1 IPC interface (interprocess communication)
2 ==========================================
3 Michael Stapelberg <michael@i3wm.org>
6 This document describes how to interface with i3 from a separate process. This
7 is useful for example to remote-control i3 (to write test cases for example) or
8 to get various information like the current workspaces to implement an external
11 The method of choice for IPC in our case is a unix socket because it has very
12 little overhead on both sides and is usually available without headaches in
13 most languages. In the default configuration file, the ipc-socket gets created
14 in +/tmp/i3-%u.XXXXXX/ipc-socket.%p+ where +%u+ is your UNIX username, +%p+ is
15 the PID of i3 and XXXXXX is a string of random characters from the portable
16 filename character set (see mkdtemp(3)). You can get the socketpath from i3 by
17 calling +i3 --get-socketpath+.
19 All i3 utilities, like +i3-msg+ and +i3-input+ will read the +I3_SOCKET_PATH+
20 X11 property, stored on the X11 root window.
23 .Use an existing library!
24 There are existing libraries for many languages. You can have a look at
25 <<libraries>> or search the web if your language of choice is not mentioned.
26 Usually, it is not necessary to implement low-level communication with i3
29 == Establishing a connection
31 To establish a connection, simply open the IPC socket. The following code
32 snippet illustrates this in Perl:
34 -------------------------------------------------------------
36 chomp(my $path = qx(i3 --get-socketpath));
37 my $sock = IO::Socket::UNIX->new(Peer => $path);
38 -------------------------------------------------------------
40 == Sending messages to i3
42 To send a message to i3, you have to format in the binary message format which
43 i3 expects. This format specifies a magic string in the beginning to ensure
44 the integrity of messages (to prevent follow-up errors). Following the magic
45 string comes the length of the payload of the message as 32-bit integer, and
46 the type of the message as 32-bit integer (the integers are not converted, so
47 they are in native byte order).
49 The magic string currently is "i3-ipc" and will only be changed when a change
50 in the IPC API is done which breaks compatibility (we hope that we don’t need
53 .Currently implemented message types
54 [options="header",cols="^10%,^20%,^20%,^50%"]
55 |======================================================
56 | Type (numeric) | Type (name) | Reply type | Purpose
57 | 0 | +RUN_COMMAND+ | <<_command_reply,COMMAND>> | Run the payload as an i3 command (like the commands you can bind to keys).
58 | 1 | +GET_WORKSPACES+ | <<_workspaces_reply,WORKSPACES>> | Get the list of current workspaces.
59 | 2 | +SUBSCRIBE+ | <<_subscribe_reply,SUBSCRIBE>> | Subscribe this IPC connection to the event types specified in the message payload. See <<events>>.
60 | 3 | +GET_OUTPUTS+ | <<_outputs_reply,OUTPUTS>> | Get the list of current outputs.
61 | 4 | +GET_TREE+ | <<_tree_reply,TREE>> | Get the i3 layout tree.
62 | 5 | +GET_MARKS+ | <<_marks_reply,MARKS>> | Gets the names of all currently set marks.
63 | 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.
64 | 7 | +GET_VERSION+ | <<_version_reply,VERSION>> | Gets the i3 version.
65 | 8 | +GET_BINDING_MODES+ | <<_binding_modes_reply,BINDING_MODES>> | Gets the names of all currently configured binding modes.
66 | 9 | +GET_CONFIG+ | <<_config_reply,CONFIG>> | Returns the last loaded i3 config.
67 | 10 | +SEND_TICK+ | <<_tick_reply,TICK>> | Sends a tick event with the specified payload.
68 | 11 | +SYNC+ | <<_sync_reply,SYNC>> | Sends an i3 sync event with the specified random value to the specified window.
69 |======================================================
71 So, a typical message could look like this:
72 --------------------------------------------------
73 "i3-ipc" <message length> <message type> <payload>
74 --------------------------------------------------
77 ------------------------------------------------------------------------------
78 00000000 69 33 2d 69 70 63 04 00 00 00 00 00 00 00 65 78 |i3-ipc........ex|
80 ------------------------------------------------------------------------------
82 To generate and send such a message, you could use the following code in Perl:
83 ------------------------------------------------------------
84 sub format_ipc_command {
87 # Get the real byte count (vs. amount of characters)
88 { use bytes; $len = length($msg); }
89 return "i3-ipc" . pack("LL", $len, 0) . $msg;
92 $sock->write(format_ipc_command("exit"));
93 ------------------------------------------------------------------------------
95 == Receiving replies from i3
97 Replies from i3 usually consist of a simple string (the length of the string
98 is the message_length, so you can consider them length-prefixed) which in turn
99 contain the JSON serialization of a data structure. For example, the
100 GET_WORKSPACES message returns an array of workspaces (each workspace is a map
101 with certain attributes).
