int msgctl(int msqid, intcmd, struct msqid_ds
*buf);
DESCRIPTION
The msgctl() 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 msgctl() function shall provide message control operations
as specified by cmd. The following values for
cmd, and the message control operations they specify, are:
IPC_STAT
Place the current value of each member of the msqid_ds data
structure associated with msqid into the structure
pointed to by buf. The contents of this structure are defined
in <sys/msg.h>.
IPC_SET
Set the value of the following members of the msqid_ds data
structure associated with msqid to the corresponding
value found in the structure pointed to by buf:
IPC_SET can only be executed by a process with appropriate privileges
or that has an effective user ID equal to the value of
msg_perm.cuid or msg_perm.uid in the msqid_ds data
structure associated with msqid. Only a process with
appropriate privileges can raise the value of msg_qbytes.
IPC_RMID
Remove the message queue identifier specified by msqid from
the system and destroy the message queue and msqid_ds
data structure associated with it. IPC_RMD can only be executed by
a process with appropriate privileges or one that has an
effective user ID equal to the value of msg_perm.cuid or msg_perm.uid
in the msqid_ds data structure
associated with msqid.
RETURN VALUE
Upon successful completion, msgctl() shall return 0; otherwise,
it shall return -1 and set errno to indicate the
error.
ERRORS
The msgctl() function shall fail if:
EACCES
The argument cmd is IPC_STAT and the calling process does not
have read permission; see XSI Interprocess Communication .
EINVAL
The value of msqid is not a valid message queue identifier;
or the value of cmd is not a valid command.
EPERM
The argument cmd is IPC_RMID or IPC_SET and the effective user
ID of the calling process is not equal to that of a
process with appropriate privileges and it is not equal to the value
of msg_perm.cuid or msg_perm.uid in the data
structure associated with msqid.
EPERM
The argument cmd is IPC_SET, an attempt is being made to increase
to the value of msg_qbytes, and the effective
user ID of the calling process does not have appropriate privileges.
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() , msgget()
, 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 .
