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 (which must be greater than 0).
If addr is NULL,
then the kernel chooses the (page-aligned) 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 kernel will pick a nearby page boundary (but
always above or equal to the value specified by /proc/sys/vm/mmap_min_addr) and attempt to
create the mapping there. If another mapping already exists
there, the kernel picks a new address that may or may not
depend on the hint. 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).
After the mmap() call has
returned, the file descriptor, fd, can be closed immediately
without invalidating the mapping.
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 mapping the same region, and (in the case of file-backed mappings) are carried through to the underlying file. (To precisely control when updates are carried through to the underlying file requires the use of msync(2).)
MAP_SHARED_VALIDATE (since Linux
4.15)This flag provides the same behavior as
MAP_SHARED except that
MAP_SHARED mappings
ignore unknown flags in flags. By contrast,
when creating a mapping using MAP_SHARED_VALIDATE, the kernel
verifies all passed flags are known and fails the
mapping with the error EOPNOTSUPP for unknown flags. This
mapping type is also required to be able to use some
mapping flags (e.g., MAP_SYNC).
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 MAP_SHARED and
MAP_PRIVATE are described in
POSIX.1-2001 and POSIX.1-2008. MAP_SHARED_VALIDATE is a Linux
extension.
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
2 GB 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; provided for
compatibility with other implementations.
MAP_ANONYMOUSThe mapping is not backed by any file; its
contents are initialized to zero. The fd argument is ignored;
however, some implementations require fd to be −1 if
MAP_ANONYMOUS (or
MAP_ANON) is specified,
and portable applications should ensure this. The
offset
argument should be zero. 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—Linux 2.0 and earlier—it signaled that attempts to write to the underlying file should fail with ETXTBSY. 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 suitably
aligned: for most architectures a multiple of the
page size is sufficient; however, some architectures
may impose additional restrictions. 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.
Software that aspires to be portable should use
the MAP_FIXED flag with
care, keeping in mind that the exact layout of a
process's memory mappings is allowed to change
significantly between kernel versions, C library
versions, and operating system releases. Carefully read the discussion of this
flag in NOTES!
MAP_FIXED_NOREPLACE (since Linux
4.17)This flag provides behavior that is similar to
MAP_FIXED with respect
to the addr
enforcement, but differs in that MAP_FIXED_NOREPLACE never clobbers
a preexisting mapped range. If the requested range
would collide with an existing mapping, then this
call fails with the error EEXIST. This flag can
therefore be used as a way to atomically (with
respect to other threads) attempt to map an address
range: one thread will succeed; all others will
report failure.
Note that older kernels which do not recognize the
MAP_FIXED_NOREPLACE
flag will typically (upon detecting a collision with
a preexisting mapping) fall back to a "non-MAP_FIXED " type of
behavior: they will return an address that is
different from the requested address. Therefore,
backward-compatible software should check the
returned address against the requested address.
MAP_GROWSDOWNThis flag is used for stacks. It indicates to the
kernel virtual memory system that the mapping should
extend downward in memory. The return address is one
page lower than the memory area that is actually
created in the process's virtual address space.
Touching an address in the "guard" page below the
mapping will cause the mapping to grow by a page.
This growth can be repeated until the mapping grows
to within a page of the high end of the next lower
mapping, at which point touching the "guard" page
will result in a SIGSEGV signal.
MAP_HUGETLB (since Linux
2.6.32)Allocate the mapping using "huge" pages. See the
Linux kernel source file Documentation/admin−guide/mm/hugetlbpage.rst
for further information, as well as NOTES, below.
MAP_HUGE_2MB, MAP_HUGE_1GB (since Linux
3.8)Used in conjunction with MAP_HUGETLB to select alternative
hugetlb page sizes (respectively, 2 MB and 1 GB) on
systems that support multiple hugetlb page sizes.
More generally, the desired huge page size can be
configured by encoding the base-2 logarithm of the
desired page size in the six bits at the offset
MAP_HUGE_SHIFT. (A
value of zero in this bit field provides the default
huge page size; the default huge page size can be
discovered via the Hugepagesize field
exposed by /proc/meminfo.) Thus, the above two
constants are defined as:
#define MAP_HUGE_2MB (21 << MAP_HUGE_SHIFT) #define MAP_HUGE_1GB (30 << MAP_HUGE_SHIFT)
The range of huge page sizes that are supported by
the system can be discovered by listing the
subdirectories in /sys/kernel/mm/hugepages.
MAP_LOCKED (since Linux
2.5.37)Mark the mapped region to be locked in the same
way as mlock(2). This
implementation will try to populate (prefault) the
whole range but the mmap() call doesn't fail with
ENOMEM if this fails.
Therefore major faults might happen later on. So the
semantic is not as strong as mlock(2). One
should use mmap() plus
mlock(2) when major
faults are not acceptable after the initialization of
the mapping. The MAP_LOCKED flag is ignored in older
kernels.
MAP_NONBLOCK (since Linux
2.5.46)This flag is meaningful only 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.
