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

NAME
       shmctl -	shared memory control

SYNOPSIS
       #include	<sys/ipc.h>

       #include	<sys/shm.h>

       int shmctl(int shmid, int cmd, struct shmid_ds *buf);

DESCRIPTION
       shmctl()	allows the user	to receive information on a shared memory seg-
       ment, set the owner, group, and permissions of a	shared memory segment,
       or  destroy  a segment. The information about the segment identified by
       shmid is	returned in a shmid_ds structure:

	   struct shmid_ds {
	       struct ipc_perm shm_perm;  /* operation perms */
	       int shm_segsz;		  /* size of segment (bytes) */
	       time_t shm_atime;	  /* last attach time */
	       time_t shm_dtime;	  /* last detach time */
	       time_t shm_ctime;	  /* last change time */
	       unsigned	short shm_cpid;	  /* pid of creator */
	       unsigned	short shm_lpid;	  /* pid of last operator */
	       short shm_nattch;	  /* no. of current attaches */
	       ...
	   };

       The highlighted fields in the member shm_perm can be set:

	   struct ipc_perm {
	       key_t  key;
	       ushort uid;   /*	owner euid and egid */
	       ushort gid;
	       ushort cuid;  /*	creator	euid and egid */
	       ushort cgid;
	       ushort mode;  /*	lower 9	bits of	access modes */
	       ushort seq;   /*	sequence number	*/
	   };

       The following cmds are available:

       IPC_STAT	   is used to copy the information  about  the	shared	memory
		   segment into	the buffer buf.	The user must have read	access
		   to the shared memory	segment.

       IPC_SET	   is used to apply the	changes	the user has made to the  uid,
		   gid,	or mode	members	of the shm_perms field.	 Only the low-
		   est 9 bits of mode are used.	 The shm_ctime member is  also
		   updated.   The  user	 must  be  the	owner, creator,	or the
		   super-user.

       IPC_RMID	   is used to mark the segment as destroyed. It	will  actually
		   be  destroyed  after	 the  last  detach.   (I.e.,  when the
		   shm_nattch member of	the associated structure  shmid_ds  is
		   zero.)   The	user must be the owner,	creator, or the	super-
		   user.

       The user	must ensure that a segment is eventually destroyed;  otherwise
       its pages that were faulted in will remain in memory or swap.

       In  addition,  the super-user can prevent or allow swapping of a	shared
       memory segment with the following cmds: (Linux only)

       SHM_LOCK	   prevents swapping of	a shared memory	segment. The user must
		   fault  in  any  pages that are required to be present after
		   locking is enabled.

       SHM_UNLOCK  allows the shared memory segment to be swapped out.

       The IPC_INFO, SHM_STAT and SHM_INFO  control  calls  are	 used  by  the
       ipcs(8)	program	to provide information on allocated resources.	In the
       future, these may be modified as	needed or moved	to a proc file	system
       interface.

RETURN VALUE
       0 is returned on	success, -1 on error.

ERRORS
       On error, errno will be set to one of the following:

       EACCES	   is  returned	 if  IPC_STAT  is requested and	shm_perm.modes
		   does	not allow read access for shmid.

       EFAULT	   The argument	cmd has	value  IPC_SET	or  IPC_STAT  but  the
		   address pointed to by buf isn't accessible.

       EINVAL	   is  returned	 if shmid is not a valid identifier, or	cmd is
		   not a valid command.

       EIDRM	   is returned if shmid	points to a removed identifier.

       EPERM	   is returned if IPC_SET or IPC_RMID is  attempted,  and  the
		   effective user ID of	the calling process is not the creator
		   (as	found  in  shm_perm.cuid),  the	 owner	(as  found  in
		   shm_perm.uid), or the super-user.

       EOVERFLOW   is  returned	 if  IPC_STAT is attempted, and	the gid	or uid
		   value is too	large to be stored in the structure pointed to
		   by buf.

NOTE
       Various	fields	in  a  struct shmid_ds were shorts under Linux 2.2 and
       have become longs under Linux 2.4. To take advantage of this, a	recom-
       pilation	 under glibc-2.1.91 or later should suffice.  (The kernel dis-
       tinguishes old and new calls by a IPC_64	flag in	cmd.)

CONFORMING TO
       SVr4, SVID.  SVr4 documents additional error conditions EINVAL, ENOENT,
       ENOSPC, ENOMEM, EEXIST.	Neither	SVr4 nor SVID documents	an EIDRM error
       condition.

SEE ALSO
       shmget(2), shmop(2)

Linux 2.4.1			  2002-01-05			     SHMCTL(2)

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

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