MSGCTL(2) | System Calls Manual | MSGCTL(2) |
msgctl
— message
control operations
#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
msqid_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
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.
Upon successful completion, the value 0 is returned; otherwise the value -1 is returned and the global variable errno is set to indicate the error.
msgctl
() will fail if:
EPERM
]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
]IPC_STAT
and the caller has no read
permission for this message queue.EINVAL
]cmd is not a valid command.
EFAULT
]Message queues appeared in AT&T System V Release 1 UNIX.
December 10, 2014 | OpenBSD-5.9 |