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 the next higher 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_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 re-implemented.
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;
/* Canaqt 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 */
This page is part of release 3.14 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/.
