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<H4><A NAME="SECTION00742230000000000000">Kernel <TT>msqid_ds</TT> structure</A></H4>
<P>
Each of the three types of IPC objects has an internal data structure which is maintained
by the kernel. For message queues, this is the <TT>msqid_ds</TT> structure. The kernel creates,
stores, and maintains an instance of this structure for every message queue created on the
system. It is defined in <TT>linux/msg.h</TT> as follows:
<P>
<P>
<HR><PRE>/* one msqid structure for each queue on the system */
struct msqid_ds {
struct ipc_perm msg_perm;
struct msg *msg_first; /* first message on queue */
struct msg *msg_last; /* last message in queue */
time_t msg_stime; /* last msgsnd time */
time_t msg_rtime; /* last msgrcv time */
time_t msg_ctime; /* last change time */
struct wait_queue *wwait;
struct wait_queue *rwait;
ushort msg_cbytes;
ushort msg_qnum;
ushort msg_qbytes; /* max number of bytes on queue */
ushort msg_lspid; /* pid of last msgsnd */
ushort msg_lrpid; /* last receive pid */
};</PRE>
<HR>While you will rarely have to concern yourself with most of the members of this structure, a brief
description of each is in order to complete our tour:
<P>
<DL ><DT><STRONG><TT>msg_perm</TT></STRONG>
<DD>
<P>
An instance of the <TT>ipc_perm</TT> structure, which is defined
for us in <TT>linux/ipc.h</TT>. This holds the permission information for
the message queue, including the access permissions, and information
about the creator of the queue (uid, etc).
<P>
<DT><STRONG><TT>msg_first</TT></STRONG>
<DD>
<P>
Link to the first message in the queue (the head of the list).
<P>
<DT><STRONG><TT>msg_last</TT></STRONG>
<DD>
<P>
Link to the last message in the queue (the tail of the list).
<P>
<DT><STRONG><TT>msg_stime</TT></STRONG>
<DD>
<P>
Timestamp (<TT>time_t</TT>) of the last message that was sent to the queue.
<P>
<DT><STRONG><TT>msg_rtime</TT></STRONG>
<DD>
<P>
Timestamp of the last message retrieved from the queue.
<P>
<DT><STRONG><TT>msg_ctime</TT></STRONG>
<DD>
<P>
Timestamp of the last ``change'' made to the queue (more on this later).
<P>
<DT><STRONG><TT>wwait</TT></STRONG>
<DD>
<P>
and
<P>
<DT><STRONG><TT>rwait</TT></STRONG>
<DD>
<P>
Pointers into the kernel's <EM>wait queue</EM>. They are
used when an operation on a message queue deems the process go into a sleep
state (i.e. queue is full and the process is waiting for an opening).
<P>
<DT><STRONG><TT>msg_cbytes</TT></STRONG>
<DD>
<P>
Total number of bytes residing on the queue (sum of the sizes of all messages).
<P>
<DT><STRONG><TT>msg_qnum</TT></STRONG>
<DD>
<P>
Number of messages currently in the queue.
<P>
<DT><STRONG><TT>msg_qbytes</TT></STRONG>
<DD>
<P>
Maximum number of bytes on the queue.
<P>
<DT><STRONG><TT>msg_lspid</TT></STRONG>
<DD>
<P>
The PID of the process who sent the last message.
<P>
<DT><STRONG><TT>msg_lrpid</TT></STRONG>
<DD>
<P>
The PID of the process who retrieved the last message.
<P>
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<I>Converted on: <BR>
Fri Mar 29 14:43:04 EST 1996</I>
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