105 The reply format is identical to the normal message format. There also is
106 the magic string, then the message length, then the message type and the
109 The following reply types are implemented:
112 Confirmation/Error code for the RUN_COMMAND message.
114 Reply to the GET_WORKSPACES message.
116 Confirmation/Error code for the SUBSCRIBE message.
118 Reply to the GET_OUTPUTS message.
120 Reply to the GET_TREE message.
122 Reply to the GET_MARKS message.
124 Reply to the GET_BAR_CONFIG message.
126 Reply to the GET_VERSION message.
128 Reply to the GET_BINDING_MODES message.
130 Reply to the GET_CONFIG message.
132 Reply to the SEND_TICK message.
137 The reply consists of a list of serialized maps for each command that was
138 parsed. Each has the property +success (bool)+ and may also include a
139 human-readable error message in the property +error (string)+.
143 [{ "success": true }]
146 [[_workspaces_reply]]
149 The reply consists of a serialized list of workspaces. Each workspace has the
150 following properties:
153 The logical number of the workspace. Corresponds to the command
154 to switch to this workspace. For named workspaces, this will be -1.
156 The name of this workspace (by default num+1), as changed by the
157 user. Encoded in UTF-8.
159 Whether this workspace is currently visible on an output (multiple
160 workspaces can be visible at the same time).
162 Whether this workspace currently has the focus (only one workspace
163 can have the focus at the same time).
165 Whether a window on this workspace has the "urgent" flag set.
167 The rectangle of this workspace (equals the rect of the output it
168 is on), consists of x, y, width, height.
170 The video output this workspace is on (LVDS1, VGA1, …).
209 The reply consists of a single serialized map. The only property is
210 +success (bool)+, indicating whether the subscription was successful (the
211 default) or whether a JSON parse error occurred.
221 The reply consists of a serialized list of outputs. Each output has the
222 following properties:
225 The name of this output (as seen in +xrandr(1)+). Encoded in UTF-8.
227 Whether this output is currently active (has a valid mode).
229 Whether this output is currently the primary output.
230 current_workspace (string)::
231 The name of the current workspace that is visible on this output. +null+ if
232 the output is not active.
234 The rectangle of this output (equals the rect of the output it
235 is on), consists of x, y, width, height.
243 "current_workspace": "4",
254 "current_workspace": "1",
268 The reply consists of a serialized tree. Each node in the tree (representing
269 one container) has at least the properties listed below. While the nodes might
270 have more properties, please do not use any properties which are not documented
271 here. They are not yet finalized and will probably change!
274 The internal ID (actually a C pointer value) of this container. Do not
275 make any assumptions about it. You can use it to (re-)identify and
276 address containers when talking to i3.
278 The internal name of this container. For all containers which are part
279 of the tree structure down to the workspace contents, this is set to a
280 nice human-readable name of the container.
281 For containers that have an X11 window, the content is the title
282 (_NET_WM_NAME property) of that window.
283 For all other containers, the content is not defined (yet).
285 Type of this container. Can be one of "root", "output", "con",
286 "floating_con", "workspace" or "dockarea".
288 Can be either "normal", "none" or "pixel", depending on the
289 container’s border style.
290 current_border_width (integer)::
291 Number of pixels of the border width.
293 Can be either "splith", "splitv", "stacked", "tabbed", "dockarea" or
295 Other values might be possible in the future, should we add new
297 orientation (string)::
298 Can be either "none" (for non-split containers), "horizontal" or
300 THIS FIELD IS OBSOLETE. It is still present, but your code should not
301 use it. Instead, rely on the layout field.
303 The percentage which this container takes in its parent. A value of
304 +null+ means that the percent property does not make sense for this
305 container, for example for the root container.
