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
       function  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
              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 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
       original Standard can be obtained online at http://www.opengroup.org/unix/online.html .