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 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 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 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.
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 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 XSI Interprocess Communication can be easily
modified to use the alternative interfaces.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
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, <sys/msg.h>
COPYRIGHT
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 http://www.opengroup.org/unix/online.html .