NAME
msgctl
—
message control operations
SYNOPSIS
#include
<sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
int
msgctl
(int
msqid, int cmd,
struct msqid_ds
*buf);
DESCRIPTION
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
- 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.
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
orIPC_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.
SEE ALSO
HISTORY
Message queues appeared in AT&T System V Release 1 UNIX.