IPC interface (interprocess communication)
==========================================
Michael Stapelberg <michael+i3@stapelberg.de>
March 2010

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.

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 +i3-msg+ and
+i3-input+ use) is +/tmp/i3-ipc.sock+.

== Establishing a connection

To establish a connection, simply open the IPC socket. The following code
snippet illustrates this in Perl:

-------------------------------------------------------------
use IO::Socket::UNIX;
my $sock = IO::Socket::UNIX->new(Peer => '/tmp/i3-ipc.sock');
-------------------------------------------------------------

== Sending messages to i3

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).

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).

Currently implemented message types are the following:

0 (COMMAND)::
	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.
1 (GET_WORKSPACES)::
	Gets the current workspaces. The reply will be a JSON-encoded list of
	workspaces (see the reply section).

So, a typical message could look like this:
--------------------------------------------------
"i3-ipc" <message length> <message type> <payload>
--------------------------------------------------

Or, as a hexdump:
------------------------------------------------------------------------------
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.|
------------------------------------------------------------------------------

To generate and send such a message, you could use the following code in Perl:
------------------------------------------------------------
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->write(format_ipc_command("exit"));
------------------------------------------------------------

== Receiving replies from i3

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).

=== Reply format

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.

The following reply types are implemented:

1 (GET_WORKSPACES)::
	Reply to the GET_WORKSPACES message.

=== GET_WORKSPACES reply

The reply consists of a serialized list of workspaces. Each workspace has the
following properties:

num (integer)::
	The internal number of the workspace. Corresponds to the command
	to switch to this workspace.
name (string)::
	The name of this workspace (by default num+1), as changed by the
	user. Encoded in UTF-8.
visible (boolean)::
	Whether this workspace is currently visible on an output (multiple
	workspaces can be visible at the same time).
focused (boolean)::
	Whether this workspace currently has the focus (only one workspace
	can have the focus at the same time).
rect (map)::
	The rectangle of this workspace (equals the rect of the output it
	is on), consists of x, y, width, height.
output (string)::
	The video output this workspace is on (LVDS1, VGA1, …).

*Example:*
-------------------
[
 {
  "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"
 }
]
-------------------