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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()	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.

       If SHM_RDONLY is specified in shmflg, the segment is attached for reading and the  process
       must have read permission for the segment.  Otherwise the segment is attached for read and
       write and the process must have read and write permission for the segment.   There  is  no
       notion of a write-only shared memory segment.

       The  (Linux-specific)  SHM_REMAP flag may be specified in shmflg to indicate that the map-
       ping of the segment should replace 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()	detaches  the  shared  memory segment located at the address specified by shmaddr
       from the address space of the calling process.  The to-be-detached segment  must  be  cur-
       rently 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 follows:

	      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 segment  is  marked  for
	      deletion, the segment is deleted.

       After a fork(2) the child inherits the attached shared memory segments.

       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.

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 indicate 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  speci-
	      fied)  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
       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 *.  (Linux libc4
       and libc5 have the char * prototypes; glibc2 has void *.)

NOTES
       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 main-
       tained 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 specify this behavior and many other implemen-
       tations do not support it.

       The following system parameter affects shmat():

       SHMLBA Segment  low  boundary  address  multiple.   Must be page aligned.  For the current
	      implementation the SHMLBA value is PAGE_SIZE.

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

SEE ALSO
       brk(2), mmap(2), shmctl(2), shmget(2), capabilities(7), shm_overview(7), svipc(7)

COLOPHON
       This  page  is  part of release 3.53 of the Linux man-pages project.  A description of the
       project,    and	  information	 about	  reporting    bugs,	can    be    found     at
       http://www.kernel.org/doc/man-pages/.

Linux					    2013-02-12					 SHMOP(2)
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