int
msgctl(
int msqid
, int cmd
, struct msqid_ds *buf
)
)
system call performs control operations on the message queue specified
by
msqid
.
Each message queue has a
msqid_ds
structure associated with it which contains the following members:
struct ipc_perm msg_perm; /* msg queue permission bits */
msgqnum_t msg_qnum; /* # of msgs in the queue */
msglen_t msg_qbytes; /* max # of bytes on the queue */
pid_t msg_lspid; /* pid of last msgsnd() */
pid_t msg_lrpid; /* pid of last msgrcv() */
time_t msg_stime; /* time of last msgsnd() */
time_t msg_rtime; /* time of last msgrcv() */
time_t msg_ctime; /* time of last msgctl() */
The
ipc_perm
structure used inside the
msgid_ds
structure is defined in
<sys/ipc.h
>
and contains the following members:
uid_t cuid; /* creator user id */
gid_t cgid; /* creator group id */
uid_t uid; /* user id */
gid_t gid; /* group id */
mode_t mode; /* permission (lower 9 bits) */
The operation to be performed by
msgctl()
is specified in
cmd
and is one of:
IPC_STAT
buf
.
IPC_SET
msqid
.
The values are taken from the corresponding fields in the structure
pointed to by
buf
.
This operation can only be executed by the super-user, or a process that
has an effective user id equal to either
msg_perm.cuid
or
msg_perm.uid
in the data structure associated with the message queue.
The value of
msg_qbytes
can only be increased by the super-user.
Values for
msg_qbytes
that exceed the system limit
(
MSGMNB
from
<
sys/msg.h
>)
are silently truncated to that limit.
IPC_RMID
msqid
and destroy the data associated with it.
Only the super-user or a process with an effective uid equal to the
msg_perm.cuid
or
msg_perm.uid
values in the data structure associated with the queue can do this.
The permission to read from or write to a message queue (see msgsnd(2) and msgrcv(2)) is determined by the msg_perm.mode field in the same way as is done with files (see chmod(2)), but the effective uid can match either the msg_perm.cuid field or the msg_perm.uid field, and the effective gid can match either msg_perm.cgid or msg_perm.gid.
)
will fail if:
EPERM
]
cmd
is equal to
IPC_SET
or
IPC_RMID
and the caller is not the super-user, nor does
the effective uid match either the
msg_perm.uid
or
msg_perm.cuid
fields of the data structure associated with the message queue.
An attempt was made to increase the value of
msg_qbytes
through
IPC_SET
,
but the caller is not the super-user.
EACCES
]
cmd
is
IPC_STAT
and the caller has no read permission for this message queue.
EINVAL
]
msqid
is not a valid message queue identifier.
cmd
is not a valid command.
EFAULT
]
buf
specifies an invalid address.
AT&T
System V UNIX
.