MSGGET(3)                  POSIX Programmer's Manual                 MSGGET(3)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

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(3)
