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NAME
mmap, munmap - map or unmap files or devices into memory
SYNOPSIS
#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.
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 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_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 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_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 of these flags are described in POSIX.1-2001 and POSIX.1-2008.
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. Deprecated.
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. The use of MAP_ANONYMOUS in conjunction
with MAP_SHARED is supported on Linux only since kernel 2.4.
MAP_DENYWRITE
This 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_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 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 this option 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.
Furthermore, this option is extremely hazardous (when used on its own), because it
forcibly removes preexisting mappings, making it easy for a multithreaded process
to corrupt its own address space.
For example, thread A looks through /proc/<pid>/maps and locates an available
address range, while thread B simultaneously acquires part or all of that same
address range. Thread A then calls mmap(MAP_FIXED), effectively overwriting the
mapping that thread B created.
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⟩.
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/vm/hugetlbpage.txt 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 vie 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). 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, 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 and POSIX.1-2008.
However, most systems also support MAP_ANONYMOUS (or its synonym MAP_ANON).
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.
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 cause of the error.
On success, munmap() returns 0. On failure, it returns -1, and errno is set to indicate
the cause of 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).
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 neither MAP_PRIVATE or MAP_SHARED, or contained both of these
values.
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.
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).
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 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).
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
┌───────────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├───────────────────┼───────────────┼─────────┤
│mmap(), munmap() │ Thread safety │ MT-Safe │
└───────────────────┴───────────────┴─────────┘
CONFORMING TO
POSIX.1-2001, POSIX.1-2008, SVr4, 4.4BSD.
AVAILABILITY
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
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).
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.
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 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.
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.
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)).
EXAMPLE
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 <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);
}
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.
COLOPHON
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