1.. SPDX-License-Identifier: CC-BY-SA-2.0-UK 2 3********************** 4Yocto Project Concepts 5********************** 6 7This chapter provides explanations for Yocto Project concepts that go 8beyond the surface of "how-to" information and reference (or look-up) 9material. Concepts such as components, the :term:`OpenEmbedded Build System` 10workflow, 11cross-development toolchains, shared state cache, and so forth are 12explained. 13 14Yocto Project Components 15======================== 16 17The :term:`BitBake` task executor 18together with various types of configuration files form the 19:term:`OpenEmbedded-Core (OE-Core)`. This section 20overviews these components by describing their use and how they 21interact. 22 23BitBake handles the parsing and execution of the data files. The data 24itself is of various types: 25 26- *Recipes:* Provides details about particular pieces of software. 27 28- *Class Data:* Abstracts common build information (e.g. how to build a 29 Linux kernel). 30 31- *Configuration Data:* Defines machine-specific settings, policy 32 decisions, and so forth. Configuration data acts as the glue to bind 33 everything together. 34 35BitBake knows how to combine multiple data sources together and refers 36to each data source as a layer. For information on layers, see the 37":ref:`dev-manual/common-tasks:understanding and creating layers`" 38section of the Yocto Project Development Tasks Manual. 39 40Following are some brief details on these core components. For 41additional information on how these components interact during a build, 42see the 43":ref:`overview-manual/concepts:openembedded build system concepts`" 44section. 45 46BitBake 47------- 48 49BitBake is the tool at the heart of the :term:`OpenEmbedded Build System` 50and is responsible 51for parsing the :term:`Metadata`, generating 52a list of tasks from it, and then executing those tasks. 53 54This section briefly introduces BitBake. If you want more information on 55BitBake, see the :doc:`BitBake User Manual <bitbake:index>`. 56 57To see a list of the options BitBake supports, use either of the 58following commands:: 59 60 $ bitbake -h 61 $ bitbake --help 62 63The most common usage for BitBake is ``bitbake recipename``, where 64``recipename`` is the name of the recipe you want to build (referred 65to as the "target"). The target often equates to the first part of a 66recipe's filename (e.g. "foo" for a recipe named ``foo_1.3.0-r0.bb``). 67So, to process the ``matchbox-desktop_1.2.3.bb`` recipe file, you might 68type the following:: 69 70 $ bitbake matchbox-desktop 71 72Several different 73versions of ``matchbox-desktop`` might exist. BitBake chooses the one 74selected by the distribution configuration. You can get more details 75about how BitBake chooses between different target versions and 76providers in the 77":ref:`bitbake:bitbake-user-manual/bitbake-user-manual-execution:preferences`" section 78of the BitBake User Manual. 79 80BitBake also tries to execute any dependent tasks first. So for example, 81before building ``matchbox-desktop``, BitBake would build a cross 82compiler and ``glibc`` if they had not already been built. 83 84A useful BitBake option to consider is the ``-k`` or ``--continue`` 85option. This option instructs BitBake to try and continue processing the 86job as long as possible even after encountering an error. When an error 87occurs, the target that failed and those that depend on it cannot be 88remade. However, when you use this option other dependencies can still 89be processed. 90 91Recipes 92------- 93 94Files that have the ``.bb`` suffix are "recipes" files. In general, a 95recipe contains information about a single piece of software. This 96information includes the location from which to download the unaltered 97source, any source patches to be applied to that source (if needed), 98which special configuration options to apply, how to compile the source 99files, and how to package the compiled output. 100 101The term "package" is sometimes used to refer to recipes. However, since 102the word "package" is used for the packaged output from the OpenEmbedded 103build system (i.e. ``.ipk`` or ``.deb`` files), this document avoids 104using the term "package" when referring to recipes. 105 106Classes 107------- 108 109Class files (``.bbclass``) contain information that is useful to share 110between recipes files. An example is the 111:ref:`autotools <ref-classes-autotools>` class, 112which contains common settings for any application that Autotools uses. 113The ":ref:`ref-manual/classes:Classes`" chapter in the 114Yocto Project Reference Manual provides details about classes and how to 115use them. 116 117Configurations 118-------------- 119 120The configuration files (``.conf``) define various configuration 121variables that govern the OpenEmbedded build process. These files fall 122into several areas that define machine configuration options, 123distribution configuration options, compiler tuning options, general 124common configuration options, and user configuration options in 125``conf/local.conf``, which is found in the :term:`Build Directory`. 126 127 128Layers 129====== 130 131Layers are repositories that contain related metadata (i.e. sets of 132instructions) that tell the OpenEmbedded build system how to build a 133target. :ref:`overview-manual/yp-intro:the yocto project layer model` 134facilitates collaboration, sharing, customization, and reuse within the 135Yocto Project development environment. Layers logically separate 136information for your project. For example, you can use a layer to hold 137all the configurations for a particular piece of hardware. Isolating 138hardware-specific configurations allows you to share other metadata by 139using a different layer where that metadata might be common across 140several pieces of hardware. 141 142There are many layers working in the Yocto Project development environment. The 143:yocto_home:`Yocto Project Curated Layer Index </software-overview/layers/>` 144and :oe_layerindex:`OpenEmbedded Layer Index <>` both contain layers from 145which you can use or leverage. 146 147By convention, layers in the Yocto Project follow a specific form. 148Conforming to a known structure allows BitBake to make assumptions 149during builds on where to find types of metadata. You can find 150procedures and learn about tools (i.e. ``bitbake-layers``) for creating 151layers suitable for the Yocto Project in the 152":ref:`dev-manual/common-tasks:understanding and creating layers`" 153section of the Yocto Project Development Tasks Manual. 154 155OpenEmbedded Build System Concepts 156================================== 157 158This section takes a more detailed look inside the build process used by 159the :term:`OpenEmbedded Build System`, 160which is the build 161system specific to the Yocto Project. At the heart of the build system 162is BitBake, the task executor. 163 164The following diagram represents the high-level workflow of a build. The 165remainder of this section expands on the fundamental input, output, 166process, and metadata logical blocks that make up the workflow. 167 168.. image:: figures/YP-flow-diagram.png 169 :align: center 170 171In general, the build's workflow consists of several functional areas: 172 173- *User Configuration:* metadata you can use to control the build 174 process. 175 176- *Metadata Layers:* Various layers that provide software, machine, and 177 distro metadata. 178 179- *Source Files:* Upstream releases, local projects, and SCMs. 180 181- *Build System:* Processes under the control of 182 :term:`BitBake`. This block expands 183 on how BitBake fetches source, applies patches, completes 184 compilation, analyzes output for package generation, creates and 185 tests packages, generates images, and generates cross-development 186 tools. 187 188- *Package Feeds:* Directories containing output packages (RPM, DEB or 189 IPK), which are subsequently used in the construction of an image or 190 Software Development Kit (SDK), produced by the build system. These 191 feeds can also be copied and shared using a web server or other means 192 to facilitate extending or updating existing images on devices at 193 runtime if runtime package management is enabled. 194 195- *Images:* Images produced by the workflow. 196 197- *Application Development SDK:* Cross-development tools that are 198 produced along with an image or separately with BitBake. 199 200User Configuration 201------------------ 202 203User configuration helps define the build. Through user configuration, 204you can tell BitBake the target architecture for which you are building 205the image, where to store downloaded source, and other build properties. 206 207The following figure shows an expanded representation of the "User 208Configuration" box of the :ref:`general workflow 209figure <overview-manual/concepts:openembedded build system concepts>`: 210 211.. image:: figures/user-configuration.png 212 :align: center 213 214BitBake needs some basic configuration files in order to complete a 215build. These files are ``*.conf`` files. The minimally necessary ones 216reside as example files in the ``build/conf`` directory of the 217:term:`Source Directory`. For simplicity, 218this section refers to the Source Directory as the "Poky Directory." 219 220When you clone the :term:`Poky` Git repository 221or you download and unpack a Yocto Project release, you can set up the 222Source Directory to be named anything you want. For this discussion, the 223cloned repository uses the default name ``poky``. 224 225.. note:: 226 227 The Poky repository is primarily an aggregation of existing 228 repositories. It is not a canonical upstream source. 229 230The ``meta-poky`` layer inside Poky contains a ``conf`` directory that 231has example configuration files. These example files are used as a basis 232for creating actual configuration files when you source 233:ref:`structure-core-script`, which is the 234build environment script. 235 236Sourcing the build environment script creates a 237:term:`Build Directory` if one does not 238already exist. BitBake uses the Build Directory for all its work during 239builds. The Build Directory has a ``conf`` directory that contains 240default versions of your ``local.conf`` and ``bblayers.conf`` 241configuration files. These default configuration files are created only 242if versions do not already exist in the Build Directory at the time you 243source the build environment setup script. 244 245Because the Poky repository is fundamentally an aggregation of existing 246repositories, some users might be familiar with running the 247:ref:`structure-core-script` script in the context of separate 248:term:`OpenEmbedded-Core (OE-Core)` and BitBake 249repositories rather than a single Poky repository. This discussion 250assumes the script is executed from within a cloned or unpacked version 251of Poky. 252 253Depending on where the script is sourced, different sub-scripts are 254called to set up the Build Directory (Yocto or OpenEmbedded). 255Specifically, the script ``scripts/oe-setup-builddir`` inside the poky 256directory sets up the Build Directory and seeds the directory (if 257necessary) with configuration files appropriate for the Yocto Project 258development environment. 259 260.. note:: 261 262 The 263 scripts/oe-setup-builddir 264 script uses the 265 ``$TEMPLATECONF`` 266 variable to determine which sample configuration files to locate. 267 268The ``local.conf`` file provides many basic variables that define a 269build environment. Here is a list of a few. To see the default 270configurations in a ``local.conf`` file created by the build environment 271script, see the 272:yocto_git:`local.conf.sample </poky/tree/meta-poky/conf/local.conf.sample>` 273in the ``meta-poky`` layer: 274 275- *Target Machine Selection:* Controlled by the 276 :term:`MACHINE` variable. 277 278- *Download Directory:* Controlled by the 279 :term:`DL_DIR` variable. 280 281- *Shared State Directory:* Controlled by the 282 :term:`SSTATE_DIR` variable. 283 284- *Build Output:* Controlled by the 285 :term:`TMPDIR` variable. 286 287- *Distribution Policy:* Controlled by the 288 :term:`DISTRO` variable. 289 290- *Packaging Format:* Controlled by the 291 :term:`PACKAGE_CLASSES` 292 variable. 293 294- *SDK Target Architecture:* Controlled by the 295 :term:`SDKMACHINE` variable. 296 297- *Extra Image Packages:* Controlled by the 298 :term:`EXTRA_IMAGE_FEATURES` 299 variable. 300 301.. note:: 302 303 Configurations set in the ``conf/local.conf`` file can also be set 304 in the ``conf/site.conf`` and ``conf/auto.conf`` configuration files. 