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 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: 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.
- 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 only honored 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.
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).
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¶
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).
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);
}
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 3.44 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/.