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Add documentation for the IPC interface

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Michael Stapelberg 2010-03-12 03:05:42 +01:00
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<body>
<div id="header">
<h1>IPC interface (interprocess communication)</h1>
<span id="author">Michael Stapelberg</span><br />
<span id="email"><tt>&lt;<a href="mailto:michael+i3@stapelberg.de">michael+i3@stapelberg.de</a>&gt;</tt></span><br />
<span id="revdate">March 2010</span>
<div id="toc">
<div id="toctitle">Table of Contents</div>
<noscript><p><b>JavaScript must be enabled in your browser to display the table of contents.</b></p></noscript>
</div>
</div>
<div id="content">
<div id="preamble">
<div class="sectionbody">
<div class="paragraph"><p>This document describes how to interface with i3 from a separate process. This
is useful for example to remote-control i3 (to write test cases for example) or
to get various information like the current workspaces to implement an external
workspace bar.</p></div>
<div class="paragraph"><p>The method of choice for IPC in our case is a unix socket because it has very
little overhead on both sides and is usually available without headaches in
most languages. In the default configuration file, no ipc-socket path is
specified and thus no socket is created. The standard path (which <tt>i3-msg</tt> and
<tt>i3-input</tt> use) is <tt>/tmp/i3-ipc.sock</tt>.</p></div>
</div>
</div>
<h2 id="_establishing_a_connection">1. Establishing a connection</h2>
<div class="sectionbody">
<div class="paragraph"><p>To establish a connection, simply open the IPC socket. The following code
snippet illustrates this in Perl:</p></div>
<div class="listingblock">
<div class="content">
<pre><tt>use IO::Socket::UNIX;
my $sock = IO::Socket::UNIX-&gt;new(Peer =&gt; '/tmp/i3-ipc.sock');</tt></pre>
</div></div>
</div>
<h2 id="_sending_messages_to_i3">2. Sending messages to i3</h2>
<div class="sectionbody">
<div class="paragraph"><p>To send a message to i3, you have to format in the binary message format which
i3 expects. This format specifies a magic string in the beginning to ensure
the integrity of messages (to prevent follow-up errors). Afterwards follows
the length of the payload of the message as 32-bit integer and the type of
the message as 32-bit integer (the integers are not converted, so they are
in native byte order).</p></div>
<div class="paragraph"><p>The magic string currently is "i3-ipc" and will only be changed when a change
in the IPC API is done which breaks compatibility (we hope that we don’t need
to do that).</p></div>
<div class="paragraph"><p>Currently implemented message types are the following:</p></div>
<div class="dlist"><dl>
<dt class="hdlist1">
0 (COMMAND)
</dt>
<dd>
<p>
The payload of the message is a command for i3 (like the commands you
can bind to keys in the configuration file) and will be executed
directly after receiving it. There is no reply to this message.
</p>
</dd>
<dt class="hdlist1">
1 (GET_WORKSPACES)
</dt>
<dd>
<p>
Gets the current workspaces. The reply will be a JSON-encoded list of
workspaces (see the reply section).
</p>
</dd>
</dl></div>
<div class="paragraph"><p>So, a typical message could look like this:</p></div>
<div class="listingblock">
<div class="content">
<pre><tt>"i3-ipc" &lt;message length&gt; &lt;message type&gt; &lt;payload&gt;</tt></pre>
</div></div>
<div class="paragraph"><p>Or, as a hexdump:</p></div>
<div class="listingblock">
<div class="content">
<pre><tt>00000000 69 33 2d 69 70 63 04 00 00 00 00 00 00 00 65 78 |i3-ipc........ex|
00000010 69 74 0a |it.|</tt></pre>
</div></div>
<div class="paragraph"><p>To generate and send such a message, you could use the following code in Perl:</p></div>
<div class="listingblock">
<div class="content">
<pre><tt>sub format_ipc_command {
my ($msg) = @_;
my $len;
# Get the real byte count (vs. amount of characters)
{ use bytes; $len = length($msg); }
return "i3-ipc" . pack("LL", $len, 0) . $msg;
}
$sock-&gt;write(format_ipc_command("exit"));</tt></pre>
</div></div>
</div>
<h2 id="_receiving_replies_from_i3">3. Receiving replies from i3</h2>
<div class="sectionbody">
<div class="paragraph"><p>Replies of i3 usually consist of a simple string (the length of the string
is the message_length, so you can consider them length-prefixed) which in turn
contain the JSON serialization of a data structure. For example, the
GET_WORKSPACES message returns an array of workspaces (each workspace is a map
with certain attributes).</p></div>
<h3 id="_reply_format">3.1. Reply format</h3><div style="clear:left"></div>
<div class="paragraph"><p>The reply format is identical to the normal message format. There also is
the magic string, then the message length, then the message type and the
payload.</p></div>
<div class="paragraph"><p>The following reply types are implemented:</p></div>
<div class="dlist"><dl>
<dt class="hdlist1">
1 (GET_WORKSPACES)
</dt>
<dd>
<p>
Reply to the GET_WORKSPACES message.
</p>
</dd>
</dl></div>
<h3 id="_get_workspaces_reply">3.2. GET_WORKSPACES reply</h3><div style="clear:left"></div>
<div class="paragraph"><p>The reply consists of a serialized list of workspaces. Each workspace has the
following properties:</p></div>
<div class="dlist"><dl>
<dt class="hdlist1">
num (integer)
</dt>
<dd>
<p>
The internal number of the workspace. Corresponds to the command
to switch to this workspace.
</p>
</dd>
<dt class="hdlist1">
name (string)
</dt>
<dd>
<p>
The name of this workspace (by default num+1), as changed by the
user. Encoded in UTF-8.
</p>
</dd>
<dt class="hdlist1">
visible (boolean)
</dt>
<dd>
<p>
Whether this workspace is currently visible on an output (multiple
workspaces can be visible at the same time).
</p>
</dd>
<dt class="hdlist1">
focused (boolean)
</dt>
<dd>
<p>
Whether this workspace currently has the focus (only one workspace
can have the focus at the same time).
</p>
</dd>
<dt class="hdlist1">
rect (map)
</dt>
<dd>
<p>
The rectangle of this workspace (equals the rect of the output it
is on), consists of x, y, width, height.
</p>
</dd>
<dt class="hdlist1">
output (string)
</dt>
<dd>
<p>
The video output this workspace is on (LVDS1, VGA1, …).
</p>
</dd>
</dl></div>
<div class="paragraph"><p><strong>Example:</strong></p></div>
<div class="listingblock">
<div class="content">
<pre><tt>[
{
"num": 0,
"name": "1",
"visible": true,
"focused": true,
"rect": {
"x": 0,
"y": 0,
"width": 1280,
"height": 800
},
"output": "LVDS1"
},
{
"num": 1,
"name": "2",
"visible": false,
"focused": false,
"rect": {
"x": 0,
"y": 0,
"width": 1280,
"height": 800
},
"output": "LVDS1"
}
]</tt></pre>
</div></div>
</div>
</div>
<div id="footnotes"><hr /></div>
<div id="footer">
<div id="footer-text">
Last updated 2010-03-12 03:04:46 CEST
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