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+**********************
+Yocto Project Concepts
+**********************
+This chapter provides explanations for Yocto Project concepts that go
+beyond the surface of "how-to" information and reference (or look-up)
+material. Concepts such as components, the `OpenEmbedded build
+system <&YOCTO_DOCS_REF_URL;#build-system-term>`__ workflow,
+cross-development toolchains, shared state cache, and so forth are
+explained.
+Yocto Project Components
+========================
+The `BitBake <&YOCTO_DOCS_REF_URL;#bitbake-term>`__ task executor
+together with various types of configuration files form the
+`OpenEmbedded-Core <&YOCTO_DOCS_REF_URL;#oe-core>`__. This section
+overviews these components by describing their use and how they
+interact.
+BitBake handles the parsing and execution of the data files. The data
+itself is of various types:
+-  *Recipes:* Provides details about particular pieces of software.
+-  *Class Data:* Abstracts common build information (e.g. how to build a
+   Linux kernel).
+-  *Configuration Data:* Defines machine-specific settings, policy
+   decisions, and so forth. Configuration data acts as the glue to bind
+   everything together.
+BitBake knows how to combine multiple data sources together and refers
+to each data source as a layer. For information on layers, see the
+"`Understanding and Creating
+Layers <&YOCTO_DOCS_DEV_URL;#understanding-and-creating-layers>`__"
+section of the Yocto Project Development Tasks Manual.
+Following are some brief details on these core components. For
+additional information on how these components interact during a build,
+see the "`OpenEmbedded Build System
+Concepts <#openembedded-build-system-build-concepts>`__" section.
+.. _usingpoky-components-bitbake:
+BitBake
+-------
+BitBake is the tool at the heart of the `OpenEmbedded build
+system <&YOCTO_DOCS_REF_URL;#build-system-term>`__ and is responsible
+for parsing the `Metadata <&YOCTO_DOCS_REF_URL;#metadata>`__, generating
+a list of tasks from it, and then executing those tasks.
+This section briefly introduces BitBake. If you want more information on
+BitBake, see the `BitBake User Manual <&YOCTO_DOCS_BB_URL;>`__.
+To see a list of the options BitBake supports, use either of the
+following commands: $ bitbake -h $ bitbake --help
+The most common usage for BitBake is ``bitbake packagename``, where
+``packagename`` is the name of the package you want to build (referred
+to as the "target"). The target often equates to the first part of a
+recipe's filename (e.g. "foo" for a recipe named ``foo_1.3.0-r0.bb``).
+So, to process the ``matchbox-desktop_1.2.3.bb`` recipe file, you might
+type the following: $ bitbake matchbox-desktop Several different
+versions of ``matchbox-desktop`` might exist. BitBake chooses the one
+selected by the distribution configuration. You can get more details
+about how BitBake chooses between different target versions and
+providers in the
+"`Preferences <&YOCTO_DOCS_BB_URL;#bb-bitbake-preferences>`__" section
+of the BitBake User Manual.
+BitBake also tries to execute any dependent tasks first. So for example,
+before building ``matchbox-desktop``, BitBake would build a cross
+compiler and ``glibc`` if they had not already been built.
+A useful BitBake option to consider is the ``-k`` or ``--continue``
+option. This option instructs BitBake to try and continue processing the
+job as long as possible even after encountering an error. When an error
+occurs, the target that failed and those that depend on it cannot be
+remade. However, when you use this option other dependencies can still
+be processed.
+.. _overview-components-recipes:
+Recipes
+-------
+Files that have the ``.bb`` suffix are "recipes" files. In general, a
+recipe contains information about a single piece of software. This
+information includes the location from which to download the unaltered
+source, any source patches to be applied to that source (if needed),
+which special configuration options to apply, how to compile the source
+files, and how to package the compiled output.
+The term "package" is sometimes used to refer to recipes. However, since
+the word "package" is used for the packaged output from the OpenEmbedded
+build system (i.e. ``.ipk`` or ``.deb`` files), this document avoids
+using the term "package" when referring to recipes.
+.. _overview-components-classes:
+Classes
+-------
+Class files (``.bbclass``) contain information that is useful to share
+between recipes files. An example is the
+```autotools`` <&YOCTO_DOCS_REF_URL;#ref-classes-autotools>`__ class,
+which contains common settings for any application that Autotools uses.
+The "`Classes <&YOCTO_DOCS_REF_URL;#ref-classes>`__" chapter in the
+Yocto Project Reference Manual provides details about classes and how to
+use them.
+.. _overview-components-configurations:
+Configurations
+--------------
+The configuration files (``.conf``) define various configuration
+variables that govern the OpenEmbedded build process. These files fall
+into several areas that define machine configuration options,
+distribution configuration options, compiler tuning options, general
+common configuration options, and user configuration options in
+``conf/local.conf``, which is found in the `Build
+Directory <&YOCTO_DOCS_REF_URL;#build-directory>`__.
+.. _overview-layers:
+Layers
+======
+Layers are repositories that contain related metadata (i.e. sets of
+instructions) that tell the OpenEmbedded build system how to build a
+target. Yocto Project's `layer model <#the-yocto-project-layer-model>`__
+facilitates collaboration, sharing, customization, and reuse within the
+Yocto Project development environment. Layers logically separate
+information for your project. For example, you can use a layer to hold
+all the configurations for a particular piece of hardware. Isolating
+hardware-specific configurations allows you to share other metadata by
+using a different layer where that metadata might be common across
+several pieces of hardware.
+Many layers exist that work in the Yocto Project development
+environment. The `Yocto Project Curated Layer
+Index <https://caffelli-staging.yoctoproject.org/software-overview/layers/>`__
+and `OpenEmbedded Layer
+Index <http://layers.openembedded.org/layerindex/branch/master/layers/>`__
+both contain layers from which you can use or leverage.
+By convention, layers in the Yocto Project follow a specific form.
+Conforming to a known structure allows BitBake to make assumptions
+during builds on where to find types of metadata. You can find
+procedures and learn about tools (i.e. ``bitbake-layers``) for creating
+layers suitable for the Yocto Project in the "`Understanding and
+Creating
+Layers <&YOCTO_DOCS_DEV_URL;#understanding-and-creating-layers>`__"
+section of the Yocto Project Development Tasks Manual.
+.. _openembedded-build-system-build-concepts:
+OpenEmbedded Build System Concepts
+==================================
+This section takes a more detailed look inside the build process used by
+the `OpenEmbedded build
+system <&YOCTO_DOCS_REF_URL;#build-system-term>`__, which is the build
+system specific to the Yocto Project. At the heart of the build system
+is BitBake, the task executor.
+The following diagram represents the high-level workflow of a build. The
+remainder of this section expands on the fundamental input, output,
+process, and metadata logical blocks that make up the workflow.
+In general, the build's workflow consists of several functional areas:
+-  *User Configuration:* metadata you can use to control the build
+   process.
+-  *Metadata Layers:* Various layers that provide software, machine, and
+   distro metadata.
+-  *Source Files:* Upstream releases, local projects, and SCMs.
+-  *Build System:* Processes under the control of
+   `BitBake <&YOCTO_DOCS_REF_URL;#bitbake-term>`__. This block expands
+   on how BitBake fetches source, applies patches, completes
+   compilation, analyzes output for package generation, creates and
+   tests packages, generates images, and generates cross-development
+   tools.
+-  *Package Feeds:* Directories containing output packages (RPM, DEB or
+   IPK), which are subsequently used in the construction of an image or
+   Software Development Kit (SDK), produced by the build system. These
+   feeds can also be copied and shared using a web server or other means
+   to facilitate extending or updating existing images on devices at
+   runtime if runtime package management is enabled.
+-  *Images:* Images produced by the workflow.
+-  *Application Development SDK:* Cross-development tools that are
+   produced along with an image or separately with BitBake.
+User Configuration
+------------------
+User configuration helps define the build. Through user configuration,
+you can tell BitBake the target architecture for which you are building
+the image, where to store downloaded source, and other build properties.
+The following figure shows an expanded representation of the "User
+Configuration" box of the `general workflow
+figure <#general-workflow-figure>`__:
+BitBake needs some basic configuration files in order to complete a
+build. These files are ``*.conf`` files. The minimally necessary ones
+reside as example files in the ``build/conf`` directory of the `Source
+Directory <&YOCTO_DOCS_REF_URL;#source-directory>`__. For simplicity,
+this section refers to the Source Directory as the "Poky Directory."
+When you clone the `Poky <&YOCTO_DOCS_REF_URL;#poky>`__ Git repository
+or you download and unpack a Yocto Project release, you can set up the
+Source Directory to be named anything you want. For this discussion, the
+cloned repository uses the default name ``poky``.
+.. note::
+   The Poky repository is primarily an aggregation of existing
+   repositories. It is not a canonical upstream source.
+The ``meta-poky`` layer inside Poky contains a ``conf`` directory that
+has example configuration files. These example files are used as a basis
+for creating actual configuration files when you source
+````` <&YOCTO_DOCS_REF_URL;#structure-core-script>`__, which is the
+build environment script.
