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 .