IPC interface (interprocess communication)

Michael Stapelberg
<michael@i3wm.org>
June 2020

Table of Contents

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. By default i3 will set the path of the IPC socket based on:

The ipc-socket configuration directive if it is used
The I3SOCK environmental variable if it is set
$XDG_RUNTIME_DIR/i3/ipc-socket.%p if the directory is available where %p is the PID of i3 and XXXXXX is a string of random characters
/tmp/i3-%u.XXXXXX/ipc-socket.%p where %u is your UNIX username

You can get the socketpath from i3 by executing i3 --get-socketpath, which will print the path to the standard output (plus a newline) or by reading the I3SOCK environmental variable.

All i3 utilities, like i3-msg and i3-input will determine the path of the IPC socket frome the I3SOCK environmental variable if it is set or 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.

1. Establishing a connection

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

use IO::Socket::UNIX;
chomp(my $path = qx(i3 --get-socketpath));
my $sock = IO::Socket::UNIX->new(Peer => $path);

2. Sending messages to i3

To send a message to i3, you have to format it 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). Following the magic string comes the length of the payload of the message as a 32-bit integer, and the type of the message as a 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).

Table 1. Currently implemented message types
Type (numeric)	Type (name)	Reply type	Purpose
0	`RUN_COMMAND`	COMMAND	Run the payload as an i3 command (like the commands you can bind to keys).
1	`GET_WORKSPACES`	WORKSPACES	Get the list of current workspaces.
2	`SUBSCRIBE`	SUBSCRIBE	Subscribe this IPC connection to the event types specified in the message payload. See [events].
3	`GET_OUTPUTS`	OUTPUTS	Get the list of current outputs.
4	`GET_TREE`	TREE	Get the i3 layout tree.
5	`GET_MARKS`	MARKS	Gets the names of all currently set marks.
6	`GET_BAR_CONFIG`	BAR_CONFIG	Gets the specified bar configuration or the names of all bar configurations if payload is empty.
7	`GET_VERSION`	VERSION	Gets the i3 version.
8	`GET_BINDING_MODES`	BINDING_MODES	Gets the names of all currently configured binding modes.
9	`GET_CONFIG`	CONFIG	Returns the last loaded i3 config.
10	`SEND_TICK`	TICK	Sends a tick event with the specified payload.
11	`SYNC`	SYNC	Sends an i3 sync event with the specified random value to the specified window.
12	`GET_BINDING_STATE`	BINDING_STATE	Request the current binding state, i.e. the currently active binding mode name.

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                                             |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"));

3. Receiving replies from i3

Each message sent to i3 will cause exactly one reply to be sent in return. The order of the sent replies will always correspond to the order of the sent requests. The only exception to this is [events], which (once subscribed to) may be sent at any time (though never in the middle of another event or reply).

It is generally safe to send several messages to i3 without first waiting for a reply for each one (pipelining) — though, note that depending on the language / network library you use, writing to the socket without also reading from it may cause a deadlock due to the socket buffers getting full.

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 payload of replies from i3 usually consists 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).

Replies currently have a 1:1 correspondence to messages, with the message type of the reply corresponding to the message type of the message which caused the reply to be sent.

The following reply types are implemented:

COMMAND (0): 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.
OUTPUTS (3): Reply to the GET_OUTPUTS message.
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.
SYNC (11): Reply to the SYNC message.
GET_BINDING_STATE (12): Reply to the GET_BINDING_STATE message.

4. Messages and replies

4.1. RUN_COMMAND / COMMAND

Run the payload as an i3 command (like the commands you can bind to keys).

Message:

The message payload is the string containing the command to execute. There is no JSON encoding or trailing newline.

Reply:

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).

Note

When sending the restart command, you will get a singular reply once the restart completed. All IPC connection states (e.g. subscriptions) will reset and all but one socket will be closed. Libraries must be able to cope with this by aligning their internal states. It is also recommended that libraries close the last remaining socket(one which replied to restart command) to achieve the full reset.

Note	It is easiest to always send the `restart` command alone: due to i3’s state reset, the reply messages of preceding commands are lost, and following commands will not be executed.

Note	When processing the `exit` command, i3 will immediately exit without sending a reply. Expect the socket to be shut down.