305 306The ``bblayers.conf`` file tells BitBake what layers you want considered 307during the build. By default, the layers listed in this file include 308layers minimally needed by the build system. However, you must manually 309add any custom layers you have created. You can find more information on 310working with the ``bblayers.conf`` file in the 311":ref:`dev-manual/common-tasks:enabling your layer`" 312section in the Yocto Project Development Tasks Manual. 313 314The files ``site.conf`` and ``auto.conf`` are not created by the 315environment initialization script. If you want the ``site.conf`` file, 316you need to create that yourself. The ``auto.conf`` file is typically 317created by an autobuilder: 318 319- *site.conf:* You can use the ``conf/site.conf`` configuration 320 file to configure multiple build directories. For example, suppose 321 you had several build environments and they shared some common 322 features. You can set these default build properties here. A good 323 example is perhaps the packaging format to use through the 324 :term:`PACKAGE_CLASSES` 325 variable. 326 327 One useful scenario for using the ``conf/site.conf`` file is to 328 extend your :term:`BBPATH` variable 329 to include the path to a ``conf/site.conf``. Then, when BitBake looks 330 for Metadata using :term:`BBPATH`, it finds the ``conf/site.conf`` file 331 and applies your common configurations found in the file. To override 332 configurations in a particular build directory, alter the similar 333 configurations within that build directory's ``conf/local.conf`` 334 file. 335 336- *auto.conf:* The file is usually created and written to by an 337 autobuilder. The settings put into the file are typically the same as 338 you would find in the ``conf/local.conf`` or the ``conf/site.conf`` 339 files. 340 341You can edit all configuration files to further define any particular 342build environment. This process is represented by the "User 343Configuration Edits" box in the figure. 344 345When you launch your build with the ``bitbake target`` command, BitBake 346sorts out the configurations to ultimately define your build 347environment. It is important to understand that the 348:term:`OpenEmbedded Build System` reads the 349configuration files in a specific order: ``site.conf``, ``auto.conf``, 350and ``local.conf``. And, the build system applies the normal assignment 351statement rules as described in the 352":doc:`bitbake:bitbake-user-manual/bitbake-user-manual-metadata`" chapter 353of the BitBake User Manual. Because the files are parsed in a specific 354order, variable assignments for the same variable could be affected. For 355example, if the ``auto.conf`` file and the ``local.conf`` set variable1 356to different values, because the build system parses ``local.conf`` 357after ``auto.conf``, variable1 is assigned the value from the 358``local.conf`` file. 359 360Metadata, Machine Configuration, and Policy Configuration 361--------------------------------------------------------- 362 363The previous section described the user configurations that define 364BitBake's global behavior. This section takes a closer look at the 365layers the build system uses to further control the build. These layers 366provide Metadata for the software, machine, and policies. 367 368In general, there are three types of layer input. You can see them below 369the "User Configuration" box in the `general workflow 370figure <overview-manual/concepts:openembedded build system concepts>`: 371 372- *Metadata (.bb + Patches):* Software layers containing 373 user-supplied recipe files, patches, and append files. A good example 374 of a software layer might be the :oe_layer:`meta-qt5 layer </meta-qt5>` 375 from the :oe_layerindex:`OpenEmbedded Layer Index <>`. This layer is for 376 version 5.0 of the popular `Qt <https://wiki.qt.io/About_Qt>`__ 377 cross-platform application development framework for desktop, embedded and 378 mobile. 379 380- *Machine BSP Configuration:* Board Support Package (BSP) layers (i.e. 381 "BSP Layer" in the following figure) providing machine-specific 382 configurations. This type of information is specific to a particular 383 target architecture. A good example of a BSP layer from the 384 :ref:`overview-manual/yp-intro:reference distribution (poky)` is the 385 :yocto_git:`meta-yocto-bsp </poky/tree/meta-yocto-bsp>` 386 layer. 387 388- *Policy Configuration:* Distribution Layers (i.e. "Distro Layer" in 389 the following figure) providing top-level or general policies for the 390 images or SDKs being built for a particular distribution. For 391 example, in the Poky Reference Distribution the distro layer is the 392 :yocto_git:`meta-poky </poky/tree/meta-poky>` 393 layer. Within the distro layer is a ``conf/distro`` directory that 394 contains distro configuration files (e.g. 395 :yocto_git:`poky.conf </poky/tree/meta-poky/conf/distro/poky.conf>` 396 that contain many policy configurations for the Poky distribution. 397 398The following figure shows an expanded representation of these three 399layers from the :ref:`general workflow figure 400<overview-manual/concepts:openembedded build system concepts>`: 401 402.. image:: figures/layer-input.png 403 :align: center 404 405In general, all layers have a similar structure. They all contain a 406licensing file (e.g. ``COPYING.MIT``) if the layer is to be distributed, 407a ``README`` file as good practice and especially if the layer is to be 408distributed, a configuration directory, and recipe directories. You can 409learn about the general structure for layers used with the Yocto Project 410in the 411":ref:`dev-manual/common-tasks:creating your own layer`" 412section in the 413Yocto Project Development Tasks Manual. For a general discussion on 414layers and the many layers from which you can draw, see the 415":ref:`overview-manual/concepts:layers`" and 416":ref:`overview-manual/yp-intro:the yocto project layer model`" sections both 417earlier in this manual. 418 419If you explored the previous links, you discovered some areas where many 420layers that work with the Yocto Project exist. The :yocto_git:`Source 421Repositories <>` also shows layers categorized under "Yocto Metadata Layers." 422 423.. note:: 424 425 There are layers in the Yocto Project Source Repositories that cannot be 426 found in the OpenEmbedded Layer Index. Such layers are either 427 deprecated or experimental in nature. 428 429BitBake uses the ``conf/bblayers.conf`` file, which is part of the user 430configuration, to find what layers it should be using as part of the 431build. 432 433Distro Layer 434~~~~~~~~~~~~ 435 436The distribution layer provides policy configurations for your 437distribution. Best practices dictate that you isolate these types of 438configurations into their own layer. Settings you provide in 439``conf/distro/distro.conf`` override similar settings that BitBake finds 440in your ``conf/local.conf`` file in the Build Directory. 441 442The following list provides some explanation and references for what you 443typically find in the distribution layer: 444 445- *classes:* Class files (``.bbclass``) hold common functionality that 446 can be shared among recipes in the distribution. When your recipes 447 inherit a class, they take on the settings and functions for that 448 class. You can read more about class files in the 449 ":ref:`ref-manual/classes:Classes`" chapter of the Yocto 450 Reference Manual. 451 452- *conf:* This area holds configuration files for the layer 453 (``conf/layer.conf``), the distribution 454 (``conf/distro/distro.conf``), and any distribution-wide include 455 files. 456 457- *recipes-*:* Recipes and append files that affect common 458 functionality across the distribution. This area could include 459 recipes and append files to add distribution-specific configuration, 460 initialization scripts, custom image recipes, and so forth. Examples 461 of ``recipes-*`` directories are ``recipes-core`` and 462 ``recipes-extra``. Hierarchy and contents within a ``recipes-*`` 463 directory can vary. Generally, these directories contain recipe files 464 (``*.bb``), recipe append files (``*.bbappend``), directories that 465 are distro-specific for configuration files, and so forth. 466 467BSP Layer 468~~~~~~~~~ 469 470The BSP Layer provides machine configurations that target specific 471hardware. Everything in this layer is specific to the machine for which 472you are building the image or the SDK. A common structure or form is 473defined for BSP layers. You can learn more about this structure in the 474:doc:`/bsp-guide/index`. 475 476.. note:: 477 478 In order for a BSP layer to be considered compliant with the Yocto 479 Project, it must meet some structural requirements. 480 481The BSP Layer's configuration directory contains configuration files for 482the machine (``conf/machine/machine.conf``) and, of course, the layer 483(``conf/layer.conf``). 484 485The remainder of the layer is dedicated to specific recipes by function: 486``recipes-bsp``, ``recipes-core``, ``recipes-graphics``, 487``recipes-kernel``, and so forth. There can be metadata for multiple 488formfactors, graphics support systems, and so forth. 489 490.. note:: 491 492 While the figure shows several 493 recipes-\* 494 directories, not all these directories appear in all BSP layers. 495 496Software Layer 497~~~~~~~~~~~~~~ 498 499The software layer provides the Metadata for additional software 500packages used during the build. This layer does not include Metadata 501that is specific to the distribution or the machine, which are found in 502their respective layers. 503 504This layer contains any recipes, append files, and patches, that your 505project needs. 506 507Sources 508------- 509 510In order for the OpenEmbedded build system to create an image or any 511target, it must be able to access source files. The :ref:`general workflow 512figure <overview-manual/concepts:openembedded build system concepts>` 513represents source files using the "Upstream Project Releases", "Local 514Projects", and "SCMs (optional)" boxes. The figure represents mirrors, 515which also play a role in locating source files, with the "Source 516Materials" box. 517 518The method by which source files are ultimately organized is a function 519of the project. For example, for released software, projects tend to use 520tarballs or other archived files that can capture the state of a release 521guaranteeing that it is statically represented. On the other hand, for a 522project that is more dynamic or experimental in nature, a project might 523keep source files in a repository controlled by a Source Control Manager 524(SCM) such as Git. Pulling source from a repository allows you to 525control the point in the repository (the revision) from which you want 526to build software. A combination of the two is also possible. 527 528BitBake uses the :term:`SRC_URI` 529variable to point to source files regardless of their location. Each 530recipe must have a :term:`SRC_URI` variable that points to the source. 531 532Another area that plays a significant role in where source files come 533from is pointed to by the 534:term:`DL_DIR` variable. This area is 535a cache that can hold previously downloaded source. You can also 536instruct the OpenEmbedded build system to create tarballs from Git 537repositories, which is not the default behavior, and store them in the 538:term:`DL_DIR` by using the 539:term:`BB_GENERATE_MIRROR_TARBALLS` 540variable. 541 542Judicious use of a :term:`DL_DIR` directory can save the build system a trip 543across the Internet when looking for files. A good method for using a 544download directory is to have :term:`DL_DIR` point to an area outside of 545your Build Directory. Doing so allows you to safely delete the Build 546Directory if needed without fear of removing any downloaded source file. 547 548The remainder of this section provides a deeper look into the source 549files and the mirrors. Here is a more detailed look at the source file 550area of the :ref:`general workflow figure <overview-manual/concepts:openembedded build system concepts>`: 551 552.. image:: figures/source-input.png 553 :align: center 554 555Upstream Project Releases 556~~~~~~~~~~~~~~~~~~~~~~~~~ 557 558Upstream project releases exist anywhere in the form of an archived file 559(e.g. tarball or zip file). These files correspond to individual 560recipes. For example, the figure uses specific releases each for 561BusyBox, Qt, and Dbus. An archive file can be for any released product 562that can be built using a recipe. 563 564Local Projects 565~~~~~~~~~~~~~~ 566 567Local projects are custom bits of software the user provides. These bits 568reside somewhere local to a project - perhaps a directory into which the 569user checks in items (e.g. a local directory containing a development 570source tree used by the group). 