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MMAP(2)			    BSD	System Calls Manual		       MMAP(2)

     mmap -- allocate memory, or map files or devices into memory

     Standard C	Library	(libc, -lc)

     #include <sys/mman.h>

     void *
     mmap(void *addr, size_t len, int prot, int	flags, int fd, off_t offset);

     The mmap()	system call causes the pages starting at addr and continuing
     for at most len bytes to be mapped	from the object	described by fd,
     starting at byte offset offset.  If len is	not a multiple of the page-
     size, the mapped region may extend	past the specified range.  Any such
     extension beyond the end of the mapped object will	be zero-filled.

     If	addr is	non-zero, it is	used as	a hint to the system.  (As a conve-
     nience to the system, the actual address of the region may	differ from
     the address supplied.)  If	addr is	zero, an address will be selected by
     the system.  The actual starting address of the region is returned.  A
     successful	mmap deletes any previous mapping in the allocated address

     The protections (region accessibility) are	specified in the prot argument
     by	or'ing the following values:

     PROT_NONE	 Pages may not be accessed.
     PROT_READ	 Pages may be read.
     PROT_WRITE	 Pages may be written.
     PROT_EXEC	 Pages may be executed.

     The flags argument	specifies the type of the mapped object, mapping op-
     tions and whether modifications made to the mapped	copy of	the page are
     private to	the process or are to be shared	with other references.	Shar-
     ing, mapping type and options are specified in the	flags argument by
     or'ing the	following values:

     MAP_ANON	       Map anonymous memory not	associated with	any specific
		       file.  The file descriptor used for creating MAP_ANON
		       must be -1.  The	offset argument	is ignored.

     MAP_FIXED	       Do not permit the system	to select a different address
		       than the	one specified.	If the specified address can-
		       not be used, mmap() will	fail.  If MAP_FIXED is speci-
		       fied, addr must be a multiple of	the pagesize.  If a
		       MAP_FIXED request is successful,	the mapping estab-
		       lished by mmap()	replaces any previous mappings for the
		       process'	pages in the range from	addr to	addr + len.
		       Use of this option is discouraged.

     MAP_HASSEMAPHORE  Notify the kernel that the region may contain sema-
		       phores and that special handling	may be necessary.

     MAP_INHERIT       This flag never operated	as advertised and is no	longer
		       supported.  Please refer	to minherit(2) for further in-

     MAP_NOCORE	       Region is not included in a core	file.

     MAP_NOSYNC	       Causes data dirtied via this VM map to be flushed to
		       physical	media only when	necessary (usually by the
		       pager) rather than gratuitously.	 Typically this	pre-
		       vents the update	daemons	from flushing pages dirtied
		       through such maps and thus allows efficient sharing of
		       memory across unassociated processes using a file-
		       backed shared memory map.  Without this option any VM
		       pages you dirty may be flushed to disk every so often
		       (every 30-60 seconds usually) which can create perfor-
		       mance problems if you do	not need that to occur (such
		       as when you are using shared file-backed	mmap regions
		       for IPC purposes).  Note	that VM/file system coherency
		       is maintained whether you use MAP_NOSYNC	or not.	 This
		       option is not portable across UNIX platforms (yet),
		       though some may implement the same behavior by default.

		       WARNING!	 Extending a file with ftruncate(2), thus cre-
		       ating a big hole, and then filling the hole by modify-
		       ing a shared mmap() can lead to severe file fragmenta-
		       tion.  In order to avoid	such fragmentation you should
		       always pre-allocate the file's backing store by
		       write()ing zero's into the newly	extended area prior to
		       modifying the area via your mmap().  The	fragmentation
		       problem is especially sensitive to MAP_NOSYNC pages,
		       because pages may be flushed to disk in a totally ran-
		       dom order.

		       The same	applies	when using MAP_NOSYNC to implement a
		       file-based shared memory	store.	It is recommended that
		       you create the backing store by write()ing zero's to
		       the backing file	rather than ftruncate()ing it.	You
		       can test	file fragmentation by observing	the KB/t
		       (kilobytes per transfer)	results	from an	"iostat	1"
		       while reading a large file sequentially,	e.g. using "dd
		       if=filename of=/dev/null	bs=32k".

		       The fsync(2) system call	will flush all dirty data and
		       metadata	associated with	a file,	including dirty	NOSYNC
		       VM data,	to physical media.  The	sync(8)	command	and
		       sync(2) system call generally do	not flush dirty	NOSYNC
		       VM data.	 The msync(2) system call is obsolete since
		       BSD implements a	coherent file system buffer cache.
		       However,	it may be used to associate dirty VM pages
		       with file system	buffers	and thus cause them to be
		       flushed to physical media sooner	rather than later.

     MAP_PRIVATE       Modifications are private.

     MAP_SHARED	       Modifications are shared.

     MAP_STACK	       MAP_STACK implies MAP_ANON, and offset of 0.  The fd
		       argument	must be	-1 and prot must include at least
		       PROT_READ and PROT_WRITE.  This option creates a	memory
		       region that grows to at most len	bytes in size, start-
		       ing from	the stack top and growing down.	 The stack top
		       is the starting address returned	by the call, plus len
		       bytes.  The bottom of the stack at maximum growth is
		       the starting address returned by	the call.

     The close(2) system call does not unmap pages, see	munmap(2) for further

     The current design	does not allow a process to specify the	location of
     swap space.  In the future	we may define an additional mapping type,
     MAP_SWAP, in which	the file descriptor argument specifies a file or de-
     vice to which swapping should be done.

     Although this implementation does not impose any alignment	restrictions
     on	the offset argument, a portable	program	must only use page-aligned

     Upon successful completion, mmap()	returns	a pointer to the mapped	re-
     gion.  Otherwise, a value of MAP_FAILED is	returned and errno is set to
     indicate the error.

     The mmap()	system call will fail if:

     [EACCES]		The flag PROT_READ was specified as part of the	prot
			argument and fd	was not	open for reading.  The flags
			MAP_SHARED and PROT_WRITE were specified as part of
			the flags and prot argument and	fd was not open	for

     [EBADF]		The fd argument	is not a valid open file descriptor.

     [EINVAL]		MAP_FIXED was specified	and the	addr argument was not
			page aligned, or part of the desired address space re-
			sides out of the valid address space for a user

     [EINVAL]		The len	argument was equal to zero.

     [EINVAL]		MAP_ANON was specified and the fd argument was not -1.

     [ENODEV]		MAP_ANON has not been specified	and fd did not refer-
			ence a regular or character special file.

     [ENOMEM]		MAP_FIXED was specified	and the	addr argument was not
			available.  MAP_ANON was specified and insufficient
			memory was available.  The system has reached the per-
			process	mmap limit specified in	the vm.max_proc_mmap

     madvise(2), mincore(2), minherit(2), mlock(2), mprotect(2), msync(2),
     munlock(2), munmap(2), getpagesize(3), make.conf(5)

     The len argument is limited to the	maximum	file size or available user-
     land address space.  Files	may not	be able	to be made more	than 1TB large
     on	32 bit systems due to file systems restrictions	and bugs, but address
     space is far more restrictive.  Larger files may be possible on 64	bit

     The previous documented limit of 2GB was a	documentation bug.  That limit
     has not existed since FreeBSD 2.2.

BSD				 July 26, 2009				   BSD


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