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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	<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, and is never 0.

       The prot	argument describes the desired memory protection (and must not
       conflict	 with the open mode of the file). It is	either PROT_NONE or is
       the bitwise OR of one or	more of	the other PROT_* flags.

       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.  It  is  unspecified
		  whether  changes  made  to  the file after the mmap call are
		  visible in the mapped	region.

       You must	specify	exactly	one of MAP_SHARED and MAP_PRIVATE.

       The above three flags are described in POSIX.1b (formerly POSIX.4)  and
       SUSv2.  Linux also knows	about the following non-standard flags:

       MAP_DENYWRITE
	      This  flag is ignored.  (Long ago, it signalled that attempts to
	      write to the underlying file should fail with ETXTBUSY. But this
	      was a source of denial-of-service	attacks.)

       MAP_EXECUTABLE
	      This flag	is ignored.

       MAP_NORESERVE
	      (Used  together  with  MAP_PRIVATE.)  Do	not reserve swap space
	      pages for	this mapping. When swap	space is reserved, one has the
	      guarantee	 that  it  is possible to modify this private copy-on-
	      write region.  When it is	not reserved  one  might  get  SIGSEGV
	      upon a write when	no memory is available.

       MAP_LOCKED
	      This flag	is ignored.

       MAP_GROWSDOWN
	      Used for stacks. Indicates to the	kernel VM system that the map-
	      ping should extend downwards in memory.

       MAP_ANONYMOUS
	      The mapping is not backed	by any file; the fd and	 offset	 argu-
	      ments  are ignored.  This	flag in	conjunction with MAP_SHARED is
	      implemented since	Linux 2.4.

       MAP_ANON
	      Alias for	MAP_ANONYMOUS. Deprecated.

       MAP_FILE
	      Compatibility flag. Ignored.

       Some systems document the additional flags MAP_AUTOGROW,	MAP_AUTORESRV,
       MAP_COPY, and MAP_LOCAL.

       fd  should  be a	valid file descriptor, unless MAP_ANONYMOUS is set, in
       which case the argument is ignored.

       offset should be	a multiple of the page size as	returned  by  getpage-
       size(2).

       Memory  mapped  by  mmap	 is  preserved	across	fork(2), with the same
       attributes.

       A file is mapped	in multiples of	the page size. For a file that is  not
       a  multiple  of	the  page  size,  the  remaining memory	is zeroed when
       mapped, and writes to that region are not written out to	the file.  The
       effect  of changing the size of the underlying file of a	mapping	on the
       pages that correspond to	added  or  removed  regions  of	 the  file  is
       unspecified.

       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.

       The  address  start must	be a multiple of the page size.	All pages con-
       taining a part of the indicated range are unmapped, and subsequent ref-
       erences to these	pages will generate SIGSEGV. It	is not an error	if the
       indicated range does not	contain	any mapped pages.

       For file-backed mappings, the st_atime field for	the mapped file	may be
       updated at any time between the mmap() and the corresponding unmapping;
       the first reference to a	mapped page will update	the field  if  it  has
       not been	already.

       The  st_ctime  and st_mtime field for a file mapped with	PROT_WRITE and
       MAP_SHARED will be updated after	a write	 to  the  mapped  region,  and
       before  a  subsequent msync() with the MS_SYNC or MS_ASYNC flag,	if one
       occurs.

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 A	file descriptor	refers to a non-regular	file.  Or  MAP_PRIVATE
	      was  requested,  but  fd is not open for reading.	 Or MAP_SHARED
	      was requested and	PROT_WRITE is set,  but	 fd  is	 not  open  in
	      read/write (O_RDWR) mode.	 Or PROT_WRITE is set, but the file is
	      append-only.

       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, or the process's maximum number of  map-
	      pings would have been exceeded.

       ENODEV The underlying filesystem	of the specified file does not support
	      memory mapping.

       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, SUSv2.  SVr4 documents addi-
       tional error codes ENXIO	and ENODEV.  SUSv2 documents additional	 error
       codes EMFILE and	EOVERFLOW.

SEE ALSO
       getpagesize(2),	 mmap2(2),   mremap(2),	 msync(2),  shm_open(2),  B.O.
       Gallmeister, POSIX.4, O'Reilly, pp. 128-129 and 389-391.

Linux 2.3.51			  2000-03-25			       MMAP(2)

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

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