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

NAME
     mmap - map files or devices into memory

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include 
     #include 

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

DESCRIPTION
     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
     range.

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

     PROT_EXEC	 Pages may be executed.

     PROT_READ	 Pages may be read.

     PROT_WRITE  Pages may be written.

     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 cannot be used,
		 mmap() will fail.  If MAP_FIXED is specified, addr must be a
		 multiple of the pagesize.  Use of this option is discouraged.

     MAP_HASSEMAPHORE
		 Notify the kernel that the region may contain semaphores and
		 that special handling may be necessary.

     MAP_INHERIT
		 Permit regions to be inherited across execve(2) system calls.

     MAP_PRIVATE
		 Modifications are private.


     MAP_SHARED  Modifications are shared.

     MAP_STACK	 This option is only available if your system has been com-
		 piled with VM_STACK defined when compiling the kernel.  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 starting address returned by
		 the call.

     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 prevents the update daemons
		 from flushing pages dirtied through such maps and thus allows
		 efficient sharing of memory across unassociated processes us-
		 ing a file-backed shared memory map.  Without this option any
		 VM pages you dirty may be flushed to disk every so often (ev-
		 ery 30-60 seconds usually) which can create performance prob-
		 lems 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 be-
		 havior by default.

		 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(1) command and sync(2) system call
		 generally do not flush dirty NOSYNC VM data.  The msync(2)
		 system call is obsolete since BSD UNIX implements a coherent
		 filesystem buffer cache.  However, it may be used to associ-
		 ate dirty VM pages with filesystem buffers and thus cause
		 them to be flushed to physical media sooner rather then lat-
		 er.

     MAP_NOCORE  Region is not included in a core file.

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

     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.

RETURN VALUES
     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.

ERRORS
     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
			writing.



     [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 pro-
			cess.

     [EINVAL]		Len was negative.

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

     [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, or the system has reached the per-process
			mmap limit specified in the vm.max_proc_mmap sysctl.
			MAP_ANON was specified and insufficient memory was
			available.

SEE ALSO
     madvise(2),  mincore(2),  mlock(2),  mprotect(2),	msync(2),  munlock(2),
      munmap(2),  getpagesize(3)

BUGS
     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.

     We currently can only deal with page aligned file offsets.

4th Berkeley Distribution	 May 11, 1995				     3