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
	  attach 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
	      replace any existing mapping in the range	 starting  at  shmaddr
	      and  continuing for the size of the segment.  (Normally, an EIN-
	      VAL 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 in the user namespace that governs its IPC namespace.

       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
	      tables.

       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
       POSIX.1-2001, POSIX.1-2008, SVr4.

       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  does  not  specify
       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 4.10 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
       https://www.kernel.org/doc/man-pages/.



Linux				  2016-10-08			      SHMOP(2)