msgctl man page on MirBSD

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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 some 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 determine the actions
     of msgctl(). The data structure is defined in <sys/msg.h> and contains
     (amongst others) 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 {
	     uid_t   cuid;   /* creator user id */
	     gid_t   cgid;   /* creator group id */
	     uid_t   uid;    /* user id */
	     gid_t   gid;    /* group id */
	     mode_t  mode;   /* permission (9 bits, see chmod(2)) */
	     u_short 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 superuser, 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 superuser. 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 superuser 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(2)
     and msgrcv(2)) 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 ei-
     ther the msg_perm.cuid field or the msg_perm.uid field, and the effective
     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]	   cmd is equal to IPC_SET or IPC_RMID and the caller is not
		   the superuser, nor does the effective UID match either the
		   msg_perm.uid or msg_perm.cuid fields of the data structure
		   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 superuser.

     [EACCES]	   The command is IPC_STAT and the caller has no read permis-
		   sion for this message queue.

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

		   cmd is not a valid command.

     [EFAULT]	   buf specifies an invalid address.

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

HISTORY
     Message queues appeared in the first release of AT&T Unix System V.

MirOS BSD #10-current	       August 17, 1995				     1
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