+Sourcing the build environment script creates a `Build
+Directory <&YOCTO_DOCS_REF_URL;#build-directory>`__ if one does not
+already exist. BitBake uses the Build Directory for all its work during
+builds. The Build Directory has a ``conf`` directory that contains
+default versions of your ``local.conf`` and ``bblayers.conf``
+configuration files. These default configuration files are created only
+if versions do not already exist in the Build Directory at the time you
+source the build environment setup script.
+Because the Poky repository is fundamentally an aggregation of existing
+repositories, some users might be familiar with running the ```` script
+in the context of separate
+`OpenEmbedded-Core <&YOCTO_DOCS_REF_URL;#oe-core>`__ and BitBake
+repositories rather than a single Poky repository. This discussion
+assumes the script is executed from within a cloned or unpacked version
+of Poky.
+Depending on where the script is sourced, different sub-scripts are
+called to set up the Build Directory (Yocto or OpenEmbedded).
+Specifically, the script ``scripts/oe-setup-builddir`` inside the poky
+directory sets up the Build Directory and seeds the directory (if
+necessary) with configuration files appropriate for the Yocto Project
+development environment.
+.. note::
+   The
+   scripts/oe-setup-builddir
+   script uses the
+   $TEMPLATECONF
+   variable to determine which sample configuration files to locate.
+The ``local.conf`` file provides many basic variables that define a
+build environment. Here is a list of a few. To see the default
+configurations in a ``local.conf`` file created by the build environment
+script, see the
+```local.conf.sample`` <&YOCTO_GIT_URL;/cgit/cgit.cgi/poky/tree/meta-poky/conf/local.conf.sample>`__
+in the ``meta-poky`` layer:
+-  *Target Machine Selection:* Controlled by the
+   ```MACHINE`` <&YOCTO_DOCS_REF_URL;#var-MACHINE>`__ variable.
+-  *Download Directory:* Controlled by the
+   ```DL_DIR`` <&YOCTO_DOCS_REF_URL;#var-DL_DIR>`__ variable.
+-  *Shared State Directory:* Controlled by the
+   ```SSTATE_DIR`` <&YOCTO_DOCS_REF_URL;#var-SSTATE_DIR>`__ variable.
+-  *Build Output:* Controlled by the
+   ```TMPDIR`` <&YOCTO_DOCS_REF_URL;#var-TMPDIR>`__ variable.
+-  *Distribution Policy:* Controlled by the
+   ```DISTRO`` <&YOCTO_DOCS_REF_URL;#var-DISTRO>`__ variable.
+-  *Packaging Format:* Controlled by the
+   ```PACKAGE_CLASSES`` <&YOCTO_DOCS_REF_URL;#var-PACKAGE_CLASSES>`__
+   variable.
+-  *SDK Target Architecture:* Controlled by the
+   ```SDKMACHINE`` <&YOCTO_DOCS_REF_URL;#var-SDKMACHINE>`__ variable.
+-  *Extra Image Packages:* Controlled by the
+   ```EXTRA_IMAGE_FEATURES`` <&YOCTO_DOCS_REF_URL;#var-EXTRA_IMAGE_FEATURES>`__
+   variable.
+.. note::
+   Configurations set in the
+   conf/local.conf
+   file can also be set in the
+   conf/site.conf
+   and
+   conf/auto.conf
+   configuration files.
+The ``bblayers.conf`` file tells BitBake what layers you want considered
+during the build. By default, the layers listed in this file include
+layers minimally needed by the build system. However, you must manually
+add any custom layers you have created. You can find more information on
+working with the ``bblayers.conf`` file in the "`Enabling Your
+Layer <&YOCTO_DOCS_DEV_URL;#enabling-your-layer>`__" section in the
+Yocto Project Development Tasks Manual.
+The files ``site.conf`` and ``auto.conf`` are not created by the
+environment initialization script. If you want the ``site.conf`` file,
+you need to create that yourself. The ``auto.conf`` file is typically
+created by an autobuilder:
+-  *``site.conf``:* You can use the ``conf/site.conf`` configuration
+   file to configure multiple build directories. For example, suppose
+   you had several build environments and they shared some common
+   features. You can set these default build properties here. A good
+   example is perhaps the packaging format to use through the
+   ```PACKAGE_CLASSES`` <&YOCTO_DOCS_REF_URL;#var-PACKAGE_CLASSES>`__
+   variable.
+   One useful scenario for using the ``conf/site.conf`` file is to
+   extend your ```BBPATH`` <&YOCTO_DOCS_REF_URL;#var-BBPATH>`__ variable
+   to include the path to a ``conf/site.conf``. Then, when BitBake looks
+   for Metadata using ``BBPATH``, it finds the ``conf/site.conf`` file
+   and applies your common configurations found in the file. To override
+   configurations in a particular build directory, alter the similar
+   configurations within that build directory's ``conf/local.conf``
+   file.
+-  *``auto.conf``:* The file is usually created and written to by an
+   autobuilder. The settings put into the file are typically the same as
+   you would find in the ``conf/local.conf`` or the ``conf/site.conf``
+   files.
+You can edit all configuration files to further define any particular
+build environment. This process is represented by the "User
+Configuration Edits" box in the figure.
+When you launch your build with the ``bitbake target`` command, BitBake
+sorts out the configurations to ultimately define your build
+environment. It is important to understand that the `OpenEmbedded build
+system <&YOCTO_DOCS_REF_URL;#build-system-term>`__ reads the
+configuration files in a specific order: ``site.conf``, ``auto.conf``,
+and ``local.conf``. And, the build system applies the normal assignment
+statement rules as described in the "`Syntax and
+Operators <&YOCTO_DOCS_BB_URL;#bitbake-user-manual-metadata>`__" chapter
+of the BitBake User Manual. Because the files are parsed in a specific
+order, variable assignments for the same variable could be affected. For
+example, if the ``auto.conf`` file and the ``local.conf`` set variable1
+to different values, because the build system parses ``local.conf``
+after ``auto.conf``, variable1 is assigned the value from the
+``local.conf`` file.
+Metadata, Machine Configuration, and Policy Configuration
+---------------------------------------------------------
+The previous section described the user configurations that define
+BitBake's global behavior. This section takes a closer look at the
+layers the build system uses to further control the build. These layers
+provide Metadata for the software, machine, and policies.
+In general, three types of layer input exists. You can see them below
+the "User Configuration" box in the `general workflow
+figure <#general-workflow-figure>`__:
+-  *Metadata (``.bb`` + Patches):* Software layers containing
+   user-supplied recipe files, patches, and append files. A good example
+   of a software layer might be the
+   ```meta-qt5`` <https://github.com/meta-qt5/meta-qt5>`__ layer from
+   the `OpenEmbedded Layer
+   Index <http://layers.openembedded.org/layerindex/branch/master/layers/>`__.
+   This layer is for version 5.0 of the popular
+   `Qt <https://wiki.qt.io/About_Qt>`__ cross-platform application
+   development framework for desktop, embedded and mobile.
+-  *Machine BSP Configuration:* Board Support Package (BSP) layers (i.e.
+   "BSP Layer" in the following figure) providing machine-specific
+   configurations. This type of information is specific to a particular
+   target architecture. A good example of a BSP layer from the `Poky
+   Reference Distribution <#gs-reference-distribution-poky>`__ is the
+   ```meta-yocto-bsp`` <&YOCTO_GIT_URL;/cgit/cgit.cgi/poky/tree/meta-yocto-bsp>`__
+   layer.
+-  *Policy Configuration:* Distribution Layers (i.e. "Distro Layer" in
+   the following figure) providing top-level or general policies for the
+   images or SDKs being built for a particular distribution. For
+   example, in the Poky Reference Distribution the distro layer is the
+   ```meta-poky`` <&YOCTO_GIT_URL;/cgit/cgit.cgi/poky/tree/meta-poky>`__
+   layer. Within the distro layer is a ``conf/distro`` directory that
+   contains distro configuration files (e.g.
+   ```poky.conf`` <&YOCTO_GIT_URL;/cgit/cgit.cgi/poky/tree/meta-poky/conf/distro/poky.conf>`__
+   that contain many policy configurations for the Poky distribution.
+The following figure shows an expanded representation of these three
+layers from the `general workflow figure <#general-workflow-figure>`__:
+In general, all layers have a similar structure. They all contain a
+licensing file (e.g. ``COPYING.MIT``) if the layer is to be distributed,
+a ``README`` file as good practice and especially if the layer is to be
+distributed, a configuration directory, and recipe directories. You can
+learn about the general structure for layers used with the Yocto Project
+in the "`Creating Your Own
+Layer <&YOCTO_DOCS_DEV_URL;#creating-your-own-layer>`__" section in the
+Yocto Project Development Tasks Manual. For a general discussion on
+layers and the many layers from which you can draw, see the
+"`Layers <#overview-layers>`__" and "`The Yocto Project Layer
+Model <#the-yocto-project-layer-model>`__" sections both earlier in this
+manual.