571 572The canonical method through which to include a local project is to use 573the :ref:`externalsrc <ref-classes-externalsrc>` 574class to include that local project. You use either the ``local.conf`` 575or a recipe's append file to override or set the recipe to point to the 576local directory on your disk to pull in the whole source tree. 577 578Source Control Managers (Optional) 579~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 580 581Another place from which the build system can get source files is with 582:ref:`bitbake:bitbake-user-manual/bitbake-user-manual-fetching:fetchers` employing various Source 583Control Managers (SCMs) such as Git or Subversion. In such cases, a 584repository is cloned or checked out. The 585:ref:`ref-tasks-fetch` task inside 586BitBake uses the :term:`SRC_URI` 587variable and the argument's prefix to determine the correct fetcher 588module. 589 590.. note:: 591 592 For information on how to have the OpenEmbedded build system generate 593 tarballs for Git repositories and place them in the 594 DL_DIR 595 directory, see the :term:`BB_GENERATE_MIRROR_TARBALLS` 596 variable in the Yocto Project Reference Manual. 597 598When fetching a repository, BitBake uses the 599:term:`SRCREV` variable to determine 600the specific revision from which to build. 601 602Source Mirror(s) 603~~~~~~~~~~~~~~~~ 604 605There are two kinds of mirrors: pre-mirrors and regular mirrors. The 606:term:`PREMIRRORS` and 607:term:`MIRRORS` variables point to 608these, respectively. BitBake checks pre-mirrors before looking upstream 609for any source files. Pre-mirrors are appropriate when you have a shared 610directory that is not a directory defined by the 611:term:`DL_DIR` variable. A Pre-mirror 612typically points to a shared directory that is local to your 613organization. 614 615Regular mirrors can be any site across the Internet that is used as an 616alternative location for source code should the primary site not be 617functioning for some reason or another. 618 619Package Feeds 620------------- 621 622When the OpenEmbedded build system generates an image or an SDK, it gets 623the packages from a package feed area located in the 624:term:`Build Directory`. The :ref:`general workflow figure 625<overview-manual/concepts:openembedded build system concepts>` 626shows this package feeds area in the upper-right corner. 627 628This section looks a little closer into the package feeds area used by 629the build system. Here is a more detailed look at the area: 630 631.. image:: figures/package-feeds.png 632 :align: center 633 634Package feeds are an intermediary step in the build process. The 635OpenEmbedded build system provides classes to generate different package 636types, and you specify which classes to enable through the 637:term:`PACKAGE_CLASSES` 638variable. Before placing the packages into package feeds, the build 639process validates them with generated output quality assurance checks 640through the :ref:`insane <ref-classes-insane>` 641class. 642 643The package feed area resides in the Build Directory. The directory the 644build system uses to temporarily store packages is determined by a 645combination of variables and the particular package manager in use. See 646the "Package Feeds" box in the illustration and note the information to 647the right of that area. In particular, the following defines where 648package files are kept: 649 650- :term:`DEPLOY_DIR`: Defined as 651 ``tmp/deploy`` in the Build Directory. 652 653- ``DEPLOY_DIR_*``: Depending on the package manager used, the package 654 type sub-folder. Given RPM, IPK, or DEB packaging and tarball 655 creation, the 656 :term:`DEPLOY_DIR_RPM`, 657 :term:`DEPLOY_DIR_IPK`, 658 :term:`DEPLOY_DIR_DEB`, or 659 :term:`DEPLOY_DIR_TAR`, 660 variables are used, respectively. 661 662- :term:`PACKAGE_ARCH`: Defines 663 architecture-specific sub-folders. For example, packages could be 664 available for the i586 or qemux86 architectures. 665 666BitBake uses the 667:ref:`do_package_write_* <ref-tasks-package_write_deb>` 668tasks to generate packages and place them into the package holding area 669(e.g. ``do_package_write_ipk`` for IPK packages). See the 670":ref:`ref-tasks-package_write_deb`", 671":ref:`ref-tasks-package_write_ipk`", 672":ref:`ref-tasks-package_write_rpm`", 673and 674":ref:`ref-tasks-package_write_tar`" 675sections in the Yocto Project Reference Manual for additional 676information. As an example, consider a scenario where an IPK packaging 677manager is being used and there is package architecture support for both 678i586 and qemux86. Packages for the i586 architecture are placed in 679``build/tmp/deploy/ipk/i586``, while packages for the qemux86 680architecture are placed in ``build/tmp/deploy/ipk/qemux86``. 681 682BitBake Tool 683------------ 684 685The OpenEmbedded build system uses 686:term:`BitBake` to produce images and 687Software Development Kits (SDKs). You can see from the :ref:`general workflow 688figure <overview-manual/concepts:openembedded build system concepts>`, 689the BitBake area consists of several functional areas. This section takes a 690closer look at each of those areas. 691 692.. note:: 693 694 Documentation for the BitBake tool is available separately. See the 695 BitBake User Manual 696 for reference material on BitBake. 697 698Source Fetching 699~~~~~~~~~~~~~~~ 700 701The first stages of building a recipe are to fetch and unpack the source 702code: 703 704.. image:: figures/source-fetching.png 705 :align: center 706 707The :ref:`ref-tasks-fetch` and 708:ref:`ref-tasks-unpack` tasks fetch 709the source files and unpack them into the 710:term:`Build Directory`. 711 712.. note:: 713 714 For every local file (e.g. 715 file:// 716 ) that is part of a recipe's 717 SRC_URI 718 statement, the OpenEmbedded build system takes a checksum of the file 719 for the recipe and inserts the checksum into the signature for the 720 do_fetch 721 task. If any local file has been modified, the 722 do_fetch 723 task and all tasks that depend on it are re-executed. 724 725By default, everything is accomplished in the Build Directory, which has 726a defined structure. For additional general information on the Build 727Directory, see the ":ref:`structure-core-build`" section in 728the Yocto Project Reference Manual. 729 730Each recipe has an area in the Build Directory where the unpacked source 731code resides. The :term:`S` variable points 732to this area for a recipe's unpacked source code. The name of that 733directory for any given recipe is defined from several different 734variables. The preceding figure and the following list describe the 735Build Directory's hierarchy: 736 737- :term:`TMPDIR`: The base directory 738 where the OpenEmbedded build system performs all its work during the 739 build. The default base directory is the ``tmp`` directory. 740 741- :term:`PACKAGE_ARCH`: The 742 architecture of the built package or packages. Depending on the 743 eventual destination of the package or packages (i.e. machine 744 architecture, :term:`Build Host`, SDK, or 745 specific machine), :term:`PACKAGE_ARCH` varies. See the variable's 746 description for details. 747 748- :term:`TARGET_OS`: The operating 749 system of the target device. A typical value would be "linux" (e.g. 750 "qemux86-poky-linux"). 751 752- :term:`PN`: The name of the recipe used 753 to build the package. This variable can have multiple meanings. 754 However, when used in the context of input files, :term:`PN` represents 755 the name of the recipe. 756 757- :term:`WORKDIR`: The location 758 where the OpenEmbedded build system builds a recipe (i.e. does the 759 work to create the package). 760 761 - :term:`PV`: The version of the 762 recipe used to build the package. 763 764 - :term:`PR`: The revision of the 765 recipe used to build the package. 766 767- :term:`S`: Contains the unpacked source 768 files for a given recipe. 769 770 - :term:`BPN`: The name of the recipe 771 used to build the package. The :term:`BPN` variable is a version of 772 the :term:`PN` variable but with common prefixes and suffixes removed. 773 774 - :term:`PV`: The version of the 775 recipe used to build the package. 776 777.. note:: 778 779 In the previous figure, notice that there are two sample hierarchies: 780 one based on package architecture (i.e. :term:`PACKAGE_ARCH`) 781 and one based on a machine (i.e. :term:`MACHINE`). 782 The underlying structures are identical. The differentiator being 783 what the OpenEmbedded build system is using as a build target (e.g. 784 general architecture, a build host, an SDK, or a specific machine). 785 786Patching 787~~~~~~~~ 788 789Once source code is fetched and unpacked, BitBake locates patch files 790and applies them to the source files: 791 792.. image:: figures/patching.png 793 :align: center 794 795The :ref:`ref-tasks-patch` task uses a 796recipe's :term:`SRC_URI` statements 797and the :term:`FILESPATH` variable 798to locate applicable patch files. 799 800Default processing for patch files assumes the files have either 801``*.patch`` or ``*.diff`` file types. You can use :term:`SRC_URI` parameters 802to change the way the build system recognizes patch files. See the 803:ref:`ref-tasks-patch` task for more 804information. 805 806BitBake finds and applies multiple patches for a single recipe in the 807order in which it locates the patches. The :term:`FILESPATH` variable 808defines the default set of directories that the build system uses to 809search for patch files. Once found, patches are applied to the recipe's 810source files, which are located in the 811:term:`S` directory. 812 813For more information on how the source directories are created, see the 814":ref:`overview-manual/concepts:source fetching`" section. For 815more information on how to create patches and how the build system 816processes patches, see the 817":ref:`dev-manual/common-tasks:patching code`" 818section in the 819Yocto Project Development Tasks Manual. You can also see the 820":ref:`sdk-manual/extensible:use \`\`devtool modify\`\` to modify the source of an existing component`" 821section in the Yocto Project Application Development and the Extensible 822Software Development Kit (SDK) manual and the 823":ref:`kernel-dev/common:using traditional kernel development to patch the kernel`" 824section in the Yocto Project Linux Kernel Development Manual. 825 826Configuration, Compilation, and Staging 827~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 828 829After source code is patched, BitBake executes tasks that configure and 830compile the source code. Once compilation occurs, the files are copied 831to a holding area (staged) in preparation for packaging: 832 833.. image:: figures/configuration-compile-autoreconf.png 834 :align: center 835 836This step in the build process consists of the following tasks: 837 838- :ref:`ref-tasks-prepare_recipe_sysroot`: 839 This task sets up the two sysroots in 840 ``${``\ :term:`WORKDIR`\ ``}`` 841 (i.e. ``recipe-sysroot`` and ``recipe-sysroot-native``) so that 842 during the packaging phase the sysroots can contain the contents of 843 the 844 :ref:`ref-tasks-populate_sysroot` 845 tasks of the recipes on which the recipe containing the tasks 846 depends. A sysroot exists for both the target and for the native 847 binaries, which run on the host system. 848 849- *do_configure*: This task configures the source by enabling and 850 disabling any build-time and configuration options for the software 851 being built. Configurations can come from the recipe itself as well 852 as from an inherited class. Additionally, the software itself might 853 configure itself depending on the target for which it is being built. 854 855 The configurations handled by the 856 :ref:`ref-tasks-configure` task 857 are specific to configurations for the source code being built by the 858 recipe. 859 860 If you are using the 861 :ref:`autotools <ref-classes-autotools>` class, 862 you can add additional configuration options by using the 863 :term:`EXTRA_OECONF` or 864 :term:`PACKAGECONFIG_CONFARGS` 865 variables. For information on how this variable works within that 866 class, see the 867 :ref:`autotools <ref-classes-autotools>` class 868 :yocto_git:`here </poky/tree/meta/classes/autotools.bbclass>`. 869 870- *do_compile*: Once a configuration task has been satisfied, 871 BitBake compiles the source using the 872 :ref:`ref-tasks-compile` task. 873 Compilation occurs in the directory pointed to by the 874 :term:`B` variable. Realize that the 875 :term:`B` directory is, by default, the same as the 876 :term:`S` directory. 