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@gmail.com>
.\"	MAP_LOCKED works from 2.5.37
.\" Modified 2004-06-17 by Michael Kerrisk <mtk.manpages@gmail.com>
.\" 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
.\" Modified 2006-12-04, mtk, various parts rewritten
.\" 2007-07-10, mtk, Added an example program.
.\"
.TH MMAP 2 2008-06-05 "Linux" "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 *" addr ", size_t " length \
", int " prot ", int " flags ,
.BI "           int " fd ", off_t " offset );
.BI "int munmap(void *" addr ", size_t " length );
.fi
.SH DESCRIPTION
.BR mmap ()
creates a new mapping in the virtual address space of
the calling process.
The starting address for the new mapping is specified in
.IR addr .
The
.I length
argument specifies the length of the mapping.

If
.I addr
is NULL,
then the kernel chooses the address at which to create the mapping;
this is the most portable method of creating a new mapping.
If
.I addr
is not NULL,
then the kernel takes it as a hint about where to place the mapping;
on Linux, the mapping will be created at the next higher page boundary.
The address of the new mapping is returned as the result of the call.

The contents of a file mapping (as opposed to an anonymous mapping; see
.B MAP_ANONYMOUS
below), are initialized using
.I length
bytes starting at offset
.I offset
in the file (or other object) referred to by the file descriptor
.IR fd .
.I offset
must be a multiple of the page size as returned by
.IR sysconf(_SC_PAGE_SIZE) .
.LP
The
.I prot
argument describes the desired memory protection of the mapping
(and must not conflict with the open mode of the file).
It is either
.B PROT_NONE
or the bitwise OR of one or more of the following 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
argument determines whether updates to the mapping
are visible to other processes mapping the same region,
and whether updates are carried through to the underlying file.
This behavior is determined by including exactly one
of the following values in
.IR flags :
.TP 1.1i
.B MAP_SHARED
Share this mapping.
Updates to the mapping are visible to other processes that map this file,
and are carried through to the underlying file.
The file may not actually be updated until
.BR msync (2)
or
.BR munmap ()
is called.
.TP
.B MAP_PRIVATE
Create a private copy-on-write mapping.
Updates to the mapping are not visible to other processes
mapping the same file, and are not carried through to
the underlying file.
It is unspecified whether changes made to the file after the
.BR mmap ()
call are visible in the mapped region.
.LP
Both of these flags are described in POSIX.1-2001.

In addition, zero or more of the following values can be ORed in
.IR flags :
.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 64-bit programs.
.TP
.B MAP_ANON
Synonym for
.BR MAP_ANONYMOUS .
Deprecated.
.TP
.B MAP_ANONYMOUS
The mapping is not backed by any file;
its contents are initialized to zero.
The
.I fd
and
.I offset
arguments are ignored;
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.
The use of
.B MAP_ANONYMOUS
in conjunction with
.B MAP_SHARED
is only supported on Linux since kernel 2.4.
.TP
.B MAP_DENYWRITE
This flag is ignored.
.\" Introduced in 1.1.36, removed in 1.3.24.
(Long ago, it signaled 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 signaled 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_FILE
Compatibility flag.
Ignored.
.\" On some systems, this was required as the opposite of
.\" MAP_ANONYMOUS -- mtk, 1 May 2007
.TP
.B MAP_FIXED
Don't interpret
.I addr
as a hint: place the mapping at exactly that address.
.I addr
must be a multiple of the page size.
If the memory region specified by
.I addr
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.
Because requiring a fixed address for a mapping is less portable,
the use of this option is discouraged.
.TP
.B MAP_GROWSDOWN
Used for stacks.
Indicates to the kernel virtual memory system that the mapping
should extend downwards in memory.
.TP
.BR MAP_LOCKED " (since Linux 2.5.37)"
Lock the pages of the mapped region into memory in the manner of
.BR mlock (2).
This flag is ignored in older kernels.
.\" If set, the mapped pages will not be swapped out.
.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.
Since Linux 2.6.23,
this flags causes
.BR MAP_POPULATE
to do nothing.
One day the combination of
.BR MAP_POPULATE
and
.BR MAP_NONBLOCK
may be re-implemented.
.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
.B 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_POPULATE " (since Linux 2.5.46)"
Populate (prefault) page tables for a mapping.
For a file mapping, this causes read-ahead on the file.
Later accesses to the mapping will not be blocked by page faults.
.BR MAP_POPULATE
is only supported for private mappings since Linux 2.6.23.
.LP
Of the above flags, only
.B MAP_FIXED
is specified in POSIX.1-2001.
However, most systems also support
.B MAP_ANONYMOUS
(or its synonym
.BR MAP_ANON ).
.LP
Some systems document the additional flags
.BR MAP_AUTOGROW ,
.BR MAP_AUTORESRV ,
.BR MAP_COPY ,
and
.BR MAP_LOCAL .
.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.
.SS munmap()
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 addr
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
.BR SIGSEGV .
It is not an error if the
indicated range does not contain any mapped pages.
.SS Timestamps changes for file-backed mappings
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 (2)
with the
.B MS_SYNC
or
.B 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,
.IR "(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 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
.RB ( 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 addr ,
.IR length ,
or
.I 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 file system 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 file system 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 "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 AVAILABILITY
On POSIX systems on which
.BR mmap (),
.BR msync (2)
and
.BR munmap ()
are available,
.B _POSIX_MAPPED_FILES
is defined in \fI<unistd.h>\fP 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 NOTES
Since kernel 2.4, this system call has been superseded by
.BR mmap2 (2).
Nowadays,
.\" Since around glibc 2.1/2.2, depending on the platform.
the glibc
.BR mmap ()
wrapper function invokes
.BR mmap2 (2)
with a suitably adjusted value for
.IR offset .

On some hardware architectures (e.g., i386),
.B PROT_WRITE
implies
.BR PROT_READ .
It is architecture dependent whether
.B PROT_READ
implies
.B PROT_EXEC
or not.
Portable programs should always set
.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 extant mapping,
and will not be 0.
If the
.B MAP_FIXED
flag is specified, and
.I addr
is 0 (NULL), then the mapped adddress 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't 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 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