1<?xml version='1.0' encoding='utf-8' ?> 2<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN" "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [ 3<!ENTITY % BOOK_ENTITIES SYSTEM "Wayland.ent"> 4%BOOK_ENTITIES; 5]> 6<chapter id="chap-Protocol"> 7 <title>Wayland Protocol and Model of Operation</title> 8 <section id="sect-Protocol-Basic-Principles"> 9 <title>Basic Principles</title> 10 <para> 11 The Wayland protocol is an asynchronous object oriented protocol. All 12 requests are method invocations on some object. The requests include 13 an object ID that uniquely identifies an object on the server. Each 14 object implements an interface and the requests include an opcode that 15 identifies which method in the interface to invoke. 16 </para> 17 <para> 18 The protocol is message-based. A message sent by a client to the server 19 is called request. A message from the server to a client is called event. 20 A message has a number of arguments, each of which has a certain type (see 21 <xref linkend="sect-Protocol-Wire-Format"/> for a list of argument types). 22 </para> 23 <para> 24 Additionally, the protocol can specify <type>enum</type>s which associate 25 names to specific numeric enumeration values. These are primarily just 26 descriptive in nature: at the wire format level enums are just integers. 27 But they also serve a secondary purpose to enhance type safety or 28 otherwise add context for use in language bindings or other such code. 29 This latter usage is only supported so long as code written before these 30 attributes were introduced still works after; in other words, adding an 31 enum should not break API, otherwise it puts backwards compatibility at 32 risk. 33 </para> 34 <para> 35 <type>enum</type>s can be defined as just a set of integers, or as 36 bitfields. This is specified via the <type>bitfield</type> boolean 37 attribute in the <type>enum</type> definition. If this attribute is true, 38 the enum is intended to be accessed primarily using bitwise operations, 39 for example when arbitrarily many choices of the enum can be ORed 40 together; if it is false, or the attribute is omitted, then the enum 41 arguments are a just a sequence of numerical values. 42 </para> 43 <para> 44 The <type>enum</type> attribute can be used on either <type>uint</type> 45 or <type>int</type> arguments, however if the <type>enum</type> is 46 defined as a <type>bitfield</type>, it can only be used on 47 <type>uint</type> args. 48 </para> 49 <para> 50 The server sends back events to the client, each event is emitted from 51 an object. Events can be error conditions. The event includes the 52 object ID and the event opcode, from which the client can determine 53 the type of event. Events are generated both in response to requests 54 (in which case the request and the event constitutes a round trip) or 55 spontaneously when the server state changes. 56 </para> 57 <para> 58 <itemizedlist> 59 <listitem> 60 <para> 61 State is broadcast on connect, events are sent 62 out when state changes. Clients must listen for 63 these changes and cache the state. 64 There is no need (or mechanism) to query server state. 65 </para> 66 </listitem> 67 <listitem> 68 <para> 69 The server will broadcast the presence of a number of global objects, 70 which in turn will broadcast their current state. 71 </para> 72 </listitem> 73 </itemizedlist> 74 </para> 75 </section> 76 <section id="sect-Protocol-Code-Generation"> 77 <title>Code Generation</title> 78 <para> 79 The interfaces, requests and events are defined in 80 <filename>protocol/wayland.xml</filename>. 81 This xml is used to generate the function prototypes that can be used by 82 clients and compositors. 83 </para> 84 <para> 85 The protocol entry points are generated as inline functions which just 86 wrap the <function>wl_proxy_*</function> functions. The inline functions aren't 87 part of the library ABI and language bindings should generate their 88 own stubs for the protocol entry points from the xml. 89 </para> 90 </section> 91 <section id="sect-Protocol-Wire-Format"> 92 <title>Wire Format</title> 93 <para> 94 The protocol is sent over a UNIX domain stream socket, where the endpoint 95 usually is named <systemitem class="service">wayland-0</systemitem> 96 (although it can be changed via <emphasis>WAYLAND_DISPLAY</emphasis> 97 in the environment). Beginning in Wayland 1.15, implementations can 98 optionally support server socket endpoints located at arbitrary 99 locations in the filesystem by setting <emphasis>WAYLAND_DISPLAY</emphasis> 100 to the absolute path at which the server endpoint listens. 101 </para> 102 <para> 103 Every message is structured as 32-bit words; values are represented in the 104 host's byte-order. The message header has 2 words in it: 105 <itemizedlist> 106 <listitem> 107 <para> 108 The first word is the sender's object ID (32-bit). 109 </para> 110 </listitem> 111 <listitem> 112 <para> 113 The second has 2 parts of 16-bit. The upper 16-bits are the message 114 size in bytes, starting at the header (i.e. it has a minimum value of 8).The lower is the request/event opcode. 115 </para> 116 </listitem> 117 </itemizedlist> 118 The payload describes the request/event arguments. Every argument is always 119 aligned to 32-bits. Where padding is required, the value of padding bytes is 120 undefined. There is no prefix that describes the type, but it is 121 inferred implicitly from the xml specification. 122 </para> 123 <para> 124 125 The representation of argument types are as follows: 126 <variablelist> 127 <varlistentry> 128 <term>int</term> 129 <term>uint</term> 130 <listitem> 131 <para> 132 The value is the 32-bit value of the signed/unsigned 133 int. 134 </para> 135 </listitem> 136 </varlistentry> 137 <varlistentry> 138 <term>fixed</term> 139 <listitem> 140 <para> 141 Signed 24.8 decimal numbers. It is a signed decimal type which 142 offers a sign bit, 23 bits of integer precision and 8 bits of 143 decimal precision. This is exposed as an opaque struct with 144 conversion helpers to and from double and int on the C API side. 145 </para> 146 </listitem> 147 </varlistentry> 148 <varlistentry> 149 <term>string</term> 150 <listitem> 151 <para> 152 Starts with an unsigned 32-bit length, followed by the 153 string contents, including terminating null byte, then padding 154 to a 32-bit boundary. 155 </para> 156 </listitem> 157 </varlistentry> 158 <varlistentry> 159 <term>object</term> 160 <listitem> 161 <para> 162 32-bit object ID. 163 </para> 164 </listitem> 165 </varlistentry> 166 <varlistentry> 167 <term>new_id</term> 168 <listitem> 169 <para> 170 The 32-bit object ID. Generally, the interface used for the new 171 object is inferred from the xml, but in the case where it's not 172 specified, a new_id is preceded by a <code>string</code> specifying 173 the interface name, and a <code>uint</code> specifying the version. 174 </para> 175 </listitem> 176 </varlistentry> 177 <varlistentry> 178 <term>array</term> 179 <listitem> 180 <para> 181 Starts with 32-bit array size in bytes, followed by the array 182 contents verbatim, and finally padding to a 32-bit boundary. 183 </para> 184 </listitem> 185 </varlistentry> 186 <varlistentry> 187 <term>fd</term> 188 <listitem> 189 <para> 190 The file descriptor is not stored in the message buffer, but in 191 the ancillary data of the UNIX domain socket message (msg_control). 192 </para> 193 </listitem> 194 </varlistentry> 195 </variablelist> 196 </para> 197 </section> 198 <xi:include href="ProtocolInterfaces.xml" xmlns:xi="http://www.w3.org/2001/XInclude"/> 199 <section id="sect-Protocol-Versioning"> 200 <title>Versioning</title> 201 <para> 202 Every interface is versioned and every protocol object implements a 203 particular version of its interface. For global objects, the maximum 204 version supported by the server is advertised with the global and the 205 actual version of the created protocol object is determined by the 206 version argument passed to wl_registry.bind(). For objects that are 207 not globals, their version is inferred from the object that created 208 them. 209 </para> 210 <para> 211 In order to keep things sane, this has a few implications for 212 interface versions: 213 <itemizedlist> 214 <listitem> 215 <para> 216 The object creation hierarchy must be a tree. Otherwise, 217 inferring object versions from the parent object becomes a much 218 more difficult to properly track. 219 </para> 220 </listitem> 221 <listitem> 222 <para> 223 When the version of an interface increases, so does the version 224 of its parent (recursively until you get to a global interface) 225 </para> 226 </listitem> 227 <listitem> 228 <para> 229 A global interface's version number acts like a counter for all 230 of its child interfaces. Whenever a child interface gets 231 modified, the global parent's interface version number also 232 increases (see above). The child interface then takes on the 233 same version number as the new version of its parent global 234 interface. 235 </para> 236 </listitem> 237 </itemizedlist> 238 </para> 239 <para> 240 To illustrate the above, consider the wl_compositor interface. It 241 has two children, wl_surface and wl_region. As of wayland version 242 1.2, wl_surface and wl_compositor are both at version 3. If 243 something is added to the wl_region interface, both wl_region and 244 wl_compositor will get bumpped to version 4. If, afterwards, 245 wl_surface is changed, both wl_compositor and wl_surface will be at 246 version 5. In this way the global interface version is used as a 247 sort of "counter" for all of its child interfaces. This makes it 248 very simple to know the version of the child given the version of its 249 parent. The child is at the highest possible interface version that 250 is less than or equal to its parent's version. 251 </para> 252 <para> 253 It is worth noting a particular exception to the above versioning 254 scheme. The wl_display (and, by extension, wl_registry) interface 255 cannot change because it is the core protocol object and its version 256 is never advertised nor is there a mechanism to request a different 257 version. 258 </para> 259 </section> 260 <section id="sect-Protocol-Connect-Time"> 261 <title>Connect Time</title> 262 <para> 263 There is no fixed connection setup information, the server emits 264 multiple events at connect time, to indicate the presence and 265 properties of global objects: outputs, compositor, input devices. 266 </para> 267 </section> 268 <section id="sect-Protocol-Security-and-Authentication"> 269 <title>Security and Authentication</title> 270 <para> 271 <itemizedlist> 272 <listitem> 273 <para> 274 mostly about access to underlying buffers, need new drm auth 275 mechanism (the grant-to ioctl idea), need to check the cmd stream? 276 </para> 277 </listitem> 278 <listitem> 279 <para> 280 getting the server socket depends on the compositor type, could 281 be a system wide name, through fd passing on the session dbus. 282 or the client is forked by the compositor and the fd is 283 already opened. 284 </para> 285 </listitem> 286 </itemizedlist> 287 </para> 288 </section> 289 <section id="sect-Protocol-Creating-Objects"> 290 <title>Creating Objects</title> 291 <para> 292 Each object has a unique ID. The IDs are allocated by the entity 293 creating the object (either client or server). IDs allocated by the 294 client are in the range [1, 0xfeffffff] while IDs allocated by the 295 server are in the range [0xff000000, 0xffffffff]. The 0 ID is 296 reserved to represent a null or non-existent object. 297 298 For efficiency purposes, the IDs are densely packed in the sense that 299 the ID N will not be used until N-1 has been used. Any ID allocation 300 algorithm that does not maintain this property is incompatible with 301 the implementation in libwayland. 302 </para> 303 </section> 304 <section id="sect-Protocol-Compositor"> 305 <title>Compositor</title> 306 <para> 307 The compositor is a global object, advertised at connect time. 308 </para> 309 <para> 310 See <xref linkend="protocol-spec-wl_compositor"/> for the 311 protocol description. 312 </para> 313 </section> 314 <section id="sect-Protocol-Surface"> 315 <title>Surfaces</title> 316 <para> 317 A surface manages a rectangular grid of pixels that clients create 318 for displaying their content to the screen. Clients don't know 319 the global position of their surfaces, and cannot access other 320 clients' surfaces. 321 </para> 322 <para> 323 Once the client has finished writing pixels, it 'commits' the 324 buffer; this permits the compositor to access the buffer and read 325 the pixels. When the compositor is finished, it releases the 326 buffer back to the client. 327 </para> 328 <para> 329 See <xref linkend="protocol-spec-wl_surface"/> for the protocol 330 description. 331 </para> 332 </section> 333 <section id="sect-Protocol-Input"> 334 <title>Input</title> 335 <para> 336 A seat represents a group of input devices including mice, 337 keyboards and touchscreens. It has a keyboard and pointer 338 focus. Seats are global objects. Pointer events are delivered 339 in surface-local coordinates. 340 </para> 341 <para> 342 The compositor maintains an implicit grab when a button is 343 pressed, to ensure that the corresponding button release 344 event gets delivered to the same surface. But there is no way 345 for clients to take an explicit grab. Instead, surfaces can 346 be mapped as 'popup', which combines transient window semantics 347 with a pointer grab. 348 </para> 349 <para> 350 To avoid race conditions, input events that are likely to 351 trigger further requests (such as button presses, key events, 352 pointer motions) carry serial numbers, and requests such as 353 wl_surface.set_popup require that the serial number of the 354 triggering event is specified. The server maintains a 355 monotonically increasing counter for these serial numbers. 356 </para> 357 <para> 358 Input events also carry timestamps with millisecond granularity. 359 Their base is undefined, so they can't be compared against 360 system time (as obtained with clock_gettime or gettimeofday). 361 They can be compared with each other though, and for instance 362 be used to identify sequences of button presses as double 363 or triple clicks. 364 </para> 365 <para> 366 See <xref linkend="protocol-spec-wl_seat"/> for the 367 protocol description. 368 </para> 369 <para> 370 Talk about: 371 372 <itemizedlist> 373 <listitem> 374 <para> 375 keyboard map, change events 376 </para> 377 </listitem> 378 <listitem> 379 <para> 380 xkb on Wayland 381 </para> 382 </listitem> 383 <listitem> 384 <para> 385 multi pointer Wayland 386 </para> 387 </listitem> 388 </itemizedlist> 389 </para> 390 <para> 391 A surface can change the pointer image when the surface is the pointer 392 focus of the input device. Wayland doesn't automatically change the 393 pointer image when a pointer enters a surface, but expects the 394 application to set the cursor it wants in response to the pointer 395 focus and motion events. The rationale is that a client has to manage 396 changing pointer images for UI elements within the surface in response 397 to motion events anyway, so we'll make that the only mechanism for 398 setting or changing the pointer image. If the server receives a request 399 to set the pointer image after the surface loses pointer focus, the 400 request is ignored. To the client this will look like it successfully 401 set the pointer image. 402 </para> 403 <para> 404 Setting the pointer image to NULL causes the cursor to be hidden. 405 </para> 406 <para> 407 The compositor will revert the pointer image back to a default image 408 when no surface has the pointer focus for that device. 409 </para> 410 <para> 411 What if the pointer moves from one window which has set a special 412 pointer image to a surface that doesn't set an image in response to 413 the motion event? The new surface will be stuck with the special 414 pointer image. We can't just revert the pointer image on leaving a 415 surface, since if we immediately enter a surface that sets a different 416 image, the image will flicker. If a client does not set a pointer image 417 when the pointer enters a surface, the pointer stays with the image set 418 by the last surface that changed it, possibly even hidden. Such a client 419 is likely just broken. 420 </para> 421 </section> 422 <section id="sect-Protocol-Output"> 423 <title>Output</title> 424 <para> 425 An output is a global object, advertised at connect time or as it 426 comes and goes. 427 </para> 428 <para> 429 See <xref linkend="protocol-spec-wl_output"/> for the protocol 430 description. 431 </para> 432 <para> 433 </para> 434 <itemizedlist> 435 <listitem> 436 <para> 437 laid out in a big (compositor) coordinate system 438 </para> 439 </listitem> 440 <listitem> 441 <para> 442 basically xrandr over Wayland 443 </para> 444 </listitem> 445 <listitem> 446 <para> 447 geometry needs position in compositor coordinate system 448 </para> 449 </listitem> 450 <listitem> 451 <para> 452 events to advertise available modes, requests to move and change 453 modes 454 </para> 455 </listitem> 456 </itemizedlist> 457 </section> 458 <section id="sect-Protocol-data-sharing"> 459 <title>Data sharing between clients</title> 460 <para> 461 The Wayland protocol provides clients a mechanism for sharing 462 data that allows the implementation of copy-paste and 463 drag-and-drop. The client providing the data creates a 464 <function>wl_data_source</function> object and the clients 465 obtaining the data will see it as <function>wl_data_offer</function> 466 object. This interface allows the clients to agree on a mutually 467 supported mime type and transfer the data via a file descriptor 468 that is passed through the protocol. 