Example:

[{ "success": true }]

When the specified command cannot be parsed, success will be false and parse_error will be true:

Example:

[{ "success": false, "parse_error": true }]

4.2. GET_WORKSPACES / WORKSPACES

Get the list of current workspaces.

Message:

No payload.

Reply:

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

id (integer): The internal ID (actually a C pointer value) of this container. Do not make any assumptions about it. You can use it to (re-)identify and address containers when talking to i3.
num (integer): The logical number of the workspace. Corresponds to the command to switch to this workspace. For named workspaces, this will be -1.
name (string): The name of this workspace if changed by the user, otherwise defaults to the string representation of the num field). 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).
urgent (boolean): Whether a window on this workspace has the "urgent" flag set.
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,
  "urgent": false,
  "rect": {
   "x": 0,
   "y": 0,
   "width": 1280,
   "height": 800
  },
  "output": "LVDS1"
 },
 {
  "num": 1,
  "name": "2",
  "visible": false,
  "focused": false,
  "urgent": false,
  "rect": {
   "x": 0,
   "y": 0,
   "width": 1280,
   "height": 800
  },
  "output": "LVDS1"
 }
]

4.3. SUBSCRIBE

Subscribe this IPC connection to the event types specified in the message payload. See [events].

Message:

A JSON-encoded array of event types to subscribe to.

Reply:

The reply consists of a single serialized map. The only property is success (bool), indicating whether the subscription was successful (the default) or whether a JSON parse error occurred.

Example:

{ "success": true }

4.4. GET_OUTPUTS / OUTPUTS

Get the list of current outputs.

Message:

No payload.

Reply:

The reply consists of a serialized list of outputs. Each output has the following properties:

name (string): 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).
primary (boolean): Whether this output is currently the primary output.
current_workspace (string or null): 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.

Example:

[
 {
  "name": "LVDS1",
  "active": true,
  "current_workspace": "4",
  "rect": {
   "x": 0,
   "y": 0,
   "width": 1280,
   "height": 800
  }
 },
 {
  "name": "VGA1",
  "active": true,
  "current_workspace": "1",
  "rect": {
   "x": 1280,
   "y": 0,
   "width": 1280,
   "height": 1024
  }
 }
]

4.5. GET_TREE / TREE

Get the i3 layout tree.

Message:

No payload.

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 have more properties, please do not use any properties which are not documented here. They are not yet finalized and will probably change!

id (integer): The internal ID (actually a C pointer value) of this container. Do not make any assumptions about it. You can use it to (re-)identify and address containers when talking to i3.
name (string): 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 "pixel", depending on the container’s border style.
current_border_width (integer): Number of pixels of the border width.
layout (string): 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 or null): The percentage which this container takes in its parent. A value of null means that the percent property does not make sense for this container, for example for the root container.
rect (map): The absolute display coordinates for this container. Display coordinates means that when you have two 1600x1200 monitors on a single X11 Display (the standard way), the coordinates of the first window on the second monitor are { "x": 1600, "y": 0, "width": 1600, "height": 1200 }.
window_rect (map): The coordinates of the actual client window inside its container. These coordinates are relative to the container and do not include the window decoration (which is actually rendered on the parent container). 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.
actual_deco_rect (map): See deco_rect. i3 v4.22 changed the way title bars are rendered. Before i3 v4.22, the deco_rect was always relative to the parent coordinates. Starting with i3 v4.22, this remains true for tabbed/stacked containers (actual_deco_rect is identical to deco_rect), but for normal-border leaf containers within vertical/horizontal split containers, actual_deco_rect is relative to the container itself. For more background, see https://github.com/i3/i3/issues/1966
geometry (map): The original geometry the window specified when i3 mapped it. Used when switching a window to floating mode, for example.
window (integer or null): 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).
window_properties (map): This optional field contains all available X11 window properties from the following list: title, instance, class, window_role, machine and transient_for.
window_type (string): The window type (_NET_WM_WINDOW_TYPE). Possible values are undefined, unknown, normal, dialog, utility, toolbar, splash, menu, dropdown_menu, popup_menu, tooltip and notification.
urgent (bool): 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.
marks (array of string): List of marks assigned to container
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.
sticky (bool): Whether this window is "sticky". If it is also floating, this window will be present on all workspaces on the same output.
fullscreen_mode (integer): Whether this container is in fullscreen state or not. Possible values are 0 (no fullscreen), 1 (fullscreened on output) or 2 (fullscreened globally). Note that all workspaces are considered fullscreened on their respective output.
floating (string): Floating state of container. Can be either "auto_on", "auto_off", "user_on" or "user_off"
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.
scratchpad_state (string): Whether the window is not in the scratchpad ("none"), freshly moved to the scratchpad but not yet resized ("fresh") or moved to the scratchpad and resized ("changed").

