MSGCTL(2) | System Calls Manual | MSGCTL(2) |
msgctl
—
#include <sys/msg.h>
int
msgctl
(int
msqid, int cmd,
struct msqid_ds
*buf);
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
IPC_SET
MSGMNB
from
<sys/msg.h>
) are silently
truncated to that limit.IPC_RMID
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.
msgctl
() will fail if:
EPERM
]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
]IPC_STAT
and the
caller has no read permission for this message queue.EINVAL
]cmd is not a valid command.
EFAULT
]msgctl
system call conforms to
X/Open System Interfaces and Headers Issue 5
(“XSH5”).
August 25, 1999 | NetBSD 9.2 |