msgctl - message control operations
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
int
msgctl(int msqid, int cmd, struct msqid_ds *buf);
The msgctl() system call performs some control operations on
the message
queue specified by msqid.
Each message queue has a data structure associated with it,
parts of
which may be altered by msgctl() and parts of which determine the actions
of msgctl(). The data structure is defined in <sys/msg.h>
and contains
(amongst others) the following members:
struct msqid_ds {
struct ipc_perm msg_perm; /* msg queue permission bits */
u_long msg_cbytes; /* # of bytes in use
on the queue */
u_long msg_qnum; /* # of msgs in the
queue */
u_long 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 shmid_ds structure
is defined in
<sys/ipc.h> and looks like this:
struct ipc_perm {
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 (9 bits, see chmod(2))
*/
u_short seq; /* sequence # (to generate unique
id) */
key_t key; /* user specified msg/sem/shm key */
};
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 superuser, 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 superuser. 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 superuser 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.
Upon successful completion, a value of 0 is returned. Otherwise, -1 is
returned and the global variable errno is set to indicate
the error.
msgctl() will fail if:
[EPERM] cmd is equal to IPC_SET or IPC_RMID and the
caller is not
the superuser, 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 is made to increase the value of
msg_qbytes
through IPC_SET but the caller is not the superuser.
[EACCES] The command 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.
msgget(2), msgrcv(2), msgsnd(2)
Message queues appeared in the first release of AT&T Unix
System V.
OpenBSD 3.6 August 17, 1995
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