shmctl

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
       segment, set the owner, group, and permissions of a shared memory seg-
       ment,  or destroy a segment. The information about the segment identi-
       fied by shmid is returned in a shmid_ds structure:

	   struct shmid_ds {
	       struct ipc_perm shm_perm;  /* operation perms */
	       size_t 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
		   lowest 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,  processes	 with  appropriate  privileges 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.

       Processes are permitted to use SHM_LOCK and SHM_UNLOCK if they running
       with the CAP_IPC_LOCK capability (normally only true for root)  or  if
       their current RLIMIT_MEMLOCK resource limit is non-zero.

       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	 cre-
		   ator	 (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			  2003-07-11			    SHMCTL(2)