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

       msgget — get the XSI message queue identifier

SYNOPSIS

       #include <sys/msg.h>

       int msgget(key_t key, int msgflg);

DESCRIPTION

       The  msgget()  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 msgget() function shall return the message queue identifier associated with the argument key.

       A  message  queue  identifier,  associated  message queue, and data structure (see <sys/msg.h>), shall be
       created for the argument key if one of the following is true:

        *  The argument key is equal to IPC_PRIVATE.

        *  The argument key does not already have a message queue identifier associated with it, and  (msgflg  &
           IPC_CREAT) is non-zero.

       Upon  creation,  the data structure associated with the new message queue identifier shall be initialized
       as follows:

        *  msg_perm.cuid, msg_perm.uid, msg_perm.cgid, and msg_perm.gid shall be set to the  effective  user  ID
           and effective group ID, respectively, of the calling process.

        *  The low-order 9 bits of msg_perm.mode shall be set to the low-order 9 bits of msgflg.

        *  msg_qnum, msg_lspid, msg_lrpid, msg_stime, and msg_rtime shall be set to 0.

        *  msg_ctime shall be set to the current time, as described in Section 2.7.1, IPC General Description.

        *  msg_qbytes shall be set to the system limit.

RETURN VALUE

       Upon  successful  completion,  msgget()  shall  return  a  non-negative  integer,  namely a message queue
       identifier.  Otherwise, it shall return -1 and set errno to indicate the error.

ERRORS

       The msgget() function shall fail if:

       EACCES A message queue identifier exists for the argument key, but operation permission as  specified  by
              the  low-order  9  bits  of  msgflg  would  not  be  granted;  see  Section  2.7, XSI Interprocess
              Communication.

       EEXIST A message queue identifier exists for the argument key but ((msgflg  &  IPC_CREAT)  &&  (msgflg  &
              IPC_EXCL)) is non-zero.

       ENOENT A message queue identifier does not exist for the argument key and (msgflg & IPC_CREAT) is 0.

       ENOSPC A  message queue identifier is to be created but the system-imposed limit on the maximum number of
              allowed message queue identifiers system-wide would be exceeded.

       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,  ftok(),  mq_close(),  mq_getattr(),
       mq_notify(), mq_open(), mq_receive(), mq_send(), mq_setattr(), mq_unlink(), msgctl(), 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                                        MSGGET(3POSIX)