877 878- *do_install*: After compilation completes, BitBake executes the 879 :ref:`ref-tasks-install` task. 880 This task copies files from the :term:`B` directory and places them in a 881 holding area pointed to by the :term:`D` 882 variable. Packaging occurs later using files from this holding 883 directory. 884 885Package Splitting 886~~~~~~~~~~~~~~~~~ 887 888After source code is configured, compiled, and staged, the build system 889analyzes the results and splits the output into packages: 890 891.. image:: figures/analysis-for-package-splitting.png 892 :align: center 893 894The :ref:`ref-tasks-package` and 895:ref:`ref-tasks-packagedata` 896tasks combine to analyze the files found in the 897:term:`D` directory and split them into 898subsets based on available packages and files. Analysis involves the 899following as well as other items: splitting out debugging symbols, 900looking at shared library dependencies between packages, and looking at 901package relationships. 902 903The ``do_packagedata`` task creates package metadata based on the 904analysis such that the build system can generate the final packages. The 905:ref:`ref-tasks-populate_sysroot` 906task stages (copies) a subset of the files installed by the 907:ref:`ref-tasks-install` task into 908the appropriate sysroot. Working, staged, and intermediate results of 909the analysis and package splitting process use several areas: 910 911- :term:`PKGD`: The destination 912 directory (i.e. ``package``) for packages before they are split into 913 individual packages. 914 915- :term:`PKGDESTWORK`: A 916 temporary work area (i.e. ``pkgdata``) used by the ``do_package`` 917 task to save package metadata. 918 919- :term:`PKGDEST`: The parent 920 directory (i.e. ``packages-split``) for packages after they have been 921 split. 922 923- :term:`PKGDATA_DIR`: A shared, 924 global-state directory that holds packaging metadata generated during 925 the packaging process. The packaging process copies metadata from 926 :term:`PKGDESTWORK` to the :term:`PKGDATA_DIR` area where it becomes globally 927 available. 928 929- :term:`STAGING_DIR_HOST`: 930 The path for the sysroot for the system on which a component is built 931 to run (i.e. ``recipe-sysroot``). 932 933- :term:`STAGING_DIR_NATIVE`: 934 The path for the sysroot used when building components for the build 935 host (i.e. ``recipe-sysroot-native``). 936 937- :term:`STAGING_DIR_TARGET`: 938 The path for the sysroot used when a component that is built to 939 execute on a system and it generates code for yet another machine 940 (e.g. cross-canadian recipes). 941 942The :term:`FILES` variable defines the 943files that go into each package in 944:term:`PACKAGES`. If you want 945details on how this is accomplished, you can look at 946:yocto_git:`package.bbclass </poky/tree/meta/classes/package.bbclass>`. 947 948Depending on the type of packages being created (RPM, DEB, or IPK), the 949:ref:`do_package_write_* <ref-tasks-package_write_deb>` 950task creates the actual packages and places them in the Package Feed 951area, which is ``${TMPDIR}/deploy``. You can see the 952":ref:`overview-manual/concepts:package feeds`" section for more detail on 953that part of the build process. 954 955.. note:: 956 957 Support for creating feeds directly from the ``deploy/*`` 958 directories does not exist. Creating such feeds usually requires some 959 kind of feed maintenance mechanism that would upload the new packages 960 into an official package feed (e.g. the Ångström distribution). This 961 functionality is highly distribution-specific and thus is not 962 provided out of the box. 963 964Image Generation 965~~~~~~~~~~~~~~~~ 966 967Once packages are split and stored in the Package Feeds area, the build 968system uses BitBake to generate the root filesystem image: 969 970.. image:: figures/image-generation.png 971 :align: center 972 973The image generation process consists of several stages and depends on 974several tasks and variables. The 975:ref:`ref-tasks-rootfs` task creates 976the root filesystem (file and directory structure) for an image. This 977task uses several key variables to help create the list of packages to 978actually install: 979 980- :term:`IMAGE_INSTALL`: Lists 981 out the base set of packages from which to install from the Package 982 Feeds area. 983 984- :term:`PACKAGE_EXCLUDE`: 985 Specifies packages that should not be installed into the image. 986 987- :term:`IMAGE_FEATURES`: 988 Specifies features to include in the image. Most of these features 989 map to additional packages for installation. 990 991- :term:`PACKAGE_CLASSES`: 992 Specifies the package backend (e.g. RPM, DEB, or IPK) to use and 993 consequently helps determine where to locate packages within the 994 Package Feeds area. 995 996- :term:`IMAGE_LINGUAS`: 997 Determines the language(s) for which additional language support 998 packages are installed. 999 1000- :term:`PACKAGE_INSTALL`: 1001 The final list of packages passed to the package manager for 1002 installation into the image. 1003 1004With :term:`IMAGE_ROOTFS` 1005pointing to the location of the filesystem under construction and the 1006:term:`PACKAGE_INSTALL` variable providing the final list of packages to 1007install, the root file system is created. 1008 1009Package installation is under control of the package manager (e.g. 1010dnf/rpm, opkg, or apt/dpkg) regardless of whether or not package 1011management is enabled for the target. At the end of the process, if 1012package management is not enabled for the target, the package manager's 1013data files are deleted from the root filesystem. As part of the final 1014stage of package installation, post installation scripts that are part 1015of the packages are run. Any scripts that fail to run on the build host 1016are run on the target when the target system is first booted. If you are 1017using a 1018:ref:`read-only root filesystem <dev-manual/common-tasks:creating a read-only root filesystem>`, 1019all the post installation scripts must succeed on the build host during 1020the package installation phase since the root filesystem on the target 1021is read-only. 1022 1023The final stages of the ``do_rootfs`` task handle post processing. Post 1024processing includes creation of a manifest file and optimizations. 1025 1026The manifest file (``.manifest``) resides in the same directory as the 1027root filesystem image. This file lists out, line-by-line, the installed 1028packages. The manifest file is useful for the 1029:ref:`testimage <ref-classes-testimage*>` class, 1030for example, to determine whether or not to run specific tests. See the 1031:term:`IMAGE_MANIFEST` 1032variable for additional information. 1033 1034Optimizing processes that are run across the image include ``mklibs`` 1035and any other post-processing commands as defined by the 1036:term:`ROOTFS_POSTPROCESS_COMMAND` 1037variable. The ``mklibs`` process optimizes the size of the libraries. 1038 1039After the root filesystem is built, processing begins on the image 1040through the :ref:`ref-tasks-image` 1041task. The build system runs any pre-processing commands as defined by 1042the 1043:term:`IMAGE_PREPROCESS_COMMAND` 1044variable. This variable specifies a list of functions to call before the 1045build system creates the final image output files. 1046 1047The build system dynamically creates ``do_image_*`` tasks as needed, 1048based on the image types specified in the 1049:term:`IMAGE_FSTYPES` variable. 1050The process turns everything into an image file or a set of image files 1051and can compress the root filesystem image to reduce the overall size of 1052the image. The formats used for the root filesystem depend on the 1053:term:`IMAGE_FSTYPES` variable. Compression depends on whether the formats 1054support compression. 1055 1056As an example, a dynamically created task when creating a particular 1057image type would take the following form:: 1058 1059 do_image_type 1060 1061So, if the type 1062as specified by the :term:`IMAGE_FSTYPES` were ``ext4``, the dynamically 1063generated task would be as follows:: 1064 1065 do_image_ext4 1066 1067The final task involved in image creation is the 1068:ref:`do_image_complete <ref-tasks-image-complete>` 1069task. This task completes the image by applying any image post 1070processing as defined through the 1071:term:`IMAGE_POSTPROCESS_COMMAND` 1072variable. The variable specifies a list of functions to call once the 1073build system has created the final image output files. 1074 1075.. note:: 1076 1077 The entire image generation process is run under 1078 Pseudo. Running under Pseudo ensures that the files in the root filesystem 1079 have correct ownership. 1080 1081SDK Generation 1082~~~~~~~~~~~~~~ 1083 1084The OpenEmbedded build system uses BitBake to generate the Software 1085Development Kit (SDK) installer scripts for both the standard SDK and 1086the extensible SDK (eSDK): 1087 1088.. image:: figures/sdk-generation.png 1089 :align: center 1090 1091.. note:: 1092 1093 For more information on the cross-development toolchain generation, 1094 see the ":ref:`overview-manual/concepts:cross-development toolchain generation`" 1095 section. For information on advantages gained when building a 1096 cross-development toolchain using the do_populate_sdk task, see the 1097 ":ref:`sdk-manual/appendix-obtain:building an sdk installer`" section in 1098 the Yocto Project Application Development and the Extensible Software 1099 Development Kit (eSDK) manual. 1100 1101Like image generation, the SDK script process consists of several stages 1102and depends on many variables. The 1103:ref:`ref-tasks-populate_sdk` 1104and 1105:ref:`ref-tasks-populate_sdk_ext` 1106tasks use these key variables to help create the list of packages to 1107actually install. For information on the variables listed in the figure, 1108see the ":ref:`overview-manual/concepts:application development sdk`" 1109section. 1110 1111The ``do_populate_sdk`` task helps create the standard SDK and handles 1112two parts: a target part and a host part. The target part is the part 1113built for the target hardware and includes libraries and headers. The 1114host part is the part of the SDK that runs on the 1115:term:`SDKMACHINE`. 1116 1117The ``do_populate_sdk_ext`` task helps create the extensible SDK and 1118handles host and target parts differently than its counter part does for 1119the standard SDK. For the extensible SDK, the task encapsulates the 1120build system, which includes everything needed (host and target) for the 1121SDK. 1122 1123Regardless of the type of SDK being constructed, the tasks perform some 1124cleanup after which a cross-development environment setup script and any 1125needed configuration files are created. The final output is the 1126Cross-development toolchain installation script (``.sh`` file), which 1127includes the environment setup script. 1128 1129Stamp Files and the Rerunning of Tasks 1130~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 1131 1132For each task that completes successfully, BitBake writes a stamp file 1133into the :term:`STAMPS_DIR` 1134directory. The beginning of the stamp file's filename is determined by 1135the :term:`STAMP` variable, and the end 1136of the name consists of the task's name and current :ref:`input 1137checksum <overview-manual/concepts:checksums (signatures)>`. 1138 1139.. note:: 1140 1141 This naming scheme assumes that 1142 BB_SIGNATURE_HANDLER 1143 is "OEBasicHash", which is almost always the case in current 1144 OpenEmbedded. 1145 1146To determine if a task needs to be rerun, BitBake checks if a stamp file 1147with a matching input checksum exists for the task. In this case, 1148the task's output is assumed to exist and still be valid. Otherwise, 1149the task is rerun. 1150 1151.. note:: 1152 1153 The stamp mechanism is more general than the shared state (sstate) 1154 cache mechanism described in the 1155 ":ref:`overview-manual/concepts:setscene tasks and shared state`" section. 1156 BitBake avoids rerunning any task that has a valid stamp file, not just 1157 tasks that can be accelerated through the sstate cache. 1158 1159 However, you should realize that stamp files only serve as a marker 1160 that some work has been done and that these files do not record task 1161 output. The actual task output would usually be somewhere in 1162 :term:`TMPDIR` (e.g. in some 1163 recipe's :term:`WORKDIR`.) What 1164 the sstate cache mechanism adds is a way to cache task output that 1165 can then be shared between build machines. 1166 1167Since :term:`STAMPS_DIR` is usually a subdirectory of :term:`TMPDIR`, removing 1168:term:`TMPDIR` will also remove :term:`STAMPS_DIR`, which means tasks will 1169properly be rerun to repopulate :term:`TMPDIR`. 