MSGGET(3P) POSIX Programmer's Manual MSGGET(3P)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.
NAMEmsgget — get the XSI message queue identifier
int msgget(key_t key, int msgflg);
The msgget() function operates on XSI message queues (see the Base Def‐
initions volume of POSIX.1‐2008, Section 3.225, 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 associ‐
ated with the argument key.
A message queue identifier, associated message queue, and data struc‐
ture (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, respec‐
tively, 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.
Upon successful completion, msgget() shall return a non-negative inte‐
ger, namely a message queue identifier. Otherwise, it shall return −1
and set errno to indicate the error.
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
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.
The POSIX Realtime Extension defines alternative interfaces for inter‐
process 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.
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.1‐2008, Section 3.225, Message
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
cal and Electronics Engineers, Inc and The Open Group. (This is
POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/online.html .
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IEEE/The Open Group 2013 MSGGET(3P)