libc/sys/msgctl.2

.\"	$NetBSD: msgctl.2,v 1.1 1995/10/16 23:49:15 jtc Exp $
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.\" Copyright (c) 1995 Frank van der Linden
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.Dd August 17, 1995
.Dt MSGCTL 2
.Os NetBSD
.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 */
    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

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; /* permission (9 bits, see chmod(2)) */
    ushort seq;	 /* sequence # (to generate unique id) */
    key_t key;	 /* user specified msg/sem/shm key */
};
.Ed

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 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
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 EACCESS
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 msgsnd 2 ,
.Xr msgrcv 2 ,
.Xr msgget 2
.Sh HISTORY
Message queues appeared in the first release of AT&T Unix System V.