1170 1171If you want some task to always be considered "out of date", you can 1172mark it with the :ref:`nostamp <bitbake:bitbake-user-manual/bitbake-user-manual-metadata:variable flags>` 1173varflag. If some other task depends on such a task, then that task will 1174also always be considered out of date, which might not be what you want. 1175 1176For details on how to view information about a task's signature, see the 1177":ref:`dev-manual/common-tasks:viewing task variable dependencies`" 1178section in the Yocto Project Development Tasks Manual. 1179 1180Setscene Tasks and Shared State 1181~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 1182 1183The description of tasks so far assumes that BitBake needs to build 1184everything and no available prebuilt objects exist. BitBake does support 1185skipping tasks if prebuilt objects are available. These objects are 1186usually made available in the form of a shared state (sstate) cache. 1187 1188.. note:: 1189 1190 For information on variables affecting sstate, see the 1191 :term:`SSTATE_DIR` 1192 and 1193 :term:`SSTATE_MIRRORS` 1194 variables. 1195 1196The idea of a setscene task (i.e ``do_taskname_setscene``) is a 1197version of the task where instead of building something, BitBake can 1198skip to the end result and simply place a set of files into specific 1199locations as needed. In some cases, it makes sense to have a setscene 1200task variant (e.g. generating package files in the 1201:ref:`do_package_write_* <ref-tasks-package_write_deb>` 1202task). In other cases, it does not make sense (e.g. a 1203:ref:`ref-tasks-patch` task or a 1204:ref:`ref-tasks-unpack` task) since 1205the work involved would be equal to or greater than the underlying task. 1206 1207In the build system, the common tasks that have setscene variants are 1208:ref:`ref-tasks-package`, 1209``do_package_write_*``, 1210:ref:`ref-tasks-deploy`, 1211:ref:`ref-tasks-packagedata`, and 1212:ref:`ref-tasks-populate_sysroot`. 1213Notice that these tasks represent most of the tasks whose output is an 1214end result. 1215 1216The build system has knowledge of the relationship between these tasks 1217and other preceding tasks. For example, if BitBake runs 1218``do_populate_sysroot_setscene`` for something, it does not make sense 1219to run any of the ``do_fetch``, ``do_unpack``, ``do_patch``, 1220``do_configure``, ``do_compile``, and ``do_install`` tasks. However, if 1221``do_package`` needs to be run, BitBake needs to run those other tasks. 1222 1223It becomes more complicated if everything can come from an sstate cache 1224because some objects are simply not required at all. For example, you do 1225not need a compiler or native tools, such as quilt, if there isn't anything 1226to compile or patch. If the ``do_package_write_*`` packages are available 1227from sstate, BitBake does not need the ``do_package`` task data. 1228 1229To handle all these complexities, BitBake runs in two phases. The first 1230is the "setscene" stage. During this stage, BitBake first checks the 1231sstate cache for any targets it is planning to build. BitBake does a 1232fast check to see if the object exists rather than doing a complete download. 1233If nothing exists, the second phase, which is the setscene stage, 1234completes and the main build proceeds. 1235 1236If objects are found in the sstate cache, the build system works 1237backwards from the end targets specified by the user. For example, if an 1238image is being built, the build system first looks for the packages 1239needed for that image and the tools needed to construct an image. If 1240those are available, the compiler is not needed. Thus, the compiler is 1241not even downloaded. If something was found to be unavailable, or the 1242download or setscene task fails, the build system then tries to install 1243dependencies, such as the compiler, from the cache. 1244 1245The availability of objects in the sstate cache is handled by the 1246function specified by the :term:`BB_HASHCHECK_FUNCTION` 1247variable and returns a list of available objects. The function specified 1248by the :term:`BB_SETSCENE_DEPVALID` 1249variable is the function that determines whether a given dependency 1250needs to be followed, and whether for any given relationship the 1251function needs to be passed. The function returns a True or False value. 1252 1253Images 1254------ 1255 1256The images produced by the build system are compressed forms of the root 1257filesystem and are ready to boot on a target device. You can see from 1258the :ref:`general workflow figure 1259<overview-manual/concepts:openembedded build system concepts>` that BitBake 1260output, in part, consists of images. This section takes a closer look at 1261this output: 1262 1263.. image:: figures/images.png 1264 :align: center 1265 1266.. note:: 1267 1268 For a list of example images that the Yocto Project provides, see the 1269 ":doc:`/ref-manual/images`" chapter in the Yocto Project Reference 1270 Manual. 1271 1272The build process writes images out to the :term:`Build Directory` 1273inside the 1274``tmp/deploy/images/machine/`` folder as shown in the figure. This 1275folder contains any files expected to be loaded on the target device. 1276The :term:`DEPLOY_DIR` variable 1277points to the ``deploy`` directory, while the 1278:term:`DEPLOY_DIR_IMAGE` 1279variable points to the appropriate directory containing images for the 1280current configuration. 1281 1282- kernel-image: A kernel binary file. The 1283 :term:`KERNEL_IMAGETYPE` 1284 variable determines the naming scheme for the kernel image file. 1285 Depending on this variable, the file could begin with a variety of 1286 naming strings. The ``deploy/images/``\ machine directory can contain 1287 multiple image files for the machine. 1288 1289- root-filesystem-image: Root filesystems for the target device (e.g. 1290 ``*.ext3`` or ``*.bz2`` files). The 1291 :term:`IMAGE_FSTYPES` 1292 variable determines the root filesystem image type. The 1293 ``deploy/images/``\ machine directory can contain multiple root 1294 filesystems for the machine. 1295 1296- kernel-modules: Tarballs that contain all the modules built for the 1297 kernel. Kernel module tarballs exist for legacy purposes and can be 1298 suppressed by setting the 1299 :term:`MODULE_TARBALL_DEPLOY` 1300 variable to "0". The ``deploy/images/``\ machine directory can 1301 contain multiple kernel module tarballs for the machine. 1302 1303- bootloaders: If applicable to the target machine, bootloaders 1304 supporting the image. The ``deploy/images/``\ machine directory can 1305 contain multiple bootloaders for the machine. 1306 1307- symlinks: The ``deploy/images/``\ machine folder contains a symbolic 1308 link that points to the most recently built file for each machine. 1309 These links might be useful for external scripts that need to obtain 1310 the latest version of each file. 1311 1312Application Development SDK 1313--------------------------- 1314 1315In the :ref:`general workflow figure 1316<overview-manual/concepts:openembedded build system concepts>`, the 1317output labeled "Application Development SDK" represents an SDK. The SDK 1318generation process differs depending on whether you build an extensible 1319SDK (e.g. ``bitbake -c populate_sdk_ext`` imagename) or a standard SDK 1320(e.g. ``bitbake -c populate_sdk`` imagename). This section takes a 1321closer look at this output: 1322 1323.. image:: figures/sdk.png 1324 :align: center 1325 1326The specific form of this output is a set of files that includes a 1327self-extracting SDK installer (``*.sh``), host and target manifest 1328files, and files used for SDK testing. When the SDK installer file is 1329run, it installs the SDK. The SDK consists of a cross-development 1330toolchain, a set of libraries and headers, and an SDK environment setup 1331script. Running this installer essentially sets up your 1332cross-development environment. You can think of the cross-toolchain as 1333the "host" part because it runs on the SDK machine. You can think of the 1334libraries and headers as the "target" part because they are built for 1335the target hardware. The environment setup script is added so that you 1336can initialize the environment before using the tools. 1337 1338.. note:: 1339 1340 - The Yocto Project supports several methods by which you can set up 1341 this cross-development environment. These methods include 1342 downloading pre-built SDK installers or building and installing 1343 your own SDK installer. 1344 1345 - For background information on cross-development toolchains in the 1346 Yocto Project development environment, see the 1347 ":ref:`overview-manual/concepts:cross-development toolchain generation`" 1348 section. 1349 1350 - For information on setting up a cross-development environment, see 1351 the :doc:`/sdk-manual/index` manual. 1352 1353All the output files for an SDK are written to the ``deploy/sdk`` folder 1354inside the :term:`Build Directory` as 1355shown in the previous figure. Depending on the type of SDK, there are 1356several variables to configure these files. Here are the variables 1357associated with an extensible SDK: 1358 1359- :term:`DEPLOY_DIR`: Points to 1360 the ``deploy`` directory. 1361 1362- :term:`SDK_EXT_TYPE`: 1363 Controls whether or not shared state artifacts are copied into the 1364 extensible SDK. By default, all required shared state artifacts are 1365 copied into the SDK. 1366 1367- :term:`SDK_INCLUDE_PKGDATA`: 1368 Specifies whether or not packagedata is included in the extensible 1369 SDK for all recipes in the "world" target. 1370 1371- :term:`SDK_INCLUDE_TOOLCHAIN`: 1372 Specifies whether or not the toolchain is included when building the 1373 extensible SDK. 1374 1375- :term:`ESDK_LOCALCONF_ALLOW`: 1376 A list of variables allowed through from the build system 1377 configuration into the extensible SDK configuration. 1378 1379- :term:`ESDK_LOCALCONF_REMOVE`: 1380 A list of variables not allowed through from the build system 1381 configuration into the extensible SDK configuration. 1382 1383- :term:`ESDK_CLASS_INHERIT_DISABLE`: 1384 A list of classes to remove from the 1385 :term:`INHERIT` value globally 1386 within the extensible SDK configuration. 1387 1388This next list, shows the variables associated with a standard SDK: 1389 1390- :term:`DEPLOY_DIR`: Points to 1391 the ``deploy`` directory. 1392 1393- :term:`SDKMACHINE`: Specifies 1394 the architecture of the machine on which the cross-development tools 1395 are run to create packages for the target hardware. 1396 1397- :term:`SDKIMAGE_FEATURES`: 1398 Lists the features to include in the "target" part of the SDK. 1399 1400- :term:`TOOLCHAIN_HOST_TASK`: 1401 Lists packages that make up the host part of the SDK (i.e. the part 1402 that runs on the :term:`SDKMACHINE`). When you use 1403 ``bitbake -c populate_sdk imagename`` to create the SDK, a set of 1404 default packages apply. This variable allows you to add more 1405 packages. 1406 1407- :term:`TOOLCHAIN_TARGET_TASK`: 1408 Lists packages that make up the target part of the SDK (i.e. the part 1409 built for the target hardware). 1410 1411- :term:`SDKPATH`: Defines the 1412 default SDK installation path offered by the installation script. 1413 1414- :term:`SDK_HOST_MANIFEST`: 1415 Lists all the installed packages that make up the host part of the 1416 SDK. This variable also plays a minor role for extensible SDK 1417 development as well. However, it is mainly used for the standard SDK. 1418 1419- :term:`SDK_TARGET_MANIFEST`: 1420 Lists all the installed packages that make up the target part of the 1421 SDK. This variable also plays a minor role for extensible SDK 1422 development as well. However, it is mainly used for the standard SDK. 1423 1424Cross-Development Toolchain Generation 1425====================================== 1426 1427The Yocto Project does most of the work for you when it comes to 1428creating :ref:`sdk-manual/intro:the cross-development toolchain`. This 1429section provides some technical background on how cross-development 1430toolchains are created and used. For more information on toolchains, you 1431can also see the :doc:`/sdk-manual/index` manual. 