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MMAP(2)			  FreeBSD System Calls Manual		       MMAP(2)

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

LIBRARY
     Standard C	Library	(libc, -lc)

SYNOPSIS
     #include <sys/mman.h>

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

DESCRIPTION
     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
     range.

     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
     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.	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.  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
		       information.

     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
     information.

     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
     device to which swapping should be	done.

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

ERRORS
     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
			writing.

     [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
			resides	out of the valid address space for a user
			process.

     [EINVAL]		The len	argument was negative.

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

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

     [EINVAL]		The offset argument was	not page-aligned.  (See	BUGS
			below.)

     [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
			sysctl.

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

BUGS
     The len argument is limited to 2GB.  Mmapping slightly more than 2GB does
     not work, but it is possible to map a window of size (filesize % 2GB) for
     file sizes	of slightly less than 2G, 4GB, 6GB and 8GB.

     The limit is imposed for a	variety	of reasons.  Most of them have to do
     with FreeBSD not wanting to use 64	bit offsets in the VM system due to
     the extreme performance penalty.  So FreeBSD uses 32bit page indexes and
     this gives	FreeBSD	a maximum of 8TB filesizes.  It	is actually bugs in
     the file system code that causes the limit	to be further restricted to
     1TB (loss of precision when doing blockno calculations).

     Another reason for	the 2GB	limit is that file system metadata can reside
     at	negative offsets.

FreeBSD	9.2		       November	17, 2001		   FreeBSD 9.2

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | SEE ALSO | BUGS

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