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 be by far too long (it already is quite long, despite showing only 1 window and one dock window).

It is useful to have an overview of the structure before taking a look at the JSON dump:

root
- LVDS1
  - topdock
  - content
    
    workspace 1
    
    window 1
  - bottomdock
    
    dock window 1
- VGA1

Example:

{
 "id": 6875648,
 "name": "root",
 "rect": {
   "x": 0,
   "y": 0,
   "width": 1280,
   "height": 800
 },
 "nodes": [

   {
    "id": 6878320,
    "name": "LVDS1",
    "layout": "output",
    "rect": {
      "x": 0,
      "y": 0,
      "width": 1280,
      "height": 800
    },
    "nodes": [

      {
       "id": 6878784,
       "name": "topdock",
       "layout": "dockarea",
       "orientation": "vertical",
       "rect": {
         "x": 0,
         "y": 0,
         "width": 1280,
         "height": 0
       }
      },

      {
       "id": 6879344,
       "name": "content",
       "rect": {
         "x": 0,
         "y": 0,
         "width": 1280,
         "height": 782
       },
       "nodes": [

         {
          "id": 6880464,
          "name": "1",
          "orientation": "horizontal",
          "rect": {
            "x": 0,
            "y": 0,
            "width": 1280,
            "height": 782
          },
          "window_properties": {
            "class": "Evince",
            "instance": "evince",
            "title": "Properties",
            "transient_for": 52428808
          },
          "floating_nodes": [],
          "nodes": [

            {
             "id": 6929968,
             "name": "#aa0000",
             "border": "normal",
             "percent": 1,
             "rect": {
               "x": 0,
               "y": 18,
               "width": 1280,
               "height": 782
             }
            }

          ]
         }

       ]
      },

      {
       "id": 6880208,
       "name": "bottomdock",
       "layout": "dockarea",
       "orientation": "vertical",
       "rect": {
         "x": 0,
         "y": 782,
         "width": 1280,
         "height": 18
       },
       "nodes": [

         {
          "id": 6931312,
          "name": "#00aa00",
          "percent": 1,
          "rect": {
            "x": 0,
            "y": 782,
            "width": 1280,
            "height": 18
          }
         }

       ]
      }
    ]
   }
 ]
}

4.6. GET_MARKS / MARKS

Gets the names of all currently set marks.

Message:

No payload.

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 [].

4.7. GET_BAR_CONFIG / BAR_CONFIG

Gets the specified bar configuration or the names of all bar configurations if payload is empty.

Message:

No payload, or the ID of the bar whose configuration to retrieve.

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"
 }
}

4.8. GET_VERSION / VERSION

Gets the i3 version.

Message:

No payload.

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
}

4.9. GET_BINDING_MODES / BINDING_MODES

Gets the names of all currently configured binding modes.

Message:

No payload.

Reply:

The reply consists of an array of all currently configured binding modes.

Example:

["default", "resize"]

4.10. GET_CONFIG / CONFIG

Returns the last loaded i3 config.

Message:

No payload.

Reply:

The config reply is a map which contains the following fields:

config (string): The top-level config file contents that i3 has loaded most recently. This field is kept for backwards compatibility. See included_configs instead.
included_configs (array of maps): i3 adds one entry to this array for each config file it loads, in order. The first entry’s raw_contents are identical to the config field.

Each included_configs entry contains the following fields

path (string): Absolute path name to the config file that i3 loaded.
raw_contents (string): The raw contents of the file as i3 read them.
variable_replaced_contents (string): The contents of the file after i3 replaced all variables. This is useful for debugging variable replacement.

Example:

{
  "config": "include font.cfg\n",
  "included_configs": [
    {
      "path": "/home/michael/configfiles/i3/config",
      "raw_contents": "include font.cfg\n",
      "variable_replaced_contents": "include font.cfg\n"
    },
    {
      "path": "/home/michael/configfiles/i3/font.cfg",
      "raw_contents": "set $font pango:monospace 8\nfont $font",
      "variable_replaced_contents": "set pango:monospace 8 pango:monospace 8\nfont pango:monospace 8\n"
    }
  ],
}

4.11. SEND_TICK / TICK

Sends a tick event with the specified payload.