1432 1433In the Yocto Project development environment, cross-development 1434toolchains are used to build images and applications that run on the 1435target hardware. With just a few commands, the OpenEmbedded build system 1436creates these necessary toolchains for you. 1437 1438The following figure shows a high-level build environment regarding 1439toolchain construction and use. 1440 1441.. image:: figures/cross-development-toolchains.png 1442 :align: center 1443 1444Most of the work occurs on the Build Host. This is the machine used to 1445build images and generally work within the the Yocto Project 1446environment. When you run 1447:term:`BitBake` to create an image, the 1448OpenEmbedded build system uses the host ``gcc`` compiler to bootstrap a 1449cross-compiler named ``gcc-cross``. The ``gcc-cross`` compiler is what 1450BitBake uses to compile source files when creating the target image. You 1451can think of ``gcc-cross`` simply as an automatically generated 1452cross-compiler that is used internally within BitBake only. 1453 1454.. note:: 1455 1456 The extensible SDK does not use ``gcc-cross-canadian`` 1457 since this SDK ships a copy of the OpenEmbedded build system and the 1458 sysroot within it contains ``gcc-cross``. 1459 1460The chain of events that occurs when the standard toolchain is bootstrapped:: 1461 1462 binutils-cross -> linux-libc-headers -> gcc-cross -> libgcc-initial -> glibc -> libgcc -> gcc-runtime 1463 1464- ``gcc``: The compiler, GNU Compiler Collection (GCC). 1465 1466- ``binutils-cross``: The binary utilities needed in order 1467 to run the ``gcc-cross`` phase of the bootstrap operation and build the 1468 headers for the C library. 1469 1470- ``linux-libc-headers``: Headers needed for the cross-compiler and C library build. 1471 1472- ``libgcc-initial``: An initial version of the gcc support library needed 1473 to bootstrap ``glibc``. 1474 1475- ``libgcc``: The final version of the gcc support library which 1476 can only be built once there is a C library to link against. 1477 1478- ``glibc``: The GNU C Library. 1479 1480- ``gcc-cross``: The final stage of the bootstrap process for the 1481 cross-compiler. This stage results in the actual cross-compiler that 1482 BitBake uses when it builds an image for a targeted device. 1483 1484 This tool is a "native" tool (i.e. it is designed to run on 1485 the build host). 1486 1487- ``gcc-runtime``: Runtime libraries resulting from the toolchain 1488 bootstrapping process. This tool produces a binary that consists of 1489 the runtime libraries need for the targeted device. 1490 1491You can use the OpenEmbedded build system to build an installer for the 1492relocatable SDK used to develop applications. When you run the 1493installer, it installs the toolchain, which contains the development 1494tools (e.g., ``gcc-cross-canadian``, ``binutils-cross-canadian``, and 1495other ``nativesdk-*`` tools), which are tools native to the SDK (i.e. 1496native to :term:`SDK_ARCH`), you 1497need to cross-compile and test your software. The figure shows the 1498commands you use to easily build out this toolchain. This 1499cross-development toolchain is built to execute on the 1500:term:`SDKMACHINE`, which might or 1501might not be the same machine as the Build Host. 1502 1503.. note:: 1504 1505 If your target architecture is supported by the Yocto Project, you 1506 can take advantage of pre-built images that ship with the Yocto 1507 Project and already contain cross-development toolchain installers. 1508 1509Here is the bootstrap process for the relocatable toolchain:: 1510 1511 gcc -> binutils-crosssdk -> gcc-crosssdk-initial -> linux-libc-headers -> glibc-initial -> nativesdk-glibc -> gcc-crosssdk -> gcc-cross-canadian 1512 1513- ``gcc``: The build host's GNU Compiler Collection (GCC). 1514 1515- ``binutils-crosssdk``: The bare minimum binary utilities needed in 1516 order to run the ``gcc-crosssdk-initial`` phase of the bootstrap 1517 operation. 1518 1519- ``gcc-crosssdk-initial``: An early stage of the bootstrap process for 1520 creating the cross-compiler. This stage builds enough of the 1521 ``gcc-crosssdk`` and supporting pieces so that the final stage of the 1522 bootstrap process can produce the finished cross-compiler. This tool 1523 is a "native" binary that runs on the build host. 1524 1525- ``linux-libc-headers``: Headers needed for the cross-compiler. 1526 1527- ``glibc-initial``: An initial version of the Embedded GLIBC needed to 1528 bootstrap ``nativesdk-glibc``. 1529 1530- ``nativesdk-glibc``: The Embedded GLIBC needed to bootstrap the 1531 ``gcc-crosssdk``. 1532 1533- ``gcc-crosssdk``: The final stage of the bootstrap process for the 1534 relocatable cross-compiler. The ``gcc-crosssdk`` is a transitory 1535 compiler and never leaves the build host. Its purpose is to help in 1536 the bootstrap process to create the eventual ``gcc-cross-canadian`` 1537 compiler, which is relocatable. This tool is also a "native" package 1538 (i.e. it is designed to run on the build host). 1539 1540- ``gcc-cross-canadian``: The final relocatable cross-compiler. When 1541 run on the :term:`SDKMACHINE`, 1542 this tool produces executable code that runs on the target device. 1543 Only one cross-canadian compiler is produced per architecture since 1544 they can be targeted at different processor optimizations using 1545 configurations passed to the compiler through the compile commands. 1546 This circumvents the need for multiple compilers and thus reduces the 1547 size of the toolchains. 1548 1549.. note:: 1550 1551 For information on advantages gained when building a 1552 cross-development toolchain installer, see the 1553 ":ref:`sdk-manual/appendix-obtain:building an sdk installer`" appendix 1554 in the Yocto Project Application Development and the 1555 Extensible Software Development Kit (eSDK) manual. 1556 1557Shared State Cache 1558================== 1559 1560By design, the OpenEmbedded build system builds everything from scratch 1561unless :term:`BitBake` can determine 1562that parts do not need to be rebuilt. Fundamentally, building from 1563scratch is attractive as it means all parts are built fresh and there is 1564no possibility of stale data that can cause problems. When 1565developers hit problems, they typically default back to building from 1566scratch so they have a known state from the start. 1567 1568Building an image from scratch is both an advantage and a disadvantage 1569to the process. As mentioned in the previous paragraph, building from 1570scratch ensures that everything is current and starts from a known 1571state. However, building from scratch also takes much longer as it 1572generally means rebuilding things that do not necessarily need to be 1573rebuilt. 1574 1575The Yocto Project implements shared state code that supports incremental 1576builds. The implementation of the shared state code answers the 1577following questions that were fundamental roadblocks within the 1578OpenEmbedded incremental build support system: 1579 1580- What pieces of the system have changed and what pieces have not 1581 changed? 1582 1583- How are changed pieces of software removed and replaced? 1584 1585- How are pre-built components that do not need to be rebuilt from 1586 scratch used when they are available? 1587 1588For the first question, the build system detects changes in the "inputs" 1589to a given task by creating a checksum (or signature) of the task's 1590inputs. If the checksum changes, the system assumes the inputs have 1591changed and the task needs to be rerun. For the second question, the 1592shared state (sstate) code tracks which tasks add which output to the 1593build process. This means the output from a given task can be removed, 1594upgraded or otherwise manipulated. The third question is partly 1595addressed by the solution for the second question assuming the build 1596system can fetch the sstate objects from remote locations and install 1597them if they are deemed to be valid. 1598 1599.. note:: 1600 1601 - The build system does not maintain 1602 :term:`PR` information as part of 1603 the shared state packages. Consequently, there are considerations that 1604 affect maintaining shared state feeds. For information on how the 1605 build system works with packages and can track incrementing :term:`PR` 1606 information, see the ":ref:`dev-manual/common-tasks:automatically incrementing a package version number`" 1607 section in the Yocto Project Development Tasks Manual. 1608 1609 - The code in the build system that supports incremental builds is 1610 complex. For techniques that help you work around issues 1611 related to shared state code, see the 1612 ":ref:`dev-manual/common-tasks:viewing metadata used to create the input signature of a shared state task`" 1613 and 1614 ":ref:`dev-manual/common-tasks:invalidating shared state to force a task to run`" 1615 sections both in the Yocto Project Development Tasks Manual. 1616 1617The rest of this section goes into detail about the overall incremental 1618build architecture, the checksums (signatures), and shared state. 1619 1620Overall Architecture 1621-------------------- 1622 1623When determining what parts of the system need to be built, BitBake 1624works on a per-task basis rather than a per-recipe basis. You might 1625wonder why using a per-task basis is preferred over a per-recipe basis. 1626To help explain, consider having the IPK packaging backend enabled and 1627then switching to DEB. In this case, the 1628:ref:`ref-tasks-install` and 1629:ref:`ref-tasks-package` task outputs 1630are still valid. However, with a per-recipe approach, the build would 1631not include the ``.deb`` files. Consequently, you would have to 1632invalidate the whole build and rerun it. Rerunning everything is not the 1633best solution. Also, in this case, the core must be "taught" much about 1634specific tasks. This methodology does not scale well and does not allow 1635users to easily add new tasks in layers or as external recipes without 1636touching the packaged-staging core. 1637 1638Checksums (Signatures) 1639---------------------- 1640 1641The shared state code uses a checksum, which is a unique signature of a 1642task's inputs, to determine if a task needs to be run again. Because it 1643is a change in a task's inputs that triggers a rerun, the process needs 1644to detect all the inputs to a given task. For shell tasks, this turns 1645out to be fairly easy because the build process generates a "run" shell 1646script for each task and it is possible to create a checksum that gives 1647you a good idea of when the task's data changes. 1648 1649To complicate the problem, there are things that should not be included 1650in the checksum. First, there is the actual specific build path of a 1651given task - the :term:`WORKDIR`. It 1652does not matter if the work directory changes because it should not 1653affect the output for target packages. Also, the build process has the 1654objective of making native or cross packages relocatable. 1655 1656.. note:: 1657 1658 Both native and cross packages run on the 1659 build host. However, cross packages generate output for the target 1660 architecture. 1661 1662The checksum therefore needs to exclude :term:`WORKDIR`. The simplistic 1663approach for excluding the work directory is to set :term:`WORKDIR` to some 1664fixed value and create the checksum for the "run" script. 1665 1666Another problem results from the "run" scripts containing functions that 1667might or might not get called. The incremental build solution contains 1668code that figures out dependencies between shell functions. This code is 1669used to prune the "run" scripts down to the minimum set, thereby 1670alleviating this problem and making the "run" scripts much more readable 1671as a bonus. 1672 1673So far, there are solutions for shell scripts. What about Python tasks? The 1674same approach applies even though these tasks are more difficult. The 1675process needs to figure out what variables a Python function accesses 1676and what functions it calls. Again, the incremental build solution 1677contains code that first figures out the variable and function 1678dependencies, and then creates a checksum for the data used as the input 1679to the task. 1680 1681Like the :term:`WORKDIR` case, there can be situations where dependencies should be 1682ignored. For these situations, you can instruct the build process to 1683ignore a dependency by using a line like the following:: 1684 1685 PACKAGE_ARCHS[vardepsexclude] = "MACHINE" 1686 1687This example ensures that the :term:`PACKAGE_ARCHS` variable 1688does not depend on the value of :term:`MACHINE`, even if it does 1689reference it. 1690 1691Equally, there are cases where you need to add dependencies BitBake is 1692not able to find. You can accomplish this by using a line like the 1693following:: 1694 1695 PACKAGE_ARCHS[vardeps] = "MACHINE" 1696 1697This example explicitly 1698adds the :term:`MACHINE` variable as a dependency for :term:`PACKAGE_ARCHS`. 