mmap, munmap - map or unmap files or devices into memory
SYNOPSIS
#include <sys/mman.h>
void * mmap(void * start , size_t length , int prot , int flags , int fd , off_t offset );
int munmap(void * start , size_t length );
DESCRIPTION
The
mmap function asks to map
length bytes starting at offset
offset from the file (or other object) specified by the file descriptor
fd into memory, preferably at address
start . This latter address is a hint only, and is usually specified as 0.
The actual place where the object is mapped is returned by
mmap , and is never 0.
The
prot argument describes the desired memory protection (and must not
conflict with the open mode of the file). It is either
PROT_NONE or is the bitwise OR of one or more of the other PROT_* 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 parameter specifies the type of the mapped object, mapping options and
whether modifications made to the mapped copy of the page are private to
the process or are to be shared with other references. It has bits
MAP_FIXED
Do not select a different address than the one specified.
If the specified address cannot be used,
mmap will fail. If MAP_FIXED is specified,
start must be a multiple of the pagesize. Use of this option is discouraged.
MAP_SHARED
Share this mapping with all other processes that map this object.
Storing to the region is equivalent to writing to the file.
The file may not actually be updated until
msync(2) or
munmap(2) are called.
MAP_PRIVATE
Create a private copy-on-write mapping.
Stores to the region do not affect the original file.
It is unspecified whether changes made to the file after the
mmap call are visible in the mapped region.
You must specify exactly one of MAP_SHARED and MAP_PRIVATE.
The above three flags are described in POSIX.1b (formerly POSIX.4) and SUSv2.
Linux also knows about the following non-standard flags:
MAP_DENYWRITE
This flag is ignored.
(Long ago, it signalled 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_NORESERVE
(Used together with MAP_PRIVATE.) Do not reserve swap space pages for
this mapping. When swap space is reserved, one has the guarantee
that it is possible to modify this private copy-on-write region.
When it is not reserved one might get SIGSEGV upon a write
when no memory is available.
MAP_LOCKED
(Linux 2.5.37 and later) Lock the pages of the mapped region into
memory in the manner of
mlock() . This flag is ignored in older kernels.
MAP_GROWSDOWN
Used for stacks. Indicates to the kernel VM system that the mapping
should extend downwards in memory.
MAP_ANONYMOUS
The mapping is not backed by any file; the
fd and
offset arguments are ignored. This flag in conjunction with MAP_SHARED
is implemented since Linux 2.4.
MAP_ANON
Alias for MAP_ANONYMOUS. Deprecated.
MAP_FILE
Compatibility flag. Ignored.
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.
Some systems document the additional flags MAP_AUTOGROW, MAP_AUTORESRV,
MAP_COPY, and MAP_LOCAL.
fd should be a valid file descriptor, unless MAP_ANONYMOUS is set,
in which case the argument is ignored.
offset should be a multiple of the page size as returned by
getpagesize(2) . 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
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 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() 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).
NOTES
It is architecture dependent whether
PROT_READ includes
PROT_EXEC or not. Portable programs should always set
PROT_EXEC if they intend to execute code in the new mapping.
ERRORS
EBADF
fd is not a valid file descriptor (and MAP_ANONYMOUS was not set).
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.
EINVAL
We don't like
start or
length or
offset . (E.g., they are too large, or not aligned on a PAGESIZE boundary.)
ETXTBSY
MAP_DENYWRITE was set but the object specified by
fd is open for writing.
EAGAIN
The file has been locked, or too much memory has been locked.
ENOMEM
No memory is available, or the process's maximum number of mappings would
have been exceeded.
ENODEV
The underlying filesystem of the specified file does not support
memory mapping.
Use of a mapped region can result in these signals:
SIGSEGV
Attempted write into a region specified to mmap 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).
AVAILABILITY
On POSIX systems on which
mmap , msync and
munmap are available,
_POSIX_MAPPED_FILES is defined in <unistd.h> to a value greater than 0. (See also
sysconf(3) .)
CONFORMING TO
SVr4, POSIX.1b (formerly POSIX.4), 4.4BSD, SUSv2.
SVr4 documents additional error codes ENXIO and ENODEV.
SUSv2 documents additional error codes EMFILE and EOVERFLOW.
MAP_32BIT is a Linux extension.
