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

NAME
       shmat, shmdt - System V shared memory operations

SYNOPSIS
       #include	<sys/types.h>
       #include	<sys/shm.h>

       void *shmat(int shmid, const void *shmaddr, int shmflg);

       int shmdt(const void *shmaddr);

DESCRIPTION
   shmat()
       shmat() attaches	the System V shared memory segment identified by shmid
       to the address space of the calling process.  The attaching address  is
       specified by shmaddr with one of	the following criteria:

       *  If  shmaddr  is NULL,	the system chooses a suitable (unused) address
	  at which to attach the segment.

       *  If shmaddr isn't NULL	and SHM_RND is specified in shmflg, the	attach
	  occurs  at  the address equal	to shmaddr rounded down	to the nearest
	  multiple of SHMLBA.

       *  Otherwise, shmaddr must be a page-aligned address at which  the  at-
	  tach occurs.

       In  addition  to	 SHM_RND,  the following flags may be specified	in the
       shmflg bit-mask argument:

       SHM_EXEC	(Linux-specific; since Linux 2.6.9)
	      Allow the	contents of the	segment	to be  executed.   The	caller
	      must have	execute	permission on the segment.

       SHM_RDONLY
	      Attach  the segment for read-only	access.	 The process must have
	      read permission for the segment.	If this	flag is	not specified,
	      the  segment  is	attached  for  read  and write access, and the
	      process must have	read and write	permission  for	 the  segment.
	      There is no notion of a write-only shared	memory segment.

       SHM_REMAP (Linux-specific)
	      This  flag  specifies that the mapping of	the segment should re-
	      place any	existing mapping in the	range starting at shmaddr  and
	      continuing  for  the  size of the	segment.  (Normally, an	EINVAL
	      error would result if a mapping already exists in	 this  address
	      range.)  In this case, shmaddr must not be NULL.

       The  brk(2)  value of the calling process is not	altered	by the attach.
       The segment will	automatically be detached at process exit.   The  same
       segment	may  be	 attached  as a	read and as a read-write one, and more
       than once, in the process's address space.

       A successful shmat() call updates the members of	the shmid_ds structure
       (see shmctl(2)) associated with the shared memory segment as follows:

	      shm_atime	is set to the current time.

	      shm_lpid is set to the process-ID	of the calling process.

	      shm_nattch is incremented	by one.

   shmdt()
       shmdt() detaches	the shared memory segment located at the address spec-
       ified by	shmaddr	from the address space of the  calling	process.   The
       to-be-detached segment must be currently	attached with shmaddr equal to
       the value returned by the attaching shmat() call.

       On a successful shmdt() call, the system	updates	 the  members  of  the
       shmid_ds	 structure  associated	with the shared	memory segment as fol-
       lows:

	      shm_dtime	is set to the current time.

	      shm_lpid is set to the process-ID	of the calling process.

	      shm_nattch is decremented	by one.	 If it becomes 0 and the  seg-
	      ment is marked for deletion, the segment is deleted.

RETURN VALUE
       On  success,  shmat() returns the address of the	attached shared	memory
       segment;	on error, (void	*) -1 is returned, and errno is	set  to	 indi-
       cate the	cause of the error.

       On  success,  shmdt()  returns 0; on error -1 is	returned, and errno is
       set to indicate the cause of the	error.

ERRORS
       When shmat() fails, errno is set	to one of the following:

       EACCES The calling process does not have	the required  permissions  for
	      the  requested  attach type, and does not	have the CAP_IPC_OWNER
	      capability.

       EIDRM  shmid points to a	removed	identifier.

       EINVAL Invalid shmid  value,  unaligned	(i.e.,	not  page-aligned  and
	      SHM_RND  was  not	 specified) or invalid shmaddr value, or can't
	      attach segment  at  shmaddr,  or	SHM_REMAP  was	specified  and
	      shmaddr was NULL.

       ENOMEM Could not	allocate memory	for the	descriptor or for the page ta-
	      bles.

       When shmdt() fails, errno is set	as follows:

       EINVAL There is no shared  memory  segment  attached  at	 shmaddr;  or,
	      shmaddr is not aligned on	a page boundary.

CONFORMING TO
       SVr4, POSIX.1-2001.

       In  SVID	 3  (or	perhaps	earlier), the type of the shmaddr argument was
       changed from char * into	const void *, and the returned type of shmat()
       from char * into	void *.

NOTES
       After  a	 fork(2),  the	child inherits the attached shared memory seg-
       ments.

       After an	execve(2), all attached	shared memory  segments	 are  detached
       from the	process.

       Upon  _exit(2),	all  attached shared memory segments are detached from
       the process.

       Using shmat() with shmaddr equal	to NULL	is the preferred, portable way
       of  attaching a shared memory segment.  Be aware	that the shared	memory
       segment attached	in this	way may	be attached at different addresses  in
       different  processes.   Therefore,  any	pointers maintained within the
       shared memory must be made relative (typically to the starting  address
       of the segment),	rather than absolute.

       On  Linux,  it is possible to attach a shared memory segment even if it
       is already marked to be deleted.	 However, POSIX.1-2001 does not	 spec-
       ify this	behavior and many other	implementations	do not support it.

       The following system parameter affects shmat():

       SHMLBA Segment low boundary address multiple.  When explicitly specify-
	      ing an attach address in a call to shmat(),  the	caller	should
	      ensure  that  the	 address is a multiple of this value.  This is
	      necessary	on some	architectures, in order	either to ensure  good
	      CPU  cache  performance  or to ensure that different attaches of
	      the same segment have consistent views  within  the  CPU	cache.
	      SHMLBA  is  normally  some  multiple of the system page size (on
	      many Linux architectures,	it is the  same	 as  the  system  page
	      size).

       The  implementation places no intrinsic per-process limit on the	number
       of shared memory	segments (SHMSEG).

SEE ALSO
       brk(2),	mmap(2),  shmctl(2),  shmget(2),  capabilities(7),   shm_over-
       view(7),	svipc(7)

COLOPHON
       This  page  is  part of release 3.74 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest	  version     of     this    page,    can    be	   found    at
       http://www.kernel.org/doc/man-pages/.

Linux				  2014-07-08			      SHMOP(2)

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

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