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mmap, munmap - map or unmap files or devices into memory
void * mmap(void *start, size_t length, int prot, int flags, int fd,
int munmap(void *start, size_t length);
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 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, 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.
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:
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.)
This flag is ignored.
(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 (since Linux 2.5.37)
Lock the pages of the mapped region into memory in the manner of
mlock(). This flag is ignored in older kernels.
Used for stacks. Indicates to the kernel VM system that the
mapping should extend downwards in memory.
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.
Alias for MAP_ANONYMOUS. Deprecated.
Compatibility flag. Ignored.
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_POPULATE (since Linux 2.5.46)
Populate (prefault) pagetables.
MAP_NONBLOCK (since Linux 2.5.46)
Do not block on IO.
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
Memory mapped by mmap() is preserved across fork(2), with the same
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
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
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).
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.
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
EAGAIN The file has been locked, or too much memory has been locked
EBADF fd is not a valid file descriptor (and MAP_ANONYMOUS was not
EINVAL We don’t like start or length or offset. (E.g., they are too
large, or not aligned on a PAGESIZE boundary.)
ENFILE The system limit on the total number of open files has been
ENODEV The underlying filesystem of the specified file does not support
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.
MAP_DENYWRITE was set but the object specified by fd is open for
Use of a mapped region can result in these signals:
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
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).)
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.
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.
getpagesize(2), mlock(2), mmap2(2), mremap(2), msync(2), setrlimit(2),
B.O. Gallmeister, POSIX.4, O’Reilly, pp. 128-129 and 389-391.