NAME
msgctl —
message control
operations
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/msg.h>
int
msgctl(
int
msqid,
int cmd,
struct msqid_ds *buf);
DESCRIPTION
The
msgctl() 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
- Gather information about the message queue and place it in
the structure pointed to by buf.
-
-
IPC_SET
- Set the value of the msg_perm.uid,
msg_perm.gid, msg_perm.mode
and msg_qbytes fields in the structure associated
with 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
- Remove the message queue specified by
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.
RETURN VALUES
Upon successful completion, a value of 0 is returned. Otherwise, -1 is returned
and the global variable
errno is set to indicate the
error.
ERRORS
msgctl() 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.
SEE ALSO
msgget(2),
msgrcv(2),
msgsnd(2)
STANDARDS
The
msgctl system call conforms to
X/Open
System Interfaces and Headers Issue 5 (“XSH5”).
HISTORY
Message queues appeared in the first release of
AT&T
System V UNIX.