msgctl(2) DG/UX 5.4 Rel. 2.01 msgctl(2)
NAME
msgctl - get or set message queue attributes or destroy a message
queue
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
int msgctl (msqid, cmd, buf)
int msqid;
int cmd;
struct msqidds * buf;
where:
msqid A message queue identifier
cmd The message control operation to be performed
buf The address of a message queue attribute record (used only
if cmd is IPC_STAT or IPC_SET)
DESCRIPTION
Use msgctl to get and set message queue attributes or to destroy a
message queue. The subject message queue is identified by msqid.
The action performed by msgctl depends on the value of cmd as
follows:
IPC_STAT The user-visible msqid_ds structure is returned in buf. If
an error occurs, the contents of buf are undefined.
IPC_SET The following message queue attributes are set to the
values found in the structure pointed to by buf: user id
(msg_perm.uid), group id (msg_perm.gid), permission rights
(in msg_perm.mode), and the maximum size (msg_qbytes).
If an error occurs, the message queue remains unchanged.
Otherwise, the last change time (msg_ctime) is set to the
current time.
IPC_RMID The message queue is destroyed. All resources consumed by
the message queue are freed and the message queue
identifier is invalidated. All queued messages are lost.
If an error occurs, the message queue remains unchanged.
Note that the msqidds struct that is returned with the IPC_STAT and
IPC_SET action arguments differs in the ELF and COFF environments.
Specifically, the fields msgcbytes, msgqnum, and msgqbytes are
type unsigned long in the ELF environment and type unsigned short in
the COFF environment. In both environments, a message queue allows
the attributes reported in these fields to have values larger than
will fit in a short container. Therefore, if a COFF program uses
msgctl(2) with the IPC_STAT option, the kernel might not report the
correct values for these attributes.
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msgctl(2) DG/UX 5.4 Rel. 2.01 msgctl(2)
ACCESS CONTROL
Operation permission depends on the value of cmd as follows:
IPC_STAT The calling process is required to have read access to the
message queue.
IPC_SET The effective user id of the calling process must be equal
to the message queue's user id, the message queue creator's
user id, or that of the superuser. If the maximum size of
the message queue is increased, the effective user id of
the calling process must be the superuser.
IPC_RMID The effective user id of the calling process must be equal
to the message queue's user id, the message queue creator's
user id, or that of the superuser.
RETURN VALUE
0 Completed successfully.
-1 An error occurred. errno is set to indicate the error.
DIAGNOSTICS
Errno may be set to one of the following error codes regardless of
the value of cmd:
EINVAL msqid is not a valid message queue identifier.
EINVAL cmd is not a valid command.
If cmd is IPC_STAT, errno may be set to one of these values:
EACCES Read permission is denied to the calling process.
EFAULT buf points to an illegal address.
If cmd is IPC_SET, errno may be set to one of these values:
EPERM Permission to change the message queue attributes is denied
to the calling process.
EPERM Permission to increase to the maximum size of the message
queue is denied to the calling process.
EFAULT buf points to an illegal address.
If cmd is IPC_RMID, errno may be set to this value:
EPERM Permission to remove the message queue is denied to the
calling process.
SEE ALSO
intro(2), ipcrm(1), ipcs(1), msgget(2), msgrcv(2), msgsnd(2).
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