freebsd-dev/lib/libc/gen/msgctl.3

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.Dd November 24, 1997
.Dt MSGCTL 3
.Os FreeBSD
.Sh NAME
.Nm msgctl
.Nd message control operations
.Sh SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sys/ipc.h>
.Fd #include <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 .

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
.Aq Pa sys/msg.h
and contains (amongst others) the following members:
.Bd -literal
struct msqid_ds {
	struct	ipc_perm msg_perm;	/* msg queue permission bits */
	struct	msg *msg_first;	/* first message in the queue */
	struct	msg *msg_last;	/* last message in the queue */
	u_long	msg_cbytes;	/* number of bytes in use on the queue */
	u_long	msg_qnum;	/* number 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() */
	long	msg_pad1;
	time_t	msg_rtime;	/* time of last msgrcv() */
	long	msg_pad2;
	time_t	msg_ctime;	/* time of last msgctl() */
	long	msg_pad3;
	long	msg_pad4[4];
};
.Ed
.Pp
The
.Bf -literal
ipc_perm
.Ef
structure used inside the
.Bf -literal
shmid_ds
.Ef
structure is defined in
.Aq Pa sys/ipc.h
and looks like this:
.Bd -literal
struct ipc_perm {
	ushort	cuid;	/* creator user id */
	ushort	cgid;	/* creator group id */
	ushort	uid;	/* user id */
	ushort	gid;	/* group id */
	ushort	mode;	/* r/w permission */
	ushort	seq;	/* sequence # (to generate unique msg/sem/shm 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 super-user, 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 super-user.
Values for
.Va msg_qbytes
that exceed the system limit (MSGMNB from
.Aq Pa 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 super-user 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

The permission to read from or write to a message queue (see
.Xr msgsnd 3
and
.Xr msgrcv 3 )
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
Upon successful completion, a value of 0 is returned.
Otherwise, -1 is
returned and the global variable
.Va errno
is set to indicate the error.
.Sh ERRORS
.Fn msgctl
will fail if:
.Bl -tag -width Er
.It Bq Er EPERM
.Fa 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
.Va msg_perm.uid
or
.Va msg_perm.cuid
fields of the data structure associated with the message queue.

An attempt is made to increase the value of
.Va msg_qbytes
through IPC_SET
but the caller is not the super-user.
.It Bq Er EACCES
The command is 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.

.Va cmd
is not a valid command.
.It Bq Er EFAULT
.Fa buf
specifies an invalid address.
.El
.Sh SEE ALSO
.Xr msgget 3 ,
.Xr msgrcv 3 ,
.Xr msgsnd 3
.Sh HISTORY
Message queues appeared in the first release of
.At V .