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NAME
msgctl - XSI message control operations
SYNOPSIS
#include <sys/msg.h>
int msgctl(int msqid, int cmd, 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:
msg_perm.uid
msg_perm.gid
msg_perm.mode
msg_qbytes
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 .