mmap
NAME
mmap, munmap - map or unmap files or devices into memory
SYNOPSIS
#include <sys/mman.h> I void *mmap(void * start , size_t length \ ", int " prot ", int " flags , I int fd , off_t offset ); I int munmap(void * start , size_t length );
DESCRIPTION
R mmap ()
creates a new mapping in the virtual address space of
the calling process.
The starting address for the new mapping is specified in
R start .
The
length
argument specifies the length of the mapping.
If
start
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
start
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
R fd .
offset
must be a multiple of the page size as returned by
R 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 caried through to the underlying file.
This behavior is determined by including exactly one
of the following values in
R flags :
MAP_SHARED
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
R 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
R flags :
MAP_32BIT
Put the mapping into the first 2GB of the process address space.
Ignored when
MAP_FIXED
is set.
This flag is currently only supported on x86-64 for 64bit programs.
MAP_ANON
Synonym for
R 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
R 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
R 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
start
as a hint: place the mapping at exactly that address.
start
must be a multiple of the page size.
If the memory region specified by
start
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,
R 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.
R 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.
R MAP_NONBLOCK (since Linux 2.5.46)
Only meaningful in conjunction with
R MAP_POPULATE .
Don't perform read-ahead:
only create page tables entries for pages
that are already present in RAM.
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.
R MAP_POPULATE (since Linux 2.5.46)
Populate (prefault) page tables for a file mapping,
by performing read-ahead on the file.
Later accesses to the mapping will not be blocked by page faults.
Of the above flags, only
MAP_FIXED
is specified in POSIX.1-2001.
However, most systems also support
MAP_ANONYMOUS
(or its synonym
R MAP_ANON ).
Some systems document the additional flags MAP_AUTOGROW, MAP_AUTORESRV,
MAP_COPY, and MAP_LOCAL.
Memory mapped by
R 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
R 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
start
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
R 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
R 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
R MS_ASYNC
flag, if one occurs.
RETURN VALUE
On success,
R 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,
R munmap ()
returns 0, on failure -1, and
errno
is set (probably to
R 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
R start ,
R length ,
or
R 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
R 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 filesystem 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 filesystem 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
R mmap (),
msync(2)
and
R munmap ()
are available,
_POSIX_MAPPED_FILES
is defined in <unistd.h> to a value greater than 0. (See also
sysconf(3).)
NOTES
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.
BUGS
On Linux there are no guarantees like those suggested above under
R 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
R PROT_NONE .
SUSv3 specifies that
R mmap ()
should fail if
length
is 0.
However, in kernels before 2.6.12,
R mmap ()
succeeded in this case: no mapping was created and the call returned
R start .
Since kernel 2.6.12,
R mmap ()
fails with the error
EINVAL
for this case.
SEE ALSO
getpagesize(2),
mincore(2),
mlock(2),
mmap2(2),
mremap(2),
msync(2),
remap_file_pages(2),
setrlimit(2),
shm_open(3)
B.O. Gallmeister, POSIX.4, O'Reilly, pp. 128-129 and 389-391.
B.O. Gallmeister, POSIX.4, O'Reilly, pp. 128-129 and 389-391.
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