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mmap, munmap — map or unmap files or devices into memory
#include <sys/mman.h>
void
*mmap( |
void *addr, |
| size_t length, | |
| int prot, | |
| int flags, | |
| int fd, | |
off_t offset); |
int
munmap( |
void *addr, |
size_t length); |
See NOTES for information on feature test macro requirements.
mmap() creates a new mapping
in the virtual address space of the calling process. The
starting address for the new mapping is specified in
addr. The length argument specifies the
length of the mapping.
If 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 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 a
nearby 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 MAP_ANONYMOUS
below), are initialized using length bytes starting at offset
offset in the file
(or other object) referred to by the file descriptor
fd. offset must be a multiple of
the page size as returned by sysconf(_SC_PAGE_SIZE).
The prot argument
describes the desired memory protection of the mapping (and
must not conflict with the open mode of the file). It is
either PROT_NONE or the bitwise
OR of one or more of the following flags:
PROT_EXECPages may be executed.
PROT_READPages may be read.
PROT_WRITEPages may be written.
PROT_NONEPages may not be accessed.
The 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 flags:
MAP_SHAREDShare 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 msync(2) or
munmap() is called.
MAP_PRIVATECreate 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 mmap() call are visible in the mapped
region.
Both of these flags are described in POSIX.1-2001.
In addition, zero or more of the following values can be
ORed in flags:
MAP_32BIT (since Linux 2.4.20,
2.6)Put the mapping into the first 2 Gigabytes of the
process address space. This flag is supported only on
x86-64, for 64-bit programs. It was added to allow
thread stacks to be allocated somewhere in the first
2GB of memory, so as to improve context-switch
performance on some early 64-bit processors. Modern
x86-64 processors no longer have this performance
problem, so use of this flag is not required on those
systems. The MAP_32BIT
flag is ignored when MAP_FIXED is set.
MAP_ANONSynonym for MAP_ANONYMOUS. Deprecated.
MAP_ANONYMOUSThe mapping is not backed by any file; its contents
are initialized to zero. The fd and offset arguments are
ignored; however, some implementations require
fd to be
−1 if MAP_ANONYMOUS
(or MAP_ANON) is
specified, and portable applications should ensure
this. The use of MAP_ANONYMOUS in conjunction with
MAP_SHARED is supported
on Linux only since kernel 2.4.
MAP_DENYWRITEThis flag is ignored. (Long ago, it signaled that
attempts to write to the underlying file should fail
with ETXTBUSY. But this
was a source of denial-of-service attacks.)
MAP_EXECUTABLEThis flag is ignored.
MAP_FILECompatibility flag. Ignored.
MAP_FIXEDDon't interpret addr as a hint: place the
mapping at exactly that address. addr must be a multiple
of the page size. If the memory region specified by
addr and
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, mmap() will fail. Because requiring a
fixed address for a mapping is less portable, the use
of this option is discouraged.
MAP_GROWSDOWNUsed for stacks. Indicates to the kernel virtual memory system that the mapping should extend downward in memory.
MAP_HUGETLB (since Linux
2.6.32)Allocate the mapping using "huge pages." See the
Linux kernel source file Documentation/vm/hugetlbpage.txt for
further information.
MAP_LOCKED (since Linux
2.5.37)Lock the pages of the mapped region into memory in the manner of mlock(2). This flag is ignored in older kernels.
MAP_NONBLOCK (since Linux
2.5.46)Only meaningful in conjunction with MAP_POPULATE. Don't perform
read-ahead: create page tables entries only for pages
that are already present in RAM. Since Linux 2.6.23,
this flag causes MAP_POPULATE to do nothing. One day
the combination of MAP_POPULATE and MAP_NONBLOCK may be
reimplemented.
MAP_NORESERVEDo 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 /proc/sys/vm/overcommit_memory in
proc(5). In kernels
before 2.6, this flag had effect only for private
writable mappings.
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. MAP_POPULATE is
supported for private mappings only since Linux
2.6.23.
MAP_STACK (since Linux
2.6.27)Allocate the mapping at an address suitable for a process or thread stack. This flag is currently a no-op, but is used in the glibc threading implementation so that if some architectures require special treatment for stack allocations, support can later be transparently implemented for glibc.
MAP_UNINITIALIZED (since Linux
2.6.33)Don't clear anonymous pages. This flag is intended
to improve performance on embedded devices. This flag
is honored only if the kernel was configured with the
CONFIG_MMAP_ALLOW_UNINITIALIZED
option. Because of the security implications, that
option is normally enabled only on embedded devices
(i.e., devices where one has complete control of the
contents of user memory).
Of the above flags, only MAP_FIXED is specified in POSIX.1-2001.
However, most systems also support MAP_ANONYMOUS (or its synonym MAP_ANON).
Some systems document the additional flags MAP_AUTOGROW, MAP_AUTORESRV, MAP_COPY, and MAP_LOCAL.
Memory mapped by mmap() is
preserved across fork(2), with the same
attributes.
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 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.
The address 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 SIGSEGV. It is not an error if the
indicated range does not contain any mapped pages.
For file-backed mappings, the st_atime field for the
mapped file may be updated at any time between the
mmap() and the corresponding
unmapping; the first reference to a mapped page will update
the field if it has not been already.
The st_ctime
and st_mtime
field for a file mapped with PROT_WRITE and MAP_SHARED will be updated after a write
to the mapped region, and before a subsequent msync(2) with the
MS_SYNC or MS_ASYNC flag, if one occurs.
