MSGCTL(P) POSIX Programmer's Manual MSGCTL(P)
msgctl - XSI message control operations
int msgctl(int msqid, int cmd, struct msqid_ds *buf);
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 func-
tion interoperates with the realtime interprocess communication facilities defined in
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:
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>.
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.
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.
Upon successful completion, msgctl() shall return 0; otherwise, it shall return -1 and set
errno to indicate the error.
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 associ-
ated 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 appro-
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 interfaces.
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>
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 orig-
inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .
IEEE/The Open Group 2003 MSGCTL(P)