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<h1><a href="../index.html">Peter's website <span id="headerSubTitle">- random notes</span></a></h1>
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<article>
<h1>Creating websocket connections via 9front's webfs</h1>
<p>With the following patch (note: work in progress, contains bugs), 9front's webfs can be used to create websocket connections. The patch is available <a href="https://files.pmikkelsen.com/websocket-webfs.patch">here</a>.</p>
<h2>The idea</h2>
<p>Creating a websocket connection is done just like creating a http request is, except the url must use scheme <code>wss://</code> or <code>ws://</code>, and the <code>body</code> file must be opened read-write to allow both sending and receiving.</p>
<p>The very first message to be read from <code>body</code> is always "websocket ready", meaning the connection has been established. After that the actual communication begins.</p>
<p>Webfs itself takes care of the control messages, so only the data messages gets delivered to the user, in a textual format that is easy to parse. Each message read and writen to <code>body</code> consists of a 22 byte header in the following format</p>
<code><pre>
b nnnnnnnnnnnnnnnnnnnn
</pre></code>
<p>or</p>
<code><pre>
t nnnnnnnnnnnnnnnnnnnn
</pre></code>
<p>for binary and text messages respectively. The 20 last bytes are a 0-padded ascii number in base 10, telling the size of the message body, which follows directly after the header.</p>
<h2>Example program</h2>
<p>To demonstrate, the following rc script connects to <code>wss://echo.websocket.org</code> which is a service that echos everything back to the user.</p>
<code><pre>
#!/bin/rc
rfork en
webfs
fn sendtextmsg{
awk 'BEGIN{printf "t %0.20d%s", length(ARGV[1]), ARGV[1]}' $1 >[1=0]
}
fn readheader{
header=`{read -c 22}
length=`{echo $header(2) p | dc}
echo $header(1) $length
}
fn handlemsg{
echo Handling message:
echo ' TYPE :' $1
echo ' LENGTH:' $2
echo ' BODY :' $3
}
<>/mnt/web/clone {
d=/mnt/web/^`{sed 1q}
echo url wss://echo.websocket.org >[1=0]
<>$d/body {
read -n 1 # Read the ready message
sendtextmsg 'First message sent'
while(header=`{readheader}){
echo Header: $header
body=`''{read -c $header(2)}
handlemsg $header(1) $header(2) $body
sleep 2
sendtextmsg 'Echo test'
}
}
}
</pre></code>
<h2>Bugs</h2>
<p>Yes. The code doesn't really handle errors or connection shutdowns yet. Also, I need to add proper cleanup code around in different places.</p>
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