89 lines
3.9 KiB
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89 lines
3.9 KiB
HTML
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<!--Converted with LaTeX2HTML 96.1-c (Feb 29, 1996) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds -->
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<TITLE>SYSTEM CALL: msgget()</TITLE>
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<A NAME="tex2html802" HREF="node35.html"><IMG WIDTH=37 HEIGHT=24 ALIGN=BOTTOM ALT="next" SRC="next_motif.gif"></A> <A NAME="tex2html800" HREF="node27.html"><IMG WIDTH=26 HEIGHT=24 ALIGN=BOTTOM ALT="up" SRC="up_motif.gif"></A> <A NAME="tex2html794" HREF="node33.html"><IMG WIDTH=63 HEIGHT=24 ALIGN=BOTTOM ALT="previous" SRC="previous_motif.gif"></A> <A NAME="tex2html804" HREF="node1.html"><IMG WIDTH=65 HEIGHT=24 ALIGN=BOTTOM ALT="contents" SRC="contents_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME="tex2html803" HREF="node35.html">SYSTEM CALL: msgsnd()</A>
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<B>Up:</B> <A NAME="tex2html801" HREF="node27.html">6.4.2 Message Queues</A>
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<B> Previous:</B> <A NAME="tex2html795" HREF="node33.html">Kernel ipc_perm structure</A>
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<BR> <P>
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<H3><A NAME="SECTION00742300000000000000">SYSTEM CALL: msgget()</A></H3>
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<P>
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In order to create a new message queue, or access an existing queue, the
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<TT>msgget()</TT> system call is used.
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<P>
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<P>
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<HR><PRE> SYSTEM CALL: msgget();
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PROTOTYPE: int msgget ( key_t key, int msgflg );
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RETURNS: message queue identifier on success
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-1 on error: errno = EACCESS (permission denied)
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EEXIST (Queue exists, cannot create)
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EIDRM (Queue is marked for deletion)
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ENOENT (Queue does not exist)
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ENOMEM (Not enough memory to create queue)
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ENOSPC (Maximum queue limit exceeded)
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NOTES:</PRE>
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<HR>The first argument to <TT>msgget()</TT> is the key value (in our case returned
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by a call to <TT>ftok()</TT>). This key value is then compared to existing key
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values that exist within the kernel for other message queues. At that point,
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the open or access operation is dependent upon the contents of the <TT>msgflg</TT>
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argument.
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<P>
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<DL ><DT><STRONG>IPC_CREAT</STRONG>
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<DD>
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<P>
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Create the queue if it doesn't already exist in the kernel.
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<P>
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<DT><STRONG>IPC_EXCL</STRONG>
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<DD>
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<P>
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When used with IPC_CREAT, fail if queue already exists.
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<P>
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</DL>
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<P>
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If <TT>IPC_CREAT</TT> is used alone, <TT>msgget()</TT> either returns the message queue
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identifier for a newly created message queue, or returns the identifier for a queue
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which exists with the same key value. If <TT>IPC_EXCL</TT> is used along with <TT>IPC_CREAT</TT>,
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then either a new queue is created, or if the queue exists, the call fails with -1.
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<TT>IPC_EXCL</TT> is useless by itself, but when combined with <TT>IPC_CREAT</TT>, it can
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be used as a facility to guarantee that no existing queue is opened for access.
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<P>
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An optional octal mode may be OR'd into the mask, since each IPC object has
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permissions that are similar in functionality to file permissions on a UNIX
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file system!
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<P>
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Let's create a quick wrapper function for opening or creating message queue:
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<P>
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<P>
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<HR><PRE>int open_queue( key_t keyval )
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{
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int qid;
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if((qid = msgget( keyval, IPC_CREAT | 0660 )) == -1)
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{
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return(-1);
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}
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return(qid);
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}</PRE>
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<HR>Note the use of the explicit permissions of <TT>0660</TT>. This small function either returns
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a message queue identifier (<TT>int</TT>), or -1 on error. The key value must be passed to
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it as its only argument.
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<P>
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<BR> <HR>
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<P><ADDRESS>
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<I>Converted on: <BR>
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Fri Mar 29 14:43:04 EST 1996</I>
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