IPC interface (interprocess communication)
==========================================
-Michael Stapelberg <michael+i3@stapelberg.de>
-March 2010
+Michael Stapelberg <michael@i3wm.org>
+September 2017
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
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, the ipc-socket gets created
-in +/tmp/i3-%u/ipc-socket.%p+ where +%u+ is your UNIX username and +%p+ is the
-PID of i3.
+in +/tmp/i3-%u.XXXXXX/ipc-socket.%p+ where +%u+ is your UNIX username, +%p+ is
+the PID of i3 and XXXXXX is a string of random characters from the portable
+filename character set (see mkdtemp(3)). You can get the socketpath from i3 by
+calling +i3 --get-socketpath+.
All i3 utilities, like +i3-msg+ and +i3-input+ will read the +I3_SOCKET_PATH+
X11 property, stored on the X11 root window.
+[WARNING]
+.Use an existing library!
+There are existing libraries for many languages. You can have a look at
+<<libraries>> or search the web if your language of choice is not mentioned.
+Usually, it is not necessary to implement low-level communication with i3
+directly.
+
== Establishing a connection
To establish a connection, simply open the IPC socket. The following code
-------------------------------------------------------------
use IO::Socket::UNIX;
-my $sock = IO::Socket::UNIX->new(Peer => '/tmp/i3-ipc.sock');
+chomp(my $path = qx(i3 --get-socketpath));
+my $sock = IO::Socket::UNIX->new(Peer => $path);
-------------------------------------------------------------
== Sending messages to i3
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:
-
-COMMAND (0)::
- 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.
-GET_WORKSPACES (1)::
- Gets the current workspaces. The reply will be a JSON-encoded list of
- workspaces (see the reply section).
-SUBSCRIBE (2)::
- Subscribes your connection to certain events. See <<events>> for a
- description of this message and the concept of events.
-GET_OUTPUTS (3)::
- Gets the current outputs. The reply will be a JSON-encoded list of outputs
- (see the reply section).
-GET_TREE (4)::
- Gets the layout tree. i3 uses a tree as data structure which includes
- every container. The reply will be the JSON-encoded tree (see the reply
- section).
+.Currently implemented message types
+[options="header",cols="^10%,^20%,^20%,^50%"]
+|======================================================
+| Type (numeric) | Type (name) | Reply type | Purpose
+| 0 | +RUN_COMMAND+ | <<_command_reply,COMMAND>> | Run the payload as an i3 command (like the commands you can bind to keys).
+| 1 | +GET_WORKSPACES+ | <<_workspaces_reply,WORKSPACES>> | Get the list of current workspaces.
+| 2 | +SUBSCRIBE+ | <<_subscribe_reply,SUBSCRIBE>> | Subscribe this IPC connection to the event types specified in the message payload. See <<events>>.
+| 3 | +GET_OUTPUTS+ | <<_outputs_reply,OUTPUTS>> | Get the list of current outputs.
+| 4 | +GET_TREE+ | <<_tree_reply,TREE>> | Get the i3 layout tree.
+| 5 | +GET_MARKS+ | <<_marks_reply,MARKS>> | Gets the names of all currently set marks.
+| 6 | +GET_BAR_CONFIG+ | <<_bar_config_reply,BAR_CONFIG>> | Gets the specified bar configuration or the names of all bar configurations if payload is empty.
+| 7 | +GET_VERSION+ | <<_version_reply,VERSION>> | Gets the i3 version.
+| 8 | +GET_BINDING_MODES+ | <<_binding_modes_reply,BINDING_MODES>> | Gets the names of all currently configured binding modes.
+| 9 | +GET_CONFIG+ | <<_config_reply,CONFIG>> | Returns the last loaded i3 config.
+| 10 | +SEND_TICK+ | <<_tick_reply,TICK>> | Sends a tick event with the specified payload.
+|======================================================
So, a typical message could look like this:
--------------------------------------------------
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.|
+00000010 69 74 |it|
------------------------------------------------------------------------------
To generate and send such a message, you could use the following code in Perl:
The following reply types are implemented:
COMMAND (0)::
- Confirmation/Error code for the COMMAND message.
-GET_WORKSPACES (1)::
+ Confirmation/Error code for the RUN_COMMAND message.
+WORKSPACES (1)::
Reply to the GET_WORKSPACES message.
SUBSCRIBE (2)::
Confirmation/Error code for the SUBSCRIBE message.