+If you explored the previous links, you discovered some areas where many
+layers that work with the Yocto Project exist. The `Source
+Repositories <http://git.yoctoproject.org/>`__ also shows layers
+categorized under "Yocto Metadata Layers."
+.. note::
+   Layers exist in the Yocto Project Source Repositories that cannot be
+   found in the OpenEmbedded Layer Index. These layers are either
+   deprecated or experimental in nature.
+BitBake uses the ``conf/bblayers.conf`` file, which is part of the user
+configuration, to find what layers it should be using as part of the
+build.
+Distro Layer
+~~~~~~~~~~~~
+The distribution layer provides policy configurations for your
+distribution. Best practices dictate that you isolate these types of
+configurations into their own layer. Settings you provide in
+``conf/distro/distro.conf`` override similar settings that BitBake finds
+in your ``conf/local.conf`` file in the Build Directory.
+The following list provides some explanation and references for what you
+typically find in the distribution layer:
+-  *classes:* Class files (``.bbclass``) hold common functionality that
+   can be shared among recipes in the distribution. When your recipes
+   inherit a class, they take on the settings and functions for that
+   class. You can read more about class files in the
+   "`Classes <&YOCTO_DOCS_REF_URL;#ref-classes>`__" chapter of the Yocto
+   Reference Manual.
+-  *conf:* This area holds configuration files for the layer
+   (``conf/layer.conf``), the distribution
+   (``conf/distro/distro.conf``), and any distribution-wide include
+   files.
+-  *recipes-*:* Recipes and append files that affect common
+   functionality across the distribution. This area could include
+   recipes and append files to add distribution-specific configuration,
+   initialization scripts, custom image recipes, and so forth. Examples
+   of ``recipes-*`` directories are ``recipes-core`` and
+   ``recipes-extra``. Hierarchy and contents within a ``recipes-*``
+   directory can vary. Generally, these directories contain recipe files
+   (``*.bb``), recipe append files (``*.bbappend``), directories that
+   are distro-specific for configuration files, and so forth.
+BSP Layer
+~~~~~~~~~
+The BSP Layer provides machine configurations that target specific
+hardware. Everything in this layer is specific to the machine for which
+you are building the image or the SDK. A common structure or form is
+defined for BSP layers. You can learn more about this structure in the
+`Yocto Project Board Support Package (BSP) Developer's
+Guide <&YOCTO_DOCS_BSP_URL;>`__.
+.. note::
+   In order for a BSP layer to be considered compliant with the Yocto
+   Project, it must meet some structural requirements.
+The BSP Layer's configuration directory contains configuration files for
+the machine (``conf/machine/machine.conf``) and, of course, the layer
+(``conf/layer.conf``).
+The remainder of the layer is dedicated to specific recipes by function:
+``recipes-bsp``, ``recipes-core``, ``recipes-graphics``,
+``recipes-kernel``, and so forth. Metadata can exist for multiple
+formfactors, graphics support systems, and so forth.
+.. note::
+   While the figure shows several
+   recipes-\*
+   directories, not all these directories appear in all BSP layers.
+Software Layer
+~~~~~~~~~~~~~~
+The software layer provides the Metadata for additional software
+packages used during the build. This layer does not include Metadata
+that is specific to the distribution or the machine, which are found in
+their respective layers.
+This layer contains any recipes, append files, and patches, that your
+project needs.
+.. _sources-dev-environment:
+Sources
+-------
+In order for the OpenEmbedded build system to create an image or any
+target, it must be able to access source files. The `general workflow
+figure <#general-workflow-figure>`__ represents source files using the
+"Upstream Project Releases", "Local Projects", and "SCMs (optional)"
+boxes. The figure represents mirrors, which also play a role in locating
+source files, with the "Source Materials" box.
+The method by which source files are ultimately organized is a function
+of the project. For example, for released software, projects tend to use
+tarballs or other archived files that can capture the state of a release
+guaranteeing that it is statically represented. On the other hand, for a
+project that is more dynamic or experimental in nature, a project might
+keep source files in a repository controlled by a Source Control Manager
+(SCM) such as Git. Pulling source from a repository allows you to
+control the point in the repository (the revision) from which you want
+to build software. Finally, a combination of the two might exist, which
+would give the consumer a choice when deciding where to get source
+files.
+BitBake uses the ```SRC_URI`` <&YOCTO_DOCS_REF_URL;#var-SRC_URI>`__
+variable to point to source files regardless of their location. Each
+recipe must have a ``SRC_URI`` variable that points to the source.
+Another area that plays a significant role in where source files come
+from is pointed to by the
+```DL_DIR`` <&YOCTO_DOCS_REF_URL;#var-DL_DIR>`__ variable. This area is
+a cache that can hold previously downloaded source. You can also
+instruct the OpenEmbedded build system to create tarballs from Git
+repositories, which is not the default behavior, and store them in the
+``DL_DIR`` by using the
+```BB_GENERATE_MIRROR_TARBALLS`` <&YOCTO_DOCS_REF_URL;#var-BB_GENERATE_MIRROR_TARBALLS>`__
+variable.
+Judicious use of a ``DL_DIR`` directory can save the build system a trip
+across the Internet when looking for files. A good method for using a
+download directory is to have ``DL_DIR`` point to an area outside of
+your Build Directory. Doing so allows you to safely delete the Build
+Directory if needed without fear of removing any downloaded source file.
+The remainder of this section provides a deeper look into the source
+files and the mirrors. Here is a more detailed look at the source file
+area of the `general workflow figure <#general-workflow-figure>`__:
+Upstream Project Releases
+~~~~~~~~~~~~~~~~~~~~~~~~~
+Upstream project releases exist anywhere in the form of an archived file
+(e.g. tarball or zip file). These files correspond to individual
+recipes. For example, the figure uses specific releases each for
+BusyBox, Qt, and Dbus. An archive file can be for any released product
+that can be built using a recipe.
+Local Projects
+~~~~~~~~~~~~~~
+Local projects are custom bits of software the user provides. These bits
+reside somewhere local to a project - perhaps a directory into which the
+user checks in items (e.g. a local directory containing a development
+source tree used by the group).
+The canonical method through which to include a local project is to use
+the ```externalsrc`` <&YOCTO_DOCS_REF_URL;#ref-classes-externalsrc>`__
+class to include that local project. You use either the ``local.conf``
+or a recipe's append file to override or set the recipe to point to the
+local directory on your disk to pull in the whole source tree.
+.. _scms:
+Source Control Managers (Optional)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Another place from which the build system can get source files is with
+`fetchers <&YOCTO_DOCS_BB_URL;#bb-fetchers>`__ employing various Source
+Control Managers (SCMs) such as Git or Subversion. In such cases, a
+repository is cloned or checked out. The
+```do_fetch`` <&YOCTO_DOCS_REF_URL;#ref-tasks-fetch>`__ task inside
+BitBake uses the ```SRC_URI`` <&YOCTO_DOCS_REF_URL;#var-SRC_URI>`__
+variable and the argument's prefix to determine the correct fetcher
+module.
+.. note::
+   For information on how to have the OpenEmbedded build system generate
+   tarballs for Git repositories and place them in the
+   DL_DIR
+   directory, see the
+   BB_GENERATE_MIRROR_TARBALLS
+   variable in the Yocto Project Reference Manual.
+When fetching a repository, BitBake uses the
+```SRCREV`` <&YOCTO_DOCS_REF_URL;#var-SRCREV>`__ variable to determine
+the specific revision from which to build.
+Source Mirror(s)
+~~~~~~~~~~~~~~~~
+Two kinds of mirrors exist: pre-mirrors and regular mirrors. The
+```PREMIRRORS`` <&YOCTO_DOCS_REF_URL;#var-PREMIRRORS>`__ and
+```MIRRORS`` <&YOCTO_DOCS_REF_URL;#var-MIRRORS>`__ variables point to
+these, respectively. BitBake checks pre-mirrors before looking upstream
+for any source files. Pre-mirrors are appropriate when you have a shared
+directory that is not a directory defined by the
+```DL_DIR`` <&YOCTO_DOCS_REF_URL;#var-DL_DIR>`__ variable. A Pre-mirror
+typically points to a shared directory that is local to your
+organization.
+Regular mirrors can be any site across the Internet that is used as an
+alternative location for source code should the primary site not be
+functioning for some reason or another.
+.. _package-feeds-dev-environment:
+Package Feeds
+-------------
+When the OpenEmbedded build system generates an image or an SDK, it gets
+the packages from a package feed area located in the `Build
+Directory <&YOCTO_DOCS_REF_URL;#build-directory>`__. The `general
+workflow figure <#general-workflow-figure>`__ shows this package feeds
+area in the upper-right corner.
+This section looks a little closer into the package feeds area used by
+the build system. Here is a more detailed look at the area:
+Package feeds are an intermediary step in the build process. The
+OpenEmbedded build system provides classes to generate different package
+types, and you specify which classes to enable through the
+```PACKAGE_CLASSES`` <&YOCTO_DOCS_REF_URL;#var-PACKAGE_CLASSES>`__
+variable. Before placing the packages into package feeds, the build
+process validates them with generated output quality assurance checks
+through the ```insane`` <&YOCTO_DOCS_REF_URL;#ref-classes-insane>`__
+class.
+The package feed area resides in the Build Directory. The directory the
+build system uses to temporarily store packages is determined by a
+combination of variables and the particular package manager in use. See
+the "Package Feeds" box in the illustration and note the information to
+the right of that area. In particular, the following defines where
+package files are kept:
+-  ```DEPLOY_DIR`` <&YOCTO_DOCS_REF_URL;#var-DEPLOY_DIR>`__: Defined as
+   ``tmp/deploy`` in the Build Directory.
+-  ``DEPLOY_DIR_*``: Depending on the package manager used, the package
+   type sub-folder. Given RPM, IPK, or DEB packaging and tarball
+   creation, the
+   ```DEPLOY_DIR_RPM`` <&YOCTO_DOCS_REF_URL;#var-DEPLOY_DIR_RPM>`__,
+   ```DEPLOY_DIR_IPK`` <&YOCTO_DOCS_REF_URL;#var-DEPLOY_DIR_IPK>`__,
+   ```DEPLOY_DIR_DEB`` <&YOCTO_DOCS_REF_URL;#var-DEPLOY_DIR_DEB>`__, or
+   ```DEPLOY_DIR_TAR`` <&YOCTO_DOCS_REF_URL;#var-DEPLOY_DIR_TAR>`__,
+   variables are used, respectively.
+-  ```PACKAGE_ARCH`` <&YOCTO_DOCS_REF_URL;#var-PACKAGE_ARCH>`__: Defines
+   architecture-specific sub-folders. For example, packages could exist
+   for the i586 or qemux86 architectures.
+BitBake uses the
+```do_package_write_*`` <&YOCTO_DOCS_REF_URL;#ref-tasks-package_write_deb>`__
+tasks to generate packages and place them into the package holding area
+(e.g. ``do_package_write_ipk`` for IPK packages). See the
+"```do_package_write_deb`` <&YOCTO_DOCS_REF_URL;#ref-tasks-package_write_deb>`__",
+"```do_package_write_ipk`` <&YOCTO_DOCS_REF_URL;#ref-tasks-package_write_ipk>`__",
+"```do_package_write_rpm`` <&YOCTO_DOCS_REF_URL;#ref-tasks-package_write_rpm>`__",
+and
+"```do_package_write_tar`` <&YOCTO_DOCS_REF_URL;#ref-tasks-package_write_tar>`__"
+sections in the Yocto Project Reference Manual for additional
+information. As an example, consider a scenario where an IPK packaging
+manager is being used and package architecture support for both i586 and
+qemux86 exist. Packages for the i586 architecture are placed in
+``build/tmp/deploy/ipk/i586``, while packages for the qemux86
+architecture are placed in ``build/tmp/deploy/ipk/qemux86``.
+.. _bitbake-dev-environment:
+BitBake
+-------
+The OpenEmbedded build system uses
+`BitBake <&YOCTO_DOCS_REF_URL;#bitbake-term>`__ to produce images and
+Software Development Kits (SDKs). You can see from the `general workflow
+figure <#general-workflow-figure>`__, the BitBake area consists of
+several functional areas. This section takes a closer look at each of
+those areas.
+.. note::
+   Separate documentation exists for the BitBake tool. See the
+   BitBake User Manual
+   for reference material on BitBake.
+.. _source-fetching-dev-environment:
+Source Fetching
+~~~~~~~~~~~~~~~
+The first stages of building a recipe are to fetch and unpack the source
+code:
+The ```do_fetch`` <&YOCTO_DOCS_REF_URL;#ref-tasks-fetch>`__ and
+```do_unpack`` <&YOCTO_DOCS_REF_URL;#ref-tasks-unpack>`__ tasks fetch
+the source files and unpack them into the `Build
+Directory <&YOCTO_DOCS_REF_URL;#build-directory>`__.
+.. note::
+   For every local file (e.g.
+   file://
+   ) that is part of a recipe's
+   SRC_URI
+   statement, the OpenEmbedded build system takes a checksum of the file
+   for the recipe and inserts the checksum into the signature for the
+   do_fetch
+   task. If any local file has been modified, the
+   do_fetch
+   task and all tasks that depend on it are re-executed.
+By default, everything is accomplished in the Build Directory, which has
+a defined structure. For additional general information on the Build
+Directory, see the
+"```build/`` <&YOCTO_DOCS_REF_URL;#structure-core-build>`__" section in
+the Yocto Project Reference Manual.
+Each recipe has an area in the Build Directory where the unpacked source
+code resides. The ```S`` <&YOCTO_DOCS_REF_URL;#var-S>`__ variable points
+to this area for a recipe's unpacked source code. The name of that
+directory for any given recipe is defined from several different
+variables. The preceding figure and the following list describe the
+Build Directory's hierarchy:
+-  ```TMPDIR`` <&YOCTO_DOCS_REF_URL;#var-TMPDIR>`__: The base directory
+   where the OpenEmbedded build system performs all its work during the
+   build. The default base directory is the ``tmp`` directory.
+-  ```PACKAGE_ARCH`` <&YOCTO_DOCS_REF_URL;#var-PACKAGE_ARCH>`__: The
+   architecture of the built package or packages. Depending on the
+   eventual destination of the package or packages (i.e. machine
+   architecture, `build
+   host <&YOCTO_DOCS_REF_URL;#hardware-build-system-term>`__, SDK, or
+   specific machine), ``PACKAGE_ARCH`` varies. See the variable's
+   description for details.
+-  ```TARGET_OS`` <&YOCTO_DOCS_REF_URL;#var-TARGET_OS>`__: The operating
+   system of the target device. A typical value would be "linux" (e.g.
+   "qemux86-poky-linux").
+-  ```PN`` <&YOCTO_DOCS_REF_URL;#var-PN>`__: The name of the recipe used
+   to build the package. This variable can have multiple meanings.
+   However, when used in the context of input files, ``PN`` represents
+   the the name of the recipe.
+-  ```WORKDIR`` <&YOCTO_DOCS_REF_URL;#var-WORKDIR>`__: The location
+   where the OpenEmbedded build system builds a recipe (i.e. does the
+   work to create the package).
+   -  ```PV`` <&YOCTO_DOCS_REF_URL;#var-PV>`__: The version of the
+      recipe used to build the package.
+   -  ```PR`` <&YOCTO_DOCS_REF_URL;#var-PR>`__: The revision of the
+      recipe used to build the package.
+-  ```S`` <&YOCTO_DOCS_REF_URL;#var-S>`__: Contains the unpacked source
+   files for a given recipe.
+   -  ```BPN`` <&YOCTO_DOCS_REF_URL;#var-BPN>`__: The name of the recipe
+      used to build the package. The ``BPN`` variable is a version of
+      the ``PN`` variable but with common prefixes and suffixes removed.
+   -  ```PV`` <&YOCTO_DOCS_REF_URL;#var-PV>`__: The version of the
+      recipe used to build the package.
+.. note::
+   In the previous figure, notice that two sample hierarchies exist: one
+   based on package architecture (i.e.
+   PACKAGE_ARCH
+   ) and one based on a machine (i.e.
+   MACHINE
+   ). The underlying structures are identical. The differentiator being
+   what the OpenEmbedded build system is using as a build target (e.g.
+   general architecture, a build host, an SDK, or a specific machine).
+.. _patching-dev-environment:
+Patching
+~~~~~~~~
+Once source code is fetched and unpacked, BitBake locates patch files
+and applies them to the source files:
+The ```do_patch`` <&YOCTO_DOCS_REF_URL;#ref-tasks-patch>`__ task uses a
+recipe's ```SRC_URI`` <&YOCTO_DOCS_REF_URL;#var-SRC_URI>`__ statements
+and the ```FILESPATH`` <&YOCTO_DOCS_REF_URL;#var-FILESPATH>`__ variable
+to locate applicable patch files.
+Default processing for patch files assumes the files have either
+``*.patch`` or ``*.diff`` file types. You can use ``SRC_URI`` parameters
+to change the way the build system recognizes patch files. See the
+```do_patch`` <&YOCTO_DOCS_REF_URL;#ref-tasks-patch>`__ task for more
+information.
+BitBake finds and applies multiple patches for a single recipe in the
+order in which it locates the patches. The ``FILESPATH`` variable
+defines the default set of directories that the build system uses to
+search for patch files. Once found, patches are applied to the recipe's
+source files, which are located in the
+```S`` <&YOCTO_DOCS_REF_URL;#var-S>`__ directory.
