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);
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.
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 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_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.
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 only supported 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_ANON
Synonym for MAP_ANONYMOUS. Deprecated.
MAP_ANONYMOUS
The 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 only supported on Linux 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 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_GROWSDOWN
Used for stacks. Indicates to the kernel virtual memory system
that the mapping should extend downwards in memory.
MAP_HUGETLB (since Linux 2.6.32)
Allocate the mapping using "huge pages." See the 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: only create page tables entries 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 only had effect
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
only supported for private mappings since Linux 2.6.23.
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).
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.
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.
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. 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.
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.
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
appropriately. On success, munmap() returns 0, on failure -1, and
errno is set (probably to EINVAL).
ERRORS
EACCES 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.
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 limit on the total number of open files has been
reached.
ENODEV The underlying file system of the specified file does not
support memory mapping.
ENOMEM No memory is available, or the process’s maximum number of
mappings would have been exceeded.
EPERM The prot argument asks for PROT_EXEC but the mapped area belongs
to a file on a file system that was mounted no-exec.
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).
CONFORMING TO
SVr4, 4.4BSD, POSIX.1-2001.
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
Since kernel 2.4, this system call has been superseded by mmap2(2).
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).
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 only has effect 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.
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.
#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 */
SEE ALSO
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)
B.O. Gallmeister, POSIX.4, O’Reilly, pp. 128-129 and 389-391.
COLOPHON
This page is part of release 3.24 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/.