-GET_OUTPUTS (3)::
+OUTPUTS (3)::
Reply to the GET_OUTPUTS message.
-GET_TREE (4)::
+TREE (4)::
Reply to the GET_TREE message.
-
+MARKS (5)::
+ Reply to the GET_MARKS message.
+BAR_CONFIG (6)::
+ Reply to the GET_BAR_CONFIG message.
+VERSION (7)::
+ Reply to the GET_VERSION message.
+BINDING_MODES (8)::
+ Reply to the GET_BINDING_MODES message.
+GET_CONFIG (9)::
+ Reply to the GET_CONFIG message.
+TICK (10)::
+ Reply to the SEND_TICK message.
+
+[[_command_reply]]
=== COMMAND reply
-The reply consists of a single serialized map. At the moment, the only
-property is +success (bool)+, but this will be expanded in future versions.
+The reply consists of a list of serialized maps for each command that was
+parsed. Each has the property +success (bool)+ and may also include a
+human-readable error message in the property +error (string)+.
*Example:*
-------------------
-{ "success": true }
+[{ "success": true }]
-------------------
-=== GET_WORKSPACES reply
+[[_workspaces_reply]]
+=== WORKSPACES reply
The reply consists of a serialized list of workspaces. Each workspace has the
following properties:
num (integer)::
The logical number of the workspace. Corresponds to the command
- to switch to this workspace.
+ to switch to this workspace. For named workspaces, this will be -1.
name (string)::
The name of this workspace (by default num+1), as changed by the
user. Encoded in UTF-8.
]
-------------------
+[[_subscribe_reply]]
=== SUBSCRIBE reply
The reply consists of a single serialized map. The only property is
{ "success": true }
-------------------
-=== GET_OUTPUTS reply
+[[_outputs_reply]]
+=== OUTPUTS reply
The reply consists of a serialized list of outputs. Each output has the
following properties:
The name of this output (as seen in +xrandr(1)+). Encoded in UTF-8.
active (boolean)::
Whether this output is currently active (has a valid mode).
-current_workspace (integer)::
- The current workspace which is visible on this output. +null+ if the
- output is not active.
+primary (boolean)::
+ Whether this output is currently the primary output.
+current_workspace (string)::
+ The name of the current workspace that is visible on this output. +null+ if
+ the output is not active.
rect (map)::
The rectangle of this output (equals the rect of the output it
is on), consists of x, y, width, height.
{
"name": "LVDS1",
"active": true,
- "current_workspace": 4,
+ "current_workspace": "4",
"rect": {
"x": 0,
"y": 0,
{
"name": "VGA1",
"active": true,
- "current_workspace": 1,
+ "current_workspace": "1",
"rect": {
"x": 1280,
"y": 0,
"width": 1280,
"height": 1024
- },
+ }
}
]
-------------------
-=== GET_TREE reply
+[[_tree_reply]]
+=== TREE reply
The reply consists of a serialized tree. Each node in the tree (representing
one container) has at least the properties listed below. While the nodes might
The internal name of this container. For all containers which are part
of the tree structure down to the workspace contents, this is set to a
nice human-readable name of the container.
+ For containers that have an X11 window, the content is the title
+ (_NET_WM_NAME property) of that window.
For all other containers, the content is not defined (yet).
+type (string)::
+ Type of this container. Can be one of "root", "output", "con",
+ "floating_con", "workspace" or "dockarea".
border (string)::
- Can be either "normal", "none" or "1pixel", dependending on the
+ Can be either "normal", "none" or "pixel", depending on the
container’s border style.
+current_border_width (integer)::
+ Number of pixels of the border width.
layout (string)::
- Can be either "default", "stacked", "tabbed", "dockarea" or "output".
+ Can be either "splith", "splitv", "stacked", "tabbed", "dockarea" or
+ "output".
Other values might be possible in the future, should we add new
layouts.
orientation (string)::
Can be either "none" (for non-split containers), "horizontal" or
"vertical".
+ THIS FIELD IS OBSOLETE. It is still present, but your code should not
+ use it. Instead, rely on the layout field.
percent (float)::
The percentage which this container takes in its parent. A value of
+null+ means that the percent property does not make sense for this
So, when using the +default+ layout, you will have a 2 pixel border on
each side, making the window_rect +{ "x": 2, "y": 0, "width": 632,
"height": 366 }+ (for example).
