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MMAP(2)			   Linux Programmer's Manual		       MMAP(2)

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	the file descriptor 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 (and must not	conflict with the open mode of
       the file). 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.   Storing  to	the region is equivalent to writing to
		  the file.  The  file	may  not  actually  be	updated	 until
		  msync(2) or munmap(2)	are called.

       MAP_PRIVATE
		  Create  a  private  copy-on-write  mapping.	Stores	to the
		  region do not	affect the original file.

       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,	MAP_NORESERVE,
       MAP_LOCKED, MAP_GROWSDOWN and MAP_ANON(YMOUS).

       offset should ordinarily	be a multiple of the  page  size  returned  by
       getpagesize(2).

       The  munmap  system call	deletes	the mappings for the specified address
       range, and causes further references to addresses within	the  range  to
       generate	 invalid  memory references.  The region is also automatically
       unmapped	when the process is terminated.	 On the	 other	hand,  closing
       the file	descriptor does	not unmap the region.

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  suc-
       cess,  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  in
	      read/write (O_RDWR) mode.

       EINVAL We  don't	 like  start or	length or offset.  (E.g., they are too
	      large, or	not aligned on a PAGESIZE boundary.)

       ETXTBSY
	      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.

       Use of a	mapped region can result in these signals:

       SIGSEGV
	      Attempted	write into a region specified to mmap as read-only.

       SIGBUS Attempted	access to a portion of the buffer that does not	corre-
	      spond to the file	(for example, beyond  the  end	of  the	 file,
	      including	 the  case  where  another  process  has truncated the
	      file).

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.

Linux 2.3.51			 25 March 2000			       MMAP(2)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | SEE ALSO

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