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<title>flush(9P) - Plan 9 from User Space</title>
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<tr><td width=20><td><b>FLUSH(9P)</b><td align=right><b>FLUSH(9P)</b>
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<p><font size=+1><b>NAME </b></font><br>
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flush – abort a message<br>
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<p><font size=+1><b>SYNOPSIS </b></font><br>
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<i>size</i>[4] <tt><font size=+1>Tflush</font></tt> <i>tag</i>[2] <i>oldtag</i>[2]<br>
<i>size</i>[4] <tt><font size=+1>Rflush</font></tt> <i>tag</i>[2]<br>
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<p><font size=+1><b>DESCRIPTION </b></font><br>
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When the response to a request is no longer needed, such as when
a user interrupts a process doing a <i>read</i>(9p), a <tt><font size=+1>Tflush</font></tt> request
is sent to the server to purge the pending response. The message
being flushed is identified by <i>oldtag</i>. The semantics of <tt><font size=+1>flush</font></tt>
depends on messages arriving in order.
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The server should answer the <tt><font size=+1>flush</font></tt> message immediately. If it
recognizes <i>oldtag</i> as the tag of a pending transaction, it should
abort any pending response and discard that tag. In either case,
it should respond with an <tt><font size=+1>Rflush</font></tt> echoing the <i>tag</i> (not <i>oldtag</i>)
of the <tt><font size=+1>Tflush</font></tt> message. A <tt><font size=+1>Tflush</font></tt> can never be
responded to by an <tt><font size=+1>Rerror</font></tt> message.
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The server may respond to the pending request before responding
to the <tt><font size=+1>Tflush</font></tt>. It is possible for a client to send multiple <tt><font size=+1>Tflush</font></tt>
messages for a particular pending request. Each subsequent <tt><font size=+1>Tflush</font></tt>
must contain as <i>oldtag</i> the tag of the pending request (not a previous
<tt><font size=+1>Tflush</font></tt>). Should multiple <tt><font size=+1>Tflush</font></tt>es be
received for a pending request, they must be answered in order.
A <tt><font size=+1>Rflush</font></tt> for any of the multiple <tt><font size=+1>Tflush</font></tt>es implies an answer for
all previous ones. Therefore, should a server receive a request
and then multiple flushes for that request, it need respond only
to the last flush.
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When the client sends a <tt><font size=+1>Tflush</font></tt>, it must wait to receive the corresponding
<tt><font size=+1>Rflush</font></tt> before reusing <i>oldtag</i> for subsequent messages. If a response
to the flushed request is received before the <tt><font size=+1>Rflush</font></tt>, the client
must honor the response as if it had not been flushed, since the
completed request may signify a state
change in the server. For instance, <tt><font size=+1>Tcreate</font></tt> may have created a
file and <tt><font size=+1>Twalk</font></tt> may have allocated a fid. If no response is received
before the <tt><font size=+1>Rflush</font></tt>, the flushed transaction is considered to have
been canceled, and should be treated as though it had never been
sent.
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Several exceptional conditions are handled correctly by the above
specification: sending multiple flushes for a single tag, flushing
after a transaction is completed, flushing a <tt><font size=+1>Tflush</font></tt>, and flushing
an invalid tag.<br>
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<p><font size=+1><b>ENTRY POINTS </b></font><br>
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The <a href="../man3/9pclient.html"><i>9pclient</i>(3)</a> library does not generate <tt><font size=+1>flush</font></tt> transactions..
<a href="../man4/9pserve.html"><i>9pserve</i>(4)</a> generates <tt><font size=+1>flush</font></tt> transactions to cancel transactions
pending when a client hangs up.<br>
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