307 The absolute display coordinates for this container. Display
308 coordinates means that when you have two 1600x1200 monitors on a single
309 X11 Display (the standard way), the coordinates of the first window on
310 the second monitor are +{ "x": 1600, "y": 0, "width": 1600, "height":
313 The coordinates of the *actual client window* inside its container.
314 These coordinates are relative to the container and do not include the
315 window decoration (which is actually rendered on the parent container).
316 So, when using the +default+ layout, you will have a 2 pixel border on
317 each side, making the window_rect +{ "x": 2, "y": 0, "width": 632,
318 "height": 366 }+ (for example).
320 The coordinates of the *window decoration* inside its container. These
321 coordinates are relative to the container and do not include the actual
324 The original geometry the window specified when i3 mapped it. Used when
325 switching a window to floating mode, for example.
327 The X11 window ID of the *actual client window* inside this container.
328 This field is set to null for split containers or otherwise empty
329 containers. This ID corresponds to what xwininfo(1) and other
330 X11-related tools display (usually in hex).
332 Whether this container (window, split container, floating container or
333 workspace) has the urgency hint set, directly or indirectly. All parent
334 containers up until the workspace container will be marked urgent if they
335 have at least one urgent child.
337 Whether this container is currently focused.
338 focus (array of integer)::
339 List of child node IDs (see +nodes+, +floating_nodes+ and +id+) in focus
340 order. Traversing the tree by following the first entry in this array
341 will result in eventually reaching the one node with +focused+ set to
343 nodes (array of node)::
344 The tiling (i.e. non-floating) child containers of this node.
345 floating_nodes (array of node)::
346 The floating child containers of this node. Only non-empty on nodes with
349 Please note that in the following example, I have left out some keys/values
350 which are not relevant for the type of the node. Otherwise, the example would
351 be by far too long (it already is quite long, despite showing only 1 window and
354 It is useful to have an overview of the structure before taking a look at the
368 -----------------------
395 "layout": "dockarea",
396 "orientation": "vertical",
419 "orientation": "horizontal",
426 "floating_nodes": [],
450 "name": "bottomdock",
451 "layout": "dockarea",
452 "orientation": "vertical",
479 ------------------------
484 The reply consists of a single array of strings for each container that has a
485 mark. A mark can only be set on one container, so the array is unique.
486 The order of that array is undefined.
488 If no window has a mark the response will be the empty array [].
490 [[_bar_config_reply]]
493 This can be used by third-party workspace bars (especially i3bar, but others
494 are free to implement compatible alternatives) to get the +bar+ block
495 configuration from i3.
497 Depending on the input, the reply is either:
500 An array of configured bar IDs
502 A JSON map containing the configuration for the specified bar.
504 Each bar configuration has the following properties:
507 The ID for this bar. Included in case you request multiple
508 configurations and want to differentiate the different replies.
510 Either +dock+ (the bar sets the dock window type) or +hide+ (the bar
511 does not show unless a specific key is pressed).
513 Either +bottom+ or +top+ at the moment.
514 status_command (string)::
515 Command which will be run to generate a statusline. Each line on stdout
516 of this command will be displayed in the bar. At the moment, no
517 formatting is supported.
519 The font to use for text on the bar.
520 workspace_buttons (boolean)::
521 Display workspace buttons or not? Defaults to true.
522 binding_mode_indicator (boolean)::
523 Display the mode indicator or not? Defaults to true.
525 Should the bar enable verbose output for debugging? Defaults to false.
527 Contains key/value pairs of colors. Each value is a color code in hex,
528 formatted #rrggbb (like in HTML).
530 The following colors can be configured at the moment:
533 Background color of the bar.
535 Text color to be used for the statusline.
537 Text color to be used for the separator.
539 Background color of the bar on the currently focused monitor output.
541 Text color to be used for the statusline on the currently focused
544 Text color to be used for the separator on the currently focused
546 focused_workspace_text/focused_workspace_bg/focused_workspace_border::
547 Text/background/border color for a workspace button when the workspace
549 active_workspace_text/active_workspace_bg/active_workspace_border::
550 Text/background/border color for a workspace button when the workspace
551 is active (visible) on some output, but the focus is on another one.
552 You can only tell this apart from the focused workspace when you are
553 using multiple monitors.
554 inactive_workspace_text/inactive_workspace_bg/inactive_workspace_border::
555 Text/background/border color for a workspace button when the workspace
556 does not have focus and is not active (visible) on any output. This
557 will be the case for most workspaces.
