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.

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 .