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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/types.h>
     #include <sys/mman.h>

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

     The mmap()	function 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 pagesize, 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 parameter 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 parameter 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_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 then 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/filesystem 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 then 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) function 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 filesystem buffer cache.
		       However,	it may be used to associate dirty VM pages
		       with filesystem buffers and thus	cause them to be
		       flushed to physical media sooner	rather then later.

     MAP_PRIVATE       Modifications are private.

     MAP_SHARED	       Modifications are shared.

     MAP_STACK	       This option is only available if	your system has	been
		       compiled	with VM_STACK defined when compiling the ker-
		       nel.  This is the default for i386 only.	 Consider
		       adding -DVM_STACK to COPTFLAGS in your /etc/make.conf
		       to enable this option for other architechures.
		       MAP_STACK implies MAP_ANON, and offset of 0.  fd	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, starting 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	start-
		       ing address returned by the call.

     The close(2) function does	not unmap pages, see munmap(2) for further in-

     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.

     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.

     Mmap() will fail if:

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

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

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

     [EINVAL]		Len was	negative.

     [EINVAL]		MAP_ANON was specified and the fd parameter was	not

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

     [EINVAL]		Offset was not page-aligned.  (See BUGS	below.)

     [ENOMEM]		MAP_FIXED was specified	and the	addr parameter wasn't
			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), mlock(2), mprotect(2), msync(2), munlock(2),
     munmap(2),	getpagesize(3)

     len is limited to 2GB.  Mmapping slightly more than 2GB doesn't 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's actually bugs	in the
     filesystem	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 filesystem metadata can reside
     at	negative offsets.

BSD			       November	17, 2001			   BSD


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