mmap(2) System Calls Manual mmap(2)
NAME
mmap, munmap - map or unmap files or devices into memory
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <sys/mman.h>
void *mmap(size_t length;
void addr[length], size_t length, int prot, int flags,
int fd, off_t offset);
int munmap(size_t length;
void addr[length], size_t length);
See VERSIONS for information on feature test macro requirements.
DESCRIPTION
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_EXEC Pages may be executed.
PROT_READ Pages may be read.
PROT_WRITE Pages may be written.
PROT_NONE Pages may not be accessed.
The flags argument
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_SHARED
Share 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_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 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_ANON
Synonym for MAP_ANONYMOUS; provided for compatibility with other
implementations.
MAP_ANONYMOUS
The 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. Support for
MAP_ANONYMOUS in conjunction with MAP_SHARED was added in Linux
2.4.
MAP_DENYWRITE
This 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_EXECUTABLE
This flag is ignored.
MAP_FILE
Compatibility flag. Ignored.
MAP_FIXED
Don'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 length 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 Linux 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_GROWSDOWN
This 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_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 /proc/sys/vm/overcommit_memory
in proc(5). Before Linux 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. The mmap() call doesn't
fail if the mapping cannot be populated (for example, due to
limitations on the number of mapped huge pages when using
MAP_HUGETLB). Support for MAP_POPULATE in conjunction with
private mappings was added in 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).
munmap()
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.
RETURN VALUE
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).
ERRORS
EACCES 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.
EAGAIN The file has been locked, or too much memory has been locked
(see setrlimit(2)).
EBADF fd is not a valid file descriptor (and MAP_ANONYMOUS was not
set).
EEXIST MAP_FIXED_NOREPLACE was specified in flags, and the range
covered by addr and length clashes with an existing mapping.
EINVAL We don't like addr, length, or offset (e.g., they are too large,
or not aligned on a page boundary).
EINVAL (since Linux 2.6.12) length was 0.
EINVAL flags contained none of MAP_PRIVATE, MAP_SHARED, or
MAP_SHARED_VALIDATE.
ENFILE The system-wide limit on the total number of open files has been
reached.
ENODEV The underlying filesystem of the specified file does not support
memory mapping.
ENOMEM No memory is available.
ENOMEM 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.
ENOMEM (since Linux 4.7) The process's RLIMIT_DATA limit, described in
getrlimit(2), would have been exceeded.
ENOMEM We don't like addr, because it exceeds the virtual address space
of the CPU.
EOVERFLOW
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).
EPERM The prot argument asks for PROT_EXEC but the mapped area belongs
to a file on a filesystem that was mounted no-exec.
EPERM The operation was prevented by a file seal; see fcntl(2).
EPERM The MAP_HUGETLB flag was specified, but the caller was not
privileged (did not have the CAP_IPC_LOCK capability) and is not
a member of the sysctl_hugetlb_shm_group group; see the
description of /proc/sys/vm/sysctl_hugetlb_shm_group in
proc_sys(5).
ETXTBSY
MAP_DENYWRITE was set but the object specified by fd is open for
writing.
Use of a mapped region can result in these signals:
SIGSEGV
Attempted write into a region mapped as read-only.
SIGBUS Attempted 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.
ATTRIBUTES
For an explanation of the terms used in this section, see
attributes(7).
+--------------------------------------------+---------------+---------+
|Interface | Attribute | Value |
+--------------------------------------------+---------------+---------+
|mmap (), munmap () | Thread safety | MT-Safe |
+--------------------------------------------+---------------+---------+
VERSIONS
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.
C library/kernel differences
This page describes the interface provided by the glibc mmap() wrapper
function. Originally, this function invoked a system call of the same
name. Since Linux 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.
STANDARDS
POSIX.1-2008.
HISTORY
POSIX.1-2001, 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).)
NOTES
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.
An application can determine which pages of a mapping are currently
resident in the buffer/page cache using mincore(2).
Using MAP_FIXED safely
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.
Timestamps changes for file-backed mappings
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.
Huge page (Huge TLB) mappings
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.
CAVEATS
Unlike typical malloc(3) implementations, mmap() does not prevent
creating objects larger than PTRDIFF_MAX. Objects that are larger than
PTRDIFF_MAX only work in limited ways in C (in particular, pointer
subtraction results in undefined behavior if the result would be bigger
than PTRDIFF_MAX). On top of that, GCC also assumes that no object is
bigger than PTRDIFF_MAX. PTRDIFF_MAX is usually half of the address
space size; so for 32-bit processes, it is usually 0x7fffffff (almost 2
GiB).
BUGS
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.
Before Linux 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,
before Linux 2.6.12, mmap() succeeded in this case: no mapping was
created and the call returned addr. Since Linux 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)).
EXAMPLES
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.
Program source
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>
#define handle_error(msg) \
do { perror(msg); exit(EXIT_FAILURE); } while (0)
int
main(int argc, char *argv[])
{
int fd;
char *addr;
off_t offset, pa_offset;
size_t length;
ssize_t s;
struct stat sb;
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);
}
SEE ALSO
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.
Linux man-pages 6.14 2025-05-06 mmap(2)