'\" et
.TH MSGCTL "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.\"
.SH NAME
msgctl
\(em XSI message control operations
.SH SYNOPSIS
.LP
.nf
#include <sys/msg.h>
.P
int msgctl(int \fImsqid\fP, int \fIcmd\fP, struct msqid_ds *\fIbuf\fP);
.fi
.SH DESCRIPTION
The
\fImsgctl\fR()
function operates on XSI message queues (see the Base Definitions volume of POSIX.1\(hy2017,
.IR "Section 3.226" ", " "Message Queue").
It is unspecified whether this function interoperates with the
realtime interprocess communication facilities defined in
.IR "Section 2.8" ", " "Realtime".
.P
The
\fImsgctl\fR()
function shall provide message control operations as specified by
.IR cmd .
The following values for
.IR cmd ,
and the message control operations they specify, are:
.IP IPC_STAT 12
Place the current value of each member of the
.BR msqid_ds
data structure associated with
.IR msqid
into the structure pointed to by
.IR buf .
The contents of this structure are defined in
.IR <sys/msg.h> .
.IP IPC_SET 12
Set the value of the following members of the
.BR msqid_ds
data structure associated with
.IR msqid
to the corresponding value found in the structure pointed to by
.IR buf :
.RS 12 
.sp
.RS 4
.nf

msg_perm.uid
msg_perm.gid
msg_perm.mode
msg_qbytes
.fi
.P
.RE
.P
Also, the
.IR msg_ctime
timestamp shall be set to the current time, as described in
.IR "Section 2.7.1" ", " "IPC General Description".
.P
IPC_SET can only be executed by a process with appropriate privileges
or that has an effective user ID equal to the value of
.BR msg_perm.cuid
or
.BR msg_perm.uid
in the
.BR msqid_ds
data structure associated with
.IR msqid .
Only a process with appropriate privileges can raise the value of
.BR msg_qbytes .
.RE
.IP IPC_RMID 12
Remove the message queue identifier specified by
.IR msqid
from the system and destroy the message queue and
.BR msqid_ds
data structure associated with it. IPC_RMD can only be executed by a
process with appropriate privileges or one that has an effective user
ID equal to the value of
.BR msg_perm.cuid
or
.BR msg_perm.uid
in the
.BR msqid_ds
data structure associated with
.IR msqid .
.SH "RETURN VALUE"
Upon successful completion,
\fImsgctl\fR()
shall return 0; otherwise, it shall return \-1 and set
.IR errno
to indicate the error.
.SH ERRORS
The
\fImsgctl\fR()
function shall fail if:
.TP
.BR EACCES
The argument
.IR cmd
is IPC_STAT and the calling process does not have read permission; see
.IR "Section 2.7" ", " "XSI Interprocess Communication".
.TP
.BR EINVAL
The value of
.IR msqid
is not a valid message queue identifier; or the value of
.IR cmd
is not a valid command.
.TP
.BR EPERM
The argument
.IR cmd
is IPC_RMID or IPC_SET
and the effective user ID of the calling process is not equal to that
of a process with appropriate privileges and it is not equal to the
value of
.BR msg_perm.cuid
or
.BR msg_perm.uid
in the data structure associated with
.IR msqid .
.TP
.BR EPERM
The argument
.IR cmd
is IPC_SET, an attempt is being made to increase to the value of
.BR msg_qbytes ,
and the effective user ID of the calling process does not have
appropriate privileges.
.LP
.IR "The following sections are informative."
.SH EXAMPLES
None.
.SH "APPLICATION USAGE"
The POSIX Realtime Extension defines alternative interfaces for interprocess communication
(IPC). Application developers who need to use IPC should design their
applications so that modules using the IPC routines described in
.IR "Section 2.7" ", " "XSI Interprocess Communication"
can be easily modified to use the alternative interfaces.
.SH RATIONALE
None.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "Section 2.7" ", " "XSI Interprocess Communication",
.IR "Section 2.8" ", " "Realtime",
.IR "\fImq_close\fR\^(\|)",
.IR "\fImq_getattr\fR\^(\|)",
.IR "\fImq_notify\fR\^(\|)",
.IR "\fImq_open\fR\^(\|)",
.IR "\fImq_receive\fR\^(\|)",
.IR "\fImq_send\fR\^(\|)",
.IR "\fImq_setattr\fR\^(\|)",
.IR "\fImq_unlink\fR\^(\|)",
.IR "\fImsgget\fR\^(\|)",
.IR "\fImsgrcv\fR\^(\|)",
.IR "\fImsgsnd\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "Section 3.226" ", " "Message Queue",
.IR "\fB<sys_msg.h>\fP"
.\"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
.PP
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .