X-Git-Url: https://git.sur5r.net/?a=blobdiff_plain;f=docs%2Fipc.html;h=34a55eaf82ac70a6b11b9509f6e6be4b59b23250;hb=167cbbbaee792decb0c501767d9ce402df2afad3;hp=3a3cb87a47051a5f96b48a47ff39f044040d04a9;hpb=d065d5315144e44aab9288d458a57cf0a2ca8de5;p=i3%2Fi3.github.io diff --git a/docs/ipc.html b/docs/ipc.html index 3a3cb87..34a55ea 100644 --- a/docs/ipc.html +++ b/docs/ipc.html @@ -2,15 +2,15 @@ "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd"> - + - + i3: IPC interface (interprocess communication) @@ -22,16 +22,17 @@ window.onload = function(){asciidoc.footnotes(); asciidoc.toc(2);}

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.
+
@@ -60,7 +75,8 @@ snippet illustrates this in Perl:

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);
@@ -76,56 +92,88 @@ they are in native byte order).

The magic string currently is "i3-ipc" and will only be changed when a change in the IPC API is done which breaks compatibility (we hope that we don’t need to do that).

-

Currently implemented message types are the following:

-
-
-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). -

-
-
+
+ + +++++ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
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.

+

So, a typical message could look like this:

@@ -135,7 +183,7 @@ GET_TREE (4) 
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:

@@ -172,11 +220,11 @@ COMMAND (0)

- Confirmation/Error code for the COMMAND message. + Confirmation/Error code for the RUN_COMMAND message.

-GET_WORKSPACES (1) +WORKSPACES (1)

@@ -192,7 +240,7 @@ SUBSCRIBE (2)

-GET_OUTPUTS (3) +OUTPUTS (3)

@@ -200,27 +248,68 @@ GET_OUTPUTS (3)

-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. +

+

3.2. 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 }]
-

3.3. GET_WORKSPACES reply

+

3.3. WORKSPACES reply

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

@@ -230,7 +319,7 @@ 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.

@@ -333,7 +422,7 @@ default) or whether a JSON parse error occurred.

-

3.5. GET_OUTPUTS reply

+

3.5. OUTPUTS reply

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

@@ -354,12 +443,20 @@ active (boolean)

-current_workspace (integer) +primary (boolean) +
+
+

+ Whether this output is currently the primary output. +

+
+
+current_workspace (string)

- The current workspace which is visible on this output. null if the - output is not active. + The name of the current workspace that is visible on this output. null if + the output is not active.

@@ -379,7 +476,7 @@ rect (map) { "name": "LVDS1", "active": true, - "current_workspace": 4, + "current_workspace": "4", "rect": { "x": 0, "y": 0, @@ -390,19 +487,19 @@ rect (map) { "name": "VGA1", "active": true, - "current_workspace": 1, + "current_workspace": "1", "rect": { "x": 1280, "y": 0, "width": 1280, "height": 1024 - }, + } } ]
-

3.6. GET_TREE reply

+

3.6. 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 have more properties, please do not use any properties which are not documented @@ -426,24 +523,44 @@ 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 "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.

@@ -455,6 +572,8 @@ 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.

@@ -493,6 +612,16 @@ window_rect (map)

+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)
@@ -502,11 +631,25 @@ geometry (map)

+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.

@@ -517,6 +660,34 @@ 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 @@ -617,7 +788,7 @@ VGA1 "y": 0, "width": 1280, "height": 0 - }, + } }, { @@ -696,143 +867,924 @@ VGA1 }

- - -
-

4. 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). The events sent by i3 are in the same format -as replies to specific commands. However, the highest bit of the message type -is set to 1 to indicate that this is an event reply instead of a normal 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.

-

4.1. 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", "focus" ]
-
+

3.7. 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 [].

-

4.2. Available events

-

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

+

3.8. 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:

-workspace (0) +empty input

- 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). + An array of configured bar IDs

-output (1) +Bar ID

- Sent when RandR issues a change notification (of either screens, - outputs, CRTCs or output properties). + A JSON map containing the configuration for the specified bar.

-

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";
-}
-
-
-
-

4.3. workspace event

-

This event consists of a single serialized map containing a property -change (string) which indicates the type of the change ("focus", "init", -"empty", "urgent").

-

Example:

-
-
-
{ "change": "focus" }
-
-
-
-

4.4. 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. See also

-
-

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):

+

Each bar configuration has the following properties:

-C +id (string)

- i3 includes a headerfile i3/ipc.h which provides you all constants. - However, there is no library yet. + The ID for this bar. Included in case you request multiple + configurations and want to differentiate the different replies.

-Ruby +mode (string)

- http://github.com/badboy/i3-ipc + Either dock (the bar sets the dock window type) or hide (the bar + does not show unless a specific key is pressed).

-Perl +position (string)

- http://search.cpan.org/search?query=AnyEvent::I3 + Either bottom or top at the moment.

-Python +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)

- http://github.com/thepub/i3ipc + 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"
+ }
+}
+
+
+
+

3.9. 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
+}
+
+
+
+

3.10. BINDING_MODES reply

+

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

+

Example:

+
+
+
["default", "resize"]
+
+
+
+

3.11. 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" }
+
+
+
+ +
+

4. 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). The events sent by i3 are in the same format +as replies to specific commands. However, the highest bit of the message type +is set to 1 to indicate that this is an event reply instead of a normal 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.

+
+

4.1. 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" ]
+
+
+
+

4.2. 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 +

+
+
+

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";
+}
+
+
+
+

4.3. workspace event

+

This event consists of a single serialized map containing a property +change (string) which indicates the type of the change ("focus", "init", +"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",
+ "current": {
+  "id": 28489712,
+  "type": "workspace",
+  ...
+ }
+ "old": {
+  "id": 28489715,
+  "type": "workspace",
+  ...
+ }
+}
+
+
+
+

4.4. 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" }
+
+
+
+

4.5. 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
+}
+
+
+
+

4.6. 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",
+  ...
+ }
+}
+
+
+
+

4.7. 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.

+
+
+

4.8. 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 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"
+ }
+}
+
+
+
+

4.9. 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. 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 +
+
+
+
+
+C++ +
+
+
+
+
+Go +
+
+
+
+
+JavaScript +
+
+
+
+
+Lua +
+
+
+
+
+Perl +
+
+
+
+
+Python +
+
+
+
+
+Ruby +
+
+
+
+
+Rust +
+
+
+
+
+OCaml +
+
+
+
+
+
+
+
+

6. 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. +
  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} +
      [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. +

      +
      • +
    +
    3. +
  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. +
  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. +
  5. +

    +From here on out, send/receive all messages using the detected byte order. +

    +
    6. +