On success, mmap() returns a
pointer to the mapped area. On error, the value MAP_FAILED (that is, (void *) −1) is returned,
and errno is set appropriately.
On success, munmap() returns 0,
on failure −1, and errno
is set (probably to EINVAL).
A file descriptor refers to a non-regular file. Or
MAP_PRIVATE was
requested, but fd is not open for
reading. Or MAP_SHARED
was requested and PROT_WRITE is set, but fd is not open in
read/write (O_RDWR) mode.
Or PROT_WRITE is set, but
the file is append-only.
The file has been locked, or too much memory has been locked (see setrlimit(2)).
fd is not a
valid file descriptor (and MAP_ANONYMOUS was not set).
We don't like addr, length, or offset (e.g., they are
too large, or not aligned on a page boundary).
(since Linux 2.6.12) length was 0.
flags
contained neither MAP_PRIVATE or MAP_SHARED, or contained both of
these values.
The system limit on the total number of open files has been reached.
The underlying filesystem of the specified file does not support memory mapping.
No memory is available, or the process's maximum number of mappings would have been exceeded.
The prot
argument asks for PROT_EXEC but the mapped area belongs
to a file on a filesystem that was mounted no-exec.
MAP_DENYWRITE was set
but the object specified by fd is open for
writing.
On 32-bit architecture together with the large file
extension (i.e., using 64-bit off_t): the number of pages used for
length plus
number of pages used for offset would overflow
unsigned long (32 bits).
Use of a mapped region can result in these signals:
SIGSEGVAttempted write into a region mapped as read-only.
SIGBUSAttempted 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).
On POSIX systems on which mmap(), msync(2) and munmap() are available, _POSIX_MAPPED_FILES is defined in
<unistd.h>
to a value greater than 0. (See also sysconf(3).)
This page describes the interface provided by the glibc
mmap() wrapper function.
Originally, this function invoked a system call of the same
name. Since kernel 2.4, that system call has been superseded
by mmap2(2), and nowadays the
glibc mmap() wrapper function
invokes mmap2(2) with a suitably
adjusted value for offset.
On some hardware architectures (e.g., i386), PROT_WRITE implies PROT_READ. It is architecture dependent
whether PROT_READ implies
PROT_EXEC or not. Portable
programs should always set PROT_EXEC if they intend to execute code in
the new mapping.
The portable way to create a mapping is to specify
addr as 0 (NULL), and
omit MAP_FIXED from 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 MAP_FIXED flag is
specified, and addr
is 0 (NULL), then the mapped address will be 0 (NULL).
Certain flags
constants are defined only if either _BSD_SOURCE or _SVID_SOURCE is defined. (Requiring
_GNU_SOURCE also suffices, and
requiring that macro specifically would have been more
logical, since these flags are all Linux specific.) The
relevant flags are: MAP_32BIT,
MAP_ANONYMOUS (and the synonym
MAP_ANON), MAP_DENYWRITE, MAP_EXECUTABLE, MAP_FILE, MAP_GROWSDOWN, MAP_HUGETLB, MAP_LOCKED, MAP_NONBLOCK, MAP_NORESERVE, MAP_POPULATE, and MAP_STACK.
On Linux there are no guarantees like those suggested
above under 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 MAP_POPULATE flag has effect only if
prot is specified as
PROT_NONE.
SUSv3 specifies that mmap()
should fail if length
is 0. However, in kernels before 2.6.12, mmap() succeeded in this case: no mapping
was created and the call returned addr. Since kernel 2.6.12,
mmap() fails with the error
EINVAL for this case.
POSIX specifies that the system shall always zero fill any partial page at the end of the object and that system will never write any modification of the object beyond its end. On Linux, when you write data to such partial page after the end of the object, the data stays in the page cache even after the file is closed and unmapped and even though the data is never written to the file itself, subsequent mappings may see the modified content. In some cases, this could be fixed by calling msync(2) before the unmap takes place; however, this doesn't work on tmpfs (for example, when using POSIX shared memory interface documented in shm_overview(7)).
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 write(2) to output the desired bytes.
#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);
}
getpagesize(2), mincore(2), mlock(2), mmap2(2), mprotect(2), mremap(2), msync(2), remap_file_pages(2), setrlimit(2), shmat(2), shm_open(3), shm_overview(7)
The descriptions of the following files in proc(5): /proc/[pid]/maps, /proc/[pid]/map_files, and /proc/[pid]/smaps.
B.O. Gallmeister, POSIX.4, O'Reilly, pp. 128-129 and 389-391.
This page is part of release 3.54 of the Linux man-pages project. A
description of the project, and information about reporting
bugs, can be found at
http://www.kernel.org/doc/man−pages/.
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Copyright (C) 1996 Andries Brouwer <aebcwi.nl> and Copyright (C) 2006, 2007 Michael Kerrisk <mtk.manpagesgmail.com> %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. %%%LICENSE_END Modified 1997-01-31 by Eric S. Raymond <esrthyrsus.com> Modified 2000-03-25 by Jim Van Zandt <jrvvanzandt.mv.com> Modified 2001-10-04 by John Levon <mozcompsoc.man.ac.uk> Modified 2003-02-02 by Andi Kleen <akmuc.de> Modified 2003-05-21 by Michael Kerrisk <mtk.manpagesgmail.com> MAP_LOCKED works from 2.5.37 Modified 2004-06-17 by Michael Kerrisk <mtk.manpagesgmail.com> Modified 2004-09-11 by aeb Modified 2004-12-08, from Eric Estievenart <eric.estievenartfree.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. 2008-11-18, mtk, document MAP_STACK |