.\"	$OpenBSD: msgctl.2,v 1.17 2014/12/10 19:19:00 schwarze Exp $
.\"	$NetBSD: msgctl.2,v 1.2 1997/03/27 08:20:35 mikel Exp $
.\"
.\" Copyright (c) 1995 Frank van der Linden
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.Dd $Mdocdate: December 10 2014 $
.Dt MSGCTL 2
.Os
.Sh NAME
.Nm msgctl
.Nd message control operations
.Sh SYNOPSIS
.In sys/msg.h
.Ft int
.Fn msgctl "int msqid" "int cmd" "struct msqid_ds *buf"
.Sh DESCRIPTION
The
.Fn msgctl
system call performs some control operations on the message queue specified
by
.Fa msqid .
.Pp
Each message queue has a data structure associated with it, parts of which
may be altered by
.Fn msgctl
and parts of which determine the actions of
.Fn msgctl .
The data structure is defined in
.In sys/msg.h
and contains (amongst others) the following members:
.Bd -literal
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() */
};
.Ed
.Pp
The
.Fa ipc_perm
structure used inside the
.Fa msqid_ds
structure is defined in
.In sys/ipc.h
and looks like this:
.Bd -literal
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 */
};
.Ed
.Pp
The operation to be performed by
.Fn msgctl
is specified in
.Fa cmd
and is one of:
.Bl -tag -width IPC_RMIDX
.It Dv IPC_STAT
Gather information about the message queue and place it in the
structure pointed to by
.Fa buf .
.It Dv IPC_SET
Set the value of the
.Va msg_perm.uid ,
.Va msg_perm.gid ,
.Va msg_perm.mode
and
.Va msg_qbytes
fields in the structure associated with
.Fa msqid .
The values are taken from the corresponding fields in the structure
pointed to by
.Fa buf .
This operation can only be executed by the superuser, or a process that
has an effective user ID equal to either
.Va msg_perm.cuid
or
.Va msg_perm.uid
in the data structure associated with the message queue.
The value of
.Va msg_qbytes
can only be increased by the superuser.
Values for
.Va msg_qbytes
that exceed the system limit
.Pf ( Dv MSGMNB
from
.In sys/msg.h )
are silently truncated to that limit.
.It Dv IPC_RMID
Remove the message queue specified by
.Fa msqid
and destroy the data associated with it.
Only the superuser or a process with an effective UID equal to the
.Va msg_perm.cuid
or
.Va msg_perm.uid
values in the data structure associated with the queue can do this.
.El
.Pp
The permission to read from or write to a message queue (see
.Xr msgsnd 2
and
.Xr msgrcv 2 )
is determined by the
.Va msg_perm.mode
field in the same way as is
done with files (see
.Xr chmod 2 ) ,
but the effective UID can match either the
.Va msg_perm.cuid
field or the
.Va msg_perm.uid
field, and the
effective GID can match either
.Va msg_perm.cgid
or
.Va msg_perm.gid .
.Sh RETURN VALUES
.Rv -std
.Sh ERRORS
.Fn msgctl
will fail if:
.Bl -tag -width Er
.It Bq Er EPERM
.Fa cmd
is equal to
.Dv IPC_SET
or
.Dv IPC_RMID
and the caller is not the superuser, nor does
the effective UID match either the
.Va msg_perm.uid
or
.Va msg_perm.cuid
fields of the data structure associated with the message queue.
.Pp
An attempt is made to increase the value of
.Va msg_qbytes
through
.Dv IPC_SET
but the caller is not the superuser.
.It Bq Er EACCES
The command is
.Dv IPC_STAT
and the caller has no read permission for this message queue.
.It Bq Er EINVAL
.Fa msqid
is not a valid message queue identifier.
.Pp
.Va cmd
is not a valid command.
.It Bq Er EFAULT
.Fa buf
specifies an invalid address.
.El
.Sh SEE ALSO
.Xr msgget 2 ,
.Xr msgrcv 2 ,
.Xr msgsnd 2
.Sh HISTORY
Message queues appeared in
.At V.1 .