+For more information on how the source directories are created, see the
+"`Source Fetching <#source-fetching-dev-environment>`__" section. For
+more information on how to create patches and how the build system
+processes patches, see the "`Patching
+Code <&YOCTO_DOCS_DEV_URL;#new-recipe-patching-code>`__" section in the
+Yocto Project Development Tasks Manual. You can also see the "`Use
+``devtool modify`` to Modify the Source of an Existing
+Component <&YOCTO_DOCS_SDK_URL;#sdk-devtool-use-devtool-modify-to-modify-the-source-of-an-existing-component>`__"
+section in the Yocto Project Application Development and the Extensible
+Software Development Kit (SDK) manual and the "`Using Traditional Kernel
+Development to Patch the
+Kernel <&YOCTO_DOCS_KERNEL_DEV_URL;#using-traditional-kernel-development-to-patch-the-kernel>`__"
+section in the Yocto Project Linux Kernel Development Manual.
+.. _configuration-compilation-and-staging-dev-environment:
+Configuration, Compilation, and Staging
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+After source code is patched, BitBake executes tasks that configure and
+compile the source code. Once compilation occurs, the files are copied
+to a holding area (staged) in preparation for packaging:
+This step in the build process consists of the following tasks:
+-  ```do_prepare_recipe_sysroot`` <&YOCTO_DOCS_REF_URL;#ref-tasks-prepare_recipe_sysroot>`__:
+   This task sets up the two sysroots in
+   ``${``\ ```WORKDIR`` <&YOCTO_DOCS_REF_URL;#var-WORKDIR>`__\ ``}``
+   (i.e. ``recipe-sysroot`` and ``recipe-sysroot-native``) so that
+   during the packaging phase the sysroots can contain the contents of
+   the
+   ```do_populate_sysroot`` <&YOCTO_DOCS_REF_URL;#ref-tasks-populate_sysroot>`__
+   tasks of the recipes on which the recipe containing the tasks
+   depends. A sysroot exists for both the target and for the native
+   binaries, which run on the host system.
+-  *``do_configure``*: This task configures the source by enabling and
+   disabling any build-time and configuration options for the software
+   being built. Configurations can come from the recipe itself as well
+   as from an inherited class. Additionally, the software itself might
+   configure itself depending on the target for which it is being built.
+   The configurations handled by the
+   ```do_configure`` <&YOCTO_DOCS_REF_URL;#ref-tasks-configure>`__ task
+   are specific to configurations for the source code being built by the
+   recipe.
+   If you are using the
+   ```autotools`` <&YOCTO_DOCS_REF_URL;#ref-classes-autotools>`__ class,
+   you can add additional configuration options by using the
+   ```EXTRA_OECONF`` <&YOCTO_DOCS_REF_URL;#var-EXTRA_OECONF>`__ or
+   ```PACKAGECONFIG_CONFARGS`` <&YOCTO_DOCS_REF_URL;#var-PACKAGECONFIG_CONFARGS>`__
+   variables. For information on how this variable works within that
+   class, see the
+   ```autotools`` <&YOCTO_DOCS_REF_URL;#ref-classes-autotools>`__ class
+   `here <&YOCTO_GIT_URL;/cgit/cgit.cgi/poky/tree/meta/classes/autotools.bbclass>`__.
+-  *``do_compile``*: Once a configuration task has been satisfied,
+   BitBake compiles the source using the
+   ```do_compile`` <&YOCTO_DOCS_REF_URL;#ref-tasks-compile>`__ task.
+   Compilation occurs in the directory pointed to by the
+   ```B`` <&YOCTO_DOCS_REF_URL;#var-B>`__ variable. Realize that the
+   ``B`` directory is, by default, the same as the
+   ```S`` <&YOCTO_DOCS_REF_URL;#var-S>`__ directory.
+-  *``do_install``*: After compilation completes, BitBake executes the
+   ```do_install`` <&YOCTO_DOCS_REF_URL;#ref-tasks-install>`__ task.
+   This task copies files from the ``B`` directory and places them in a
+   holding area pointed to by the ```D`` <&YOCTO_DOCS_REF_URL;#var-D>`__
+   variable. Packaging occurs later using files from this holding
+   directory.
+.. _package-splitting-dev-environment:
+Package Splitting
+~~~~~~~~~~~~~~~~~
+After source code is configured, compiled, and staged, the build system
+analyzes the results and splits the output into packages:
+The ```do_package`` <&YOCTO_DOCS_REF_URL;#ref-tasks-package>`__ and
+```do_packagedata`` <&YOCTO_DOCS_REF_URL;#ref-tasks-packagedata>`__
+tasks combine to analyze the files found in the
+```D`` <&YOCTO_DOCS_REF_URL;#var-D>`__ directory and split them into
+subsets based on available packages and files. Analysis involves the
+following as well as other items: splitting out debugging symbols,
+looking at shared library dependencies between packages, and looking at
+package relationships.
+The ``do_packagedata`` task creates package metadata based on the
+analysis such that the build system can generate the final packages. The
+```do_populate_sysroot`` <&YOCTO_DOCS_REF_URL;#ref-tasks-populate_sysroot>`__
+task stages (copies) a subset of the files installed by the
+```do_install`` <&YOCTO_DOCS_REF_URL;#ref-tasks-install>`__ task into
+the appropriate sysroot. Working, staged, and intermediate results of
+the analysis and package splitting process use several areas:
+-  ```PKGD`` <&YOCTO_DOCS_REF_URL;#var-PKGD>`__: The destination
+   directory (i.e. ``package``) for packages before they are split into
+   individual packages.
+-  ```PKGDESTWORK`` <&YOCTO_DOCS_REF_URL;#var-PKGDESTWORK>`__: A
+   temporary work area (i.e. ``pkgdata``) used by the ``do_package``
+   task to save package metadata.
+-  ```PKGDEST`` <&YOCTO_DOCS_REF_URL;#var-PKGDEST>`__: The parent
+   directory (i.e. ``packages-split``) for packages after they have been
+   split.
+-  ```PKGDATA_DIR`` <&YOCTO_DOCS_REF_URL;#var-PKGDATA_DIR>`__: A shared,
+   global-state directory that holds packaging metadata generated during
+   the packaging process. The packaging process copies metadata from
+   ``PKGDESTWORK`` to the ``PKGDATA_DIR`` area where it becomes globally
+   available.
+-  ```STAGING_DIR_HOST`` <&YOCTO_DOCS_REF_URL;#var-STAGING_DIR_HOST>`__:
+   The path for the sysroot for the system on which a component is built
+   to run (i.e. ``recipe-sysroot``).
+-  ```STAGING_DIR_NATIVE`` <&YOCTO_DOCS_REF_URL;#var-STAGING_DIR_NATIVE>`__:
+   The path for the sysroot used when building components for the build
+   host (i.e. ``recipe-sysroot-native``).
+-  ```STAGING_DIR_TARGET`` <&YOCTO_DOCS_REF_URL;#var-STAGING_DIR_TARGET>`__:
+   The path for the sysroot used when a component that is built to
+   execute on a system and it generates code for yet another machine
+   (e.g. cross-canadian recipes).
+The ```FILES`` <&YOCTO_DOCS_REF_URL;#var-FILES>`__ variable defines the
+files that go into each package in
+```PACKAGES`` <&YOCTO_DOCS_REF_URL;#var-PACKAGES>`__. If you want
+details on how this is accomplished, you can look at
+```package.bbclass`` <&YOCTO_GIT_URL;/cgit/cgit.cgi/poky/tree/meta/classes/package.bbclass>`__.
+Depending on the type of packages being created (RPM, DEB, or IPK), the
+```do_package_write_*`` <&YOCTO_DOCS_REF_URL;#ref-tasks-package_write_deb>`__
+task creates the actual packages and places them in the Package Feed
+area, which is ``${TMPDIR}/deploy``. You can see the "`Package
+Feeds <#package-feeds-dev-environment>`__" section for more detail on
+that part of the build process.
+.. note::
+   Support for creating feeds directly from the
+   deploy/\*
+   directories does not exist. Creating such feeds usually requires some
+   kind of feed maintenance mechanism that would upload the new packages
+   into an official package feed (e.g. the Ångström distribution). This
+   functionality is highly distribution-specific and thus is not
+   provided out of the box.
+.. _image-generation-dev-environment:
+Image Generation
+~~~~~~~~~~~~~~~~
+Once packages are split and stored in the Package Feeds area, the build
+system uses BitBake to generate the root filesystem image:
+The image generation process consists of several stages and depends on
+several tasks and variables. The
+```do_rootfs`` <&YOCTO_DOCS_REF_URL;#ref-tasks-rootfs>`__ task creates
+the root filesystem (file and directory structure) for an image. This
+task uses several key variables to help create the list of packages to
+actually install:
+-  ```IMAGE_INSTALL`` <&YOCTO_DOCS_REF_URL;#var-IMAGE_INSTALL>`__: Lists
+   out the base set of packages from which to install from the Package
+   Feeds area.
+-  ```PACKAGE_EXCLUDE`` <&YOCTO_DOCS_REF_URL;#var-PACKAGE_EXCLUDE>`__:
+   Specifies packages that should not be installed into the image.
