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. The prot argument describes the
desired memory protection (and must not conflict with the open mode of
the file). It has bits
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 [Toc] [Back]
Create a private copy-on-write mapping. Stores to the
region do not affect the original file.
You must specify exactly one of MAP_SHARED and MAP_PRIVATE.
The above three flags are described in POSIX.1b (formerly POSIX.4).
Linux also knows about MAP_DENYWRITE, MAP_EXECUTABLE, MAP_NORESERVE,
MAP_LOCKED, MAP_GROWSDOWN and MAP_ANON(YMOUS).
offset should ordinarily be a multiple of the page size returned by
Memory mapped by mmap is preserved across fork(2), with the same
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.
On success, mmap returns a pointer to the mapped area. On error,
MAP_FAILED (-1) is returned, and errno is set appropriately. On success,
munmap returns 0, on failure -1, and errno is set (probably to
EBADF fd is not a valid file descriptor (and MAP_ANONYMOUS was not
EACCES 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.
EINVAL We don't like start or length or offset. (E.g., they are too
large, or not aligned on a PAGESIZE boundary.)
ETXTBSY [Toc] [Back]
MAP_DENYWRITE was set but the object specified by fd is open for
EAGAIN The file has been locked, or too much memory has been locked.
ENOMEM No memory is available.
Use of a mapped region can result in these signals:
SIGSEGV [Toc] [Back]
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
SVr4, POSIX.1b (formerly POSIX.4), 4.4BSD. Svr4 documents additional
error codes ENXIO and ENODEV.
getpagesize(2), msync(2), shm_open(2), B.O. Gallmeister, POSIX.4,
O'Reilly, pp. 128-129 and 389-391.
Linux 2.3.51 2000-03-25 MMAP(2)
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