+deco_rect (map)::
+ The coordinates of the *window decoration* inside its container. These
+ coordinates are relative to the container and do not include the actual
+ client window.
geometry (map)::
The original geometry the window specified when i3 mapped it. Used when
switching a window to floating mode, for example.
+window (integer)::
+ The X11 window ID of the *actual client window* inside this container.
+ This field is set to null for split containers or otherwise empty
+ containers. This ID corresponds to what xwininfo(1) and other
+ X11-related tools display (usually in hex).
urgent (bool)::
- Whether this container (window or workspace) has the urgency hint set.
+ Whether this container (window, split container, floating container or
+ workspace) has the urgency hint set, directly or indirectly. All parent
+ containers up until the workspace container will be marked urgent if they
+ have at least one urgent child.
focused (bool)::
Whether this container is currently focused.
+focus (array of integer)::
+ List of child node IDs (see +nodes+, +floating_nodes+ and +id+) in focus
+ order. Traversing the tree by following the first entry in this array
+ will result in eventually reaching the one node with +focused+ set to
+ true.
+nodes (array of node)::
+ The tiling (i.e. non-floating) child containers of this node.
+floating_nodes (array of node)::
+ The floating child containers of this node. Only non-empty on nodes with
+ type +workspace+.
Please note that in the following example, I have left out some keys/values
which are not relevant for the type of the node. Otherwise, the example would
"y": 0,
"width": 1280,
"height": 0
- },
+ }
},
{
}
------------------------
+[[_marks_reply]]
+=== MARKS reply
+
+The reply consists of a single array of strings for each container that has a
+mark. A mark can only be set on one container, so the array is unique.
+The order of that array is undefined.
+
+If no window has a mark the response will be the empty array [].
+
+[[_bar_config_reply]]
+=== BAR_CONFIG reply
+
+This can be used by third-party workspace bars (especially i3bar, but others
+are free to implement compatible alternatives) to get the +bar+ block
+configuration from i3.
+
+Depending on the input, the reply is either:
+
+empty input::
+ An array of configured bar IDs
+Bar ID::
+ A JSON map containing the configuration for the specified bar.
+
+Each bar configuration has the following properties:
+
+id (string)::
+ The ID for this bar. Included in case you request multiple
+ configurations and want to differentiate the different replies.
+mode (string)::
+ Either +dock+ (the bar sets the dock window type) or +hide+ (the bar
+ does not show unless a specific key is pressed).
+position (string)::
+ Either +bottom+ or +top+ at the moment.
+status_command (string)::
+ Command which will be run to generate a statusline. Each line on stdout
+ of this command will be displayed in the bar. At the moment, no
+ formatting is supported.
+font (string)::
+ The font to use for text on the bar.
+workspace_buttons (boolean)::
+ Display workspace buttons or not? Defaults to true.
+binding_mode_indicator (boolean)::
+ Display the mode indicator or not? Defaults to true.
+verbose (boolean)::
+ Should the bar enable verbose output for debugging? Defaults to false.
+colors (map)::
+ Contains key/value pairs of colors. Each value is a color code in hex,
+ formatted #rrggbb (like in HTML).
+
+The following colors can be configured at the moment:
+
+background::
+ Background color of the bar.
+statusline::
+ Text color to be used for the statusline.
+separator::
+ Text color to be used for the separator.
+focused_background::
+ Background color of the bar on the currently focused monitor output.
+focused_statusline::
+ Text color to be used for the statusline on the currently focused
+ monitor output.
+focused_separator::
+ Text color to be used for the separator on the currently focused
+ monitor output.
+focused_workspace_text/focused_workspace_bg/focused_workspace_border::
+ Text/background/border color for a workspace button when the workspace
+ has focus.
+active_workspace_text/active_workspace_bg/active_workspace_border::
+ Text/background/border color for a workspace button when the workspace
+ is active (visible) on some output, but the focus is on another one.
+ You can only tell this apart from the focused workspace when you are
+ using multiple monitors.
+inactive_workspace_text/inactive_workspace_bg/inactive_workspace_border::
+ Text/background/border color for a workspace button when the workspace
+ does not have focus and is not active (visible) on any output. This
+ will be the case for most workspaces.
+urgent_workspace_text/urgent_workspace_bg/urgent_workspace_border::
+ Text/background/border color for workspaces which contain at least one
+ window with the urgency hint set.
+binding_mode_text/binding_mode_bg/binding_mode_border::
+ Text/background/border color for the binding mode indicator.