+-  ```IMAGE_FEATURES`` <&YOCTO_DOCS_REF_URL;#var-IMAGE_FEATURES>`__:
+   Specifies features to include in the image. Most of these features
+   map to additional packages for installation.
+-  ```PACKAGE_CLASSES`` <&YOCTO_DOCS_REF_URL;#var-PACKAGE_CLASSES>`__:
+   Specifies the package backend (e.g. RPM, DEB, or IPK) to use and
+   consequently helps determine where to locate packages within the
+   Package Feeds area.
+-  ```IMAGE_LINGUAS`` <&YOCTO_DOCS_REF_URL;#var-IMAGE_LINGUAS>`__:
+   Determines the language(s) for which additional language support
+   packages are installed.
+-  ```PACKAGE_INSTALL`` <&YOCTO_DOCS_REF_URL;#var-PACKAGE_INSTALL>`__:
+   The final list of packages passed to the package manager for
+   installation into the image.
+With ```IMAGE_ROOTFS`` <&YOCTO_DOCS_REF_URL;#var-IMAGE_ROOTFS>`__
+pointing to the location of the filesystem under construction and the
+``PACKAGE_INSTALL`` variable providing the final list of packages to
+install, the root file system is created.
+Package installation is under control of the package manager (e.g.
+dnf/rpm, opkg, or apt/dpkg) regardless of whether or not package
+management is enabled for the target. At the end of the process, if
+package management is not enabled for the target, the package manager's
+data files are deleted from the root filesystem. As part of the final
+stage of package installation, post installation scripts that are part
+of the packages are run. Any scripts that fail to run on the build host
+are run on the target when the target system is first booted. If you are
+using a `read-only root
+filesystem <&YOCTO_DOCS_DEV_URL;#creating-a-read-only-root-filesystem>`__,
+all the post installation scripts must succeed on the build host during
+the package installation phase since the root filesystem on the target
+is read-only.
+The final stages of the ``do_rootfs`` task handle post processing. Post
+processing includes creation of a manifest file and optimizations.
+The manifest file (``.manifest``) resides in the same directory as the
+root filesystem image. This file lists out, line-by-line, the installed
+packages. The manifest file is useful for the
+```testimage`` <&YOCTO_DOCS_REF_URL;#ref-classes-testimage*>`__ class,
+for example, to determine whether or not to run specific tests. See the
+```IMAGE_MANIFEST`` <&YOCTO_DOCS_REF_URL;#var-IMAGE_MANIFEST>`__
+variable for additional information.
+Optimizing processes that are run across the image include ``mklibs``,
+``prelink``, and any other post-processing commands as defined by the
+```ROOTFS_POSTPROCESS_COMMAND`` <&YOCTO_DOCS_REF_URL;#var-ROOTFS_POSTPROCESS_COMMAND>`__
+variable. The ``mklibs`` process optimizes the size of the libraries,
+while the ``prelink`` process optimizes the dynamic linking of shared
+libraries to reduce start up time of executables.
+After the root filesystem is built, processing begins on the image
+through the ```do_image`` <&YOCTO_DOCS_REF_URL;#ref-tasks-image>`__
+task. The build system runs any pre-processing commands as defined by
+the
+```IMAGE_PREPROCESS_COMMAND`` <&YOCTO_DOCS_REF_URL;#var-IMAGE_PREPROCESS_COMMAND>`__
+variable. This variable specifies a list of functions to call before the
+build system creates the final image output files.
+The build system dynamically creates ``do_image_*`` tasks as needed,
+based on the image types specified in the
+```IMAGE_FSTYPES`` <&YOCTO_DOCS_REF_URL;#var-IMAGE_FSTYPES>`__ variable.
+The process turns everything into an image file or a set of image files
+and can compress the root filesystem image to reduce the overall size of
+the image. The formats used for the root filesystem depend on the
+``IMAGE_FSTYPES`` variable. Compression depends on whether the formats
+support compression.
+As an example, a dynamically created task when creating a particular
+image type would take the following form: do_image_type So, if the type
+as specified by the ``IMAGE_FSTYPES`` were ``ext4``, the dynamically
+generated task would be as follows: do_image_ext4
+The final task involved in image creation is the
+```do_image_complete`` <&YOCTO_DOCS_REF_URL;#ref-tasks-image-complete>`__
+task. This task completes the image by applying any image post
+processing as defined through the
+```IMAGE_POSTPROCESS_COMMAND`` <&YOCTO_DOCS_REF_URL;#var-IMAGE_POSTPROCESS_COMMAND>`__
+variable. The variable specifies a list of functions to call once the
+build system has created the final image output files.
+.. note::
+   The entire image generation process is run under
+   Pseudo
+   . Running under Pseudo ensures that the files in the root filesystem
+   have correct ownership.
+.. _sdk-generation-dev-environment:
+SDK Generation
+~~~~~~~~~~~~~~
+The OpenEmbedded build system uses BitBake to generate the Software
+Development Kit (SDK) installer scripts for both the standard SDK and
+the extensible SDK (eSDK):
+.. note::
+   For more information on the cross-development toolchain generation,
+   see the "
+   Cross-Development Toolchain Generation
+   " section. For information on advantages gained when building a
+   cross-development toolchain using the
+   do_populate_sdk
+   task, see the "
+   Building an SDK Installer
+   " section in the Yocto Project Application Development and the
+   Extensible Software Development Kit (eSDK) manual.
+Like image generation, the SDK script process consists of several stages
+and depends on many variables. The
+```do_populate_sdk`` <&YOCTO_DOCS_REF_URL;#ref-tasks-populate_sdk>`__
+and
+```do_populate_sdk_ext`` <&YOCTO_DOCS_REF_URL;#ref-tasks-populate_sdk_ext>`__
+tasks use these key variables to help create the list of packages to
+actually install. For information on the variables listed in the figure,
+see the "`Application Development SDK <#sdk-dev-environment>`__"
+section.
+The ``do_populate_sdk`` task helps create the standard SDK and handles
+two parts: a target part and a host part. The target part is the part
+built for the target hardware and includes libraries and headers. The
+host part is the part of the SDK that runs on the
+```SDKMACHINE`` <&YOCTO_DOCS_REF_URL;#var-SDKMACHINE>`__.
+The ``do_populate_sdk_ext`` task helps create the extensible SDK and
+handles host and target parts differently than its counter part does for
+the standard SDK. For the extensible SDK, the task encapsulates the
+build system, which includes everything needed (host and target) for the
+SDK.
+Regardless of the type of SDK being constructed, the tasks perform some
+cleanup after which a cross-development environment setup script and any
+needed configuration files are created. The final output is the
+Cross-development toolchain installation script (``.sh`` file), which
+includes the environment setup script.
+Stamp Files and the Rerunning of Tasks
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+For each task that completes successfully, BitBake writes a stamp file
+into the ```STAMPS_DIR`` <&YOCTO_DOCS_REF_URL;#var-STAMPS_DIR>`__
+directory. The beginning of the stamp file's filename is determined by
+the ```STAMP`` <&YOCTO_DOCS_REF_URL;#var-STAMP>`__ variable, and the end
+of the name consists of the task's name and current `input
+checksum <#overview-checksums>`__.
+.. note::
+   This naming scheme assumes that
+   BB_SIGNATURE_HANDLER
+   is "OEBasicHash", which is almost always the case in current
+   OpenEmbedded.
+To determine if a task needs to be rerun, BitBake checks if a stamp file
+with a matching input checksum exists for the task. If such a stamp file
+exists, the task's output is assumed to exist and still be valid. If the
+file does not exist, the task is rerun.
+.. note::
+   The stamp mechanism is more general than the shared state (sstate)
+   cache mechanism described in the "`Setscene Tasks and Shared
+   State <#setscene-tasks-and-shared-state>`__" section. BitBake avoids
+   rerunning any task that has a valid stamp file, not just tasks that
+   can be accelerated through the sstate cache.
+   However, you should realize that stamp files only serve as a marker
+   that some work has been done and that these files do not record task
+   output. The actual task output would usually be somewhere in
+   ```TMPDIR`` <&YOCTO_DOCS_REF_URL;#var-TMPDIR>`__ (e.g. in some
+   recipe's ```WORKDIR`` <&YOCTO_DOCS_REF_URL;#var-WORKDIR>`__.) What
+   the sstate cache mechanism adds is a way to cache task output that
+   can then be shared between build machines.
+Since ``STAMPS_DIR`` is usually a subdirectory of ``TMPDIR``, removing
+``TMPDIR`` will also remove ``STAMPS_DIR``, which means tasks will
+properly be rerun to repopulate ``TMPDIR``.
+If you want some task to always be considered "out of date", you can
+mark it with the ```nostamp`` <&YOCTO_DOCS_BB_URL;#variable-flags>`__
+varflag. If some other task depends on such a task, then that task will
+also always be considered out of date, which might not be what you want.
