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MMAP(2) Linux Programmer's Manual MMAP(2)
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_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 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_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 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 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 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_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. 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).
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.
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).
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).
SVr4, 4.4BSD, POSIX.1-2001.
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 3.51 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/.
Linux 2013-04-17 MMAP(2)
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