_FILES
is defined in <unistd.h> to a value greater than 0. (See also
.BR sysconf (3).)
.\" POSIX.1-2001: It shall be defined to -1 or 0 or 200112L.
.\" -1: unavailable, 0: ask using sysconf().
.\" glibc defines it to 1.
.SH "CONFORMING TO"
SVr4, 4.4BSD, POSIX.1-2001.
.\" SVr4 documents additional error codes ENXIO and ENODEV.
.\" SUSv2 documents additional error codes EMFILE and EOVERFLOW.
.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 start .
Since kernel 2.6.12,
.BR mmap ()
fails with the error
.B EINVAL
for this case.
.SH "SEE ALSO"
.BR getpagesize (2),
.BR mincore (2),
.BR mlock (2),
.BR mmap2 (2),
.BR mremap (2),
.BR msync (2),
.BR remap_file_pages (2),
.BR setrlimit (2),
.BR shm_open (3)
.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