558 urgent_workspace_text/urgent_workspace_bg/urgent_workspace_border::
559 Text/background/border color for workspaces which contain at least one
560 window with the urgency hint set.
561 binding_mode_text/binding_mode_bg/binding_mode_border::
562 Text/background/border color for the binding mode indicator.
565 *Example of configured bars:*
570 *Example of bar configuration:*
575 "position": "bottom",
576 "status_command": "i3status",
577 "font": "-misc-fixed-medium-r-normal--13-120-75-75-C-70-iso10646-1",
578 "workspace_buttons": true,
579 "binding_mode_indicator": true,
582 "background": "#c0c0c0",
583 "statusline": "#00ff00",
584 "focused_workspace_text": "#ffffff",
585 "focused_workspace_bg": "#000000"
593 The reply consists of a single JSON dictionary with the following keys:
596 The major version of i3, such as +4+.
598 The minor version of i3, such as +2+. Changes in the IPC interface (new
599 features) will only occur with new minor (or major) releases. However,
600 bugfixes might be introduced in patch releases, too.
602 The patch version of i3, such as +1+ (when the complete version is
603 +4.2.1+). For versions such as +4.2+, patch will be set to +0+.
604 human_readable (string)::
605 A human-readable version of i3 containing the precise git version,
606 build date and branch name. When you need to display the i3 version to
607 your users, use the human-readable version whenever possible (since
608 this is what +i3 --version+ displays, too).
609 loaded_config_file_name (string)::
610 The current config path.
615 "human_readable" : "4.2-169-gf80b877 (2012-08-05, branch \"next\")",
616 "loaded_config_file_name" : "/home/hwangcc23/.i3/config",
623 [[_binding_modes_reply]]
624 === BINDING_MODES reply
626 The reply consists of an array of all currently configured binding modes.
629 ---------------------
630 ["default", "resize"]
631 ---------------------
636 The config reply is a map which currently only contains the "config" member,
637 which is a string containing the config file as loaded by i3 most recently.
641 { "config": "font pango:monospace 8\nbindsym Mod4+q exit\n" }
647 The reply is a map containing the "success" member. After the reply was
648 received, the tick event has been written to all IPC connections which subscribe
649 to tick events. UNIX sockets are usually buffered, but you can be certain that
650 once you receive the tick event you just triggered, you must have received all
651 events generated prior to the +SEND_TICK+ message (happened-before relation).
661 The reply is a map containing the "success" member. After the reply was
662 received, the https://i3wm.org/docs/testsuite.html#i3_sync[i3 sync message] was
674 To get informed when certain things happen in i3, clients can subscribe to
675 events. Events consist of a name (like "workspace") and an event reply type
676 (like I3_IPC_EVENT_WORKSPACE). The events sent by i3 are in the same format
677 as replies to specific commands. However, the highest bit of the message type
678 is set to 1 to indicate that this is an event reply instead of a normal reply.
680 Caveat: As soon as you subscribe to an event, it is not guaranteed any longer
681 that the requests to i3 are processed in order. This means, the following
682 situation can happen: You send a GET_WORKSPACES request but you receive a
683 "workspace" event before receiving the reply to GET_WORKSPACES. If your
684 program does not want to cope which such kinds of race conditions (an
685 event based library may not have a problem here), I suggest you create a
686 separate connection to receive events.
688 === Subscribing to events
690 By sending a message of type SUBSCRIBE with a JSON-encoded array as payload
691 you can register to an event.
694 ---------------------------------
696 payload: [ "workspace", "output" ]
697 ---------------------------------
702 The numbers in parenthesis is the event type (keep in mind that you need to
703 strip the highest bit first).
706 Sent when the user switches to a different workspace, when a new
707 workspace is initialized or when a workspace is removed (because the
708 last client vanished).
710 Sent when RandR issues a change notification (of either screens,
711 outputs, CRTCs or output properties).
713 Sent whenever i3 changes its binding mode.
715 Sent when a client's window is successfully reparented (that is when i3
716 has finished fitting it into a container), when a window received input
717 focus or when certain properties of the window have changed.
718 barconfig_update (4)::
719 Sent when the hidden_state or mode field in the barconfig of any bar
720 instance was updated and when the config is reloaded.
