86 lines
4.7 KiB
HTML
86 lines
4.7 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
|
|
<!--Converted with LaTeX2HTML 96.1-c (Feb 29, 1996) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds -->
|
|
<HTML>
|
|
<HEAD>
|
|
<TITLE>Message buffer</TITLE>
|
|
<META NAME="description" CONTENT="Message buffer">
|
|
<META NAME="keywords" CONTENT="lpg">
|
|
<META NAME="resource-type" CONTENT="document">
|
|
<META NAME="distribution" CONTENT="global">
|
|
<LINK REL=STYLESHEET HREF="lpg.css">
|
|
</HEAD>
|
|
<BODY LANG="EN">
|
|
<A NAME="tex2html760" HREF="node31.html"><IMG WIDTH=37 HEIGHT=24 ALIGN=BOTTOM ALT="next" SRC="next_motif.gif"></A> <A NAME="tex2html758" HREF="node29.html"><IMG WIDTH=26 HEIGHT=24 ALIGN=BOTTOM ALT="up" SRC="up_motif.gif"></A> <A NAME="tex2html752" HREF="node29.html"><IMG WIDTH=63 HEIGHT=24 ALIGN=BOTTOM ALT="previous" SRC="previous_motif.gif"></A> <A NAME="tex2html762" HREF="node1.html"><IMG WIDTH=65 HEIGHT=24 ALIGN=BOTTOM ALT="contents" SRC="contents_motif.gif"></A> <BR>
|
|
<B> Next:</B> <A NAME="tex2html761" HREF="node31.html">Kernel msg structure</A>
|
|
<B>Up:</B> <A NAME="tex2html759" HREF="node29.html">Internal and User Data </A>
|
|
<B> Previous:</B> <A NAME="tex2html753" HREF="node29.html">Internal and User Data </A>
|
|
<BR> <P>
|
|
<H4><A NAME="SECTION00742210000000000000">Message buffer</A></H4>
|
|
<P>
|
|
The first structure we'll visit is the <TT>msgbuf</TT> structure. This particular
|
|
data structure can be thought of as a <EM>template</EM> for message data. While
|
|
it is up to the programmer to define structures of this type, it is imperative
|
|
that you understand that there <B>is</B> actually a structure of type <TT>msgbuf</TT>. It is declared in <TT>linux/msg.h</TT> as follows:
|
|
<P>
|
|
<P>
|
|
<HR><PRE>/* message buffer for msgsnd and msgrcv calls */
|
|
struct msgbuf {
|
|
long mtype; /* type of message */
|
|
char mtext[1]; /* message text */
|
|
};</PRE>
|
|
<HR>There are two members in the <TT>msgbuf</TT> structure:
|
|
<P>
|
|
<DL ><DT><STRONG><TT>mtype</TT></STRONG>
|
|
<DD>
|
|
<P>
|
|
The message <EM>type</EM>, represented in a positive number. This <EM>must</EM>
|
|
be a positive number!
|
|
<P>
|
|
<DT><STRONG><TT>mtext</TT></STRONG>
|
|
<DD>
|
|
<P>
|
|
The message data itself.
|
|
<P>
|
|
</DL>
|
|
<P>
|
|
The ability to assign a given message a <EM>type</EM>, essentially gives you the capability to
|
|
<EM>multiplex</EM> messages on a single queue. For instance, client processes could be
|
|
assigned a magic number, which could be used as the message type for messages sent from a server
|
|
process. The server itself could use some other number, which clients could use to send
|
|
messages to it. In another scenario, an application could mark error messages as having a message
|
|
type of 1, request messages could be type 2, etc. The possibilities are endless.
|
|
<P>
|
|
On another note, do not be misled by the almost too-descriptive name assigned to the message
|
|
data element (<TT>mtext</TT>). This field is not restricted to holding only arrays of characters,
|
|
but any data, in any form. The field itself is actually completely arbitrary, since this structure
|
|
gets redefined by the application programmer. Consider this redefinition:
|
|
<P>
|
|
<P>
|
|
<HR><PRE>struct my_msgbuf {
|
|
long mtype; /* Message type */
|
|
long request_id; /* Request identifier */
|
|
struct client info; /* Client information structure */
|
|
};</PRE>
|
|
<HR>Here we see the message type, as before, but the remainder of the structure has been replaced
|
|
by two other elements, one of which is another structure! This is the beauty of message queues.
|
|
The kernel makes no translations of data whatsoever. Any information can be sent.
|
|
<P>
|
|
There does exist an internal limit, however, of the maximum size of a given message. In Linux,
|
|
this is defined in <TT>linux/msg.h</TT> as follows:
|
|
<P>
|
|
<P>
|
|
<HR><PRE>#define MSGMAX 4056 /* <= 4056 */ /* max size of message (bytes) */</PRE>
|
|
<HR>Messages can be no larger than 4,056 bytes in total size, including the <TT>mtype</TT> member,
|
|
which is 4 bytes in length (<TT>long</TT>).
|
|
<P>
|
|
<HR><A NAME="tex2html760" HREF="node31.html"><IMG WIDTH=37 HEIGHT=24 ALIGN=BOTTOM ALT="next" SRC="next_motif.gif"></A> <A NAME="tex2html758" HREF="node29.html"><IMG WIDTH=26 HEIGHT=24 ALIGN=BOTTOM ALT="up" SRC="up_motif.gif"></A> <A NAME="tex2html752" HREF="node29.html"><IMG WIDTH=63 HEIGHT=24 ALIGN=BOTTOM ALT="previous" SRC="previous_motif.gif"></A> <A NAME="tex2html762" HREF="node1.html"><IMG WIDTH=65 HEIGHT=24 ALIGN=BOTTOM ALT="contents" SRC="contents_motif.gif"></A> <BR>
|
|
<B> Next:</B> <A NAME="tex2html761" HREF="node31.html">Kernel msg structure</A>
|
|
<B>Up:</B> <A NAME="tex2html759" HREF="node29.html">Internal and User Data </A>
|
|
<B> Previous:</B> <A NAME="tex2html753" HREF="node29.html">Internal and User Data </A>
|
|
<P><ADDRESS>
|
|
<I>Converted on: <BR>
|
|
Fri Mar 29 14:43:04 EST 1996</I>
|
|
</ADDRESS>
|
|
</BODY>
|
|
</HTML>
|