The msgget() function operates on XSI message queues (see the
Base Definitions volume of IEEE Std 1003.1-2001,
Section 3.224, Message Queue). It is unspecified whether this function
interoperates with the realtime interprocess communication facilities
defined in 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
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)
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 equal 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 equal to
the low-order 9 bits of msgflg.
msg_qnum, msg_lspid, msg_lrpid, msg_stime,
and msg_rtime shall be set equal to 0.
msg_ctime shall be set equal to the current time.
msg_qbytes shall be set equal to the system limit.
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.
The msgget() function shall fail if:
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 XSI Interprocess Communication
A message queue identifier exists for the argument key but ((msgflg
& IPC_CREAT) && (msgflg
& IPC_EXCL)) is non-zero.
A message queue identifier does not exist for the argument key
and (msgflg & IPC_CREAT) is 0.
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 interprocess
communication (IPC). Application developers who
need to use IPC should design their applications so that modules using
the IPC routines described in XSI Interprocess Communication
can be easily modified to use the alternative
XSI Interprocess Communication , Realtime , mq_close()
, mq_getattr() , mq_notify() , mq_open() , mq_receive()
, mq_send() , mq_setattr() , mq_unlink() , msgctl()
, msgrcv() ,
msgsnd() , the Base Definitions volume of IEEE Std 1003.1-2001,
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 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