1699 1700As an example, consider a case with in-line Python where BitBake is not 1701able to figure out dependencies. When running in debug mode (i.e. using 1702``-DDD``), BitBake produces output when it discovers something for which 1703it cannot figure out dependencies. The Yocto Project team has currently 1704not managed to cover those dependencies in detail and is aware of the 1705need to fix this situation. 1706 1707Thus far, this section has limited discussion to the direct inputs into 1708a task. Information based on direct inputs is referred to as the 1709"basehash" in the code. However, the question of a task's indirect 1710inputs still exits - items already built and present in the 1711:term:`Build Directory`. The checksum (or 1712signature) for a particular task needs to add the hashes of all the 1713tasks on which the particular task depends. Choosing which dependencies 1714to add is a policy decision. However, the effect is to generate a 1715checksum that combines the basehash and the hashes of the task's 1716dependencies. 1717 1718At the code level, there are multiple ways by which both the basehash 1719and the dependent task hashes can be influenced. Within the BitBake 1720configuration file, you can give BitBake some extra information to help 1721it construct the basehash. The following statement effectively results 1722in a list of global variable dependency excludes (i.e. variables never 1723included in any checksum):: 1724 1725 BB_BASEHASH_IGNORE_VARS ?= "TMPDIR FILE PATH PWD BB_TASKHASH BBPATH DL_DIR \\ 1726 SSTATE_DIR THISDIR FILESEXTRAPATHS FILE_DIRNAME HOME LOGNAME SHELL TERM \\ 1727 USER FILESPATH STAGING_DIR_HOST STAGING_DIR_TARGET COREBASE PRSERV_HOST \\ 1728 PRSERV_DUMPDIR PRSERV_DUMPFILE PRSERV_LOCKDOWN PARALLEL_MAKE \\ 1729 CCACHE_DIR EXTERNAL_TOOLCHAIN CCACHE CCACHE_DISABLE LICENSE_PATH SDKPKGSUFFIX" 1730 1731The previous example does not include :term:`WORKDIR` since that variable is 1732actually constructed as a path within :term:`TMPDIR`, which is included above. 1733 1734The rules for deciding which hashes of dependent tasks to include 1735through dependency chains are more complex and are generally 1736accomplished with a Python function. The code in 1737``meta/lib/oe/sstatesig.py`` shows two examples of this and also 1738illustrates how you can insert your own policy into the system if so 1739desired. This file defines the two basic signature generators 1740:term:`OpenEmbedded-Core (OE-Core)` uses: "OEBasic" and 1741"OEBasicHash". By default, a dummy "noop" signature handler is enabled 1742in BitBake. This means that behavior is unchanged from previous 1743versions. OE-Core uses the "OEBasicHash" signature handler by default 1744through this setting in the ``bitbake.conf`` file:: 1745 1746 BB_SIGNATURE_HANDLER ?= "OEBasicHash" 1747 1748The "OEBasicHash" :term:`BB_SIGNATURE_HANDLER` is the same 1749as the "OEBasic" version but adds the task hash to the :ref:`stamp 1750files <overview-manual/concepts:stamp files and the rerunning of tasks>`. This 1751results in any metadata change that changes the task hash, automatically causing 1752the task to be run again. This removes the need to bump 1753:term:`PR` values, and changes to metadata 1754automatically ripple across the build. 1755 1756It is also worth noting that the end result of these signature 1757generators is to make some dependency and hash information available to 1758the build. This information includes: 1759 1760- ``BB_BASEHASH:task-``\ taskname: The base hashes for each task in the 1761 recipe. 1762 1763- ``BB_BASEHASH_``\ filename\ ``:``\ taskname: The base hashes for each 1764 dependent task. 1765 1766- :term:`BB_TASKHASH`: The hash of the currently running task. 1767 1768Shared State 1769------------ 1770 1771Checksums and dependencies, as discussed in the previous section, solve 1772half the problem of supporting a shared state. The other half of the 1773problem is being able to use checksum information during the build and 1774being able to reuse or rebuild specific components. 1775 1776The :ref:`sstate <ref-classes-sstate>` class is a 1777relatively generic implementation of how to "capture" a snapshot of a 1778given task. The idea is that the build process does not care about the 1779source of a task's output. Output could be freshly built or it could be 1780downloaded and unpacked from somewhere. In other words, the build 1781process does not need to worry about its origin. 1782 1783Two types of output exist. One type is just about creating a directory 1784in :term:`WORKDIR`. A good example is 1785the output of either 1786:ref:`ref-tasks-install` or 1787:ref:`ref-tasks-package`. The other 1788type of output occurs when a set of data is merged into a shared 1789directory tree such as the sysroot. 1790 1791The Yocto Project team has tried to keep the details of the 1792implementation hidden in the :ref:`sstate <ref-classes-sstate>` class. From a user's perspective, 1793adding shared state wrapping to a task is as simple as this 1794:ref:`ref-tasks-deploy` example taken 1795from the :ref:`deploy <ref-classes-deploy>` class:: 1796 1797 DEPLOYDIR = "${WORKDIR}/deploy-${PN}" 1798 SSTATETASKS += "do_deploy" 1799 do_deploy[sstate-inputdirs] = "${DEPLOYDIR}" 1800 do_deploy[sstate-outputdirs] = "${DEPLOY_DIR_IMAGE}" 1801 1802 python do_deploy_setscene () { 1803 sstate_setscene(d) 1804 } 1805 addtask do_deploy_setscene 1806 do_deploy[dirs] = "${DEPLOYDIR} ${B}" 1807 do_deploy[stamp-extra-info] = "${MACHINE_ARCH}" 1808 1809The following list explains the previous example: 1810 1811- Adding "do_deploy" to ``SSTATETASKS`` adds some required 1812 sstate-related processing, which is implemented in the 1813 :ref:`sstate <ref-classes-sstate>` class, to 1814 before and after the 1815 :ref:`ref-tasks-deploy` task. 1816 1817- The ``do_deploy[sstate-inputdirs] = "${DEPLOYDIR}"`` declares that 1818 ``do_deploy`` places its output in ``${DEPLOYDIR}`` when run normally 1819 (i.e. when not using the sstate cache). This output becomes the input 1820 to the shared state cache. 1821 1822- The ``do_deploy[sstate-outputdirs] = "${DEPLOY_DIR_IMAGE}"`` line 1823 causes the contents of the shared state cache to be copied to 1824 ``${DEPLOY_DIR_IMAGE}``. 1825 1826 .. note:: 1827 1828 If ``do_deploy`` is not already in the shared state cache or if its input 1829 checksum (signature) has changed from when the output was cached, the task 1830 runs to populate the shared state cache, after which the contents of the 1831 shared state cache is copied to ${:term:`DEPLOY_DIR_IMAGE`}. If 1832 ``do_deploy`` is in the shared state cache and its signature indicates 1833 that the cached output is still valid (i.e. if no relevant task inputs 1834 have changed), then the contents of the shared state cache copies 1835 directly to ${:term:`DEPLOY_DIR_IMAGE`} by the ``do_deploy_setscene`` task 1836 instead, skipping the ``do_deploy`` task. 1837 1838- The following task definition is glue logic needed to make the 1839 previous settings effective:: 1840 1841 python do_deploy_setscene () { 1842 sstate_setscene(d) 1843 } 1844 addtask do_deploy_setscene 1845 1846 ``sstate_setscene()`` takes the flags above as input and accelerates the ``do_deploy`` task 1847 through the shared state cache if possible. If the task was 1848 accelerated, ``sstate_setscene()`` returns True. Otherwise, it 1849 returns False, and the normal ``do_deploy`` task runs. For more 1850 information, see the ":ref:`bitbake:bitbake-user-manual/bitbake-user-manual-execution:setscene`" 1851 section in the BitBake User Manual. 1852 1853- The ``do_deploy[dirs] = "${DEPLOYDIR} ${B}"`` line creates 1854 ``${DEPLOYDIR}`` and ``${B}`` before the ``do_deploy`` task runs, and 1855 also sets the current working directory of ``do_deploy`` to ``${B}``. 1856 For more information, see the ":ref:`bitbake:bitbake-user-manual/bitbake-user-manual-metadata:variable flags`" 1857 section in the BitBake 1858 User Manual. 1859 1860 .. note:: 1861 1862 In cases where ``sstate-inputdirs`` and ``sstate-outputdirs`` would be 1863 the same, you can use ``sstate-plaindirs``. For example, to preserve the 1864 ${:term:`PKGD`} and ${:term:`PKGDEST`} output from the ``do_package`` 1865 task, use the following:: 1866 1867 do_package[sstate-plaindirs] = "${PKGD} ${PKGDEST}" 1868 1869 1870- The ``do_deploy[stamp-extra-info] = "${MACHINE_ARCH}"`` line appends 1871 extra metadata to the :ref:`stamp 1872 file <overview-manual/concepts:stamp files and the rerunning of tasks>`. In 1873 this case, the metadata makes the task specific to a machine's architecture. 1874 See 1875 ":ref:`bitbake:bitbake-user-manual/bitbake-user-manual-execution:the task list`" 1876 section in the BitBake User Manual for more information on the 1877 ``stamp-extra-info`` flag. 1878 1879- ``sstate-inputdirs`` and ``sstate-outputdirs`` can also be used with 1880 multiple directories. For example, the following declares 1881 :term:`PKGDESTWORK` and ``SHLIBWORK`` as shared state input directories, 1882 which populates the shared state cache, and :term:`PKGDATA_DIR` and 1883 ``SHLIBSDIR`` as the corresponding shared state output directories:: 1884 1885 do_package[sstate-inputdirs] = "${PKGDESTWORK} ${SHLIBSWORKDIR}" 1886 do_package[sstate-outputdirs] = "${PKGDATA_DIR} ${SHLIBSDIR}" 1887 1888- These methods also include the ability to take a lockfile when 1889 manipulating shared state directory structures, for cases where file 1890 additions or removals are sensitive:: 1891 1892 do_package[sstate-lockfile] = "${PACKAGELOCK}" 1893 1894Behind the scenes, the shared state code works by looking in 1895:term:`SSTATE_DIR` and 1896:term:`SSTATE_MIRRORS` for 1897shared state files. Here is an example:: 1898 1899 SSTATE_MIRRORS ?= "\ 1900 file://.* https://someserver.tld/share/sstate/PATH;downloadfilename=PATH \ 1901 file://.* file:///some/local/dir/sstate/PATH" 1902 1903.. note:: 1904 1905 The shared state directory (:term:`SSTATE_DIR`) is organized into two-character 1906 subdirectories, where the subdirectory names are based on the first two 1907 characters of the hash. 1908 If the shared state directory structure for a mirror has the same structure 1909 as :term:`SSTATE_DIR`, you must specify "PATH" as part of the URI to enable the build 1910 system to map to the appropriate subdirectory. 1911 1912The shared state package validity can be detected just by looking at the 1913filename since the filename contains the task checksum (or signature) as 1914described earlier in this section. If a valid shared state package is 1915found, the build process downloads it and uses it to accelerate the 1916task. 1917 1918The build processes use the ``*_setscene`` tasks for the task 1919acceleration phase. BitBake goes through this phase before the main 1920execution code and tries to accelerate any tasks for which it can find 1921shared state packages. If a shared state package for a task is 1922available, the shared state package is used. This means the task and any 1923tasks on which it is dependent are not executed. 1924 1925As a real world example, the aim is when building an IPK-based image, 1926only the 1927:ref:`ref-tasks-package_write_ipk` 1928tasks would have their shared state packages fetched and extracted. 1929Since the sysroot is not used, it would never get extracted. This is 1930another reason why a task-based approach is preferred over a 1931recipe-based approach, which would have to install the output from every 1932task. 1933 1934Hash Equivalence 1935---------------- 1936 1937The above section explained how BitBake skips the execution of tasks 1938whose output can already be found in the Shared State cache. 1939 1940During a build, it may often be the case that the output / result of a task might 1941be unchanged despite changes in the task's input values. An example might be 1942whitespace changes in some input C code. In project terms, this is what we define 1943as "equivalence". 1944 1945To keep track of such equivalence, BitBake has to manage three hashes 1946for each task: 1947 1948- The *task hash* explained earlier: computed from the recipe metadata, 1949 the task code and the task hash values from its dependencies. 1950 When changes are made, these task hashes are therefore modified, 1951 causing the task to re-execute. The task hashes of tasks depending on this 1952 task are therefore modified too, causing the whole dependency 1953 chain to re-execute. 1954 1955- The *output hash*, a new hash computed from the output of Shared State tasks, 1956 tasks that save their resulting output to a Shared State tarball. 1957 The mapping between the task hash and its output hash is reported 1958 to a new *Hash Equivalence* server. This mapping is stored in a database 1959 by the server for future reference. 