+
+
+*Example of configured bars:*
+--------------
+["bar-bxuqzf"]
+--------------
+
+*Example of bar configuration:*
+--------------
+{
+ "id": "bar-bxuqzf",
+ "mode": "dock",
+ "position": "bottom",
+ "status_command": "i3status",
+ "font": "-misc-fixed-medium-r-normal--13-120-75-75-C-70-iso10646-1",
+ "workspace_buttons": true,
+ "binding_mode_indicator": true,
+ "verbose": false,
+ "colors": {
+ "background": "#c0c0c0",
+ "statusline": "#00ff00",
+ "focused_workspace_text": "#ffffff",
+ "focused_workspace_bg": "#000000"
+ }
+}
+--------------
+
+[[_version_reply]]
+=== VERSION reply
+
+The reply consists of a single JSON dictionary with the following keys:
+
+major (integer)::
+ The major version of i3, such as +4+.
+minor (integer)::
+ The minor version of i3, such as +2+. Changes in the IPC interface (new
+ features) will only occur with new minor (or major) releases. However,
+ bugfixes might be introduced in patch releases, too.
+patch (integer)::
+ The patch version of i3, such as +1+ (when the complete version is
+ +4.2.1+). For versions such as +4.2+, patch will be set to +0+.
+human_readable (string)::
+ A human-readable version of i3 containing the precise git version,
+ build date and branch name. When you need to display the i3 version to
+ your users, use the human-readable version whenever possible (since
+ this is what +i3 --version+ displays, too).
+loaded_config_file_name (string)::
+ The current config path.
+
+*Example:*
+-------------------
+{
+ "human_readable" : "4.2-169-gf80b877 (2012-08-05, branch \"next\")",
+ "loaded_config_file_name" : "/home/hwangcc23/.i3/config",
+ "minor" : 2,
+ "patch" : 0,
+ "major" : 4
+}
+-------------------
+
+[[_binding_modes_reply]]
+=== BINDING_MODES reply
+
+The reply consists of an array of all currently configured binding modes.
+
+*Example:*
+---------------------
+["default", "resize"]
+---------------------
+
+[[_config_reply]]
+=== CONFIG reply
+
+The config reply is a map which currently only contains the "config" member,
+which is a string containing the config file as loaded by i3 most recently.
+
+*Example:*
+-------------------
+{ "config": "font pango:monospace 8\nbindsym Mod4+q exit\n" }
+-------------------
+
+[[_tick_reply]]
+=== TICK reply
+
+The reply is a map containing the "success" member. After the reply was
+received, the tick event has been written to all IPC connections which subscribe
+to tick events. UNIX sockets are usually buffered, but you can be certain that
+once you receive the tick event you just triggered, you must have received all
+events generated prior to the +SEND_TICK+ message (happened-before relation).
+
+*Example:*
+-------------------
+{ "success": true }
+-------------------
== Events
*Example:*
---------------------------------
type: SUBSCRIBE
-payload: [ "workspace", "focus" ]
+payload: [ "workspace", "output" ]
---------------------------------
output (1)::
Sent when RandR issues a change notification (of either screens,
outputs, CRTCs or output properties).
+mode (2)::
+ Sent whenever i3 changes its binding mode.
+window (3)::
+ Sent when a client's window is successfully reparented (that is when i3
+ has finished fitting it into a container), when a window received input
+ focus or when certain properties of the window have changed.
+barconfig_update (4)::
+ Sent when the hidden_state or mode field in the barconfig of any bar
+ instance was updated and when the config is reloaded.
+binding (5)::
+ Sent when a configured command binding is triggered with the keyboard or
+ mouse
+shutdown (6)::
+ Sent when the ipc shuts down because of a restart or exit by user command
+tick (7)::
+ Sent when the ipc client subscribes to the tick event (with +"first":
+ true+) or when any ipc client sends a SEND_TICK message (with +"first":
+ false+).
*Example:*
--------------------------------------------------------------------
This event consists of a single serialized map containing a property
+change (string)+ which indicates the type of the change ("focus", "init",
-"empty", "urgent").
+"empty", "urgent", "reload", "rename", "restored", "move"). A
++current (object)+ property will be present with the affected workspace
+whenever the type of event affects a workspace (otherwise, it will be +null).
+
+When the change is "focus", an +old (object)+ property will be present with the
+previous workspace. When the first switch occurs (when i3 focuses the
+workspace visible at the beginning) there is no previous workspace, and the
++old+ property will be set to +null+. Also note that if the previous is empty
+it will get destroyed when switching, but will still be present in the "old"
+property.