This will help to reduce blocking on page faults
later. 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 on Linux. However,
by employing this flag, applications can ensure that
they transparently obtain support if the flag is
implemented in the future. Thus, it is used in the
glibc threading implementation to allow for the fact
that some architectures may (later) require special
treatment for stack allocations. A further reason to
employ this flag is portability: MAP_STACK exists (and has an
effect) on some other systems (e.g., some of the
BSDs).
MAP_SYNC (since Linux
4.15)This flag is available only with the MAP_SHARED_VALIDATE mapping type;
mappings of type MAP_SHARED will silently ignore
this flag. This flag is supported only for files
supporting DAX (direct mapping of persistent memory).
For other files, creating a mapping with this flag
results in an EOPNOTSUPP error.
Shared file mappings with this flag provide the guarantee that while some memory is mapped writable in the address space of the process, it will be visible in the same file at the same offset even after the system crashes or is rebooted. In conjunction with the use of appropriate CPU instructions, this provides users of such mappings with a more efficient way of making data modifications persistent.
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
and POSIX.1-2008. However, most systems also support
MAP_ANONYMOUS (or its synonym
MAP_ANON).
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 (but length need not be). 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.
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 to indicate the
error.
On success, munmap() returns
0. On failure, it returns −1, and errno is set to indicate the error (probably
to EINVAL).
A file descriptor refers to a non-regular file. Or a
file mapping 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).
MAP_FIXED_NOREPLACE
was specified in flags, and the range
covered by addr
and length
clashes with an existing mapping.
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 none of MAP_PRIVATE, MAP_SHARED, or MAP_SHARED_VALIDATE.
The system-wide 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.
The process's maximum number of mappings would have
been exceeded. This error can also occur for
munmap(), when unmapping
a region in the middle of an existing mapping, since
this results in two smaller mappings on either side of
the region being unmapped.
(since Linux 4.7) The process's RLIMIT_DATA limit, described in
getrlimit(2), would
have been exceeded.
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).
The prot
argument asks for PROT_EXEC but the mapped area belongs
to a file on a filesystem that was mounted no-exec.
The operation was prevented by a file seal; see fcntl(2).
MAP_DENYWRITE was set
but the object specified by fd is open for
writing.
Use of a mapped region can result in these signals:
SIGSEGVAttempted write into a region mapped as read-only.
SIGBUSAttempted access to a page of the buffer that lies beyond the end of the mapped file. For an explanation of the treatment of the bytes in the page that corresponds to the end of a mapped file that is not a multiple of the page size, see NOTES.
For an explanation of the terms used in this section, see attributes(7).
| Interface | Attribute | Value |
mmap(), munmap() |
Thread safety | MT-Safe |
POSIX.1-2001, POSIX.1-2008, SVr4, 4.4BSD.
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).)
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 bytes in the partial page at the end of the mapping are zeroed when mapped, and modifications 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.
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 suitable feature test macros
are defined (possibly by default): _DEFAULT_SOURCE with glibc 2.19 or later;
or _BSD_SOURCE or _SVID_SOURCE in glibc 2.19 and earlier.
(Employing _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.
An application can determine which pages of a mapping are currently resident in the buffer/page cache using mincore(2).
The only safe use for MAP_FIXED is where the address range
specified by addr
and length was
previously reserved using another mapping; otherwise, the
use of MAP_FIXED is hazardous
because it forcibly removes preexisting mappings, making it
easy for a multithreaded process to corrupt its own address
space.
For example, suppose that thread A looks through
/proc/<pid>/maps in
order to locate an unused address range that it can map
using MAP_FIXED, while thread
B simultaneously acquires part or all of that same address
range. When thread A subsequently employs mmap(MAP_FIXED), it will effectively clobber the mapping
that thread B created. In this scenario, thread B need not
create a mapping directly; simply making a library call
that, internally, uses dlopen(3) to load some
other shared library, will suffice. The dlopen(3) call will map
the library into the process's address space. Furthermore,
almost any library call may be implemented in a way that
adds memory mappings to the address space, either with this
technique, or by simply allocating memory. Examples include
brk(2), malloc(3), pthread_create(3), and
the PAM libraries http://www.linux-pam.org
Since Linux 4.17, a multithreaded program can use the
MAP_FIXED_NOREPLACE flag to
avoid the hazard described above when attempting to create
a mapping at a fixed address that has not been reserved by
a preexisting mapping.
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.
For mappings that employ huge pages, the requirements
for the arguments of mmap()
and munmap() differ somewhat
from the requirements for mappings that use the native
system page size.
For mmap(), offset must be a multiple of
the underlying huge page size. The system automatically
aligns length to be
a multiple of the underlying huge page size.
For munmap(), addr, and length must both be a
multiple of the underlying huge page size.
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 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(5) (for example, when using the 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);
}
munmap(addr, length + offset − pa_offset);
close(fd);
exit(EXIT_SUCCESS);
}
ftruncate(2), getpagesize(2), memfd_create(2), mincore(2), mlock(2), mmap2(2), mprotect(2), mremap(2), msync(2), remap_file_pages(2), setrlimit(2), shmat(2), userfaultfd(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 5.11 of the Linux man-pages project. A
description of the project, information about reporting bugs,
and the latest version of this page, can be found at
https://www.kernel.org/doc/man−pages/.
|
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 |