1960 1961- The *unihash*, a new hash, initially set to the task hash for the task. 1962 This is used to track the *unicity* of task output, and we will explain 1963 how its value is maintained. 1964 1965When Hash Equivalence is enabled, BitBake computes the task hash 1966for each task by using the unihash of its dependencies, instead 1967of their task hash. 1968 1969Now, imagine that a Shared State task is modified because of a change in 1970its code or metadata, or because of a change in its dependencies. 1971Since this modifies its task hash, this task will need re-executing. 1972Its output hash will therefore be computed again. 1973 1974Then, the new mapping between the new task hash and its output hash 1975will be reported to the Hash Equivalence server. The server will 1976let BitBake know whether this output hash is the same as a previously 1977reported output hash, for a different task hash. 1978 1979If the output hash is already known, BitBake will update the task's 1980unihash to match the original task hash that generated that output. 1981Thanks to this, the depending tasks will keep a previously recorded 1982task hash, and BitBake will be able to retrieve their output from 1983the Shared State cache, instead of re-executing them. Similarly, the 1984output of further downstream tasks can also be retrieved from Shared 1985Shate. 1986 1987If the output hash is unknown, a new entry will be created on the Hash 1988Equivalence server, matching the task hash to that output. 1989The depending tasks, still having a new task hash because of the 1990change, will need to re-execute as expected. The change propagates 1991to the depending tasks. 1992 1993To summarize, when Hash Equivalence is enabled, a change in one of the 1994tasks in BitBake's run queue doesn't have to propagate to all the 1995downstream tasks that depend on the output of this task, causing a 1996full rebuild of such tasks, and so on with the next depending tasks. 1997Instead, when the output of this task remains identical to previously 1998recorded output, BitBake can safely retrieve all the downstream 1999task output from the Shared State cache. 2000 2001.. note:: 2002 2003 Having :doc:`/test-manual/reproducible-builds` is a key ingredient for 2004 the stability of the task's output hash. Therefore, the effectiveness 2005 of Hash Equivalence strongly depends on it. 2006 2007This applies to multiple scenarios: 2008 2009- A "trivial" change to a recipe that doesn't impact its generated output, 2010 such as whitespace changes, modifications to unused code paths or 2011 in the ordering of variables. 2012 2013- Shared library updates, for example to fix a security vulnerability. 2014 For sure, the programs using such a library should be rebuilt, but 2015 their new binaries should remain identical. The corresponding tasks should 2016 have a different output hash because of the change in the hash of their 2017 library dependency, but thanks to their output being identical, Hash 2018 Equivalence will stop the propagation down the dependency chain. 2019 2020- Native tool updates. Though the depending tasks should be rebuilt, 2021 it's likely that they will generate the same output and be marked 2022 as equivalent. 2023 2024This mechanism is enabled by default in Poky, and is controlled by three 2025variables: 2026 2027- :term:`bitbake:BB_HASHSERVE`, specifying a local or remote Hash 2028 Equivalence server to use. 2029 2030- :term:`BB_HASHSERVE_UPSTREAM`, when ``BB_HASHSERVE = "auto"``, 2031 allowing to connect the local server to an upstream one. 2032 2033- :term:`bitbake:BB_SIGNATURE_HANDLER`, which must be set to ``OEEquivHash``. 2034 2035Therefore, the default configuration in Poky corresponds to the 2036below settings:: 2037 2038 BB_HASHSERVE = "auto" 2039 BB_SIGNATURE_HANDLER = "OEEquivHash" 2040 2041Rather than starting a local server, another possibility is to rely 2042on a Hash Equivalence server on a network, by setting:: 2043 2044 BB_HASHSERVE = "<HOSTNAME>:<PORT>" 2045 2046.. note:: 2047 2048 The shared Hash Equivalence server needs to be maintained together with the 2049 Shared State cache. Otherwise, the server could report Shared State hashes 2050 that only exist on specific clients. 2051 2052 We therefore recommend that one Hash Equivalence server be set up to 2053 correspond with a given Shared State cache, and to start this server 2054 in *read-only mode*, so that it doesn't store equivalences for 2055 Shared State caches that are local to clients. 2056 2057 See the :term:`BB_HASHSERVE` reference for details about starting 2058 a Hash Equivalence server. 2059 2060See the `video <https://www.youtube.com/watch?v=zXEdqGS62Wc>`__ 2061of Joshua Watt's `Hash Equivalence and Reproducible Builds 2062<https://elinux.org/images/3/37/Hash_Equivalence_and_Reproducible_Builds.pdf>`__ 2063presentation at ELC 2020 for a very synthetic introduction to the 2064Hash Equivalence implementation in the Yocto Project. 2065 2066Automatically Added Runtime Dependencies 2067======================================== 2068 2069The OpenEmbedded build system automatically adds common types of runtime 2070dependencies between packages, which means that you do not need to 2071explicitly declare the packages using 2072:term:`RDEPENDS`. There are three automatic 2073mechanisms (``shlibdeps``, ``pcdeps``, and ``depchains``) that 2074handle shared libraries, package configuration (pkg-config) modules, and 2075``-dev`` and ``-dbg`` packages, respectively. For other types of runtime 2076dependencies, you must manually declare the dependencies. 2077 2078- ``shlibdeps``: During the 2079 :ref:`ref-tasks-package` task of 2080 each recipe, all shared libraries installed by the recipe are 2081 located. For each shared library, the package that contains the 2082 shared library is registered as providing the shared library. More 2083 specifically, the package is registered as providing the 2084 `soname <https://en.wikipedia.org/wiki/Soname>`__ of the library. The 2085 resulting shared-library-to-package mapping is saved globally in 2086 :term:`PKGDATA_DIR` by the 2087 :ref:`ref-tasks-packagedata` 2088 task. 2089 2090 Simultaneously, all executables and shared libraries installed by the 2091 recipe are inspected to see what shared libraries they link against. 2092 For each shared library dependency that is found, :term:`PKGDATA_DIR` is 2093 queried to see if some package (likely from a different recipe) 2094 contains the shared library. If such a package is found, a runtime 2095 dependency is added from the package that depends on the shared 2096 library to the package that contains the library. 2097 2098 The automatically added runtime dependency also includes a version 2099 restriction. This version restriction specifies that at least the 2100 current version of the package that provides the shared library must 2101 be used, as if "package (>= version)" had been added to :term:`RDEPENDS`. 2102 This forces an upgrade of the package containing the shared library 2103 when installing the package that depends on the library, if needed. 2104 2105 If you want to avoid a package being registered as providing a 2106 particular shared library (e.g. because the library is for internal 2107 use only), then add the library to 2108 :term:`PRIVATE_LIBS` inside 2109 the package's recipe. 2110 2111- ``pcdeps``: During the ``do_package`` task of each recipe, all 2112 pkg-config modules (``*.pc`` files) installed by the recipe are 2113 located. For each module, the package that contains the module is 2114 registered as providing the module. The resulting module-to-package 2115 mapping is saved globally in :term:`PKGDATA_DIR` by the 2116 ``do_packagedata`` task. 2117 2118 Simultaneously, all pkg-config modules installed by the recipe are 2119 inspected to see what other pkg-config modules they depend on. A 2120 module is seen as depending on another module if it contains a 2121 "Requires:" line that specifies the other module. For each module 2122 dependency, :term:`PKGDATA_DIR` is queried to see if some package 2123 contains the module. If such a package is found, a runtime dependency 2124 is added from the package that depends on the module to the package 2125 that contains the module. 2126 2127 .. note:: 2128 2129 The 2130 pcdeps 2131 mechanism most often infers dependencies between 2132 -dev 2133 packages. 2134 2135- ``depchains``: If a package ``foo`` depends on a package ``bar``, 2136 then ``foo-dev`` and ``foo-dbg`` are also made to depend on 2137 ``bar-dev`` and ``bar-dbg``, respectively. Taking the ``-dev`` 2138 packages as an example, the ``bar-dev`` package might provide headers 2139 and shared library symlinks needed by ``foo-dev``, which shows the 2140 need for a dependency between the packages. 2141 2142 The dependencies added by ``depchains`` are in the form of 2143 :term:`RRECOMMENDS`. 2144 2145 .. note:: 2146 2147 By default, ``foo-dev`` also has an :term:`RDEPENDS`-style dependency on 2148 ``foo``, because the default value of ``RDEPENDS:${PN}-dev`` (set in 2149 bitbake.conf) includes "${PN}". 2150 2151 To ensure that the dependency chain is never broken, ``-dev`` and 2152 ``-dbg`` packages are always generated by default, even if the 2153 packages turn out to be empty. See the 2154 :term:`ALLOW_EMPTY` variable 2155 for more information. 2156 2157The ``do_package`` task depends on the ``do_packagedata`` task of each 2158recipe in :term:`DEPENDS` through use 2159of a ``[``\ :ref:`deptask <bitbake:bitbake-user-manual/bitbake-user-manual-metadata:variable flags>`\ ``]`` 2160declaration, which guarantees that the required 2161shared-library/module-to-package mapping information will be available 2162when needed as long as :term:`DEPENDS` has been correctly set. 2163 2164Fakeroot and Pseudo 2165=================== 2166 2167Some tasks are easier to implement when allowed to perform certain 2168operations that are normally reserved for the root user (e.g. 2169:ref:`ref-tasks-install`, 2170:ref:`do_package_write* <ref-tasks-package_write_deb>`, 2171:ref:`ref-tasks-rootfs`, and 2172:ref:`do_image* <ref-tasks-image>`). For example, 2173the ``do_install`` task benefits from being able to set the UID and GID 2174of installed files to arbitrary values. 2175 2176One approach to allowing tasks to perform root-only operations would be 2177to require :term:`BitBake` to run as 2178root. However, this method is cumbersome and has security issues. The 2179approach that is actually used is to run tasks that benefit from root 2180privileges in a "fake" root environment. Within this environment, the 2181task and its child processes believe that they are running as the root 2182user, and see an internally consistent view of the filesystem. As long 2183as generating the final output (e.g. a package or an image) does not 2184require root privileges, the fact that some earlier steps ran in a fake 2185root environment does not cause problems. 2186 2187The capability to run tasks in a fake root environment is known as 2188"`fakeroot <http://man.he.net/man1/fakeroot>`__", which is derived from 2189the BitBake keyword/variable flag that requests a fake root environment 2190for a task. 2191 2192In the :term:`OpenEmbedded Build System`, the program that implements 2193fakeroot is known as :yocto_home:`Pseudo </software-item/pseudo/>`. Pseudo 2194overrides system calls by using the environment variable ``LD_PRELOAD``, 2195which results in the illusion of running as root. To keep track of 2196"fake" file ownership and permissions resulting from operations that 2197require root permissions, Pseudo uses an SQLite 3 database. This 2198database is stored in 2199``${``\ :term:`WORKDIR`\ ``}/pseudo/files.db`` 2200for individual recipes. Storing the database in a file as opposed to in 2201memory gives persistence between tasks and builds, which is not 2202accomplished using fakeroot. 2203 2204.. note:: 2205 2206 If you add your own task that manipulates the same files or 2207 directories as a fakeroot task, then that task also needs to run 2208 under fakeroot. Otherwise, the task cannot run root-only operations, 2209 and cannot see the fake file ownership and permissions set by the 2210 other task. You need to also add a dependency on 2211 ``virtual/fakeroot-native:do_populate_sysroot``, giving the following:: 2212 2213 fakeroot do_mytask () { 2214 ... 2215 } 2216 do_mytask[depends] += "virtual/fakeroot-native:do_populate_sysroot" 2217 2218 2219For more information, see the 2220:term:`FAKEROOT* <bitbake:FAKEROOT>` variables in the 2221BitBake User Manual. You can also reference the "`Why Not 2222Fakeroot? <https://github.com/wrpseudo/pseudo/wiki/WhyNotFakeroot>`__" 2223article for background information on Fakeroot and Pseudo. 2224