Message:

The payload of the tick event to send to IPC event listeners.

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 }

4.12. SYNC

Sends an i3 sync event with the specified random value to the specified window.

Message:

A JSON-encoded map with the properties "rnd" and "window" (both integer).

Reply:

The reply is a map containing the "success" member. After the reply was received, the i3 sync message was responded to.

Example:

{ "success": true }

4.13. GET_BINDING_STATE

Request the current binding state, i.e. the currently active binding mode name.

Message:

No payload.

Reply:

The binding_state reply is a map which currently only contains the "name" member, which is the name of the currently active binding mode as a string.

Example:

{ "name": "default" }

5. Events

To get informed when certain things happen in i3, clients can subscribe to events. Events consist of a name (like "workspace") and an event reply type (like I3_IPC_EVENT_WORKSPACE). Events sent by i3 follow a format similar to replies but with the highest bit of the message type set to 1 to indicate an event reply instead of a normal reply. Note that event types and reply types do not follow the same enumeration scheme (e.g. event type 0 corresponds to the workspace event however reply type 0 corresponds to the COMMAND reply).

Caveat: As soon as you subscribe to an event, it is not guaranteed any longer that the requests to i3 are processed in order. This means, the following situation can happen: You send a GET_WORKSPACES request but you receive a "workspace" event before receiving the reply to GET_WORKSPACES. If your program does not want to cope which such kinds of race conditions (an event based library may not have a problem here), I suggest you create a separate connection to receive events.

If an event message needs to be sent and the socket is not writeable (write returns EAGAIN, happens when the socket doesn’t have enough buffer space for writing new data) then i3 uses a queue system to store outgoing messages for each client. This is combined with a timer: if the message queue for a client is not empty and no data where successfully written in the past 10 seconds, the connection is killed. Practically, this means that your client should try to always read events from the socket to avoid having its connection closed.

5.1. Subscribing to events

By sending a message of type SUBSCRIBE with a JSON-encoded array as payload you can register to an event.

Example:

type: SUBSCRIBE
payload: [ "workspace", "output" ]

5.2. Available events

The numbers in parenthesis is the event type (keep in mind that you need to strip the highest bit first).

workspace (0): Sent when the user switches to a different workspace, when a new workspace is initialized or when a workspace is removed (because the last client vanished).
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:

# the appropriate 4 bytes read from the socket are stored in $input

# unpack a 32-bit unsigned integer
my $message_type = unpack("L", $input);

# check if the highest bit is 1
my $is_event = (($message_type >> 31) == 1);

# use the other bits
my $event_type = ($message_type & 0x7F);

if ($is_event) {
  say "Received event of type $event_type";
}

5.3. workspace event

This event consists of a single serialized map containing a property change (string) which indicates the type of the change.

empty – the workspace has become empty
focus – the workspace has received input focus
init – the workspace has been created
move – the workspace has been moved to a different output
reload – i3 config has been reloaded
rename – the workspace’s name has changed
restored – the workspace’s layout has changed to a previously saved layout
urgent – the workspace has become urgent or lost its urgent status

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",
 "current": {
  "id": 28489712,
  "type": "workspace",
  ...
 }
 "old": {
  "id": 28489715,
  "type": "workspace",
  ...
 }
}

5.4. output event

This event consists of a single serialized map containing a property change (string) which indicates the type of the change (currently only "unspecified").

Example:

{ "change": "unspecified" }

5.5. 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
}

5.6. 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",
  ...
 }
}

5.7. 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.

5.8. 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 mode (string) field contains the name of the mode the binding was run in.

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"
 }
}

5.9. 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"
}

5.10. 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"
}

6. See also (existing 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.
https://github.com/acrisci/i3ipc-glib

C++

Go

JavaScript

https://github.com/acrisci/i3ipc-gjs

Lua

https://github.com/acrisci/i3ipc-lua

Perl

https://metacpan.org/module/AnyEvent::I3

Python

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

7. 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:

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.
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}
  [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 silently discarded due to the unknown message type.
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.
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.
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