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.