722 Sent when a configured command binding is triggered with the keyboard or
725 Sent when the ipc shuts down because of a restart or exit by user command
727 Sent when the ipc client subscribes to the tick event (with +"first":
728 true+) or when any ipc client sends a SEND_TICK message (with +"first":
732 --------------------------------------------------------------------
733 # the appropriate 4 bytes read from the socket are stored in $input
735 # unpack a 32-bit unsigned integer
736 my $message_type = unpack("L", $input);
738 # check if the highest bit is 1
739 my $is_event = (($message_type >> 31) == 1);
742 my $event_type = ($message_type & 0x7F);
745 say "Received event of type $event_type";
747 --------------------------------------------------------------------
751 This event consists of a single serialized map containing a property
752 +change (string)+ which indicates the type of the change ("focus", "init",
753 "empty", "urgent", "reload", "rename", "restored", "move"). A
754 +current (object)+ property will be present with the affected workspace
755 whenever the type of event affects a workspace (otherwise, it will be +null).
757 When the change is "focus", an +old (object)+ property will be present with the
758 previous workspace. When the first switch occurs (when i3 focuses the
759 workspace visible at the beginning) there is no previous workspace, and the
760 +old+ property will be set to +null+. Also note that if the previous is empty
761 it will get destroyed when switching, but will still be present in the "old"
765 ---------------------
779 ---------------------
783 This event consists of a single serialized map containing a property
784 +change (string)+ which indicates the type of the change (currently only
788 ---------------------------
789 { "change": "unspecified" }
790 ---------------------------
794 This event consists of a single serialized map containing a property
795 +change (string)+ which holds the name of current mode in use. The name
796 is the same as specified in config when creating a mode. The default
797 mode is simply named default. It contains a second property, +pango_markup+, which
798 defines whether pango markup shall be used for displaying this mode.
801 ---------------------------
806 ---------------------------
810 This event consists of a single serialized map containing a property
811 +change (string)+ which indicates the type of the change
813 * +new+ – the window has become managed by i3
814 * +close+ – the window has closed
815 * +focus+ – the window has received input focus
816 * +title+ – the window's title has changed
817 * +fullscreen_mode+ – the window has entered or exited fullscreen mode
818 * +move+ – the window has changed its position in the tree
819 * +floating+ – the window has transitioned to or from floating
820 * +urgent+ – the window has become urgent or lost its urgent status
821 * +mark+ – a mark has been added to or removed from the window
823 Additionally a +container (object)+ field will be present, which consists
824 of the window's parent container. Be aware that for the "new" event, the
825 container will hold the initial name of the newly reparented window (e.g.
826 if you run urxvt with a shell that changes the title, you will still at
827 this point get the window title as "urxvt").
830 ---------------------------
839 ---------------------------
841 === barconfig_update event
843 This event consists of a single serialized map reporting on options from the
844 barconfig of the specified bar_id that were updated in i3. This event is the
845 same as a +GET_BAR_CONFIG+ reply for the bar with the given id.
849 This event consists of a single serialized map reporting on the details of a
850 binding that ran a command because of user input. The +change (string)+ field
851 indicates what sort of binding event was triggered (right now it will always be
852 +"run"+ but may be expanded in the future).
854 The +binding (object)+ field contains details about the binding that was run:
857 The i3 command that is configured to run for this binding.
858 event_state_mask (array of strings)::
859 The group and modifier keys that were configured with this binding.
860 input_code (integer)::
861 If the binding was configured with +bindcode+, this will be the key code
862 that was given for the binding. If the binding is a mouse binding, it will be
863 the number of the mouse button that was pressed. Otherwise it will be 0.
864 symbol (string or null)::
865 If this is a keyboard binding that was configured with +bindsym+, this
866 field will contain the given symbol. Otherwise it will be +null+.
867 input_type (string)::
868 This will be +"keyboard"+ or +"mouse"+ depending on whether or not this was
869 a keyboard or a mouse binding.
872 ---------------------------
877 "event_state_mask": [
883 "input_type": "keyboard"
886 ---------------------------
890 This event is triggered when the connection to the ipc is about to shutdown
891 because of a user action such as a +restart+ or +exit+ command. The +change
892 (string)+ field indicates why the ipc is shutting down. It can be either
893 +"restart"+ or +"exit"+.
