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.TH MSGCTL 2
.SH NAME
msgctl \- message control operations
.SH SYNOPSIS
.B #include <sys/types.h>
.br
.B #include <sys/ipc.h>
.br
.B #include <sys/msg.h>
.PP
.nf
.B int msgctl (msqid, cmd, buf)
.B int msqid, cmd;
.B struct msqid_ds \(**buf;
.fi
.SH DESCRIPTION
.I Msgctl
provides a variety of message control operations as specified by
.IR cmd .
The following
.IR cmd s
are available:
.TP 15
.SM
.B IPC_STAT
Place the current value of each member of the data structure associated with
.I msqid
into the structure pointed to by
.IR buf .
The contents of this structure are defined in
.IR intro (2).
.SM {READ}
.TP
.SM
.B IPC_SET
Set the value of the following members of the data structure associated with
.I msqid
to the corresponding value found in the structure pointed to by
.IR buf :
.RS 
.RS
.nf
msg_perm.uid
msg_perm.gid
msg_perm.mode /\(** only low 9 bits \(**/
msg_qbytes
.fi
.RE
.RE
.IP
This \fIcmd\fP can only be executed by a process that has an effective user
.SM ID
equal to either that of super user or to the value of
.B msg_perm.uid
in the data structure associated with
.IR msqid .
Only super user can raise the value of
.BR msg_qbytes .
.TP
.SM
.B IPC_RMID
Remove the message queue identifier specified by
.I msqid
from the system and destroy the message queue and data structure
associated with it.
This \fIcmd\fP can only be executed by a process that has an effective user
.SM ID
equal to either that of super user or to the value of
.B msg_perm.uid
in the data structure associated with
.IR msqid .
.PP
.I Msgctl
will fail if one or more of the following are true:
.TP 15
.TP
\s-1\%[EINVAL]\s+1
.I Msqid
is not a valid message queue identifier.
.TP
\s-1\%[EINVAL]\s+1
.I Cmd
is not a valid command.
.TP
\s-1\%[EACCES]\s+1
.I Cmd
is equal to
.SM
.B IPC_STAT
and
.SM {READ}
operation permission is denied to the calling process (see
.IR intro (2)).
.TP
\s-1\%[EPERM]\s+1
.I Cmd
is equal to
.SM
.B IPC_RMID
or
.SM
.BR IPC_SET .
The effective user
.SM ID
of the calling process is not equal to that of super user and it is
not equal to the value of
.B msg_perm.uid
in the data structure associated with
.IR msqid .
.TP
\s-1\%[EPERM]\s+1
.I Cmd
is equal to
.SM
.BR IPC_SET\*S,
an attempt is being made to increase to the value of
.BR msg_qbytes,
and the effective user
.SM ID
of the calling process is not equal to that of super user.
.TP
\s-1\%[EFAULT]\s+1
.I Buf
points to an illegal address.
.SH "RETURN VALUE"
Upon successful completion, a value of 0 is returned. Otherwise, a
value of \-1 is returned and
.I errno\^
is set to indicate the error.
.SH SEE ALSO
intro(2), msgget(2), msgop(2).
.\"	@(#)msgctl.2	6.2 of 9/6/83

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