NAME
mmap, munmap - map or unmap files or devices into memory
SYNOPSIS
#include <unistd.h>
#include <sys/mman.h>
#ifdef _POSIX_MAPPED_FILES
void * mmap(void *start, size_t length, int prot , int
flags, int fd, off_t offset
int munmap(void *start, size_t length));
#endif
DESCRIPTION
The mmap function asks to map length bytes starting at
offset offset from the file (or other object) specified by
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.
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
MAP_PRIVATE
Create a private copy-on-write mapping.
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 and MAP_ANON(YMOUS).
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 refer-
ences.
RETURN VALUE
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).
ERRORS
EBADF
fd is not a valid file descriptor (and MAP_ANONYMOUS
was not set).
EACCES
MAP_PRIVATE was asked, but fd is not open for reading.
Or MAP_SHARED was asked and PROT_WRITE is set, fd is
not open for writing.
EINVAL
We don't like start or length or offset. (E.g., they
are too large, or not aligned on a PAGESIZE boundary.)
ETXTBUSY
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.
CONFORMING TO
SVr4, POSIX.1b (formerly POSIX.4), 4.4BSD. Svr4 documents
additional error codes ENXIO and ENODEV.
SEE ALSO
getpagesize(2), msync(2), shm_open(2), B.O. Gallmeister,
POSIX.4, O'Reilly, pp. 128-129 and 389-391.