896 ---------------------------
900 ---------------------------
904 This event is triggered by a subscription to tick events or by a +SEND_TICK+
907 *Example (upon subscription):*
908 --------------------------------------------------------------------------------
913 --------------------------------------------------------------------------------
915 *Example (upon +SEND_TICK+ with a payload of +arbitrary string+):*
916 --------------------------------------------------------------------------------
919 "payload": "arbitrary string"
921 --------------------------------------------------------------------------------
923 == See also (existing libraries)
927 For some languages, libraries are available (so you don’t have to implement
928 all this on your own). This list names some (if you wrote one, please let me
932 * i3 includes a headerfile +i3/ipc.h+ which provides you all constants.
933 * https://github.com/acrisci/i3ipc-glib
935 * https://github.com/drmgc/i3ipcpp
937 * https://github.com/mdirkse/i3ipc-go
938 * https://github.com/i3/go-i3
940 * https://github.com/acrisci/i3ipc-gjs
942 * https://github.com/acrisci/i3ipc-lua
944 * https://metacpan.org/module/AnyEvent::I3
946 * https://github.com/acrisci/i3ipc-python
947 * https://github.com/whitelynx/i3ipc (not maintained)
948 * https://github.com/ziberna/i3-py (not maintained)
950 * https://github.com/veelenga/i3ipc-ruby
951 * https://github.com/badboy/i3-ipc (not maintained)
953 * https://github.com/tmerr/i3ipc-rs
955 * https://github.com/Armael/ocaml-i3ipc
957 == Appendix A: Detecting byte order in memory-safe languages
959 Some programming languages such as Go don’t offer a way to serialize data in the
960 native byte order of the machine they’re running on without resorting to tricks
961 involving the +unsafe+ package.
963 The following technique can be used (and will not be broken by changes to i3) to
964 detect the byte order i3 is using:
966 1. The byte order dependent fields of an IPC message are message type and
969 * The message type +RUN_COMMAND+ (0) is the same in big and little endian, so
970 we can use it in either byte order to elicit a reply from i3.
972 * The payload length 65536 + 256 (+0x00 01 01 00+) is the same in big and
973 little endian, and also small enough to not worry about memory allocations
974 of that size. We must use payloads of length 65536 + 256 in every message
975 we send, so that i3 will be able to read the entire message regardless of
976 the byte order it uses.
978 2. Send a big endian encoded message of type +SUBSCRIBE+ (2) with payload `[]`
979 followed by 65536 + 256 - 2 +SPACE+ (ASCII 0x20) bytes.
981 * If i3 is running in big endian, this message is treated as a noop,
982 resulting in a +SUBSCRIBE+ reply with payload `{"success":true}`
983 footnote:[A small payload is important: that way, we circumvent dealing
984 with UNIX domain socket buffer sizes, whose size depends on the
985 implementation/operating system. Exhausting such a buffer results in an i3
986 deadlock unless you concurrently read and write, which — depending on the
987 programming language — makes the technique much more complicated.].
989 * If i3 is running in little endian, this message is read in its entirety due
990 to the byte order independent payload length, then
991 https://github.com/i3/i3/blob/d726d09d496577d1c337a4b97486f2c9fbc914f1/src/ipc.c#L1188[silently
992 discarded] due to the unknown message type.
994 3. Send a byte order independent message, i.e. type +RUN_COMMAND+ (0) with
995 payload +nop byte order detection. padding:+, padded to 65536 + 256 bytes
996 with +a+ (ASCII 0x61) bytes. i3 will reply to this message with a reply of
999 * The human-readable prefix is in there to not confuse readers of the i3 log.
1001 * This messages serves as a synchronization primitive so that we know whether
1002 i3 discarded the +SUBSCRIBE+ message or didn’t answer it yet.
1004 4. Receive a message header from i3, decoding the message type as big endian.
1006 * If the message’s reply type is +COMMAND+ (0), i3 is running in little
1007 endian (because the +SUBSCRIBE+ message was discarded). Decode the message
1008 payload length as little endian, receive the message payload.
1010 * If the message’s reply type is anything else, i3 is running in big endian
1011 (because our big endian encoded +SUBSCRIBE+ message was answered). Decode
1012 the message payload length in big endian, receive the message
1013 payload. Then, receive the pending +COMMAND+ message reply in big endian.
1015 5. From here on out, send/receive all messages using the detected byte order.
1017 Find an example implementation of this technique in
1018 https://github.com/i3/go-i3/blob/master/byteorder.go