mmap, munmap - map or unmap files or devices into memory
void * mmap(void *start, size_t length, int prot , int
flags, int fd, off_t offset);
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 speci
fied, 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
The above three flags are described in POSIX.1b (formerly
POSIX.4). Linux also knows about MAP_DENYWRITE, MAP_EXE
CUTABLE, MAP_NORESERVE, MAP_LOCKED, MAP_GROWSDOWN and
offset should ordinarily be a multiple of the page size
returned by getpagesize(2).
The munmap system call deletes the mappings for the speci
fied address range, and causes further references to
addresses within the range to generate invalid memory ref
erences. 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 EINVAL).
EBADF fd is not a valid file descriptor (and MAP_ANONY
MOUS was not set).
EACCES MAP_PRIVATE was asked, but fd is not open for read
ing. Or MAP_SHARED was asked and PROT_WRITE is
set, 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
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
ENOMEM No memory is available.
Use of a mapped region can result in these signals:
Attempted write into a region specified to mmap as
SIGBUS Attempted access to a portion of the buffer that
where another process has truncated the file).
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.