+For details on how to view information about a task's signature, see the
+"`Viewing Task Variable
+Dependencies <&YOCTO_DOCS_DEV_URL;#dev-viewing-task-variable-dependencies>`__"
+section in the Yocto Project Development Tasks Manual.
+Setscene Tasks and Shared State
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The description of tasks so far assumes that BitBake needs to build
+everything and no available prebuilt objects exist. BitBake does support
+skipping tasks if prebuilt objects are available. These objects are
+usually made available in the form of a shared state (sstate) cache.
+.. note::
+   For information on variables affecting sstate, see the
+   SSTATE_DIR
+   and
+   SSTATE_MIRRORS
+   variables.
+The idea of a setscene task (i.e ``do_``\ taskname\ ``_setscene``) is a
+version of the task where instead of building something, BitBake can
+skip to the end result and simply place a set of files into specific
+locations as needed. In some cases, it makes sense to have a setscene
+task variant (e.g. generating package files in the
+```do_package_write_*`` <&YOCTO_DOCS_REF_URL;#ref-tasks-package_write_deb>`__
+task). In other cases, it does not make sense (e.g. a
+```do_patch`` <&YOCTO_DOCS_REF_URL;#ref-tasks-patch>`__ task or a
+```do_unpack`` <&YOCTO_DOCS_REF_URL;#ref-tasks-unpack>`__ task) since
+the work involved would be equal to or greater than the underlying task.
+In the build system, the common tasks that have setscene variants are
+```do_package`` <&YOCTO_DOCS_REF_URL;#ref-tasks-package>`__,
+``do_package_write_*``,
+```do_deploy`` <&YOCTO_DOCS_REF_URL;#ref-tasks-deploy>`__,
+```do_packagedata`` <&YOCTO_DOCS_REF_URL;#ref-tasks-packagedata>`__, and
+```do_populate_sysroot`` <&YOCTO_DOCS_REF_URL;#ref-tasks-populate_sysroot>`__.
+Notice that these tasks represent most of the tasks whose output is an
+end result.
+The build system has knowledge of the relationship between these tasks
+and other preceding tasks. For example, if BitBake runs
+``do_populate_sysroot_setscene`` for something, it does not make sense
+to run any of the ``do_fetch``, ``do_unpack``, ``do_patch``,
+``do_configure``, ``do_compile``, and ``do_install`` tasks. However, if
+``do_package`` needs to be run, BitBake needs to run those other tasks.
+It becomes more complicated if everything can come from an sstate cache
+because some objects are simply not required at all. For example, you do
+not need a compiler or native tools, such as quilt, if nothing exists to
+compile or patch. If the ``do_package_write_*`` packages are available
+from sstate, BitBake does not need the ``do_package`` task data.
+To handle all these complexities, BitBake runs in two phases. The first
+is the "setscene" stage. During this stage, BitBake first checks the
+sstate cache for any targets it is planning to build. BitBake does a
+fast check to see if the object exists rather than a complete download.
+If nothing exists, the second phase, which is the setscene stage,
+completes and the main build proceeds.
+If objects are found in the sstate cache, the build system works
+backwards from the end targets specified by the user. For example, if an
+image is being built, the build system first looks for the packages
+needed for that image and the tools needed to construct an image. If
+those are available, the compiler is not needed. Thus, the compiler is
+not even downloaded. If something was found to be unavailable, or the
+download or setscene task fails, the build system then tries to install
+dependencies, such as the compiler, from the cache.
+The availability of objects in the sstate cache is handled by the
+function specified by the
+```BB_HASHCHECK_FUNCTION`` <&YOCTO_DOCS_BB_URL;#var-BB_HASHCHECK_FUNCTION>`__
+variable and returns a list of available objects. The function specified
+by the
+```BB_SETSCENE_DEPVALID`` <&YOCTO_DOCS_BB_URL;#var-BB_SETSCENE_DEPVALID>`__
+variable is the function that determines whether a given dependency
+needs to be followed, and whether for any given relationship the
+function needs to be passed. The function returns a True or False value.
+.. _images-dev-environment:
+Images
+------
+The images produced by the build system are compressed forms of the root
+filesystem and are ready to boot on a target device. You can see from
+the `general workflow figure <#general-workflow-figure>`__ that BitBake
+output, in part, consists of images. This section takes a closer look at
+this output:
+.. note::
+   For a list of example images that the Yocto Project provides, see the
+   "
+   Images
+   " chapter in the Yocto Project Reference Manual.
+The build process writes images out to the `Build
+Directory <&YOCTO_DOCS_REF_URL;#build-directory>`__ inside the
+``tmp/deploy/images/machine/`` folder as shown in the figure. This
+folder contains any files expected to be loaded on the target device.
+The ```DEPLOY_DIR`` <&YOCTO_DOCS_REF_URL;#var-DEPLOY_DIR>`__ variable
+points to the ``deploy`` directory, while the
+```DEPLOY_DIR_IMAGE`` <&YOCTO_DOCS_REF_URL;#var-DEPLOY_DIR_IMAGE>`__
+variable points to the appropriate directory containing images for the
+current configuration.
+-  kernel-image: A kernel binary file. The
+   ```KERNEL_IMAGETYPE`` <&YOCTO_DOCS_REF_URL;#var-KERNEL_IMAGETYPE>`__
+   variable determines the naming scheme for the kernel image file.
+   Depending on this variable, the file could begin with a variety of
+   naming strings. The ``deploy/images/``\ machine directory can contain
+   multiple image files for the machine.
+-  root-filesystem-image: Root filesystems for the target device (e.g.
+   ``*.ext3`` or ``*.bz2`` files). The
+   ```IMAGE_FSTYPES`` <&YOCTO_DOCS_REF_URL;#var-IMAGE_FSTYPES>`__
+   variable determines the root filesystem image type. The
+   ``deploy/images/``\ machine directory can contain multiple root
+   filesystems for the machine.
+-  kernel-modules: Tarballs that contain all the modules built for the
+   kernel. Kernel module tarballs exist for legacy purposes and can be
+   suppressed by setting the
+   ```MODULE_TARBALL_DEPLOY`` <&YOCTO_DOCS_REF_URL;#var-MODULE_TARBALL_DEPLOY>`__
+   variable to "0". The ``deploy/images/``\ machine directory can
+   contain multiple kernel module tarballs for the machine.
+-  bootloaders: If applicable to the target machine, bootloaders
+   supporting the image. The ``deploy/images/``\ machine directory can
+   contain multiple bootloaders for the machine.
+-  symlinks: The ``deploy/images/``\ machine folder contains a symbolic
+   link that points to the most recently built file for each machine.
+   These links might be useful for external scripts that need to obtain
+   the latest version of each file.
+.. _sdk-dev-environment:
+Application Development SDK
+---------------------------
+In the `general workflow figure <#general-workflow-figure>`__, the
+output labeled "Application Development SDK" represents an SDK. The SDK
+generation process differs depending on whether you build an extensible
+SDK (e.g. ``bitbake -c populate_sdk_ext`` imagename) or a standard SDK
+(e.g. ``bitbake -c populate_sdk`` imagename). This section takes a
+closer look at this output:
+The specific form of this output is a set of files that includes a
+self-extracting SDK installer (``*.sh``), host and target manifest
+files, and files used for SDK testing. When the SDK installer file is
+run, it installs the SDK. The SDK consists of a cross-development
+toolchain, a set of libraries and headers, and an SDK environment setup
+script. Running this installer essentially sets up your
+cross-development environment. You can think of the cross-toolchain as
+the "host" part because it runs on the SDK machine. You can think of the
+libraries and headers as the "target" part because they are built for
+the target hardware. The environment setup script is added so that you
+can initialize the environment before using the tools.
+.. note::
+   -  The Yocto Project supports several methods by which you can set up
+      this cross-development environment. These methods include
+      downloading pre-built SDK installers or building and installing
+      your own SDK installer.
+   -  For background information on cross-development toolchains in the
+      Yocto Project development environment, see the "`Cross-Development
+      Toolchain Generation <#cross-development-toolchain-generation>`__"
+      section.
+   -  For information on setting up a cross-development environment, see
+      the `Yocto Project Application Development and the Extensible
+      Software Development Kit (eSDK) <&YOCTO_DOCS_SDK_URL;>`__ manual.
+All the output files for an SDK are written to the ``deploy/sdk`` folder
+inside the `Build Directory <&YOCTO_DOCS_REF_URL;#build-directory>`__ as
+shown in the previous figure. Depending on the type of SDK, several
+variables exist that help configure these files. The following list
+shows the variables associated with an extensible SDK:
+-  ```DEPLOY_DIR`` <&YOCTO_DOCS_REF_URL;#var-DEPLOY_DIR>`__: Points to
+   the ``deploy`` directory.
+-  ```SDK_EXT_TYPE`` <&YOCTO_DOCS_REF_URL;#var-SDK_EXT_TYPE>`__:
+   Controls whether or not shared state artifacts are copied into the
+   extensible SDK. By default, all required shared state artifacts are
+   copied into the SDK.
