MSGCTL(3)                  POSIX Programmer's Manual                 MSGCTL(3)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

NAME
       msgctl -- XSI message control operations

SYNOPSIS
       #include <sys/msg.h>

       int msgctl(int msqid, int cmd, struct msqid_ds *buf);

DESCRIPTION
       The msgctl() function operates on XSI message queues (see the Base
       Definitions volume of POSIX.1-2017, Section 3.226, Message Queue).  It
       is unspecified whether this function interoperates with the realtime
       interprocess communication facilities defined in Section 2.8, Realtime.

       The msgctl() function shall provide message control operations as
       specified by cmd.  The following values for cmd, and the message
       control operations they specify, are:

       IPC_STAT    Place the current value of each member of the msqid_ds data
                   structure associated with msqid into the structure pointed
                   to by buf.  The contents of this structure are defined in
                   <sys/msg.h>.

       IPC_SET     Set the value of the following members of the msqid_ds data
                   structure associated with msqid to the corresponding value
                   found in the structure pointed to by buf:


                       msg_perm.uid
                       msg_perm.gid
                       msg_perm.mode
                       msg_qbytes

                   Also, the msg__ctime timestamp shall be set to the current
                   time, as described in Section 2.7.1, IPC General
                   Description.

                   IPC_SET can only be executed by a process with appropriate
                   privileges or that has an effective user ID equal to the
                   value of msg_perm.cuid or msg_perm.uid in the msqid_ds data
                   structure associated with msqid.  Only a process with
                   appropriate privileges can raise the value of msg_qbytes.

       IPC_RMID    Remove the message queue identifier specified by msqid from
                   the system and destroy the message queue and 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 msg_perm.cuid or
                   msg_perm.uid in the msqid_ds data structure associated with
                   msqid.

RETURN VALUE
       Upon successful completion, msgctl() shall return 0; otherwise, it
       shall return -1 and set errno to indicate the error.

ERRORS
       The msgctl() function shall fail if:

       EACCES The argument cmd is IPC_STAT and the calling process does not
              have read permission; see Section 2.7, XSI Interprocess
              Communication.

       EINVAL The value of msqid is not a valid message queue identifier; or
              the value of cmd is not a valid command.

       EPERM  The argument 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
              msg_perm.cuid or msg_perm.uid in the data structure associated
              with msqid.

       EPERM  The argument cmd is IPC_SET, an attempt is being made to
              increase to the value of msg_qbytes, and the effective user ID
              of the calling process does not have appropriate privileges.

       The following sections are informative.

EXAMPLES
       None.

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 Section 2.7, XSI Interprocess Communication can
       be easily modified to use the alternative interfaces.

RATIONALE
       None.

FUTURE DIRECTIONS
       None.

SEE ALSO
       Section 2.7, XSI Interprocess Communication, Section 2.8, Realtime,
       mq__close(), mq__getattr(), mq__notify(), mq__open(), mq__receive(),
       mq__send(), mq__setattr(), mq__unlink(), msgget(), msgrcv(), msgsnd()

       The Base Definitions volume of POSIX.1-2017, Section 3.226, Message
       Queue, <sys_msg.h>

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 .

       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 .



IEEE/The Open Group                  2017                            MSGCTL(3)