*Example:*
---------------------
-{ "change": "focus" }
+{
+ "change": "focus",
+ "current": {
+ "id": 28489712,
+ "type": "workspace",
+ ...
+ }
+ "old": {
+ "id": 28489715,
+ "type": "workspace",
+ ...
+ }
+}
---------------------
=== output event
{ "change": "unspecified" }
---------------------------
-== See also
+=== mode event
+
+This event consists of a single serialized map containing a property
++change (string)+ which holds the name of current mode in use. The name
+is the same as specified in config when creating a mode. The default
+mode is simply named default. It contains a second property, +pango_markup+, which
+defines whether pango markup shall be used for displaying this mode.
+
+*Example:*
+---------------------------
+{
+ "change": "default",
+ "pango_markup": true
+}
+---------------------------
+
+=== window event
+
+This event consists of a single serialized map containing a property
++change (string)+ which indicates the type of the change
+
+* +new+ – the window has become managed by i3
+* +close+ – the window has closed
+* +focus+ – the window has received input focus
+* +title+ – the window's title has changed
+* +fullscreen_mode+ – the window has entered or exited fullscreen mode
+* +move+ – the window has changed its position in the tree
+* +floating+ – the window has transitioned to or from floating
+* +urgent+ – the window has become urgent or lost its urgent status
+* +mark+ – a mark has been added to or removed from the window
+
+Additionally a +container (object)+ field will be present, which consists
+of the window's parent container. Be aware that for the "new" event, the
+container will hold the initial name of the newly reparented window (e.g.
+if you run urxvt with a shell that changes the title, you will still at
+this point get the window title as "urxvt").
+
+*Example:*
+---------------------------
+{
+ "change": "new",
+ "container": {
+ "id": 35569536,
+ "type": "con",
+ ...
+ }
+}
+---------------------------
+
+=== barconfig_update event
+
+This event consists of a single serialized map reporting on options from the
+barconfig of the specified bar_id that were updated in i3. This event is the
+same as a +GET_BAR_CONFIG+ reply for the bar with the given id.
+
+=== binding event
+
+This event consists of a single serialized map reporting on the details of a
+binding that ran a command because of user input. The +change (string)+ field
+indicates what sort of binding event was triggered (right now it will always be
++"run"+ but may be expanded in the future).
+
+The +binding (object)+ field contains details about the binding that was run:
+
+command (string)::
+ The i3 command that is configured to run for this binding.
+event_state_mask (array of strings)::
+ The group and modifier keys that were configured with this binding.
+input_code (integer)::
+ If the binding was configured with +bindcode+, this will be the key code
+ that was given for the binding. If the binding is a mouse binding, it will be
+ the number of the mouse button that was pressed. Otherwise it will be 0.
+symbol (string or null)::
+ If this is a keyboard binding that was configured with +bindsym+, this
+ field will contain the given symbol. Otherwise it will be +null+.
+input_type (string)::
+ This will be +"keyboard"+ or +"mouse"+ depending on whether or not this was
+ a keyboard or a mouse binding.
+
+*Example:*
+---------------------------
+{
+ "change": "run",
+ "binding": {
+ "command": "nop",
+ "event_state_mask": [
+ "shift",
+ "ctrl"
+ ],
+ "input_code": 0,
+ "symbol": "t",
+ "input_type": "keyboard"
+ }
+}
+---------------------------
+
+=== shutdown event
+
+This event is triggered when the connection to the ipc is about to shutdown
+because of a user action such as a +restart+ or +exit+ command. The +change
+(string)+ field indicates why the ipc is shutting down. It can be either
++"restart"+ or +"exit"+.
+
+*Example:*
+---------------------------
+{
+ "change": "restart"
+}
+---------------------------
+
+=== tick event
+
+This event is triggered by a subscription to tick events or by a +SEND_TICK+
+message.
+
+*Example (upon subscription):*
+--------------------------------------------------------------------------------
+{
+ "first": true,
+ "payload": ""
+}
+--------------------------------------------------------------------------------
+
+*Example (upon +SEND_TICK+ with a payload of +arbitrary string+):*
+--------------------------------------------------------------------------------
+{
+ "first": false,
+ "payload": "arbitrary string"
+}
+--------------------------------------------------------------------------------
+
+== See also (existing libraries)
+
+[[libraries]]
For some languages, libraries are available (so you don’t have to implement
all this on your own). This list names some (if you wrote one, please let me
know):
C::
- i3 includes a headerfile +i3/ipc.h+ which provides you all constants.
