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MSGCTL(2)		    BSD Programmer's Manual		     MSGCTL(2)

NAME
     msgctl - message control operations

SYNOPSIS
     #include <sys/types.h>
     #include <sys/ipc.h>
     #include <sys/msg.h>

     int
     msgctl(int msqid, int cmd, struct msqid_ds *buf);

DESCRIPTION
     The msgctl() system call performs control operations on the message queue
     specified by msqid. Each message queue has a data structure associated
     with it, parts of which may be altered by msgctl() and parts of which de-
     termine the actions of msgctl().  The data structure is defined in
     <sys/msg.h> and contains the following members:

     struct msqid_ds {
	 struct ipc_perm msg_perm;   /* msg queue permission bits */
	 u_long	     msg_cbytes;     /* # of bytes in use on the queue */
	 u_long	     msg_qnum;	     /* # of msgs in the queue */
	 u_long	     msg_qbytes;     /* max # of bytes on the queue */
	 pid_t	     msg_lspid;	     /* pid of last msgsnd() */
	 pid_t	     msg_lrpid;	     /* pid of last msgrcv() */
	 time_t	     msg_stime;	     /* time of last msgsnd() */
	 time_t	     msg_rtime;	     /* time of last msgrcv() */
	 time_t	     msg_ctime;	     /* time of last msgctl() */
     };

     The ipc_perm  structure used inside the shmid_ds  structure is defined in
     <sys/ipc.h> and looks like this:

     struct ipc_perm {
	 ushort	     cuid;	     /* creator user id */
	 ushort	     cgid;	     /* creator group id */
	 ushort	     uid;	     /* user id */
	 ushort	     gid;	     /* group id */
	 ushort	     mode;	     /* permission (9 bits, see chmod(2)) */
	 ushort	     seq;	     /* sequence # (to generate unique id) */
	 key_t	     key;	     /* user specified msg/sem/shm key */
     };

     The operation to be performed by msgctl() is specified in cmd and is one
     of:

     IPC_STAT	Gather information about the message queue and place it in the
		structure pointed to by buf.

     IPC_SET	Set the value of the msg_perm.uid, msg_perm.gid, msg_perm.mode
		and msg_qbytes fields in the structure associated with msqid.
		The values are taken from the corresponding fields in the
		structure pointed to by buf. This operation can only be exe-
		cuted by the super-user, or a process that has an effective
		user id equal to either msg_perm.cuid or msg_perm.uid in the
		data structure associated with the message queue.  The value
		of msg_qbytes can only be increased by the super-user. Values
		for msg_qbytes that exceed the system limit (MSGMNB from
		<sys/msg.h>) are silently truncated to that limit.

     IPC_RMID	Remove the message queue specified by msqid and destroy the
		data associated with it. Only the super-user or a process with
		an effective uid equal to the msg_perm.cuid or msg_perm.uid
		values in the data structure associated with the queue can do
		this.

     The permission to read from or write to a message queue (see msgsnd(3)
     and msgrcv(3))  is determined by the msg_perm.mode field in the same way
     as is done with files (see chmod(2)),  but the effective uid can match
     either the msg_perm.cuid field or the msg_perm.uid field, and the effec-
     tive gid can match either msg_perm.cgid or msg_perm.gid.

RETURN VALUES
     Upon successful completion, a value of 0 is returned. Otherwise, -1 is
     returned and the global variable errno is set to indicate the error.

ERRORS
     msgctl() will fail if:

     [EPERM]	   The cmd is equal to IPC_SET or IPC_RMID and the caller is
		   not the super-user, nor does the effective uid match either
		   the msg_perm.uid or msg_perm.cuid fields of the data struc-
		   ture associated with the message queue.

		   An attempt is made to increase the value of msg_qbytes
		   through IPC_SET but the caller is not the super-user.

     [EACCESS]	   The cmd is IPC_STAT and the caller has no read permission
		   for this message queue.

     [EINVAL]	   The msqid is not a valid message queue identifier.

		   The cmd is not a valid command.

     [EFAULT]	   The buf specifies an invalid address.

SEE ALSO
     msgsnd(2),	 msgrcv(2),  msgget(2)

4th Berkeley Distribution	 July 3, 1996				     2
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