469 </para> 470 <para> 471 The next section explains the negotiation between data source and 472 data offer objects. <xref linkend="sect-Protocol-data-sharing-devices"/> 473 explains how these objects are created and passed to different 474 clients using the <function>wl_data_device</function> interface 475 that implements copy-paste and drag-and-drop support. 476 </para> 477 <para> 478 See <xref linkend="protocol-spec-wl_data_offer"/>, 479 <xref linkend="protocol-spec-wl_data_source"/>, 480 <xref linkend="protocol-spec-wl_data_device"/> and 481 <xref linkend="protocol-spec-wl_data_device_manager"/> for 482 protocol descriptions. 483 </para> 484 <para> 485 MIME is defined in RFC's 2045-2049. A 486 <ulink url="https://www.iana.org/assignments/media-types/media-types.xhtml"> 487 registry of MIME types</ulink> is maintained by the Internet Assigned 488 Numbers Authority (IANA). 489 </para> 490 <section> 491 <title>Data negotiation</title> 492 <para> 493 A client providing data to other clients will create a <function>wl_data_source</function> 494 object and advertise the mime types for the formats it supports for 495 that data through the <function>wl_data_source.offer</function> 496 request. On the receiving end, the data offer object will generate one 497 <function>wl_data_offer.offer</function> event for each supported mime 498 type. 499 </para> 500 <para> 501 The actual data transfer happens when the receiving client sends a 502 <function>wl_data_offer.receive</function> request. This request takes 503 a mime type and a file descriptor as arguments. This request will generate a 504 <function>wl_data_source.send</function> event on the sending client 505 with the same arguments, and the latter client is expected to write its 506 data to the given file descriptor using the chosen mime type. 507 </para> 508 </section> 509 <section id="sect-Protocol-data-sharing-devices"> 510 <title>Data devices</title> 511 <para> 512 Data devices glue data sources and offers together. A data device is 513 associated with a <function>wl_seat</function> and is obtained by the clients using the 514 <function>wl_data_device_manager</function> factory object, which is also responsible for 515 creating data sources. 516 </para> 517 <para> 518 Clients are informed of new data offers through the 519 <function>wl_data_device.data_offer</function> event. After this 520 event is generated the data offer will advertise the available mime 521 types. New data offers are introduced prior to their use for 522 copy-paste or drag-and-drop. 523 </para> 524 <section> 525 <title>Selection</title> 526 <para> 527 Each data device has a selection data source. Clients create a data 528 source object using the device manager and may set it as the 529 current selection for a given data device. Whenever the current 530 selection changes, the client with keyboard focus receives a 531 <function>wl_data_device.selection</function> event. This event is 532 also generated on a client immediately before it receives keyboard 533 focus. 534 </para> 535 <para> 536 The data offer is introduced with 537 <function>wl_data_device.data_offer</function> event before the 538 selection event. 539 </para> 540 </section> 541 <section> 542 <title>Drag and Drop</title> 543 <para> 544 A drag-and-drop operation is started using the 545 <function>wl_data_device.start_drag</function> request. This 546 requests causes a pointer grab that will generate enter, motion and 547 leave events on the data device. A data source is supplied as 548 argument to start_drag, and data offers associated with it are 549 supplied to clients surfaces under the pointer in the 550 <function>wl_data_device.enter</function> event. The data offer 551 is introduced to the client prior to the enter event with the 552 <function>wl_data_device.data_offer</function> event. 553 </para> 554 <para> 555 Clients are expected to provide feedback to the data sending client 556 by calling the <function>wl_data_offer.accept</function> request with 557 a mime type it accepts. If none of the advertised mime types is 558 supported by the receiving client, it should supply NULL to the 559 accept request. The accept request causes the sending client to 560 receive a <function>wl_data_source.target</function> event with the 561 chosen mime type. 562 </para> 563 <para> 564 When the drag ends, the receiving client receives a 565 <function>wl_data_device.drop</function> event at which it is expected 566 to transfer the data using the 567 <function>wl_data_offer.receive</function> request. 568 </para> 569 </section> 570 </section> 571 </section> 572</chapter> 573