- However, there is no library yet.
-Ruby::
- http://github.com/badboy/i3-ipc
+ * i3 includes a headerfile +i3/ipc.h+ which provides you all constants.
+ * https://github.com/acrisci/i3ipc-glib
+C++::
+ * https://github.com/drmgc/i3ipcpp
+Go::
+ * https://github.com/mdirkse/i3ipc-go
+ * https://github.com/i3/go-i3
+JavaScript::
+ * https://github.com/acrisci/i3ipc-gjs
+Lua::
+ * https://github.com/acrisci/i3ipc-lua
Perl::
- http://search.cpan.org/search?query=AnyEvent::I3
+ * https://metacpan.org/module/AnyEvent::I3
Python::
- http://github.com/thepub/i3ipc
+ * https://github.com/acrisci/i3ipc-python
+ * https://github.com/whitelynx/i3ipc (not maintained)
+ * https://github.com/ziberna/i3-py (not maintained)
+Ruby::
+ * https://github.com/veelenga/i3ipc-ruby
+ * https://github.com/badboy/i3-ipc (not maintained)
+Rust::
+ * https://github.com/tmerr/i3ipc-rs
+OCaml::
+ * https://github.com/Armael/ocaml-i3ipc
+
+== Appendix A: Detecting byte order in memory-safe languages
+
+Some programming languages such as Go don’t offer a way to serialize data in the
+native byte order of the machine they’re running on without resorting to tricks
+involving the +unsafe+ package.
+
+The following technique can be used (and will not be broken by changes to i3) to
+detect the byte order i3 is using:
+
+1. The byte order dependent fields of an IPC message are message type and
+ payload length.
+
+ * The message type +RUN_COMMAND+ (0) is the same in big and little endian, so
+ we can use it in either byte order to elicit a reply from i3.
+
+ * The payload length 65536 + 256 (+0x00 01 01 00+) is the same in big and
+ little endian, and also small enough to not worry about memory allocations
+ of that size. We must use payloads of length 65536 + 256 in every message
+ we send, so that i3 will be able to read the entire message regardless of
+ the byte order it uses.
+
+2. Send a big endian encoded message of type +SUBSCRIBE+ (2) with payload `[]`
+ followed by 65536 + 256 - 2 +SPACE+ (ASCII 0x20) bytes.
+
+ * If i3 is running in big endian, this message is treated as a noop,
+ resulting in a +SUBSCRIBE+ reply with payload `{"success":true}`
+ footnote:[A small payload is important: that way, we circumvent dealing
+ with UNIX domain socket buffer sizes, whose size depends on the
+ implementation/operating system. Exhausting such a buffer results in an i3
+ deadlock unless you concurrently read and write, which — depending on the
+ programming language — makes the technique much more complicated.].
+
+ * If i3 is running in little endian, this message is read in its entirety due
+ to the byte order independent payload length, then
+ https://github.com/i3/i3/blob/d726d09d496577d1c337a4b97486f2c9fbc914f1/src/ipc.c#L1188[silently
+ discarded] due to the unknown message type.
+
+3. Send a byte order independent message, i.e. type +RUN_COMMAND+ (0) with
+ payload +nop byte order detection. padding:+, padded to 65536 + 256 bytes
+ with +a+ (ASCII 0x61) bytes. i3 will reply to this message with a reply of
+ type +COMMAND+ (0).
+
+ * The human-readable prefix is in there to not confuse readers of the i3 log.
+
+ * This messages serves as a synchronization primitive so that we know whether
+ i3 discarded the +SUBSCRIBE+ message or didn’t answer it yet.
+
+4. Receive a message header from i3, decoding the message type as big endian.
+
+ * If the message’s reply type is +COMMAND+ (0), i3 is running in little
+ endian (because the +SUBSCRIBE+ message was discarded). Decode the message
+ payload length as little endian, receive the message payload.
+
+ * If the message’s reply type is anything else, i3 is running in big endian
+ (because our big endian encoded +SUBSCRIBE+ message was answered). Decode
+ the message payload length in big endian, receive the message
+ payload. Then, receive the pending +COMMAND+ message reply in big endian.
+
+5. From here on out, send/receive all messages using the detected byte order.
+
+Find an example implementation of this technique in
+https://github.com/i3/go-i3/blob/master/byteorder.go
projects / i3 / i3.github.io / blobdiff