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

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
       IEEE Std 1003.1-2001, Section 3.224, Message Queue). It is unspecified whether this  func-
       tion  interoperates  with  the  realtime  interprocess communication facilities defined in
       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

       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 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  associ-
	      ated 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  appro-
	      priate 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 XSI Interprocess Communication can be easily
       modified to use the alternative interfaces.

RATIONALE
       None.

FUTURE DIRECTIONS
       None.

SEE ALSO
       XSI Interprocess Communication , 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 IEEE Std 1003.1-2001, <sys/msg.h>

COPYRIGHT
       Portions of this text are reprinted and	reproduced  in	electronic  form  from	IEEE  Std
       1003.1,	2003  Edition,	Standard  for Information Technology -- Portable Operating System
       Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003  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 orig-
       inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group			       2003					MSGCTL(P)
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