On Apache/2.2.15 (CentOS)
Under GNU General Public License
2013-05-20 04:28 @127.0.0.1 CrawledBy CCBot/2.0
MMAP(2) Linux Programmer's Manual MMAP(2)
NAME
mmap, mmap64, 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);
void *mmap64(void *addr, size_t length, int prot, int flags,
int fd, off64_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 map-
ping; 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_ANONY-
MOUS 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 bit-
wise 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 fol-
lowing values in flags:
MAP_SHARED Share this mapping. Updates to the mapping are visible to other pro-
cesses 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 con-
junction 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 Docu-
mentation/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 mem-
ory is available. See also the discussion of the file /proc/sys/vm/overcom-
mit_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 implemen-
tation 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.
mmap64()
The mmap64() system call operates in exactly the same way as mmap(), except that
the final argument specifies the offset as a 64-bit off64_t. This enables appli-
cations to aceess the large files.
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 termi-
nated. 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 setr-
limit(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 pro-
grams should always set PROT_EXEC if they intend to execute code in the new map-
ping.
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 map-
ping; 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 pro-
gram 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.ker-
nel.org/doc/man-pages/.
Linux 2009-09-26 MMAP(2)
Generated by $Id: phpMan.php,v 4.55 2007/09/05 04:42:51 chedong Exp $ Author: Che Dong
On Apache/2.2.15 (CentOS)
Under GNU General Public License
2013-05-20 04:28 @127.0.0.1 CrawledBy CCBot/2.0