+-  ```SDK_INCLUDE_PKGDATA`` <&YOCTO_DOCS_REF_URL;#var-SDK_INCLUDE_PKGDATA>`__:
+   Specifies whether or not packagedata is included in the extensible
+   SDK for all recipes in the "world" target.
+-  ```SDK_INCLUDE_TOOLCHAIN`` <&YOCTO_DOCS_REF_URL;#var-SDK_INCLUDE_TOOLCHAIN>`__:
+   Specifies whether or not the toolchain is included when building the
+   extensible SDK.
+-  ```SDK_LOCAL_CONF_WHITELIST`` <&YOCTO_DOCS_REF_URL;#var-SDK_LOCAL_CONF_WHITELIST>`__:
+   A list of variables allowed through from the build system
+   configuration into the extensible SDK configuration.
+-  ```SDK_LOCAL_CONF_BLACKLIST`` <&YOCTO_DOCS_REF_URL;#var-SDK_LOCAL_CONF_BLACKLIST>`__:
+   A list of variables not allowed through from the build system
+   configuration into the extensible SDK configuration.
+-  ```SDK_INHERIT_BLACKLIST`` <&YOCTO_DOCS_REF_URL;#var-SDK_INHERIT_BLACKLIST>`__:
+   A list of classes to remove from the
+   ```INHERIT`` <&YOCTO_DOCS_REF_URL;#var-INHERIT>`__ value globally
+   within the extensible SDK configuration.
+This next list, shows the variables associated with a standard SDK:
+-  ```DEPLOY_DIR`` <&YOCTO_DOCS_REF_URL;#var-DEPLOY_DIR>`__: Points to
+   the ``deploy`` directory.
+-  ```SDKMACHINE`` <&YOCTO_DOCS_REF_URL;#var-SDKMACHINE>`__: Specifies
+   the architecture of the machine on which the cross-development tools
+   are run to create packages for the target hardware.
+-  ```SDKIMAGE_FEATURES`` <&YOCTO_DOCS_REF_URL;#var-SDKIMAGE_FEATURES>`__:
+   Lists the features to include in the "target" part of the SDK.
+-  ```TOOLCHAIN_HOST_TASK`` <&YOCTO_DOCS_REF_URL;#var-TOOLCHAIN_HOST_TASK>`__:
+   Lists packages that make up the host part of the SDK (i.e. the part
+   that runs on the ``SDKMACHINE``). When you use
+   ``bitbake -c populate_sdk imagename`` to create the SDK, a set of
+   default packages apply. This variable allows you to add more
+   packages.
+-  ```TOOLCHAIN_TARGET_TASK`` <&YOCTO_DOCS_REF_URL;#var-TOOLCHAIN_TARGET_TASK>`__:
+   Lists packages that make up the target part of the SDK (i.e. the part
+   built for the target hardware).
+-  ```SDKPATH`` <&YOCTO_DOCS_REF_URL;#var-SDKPATH>`__: Defines the
+   default SDK installation path offered by the installation script.
+-  ```SDK_HOST_MANIFEST`` <&YOCTO_DOCS_REF_URL;#var-SDK_HOST_MANIFEST>`__:
+   Lists all the installed packages that make up the host part of the
+   SDK. This variable also plays a minor role for extensible SDK
+   development as well. However, it is mainly used for the standard SDK.
+-  ```SDK_TARGET_MANIFEST`` <&YOCTO_DOCS_REF_URL;#var-SDK_TARGET_MANIFEST>`__:
+   Lists all the installed packages that make up the target part of the
+   SDK. This variable also plays a minor role for extensible SDK
+   development as well. However, it is mainly used for the standard SDK.
+Cross-Development Toolchain Generation
+======================================
+The Yocto Project does most of the work for you when it comes to
+creating `cross-development
+toolchains <&YOCTO_DOCS_REF_URL;#cross-development-toolchain>`__. This
+section provides some technical background on how cross-development
+toolchains are created and used. For more information on toolchains, you
+can also see the `Yocto Project Application Development and the
+Extensible Software Development Kit (eSDK) <&YOCTO_DOCS_SDK_URL;>`__
+manual.
+In the Yocto Project development environment, cross-development
+toolchains are used to build images and applications that run on the
+target hardware. With just a few commands, the OpenEmbedded build system
+creates these necessary toolchains for you.
+The following figure shows a high-level build environment regarding
+toolchain construction and use.
+Most of the work occurs on the Build Host. This is the machine used to
+build images and generally work within the the Yocto Project
+environment. When you run
+`BitBake <&YOCTO_DOCS_REF_URL;#bitbake-term>`__ to create an image, the
+OpenEmbedded build system uses the host ``gcc`` compiler to bootstrap a
+cross-compiler named ``gcc-cross``. The ``gcc-cross`` compiler is what
+BitBake uses to compile source files when creating the target image. You
+can think of ``gcc-cross`` simply as an automatically generated
+cross-compiler that is used internally within BitBake only.
+.. note::
+   The extensible SDK does not use
+   gcc-cross-canadian
+   since this SDK ships a copy of the OpenEmbedded build system and the
+   sysroot within it contains
+   gcc-cross
+   .
+The chain of events that occurs when ``gcc-cross`` is bootstrapped is as
+follows: gcc -> binutils-cross -> gcc-cross-initial ->
+linux-libc-headers -> glibc-initial -> glibc -> gcc-cross -> gcc-runtime
+-  ``gcc``: The build host's GNU Compiler Collection (GCC).
+-  ``binutils-cross``: The bare minimum binary utilities needed in order
+   to run the ``gcc-cross-initial`` phase of the bootstrap operation.
+-  ``gcc-cross-initial``: An early stage of the bootstrap process for
+   creating the cross-compiler. This stage builds enough of the
+   ``gcc-cross``, the C library, and other pieces needed to finish
+   building the final cross-compiler in later stages. This tool is a
+   "native" package (i.e. it is designed to run on the build host).
+-  ``linux-libc-headers``: Headers needed for the cross-compiler.
+-  ``glibc-initial``: An initial version of the Embedded GNU C Library
+   (GLIBC) needed to bootstrap ``glibc``.
+-  ``glibc``: The GNU C Library.
+-  ``gcc-cross``: The final stage of the bootstrap process for the
+   cross-compiler. This stage results in the actual cross-compiler that
+   BitBake uses when it builds an image for a targeted device.
+   .. note::
+      If you are replacing this cross compiler toolchain with a custom
+      version, you must replace
+      gcc-cross
+      .
+   This tool is also a "native" package (i.e. it is designed to run on
+   the build host).
+-  ``gcc-runtime``: Runtime libraries resulting from the toolchain
+   bootstrapping process. This tool produces a binary that consists of
+   the runtime libraries need for the targeted device.
+You can use the OpenEmbedded build system to build an installer for the
+relocatable SDK used to develop applications. When you run the
+installer, it installs the toolchain, which contains the development
+tools (e.g., ``gcc-cross-canadian``, ``binutils-cross-canadian``, and
+other ``nativesdk-*`` tools), which are tools native to the SDK (i.e.
+native to ```SDK_ARCH`` <&YOCTO_DOCS_REF_URL;#var-SDK_ARCH>`__), you
+need to cross-compile and test your software. The figure shows the
+commands you use to easily build out this toolchain. This
+cross-development toolchain is built to execute on the
+```SDKMACHINE`` <&YOCTO_DOCS_REF_URL;#var-SDKMACHINE>`__, which might or
+might not be the same machine as the Build Host.
+.. note::
+   If your target architecture is supported by the Yocto Project, you
+   can take advantage of pre-built images that ship with the Yocto
+   Project and already contain cross-development toolchain installers.
+Here is the bootstrap process for the relocatable toolchain: gcc ->
+binutils-crosssdk -> gcc-crosssdk-initial -> linux-libc-headers ->
+glibc-initial -> nativesdk-glibc -> gcc-crosssdk -> gcc-cross-canadian
+-  ``gcc``: The build host's GNU Compiler Collection (GCC).
+-  ``binutils-crosssdk``: The bare minimum binary utilities needed in
+   order to run the ``gcc-crosssdk-initial`` phase of the bootstrap
+   operation.
+-  ``gcc-crosssdk-initial``: An early stage of the bootstrap process for
+   creating the cross-compiler. This stage builds enough of the
+   ``gcc-crosssdk`` and supporting pieces so that the final stage of the
+   bootstrap process can produce the finished cross-compiler. This tool
+   is a "native" binary that runs on the build host.
+-  ``linux-libc-headers``: Headers needed for the cross-compiler.
+-  ``glibc-initial``: An initial version of the Embedded GLIBC needed to
+   bootstrap ``nativesdk-glibc``.
+-  ``nativesdk-glibc``: The Embedded GLIBC needed to bootstrap the
+   ``gcc-crosssdk``.
+-  ``gcc-crosssdk``: The final stage of the bootstrap process for the
+   relocatable cross-compiler. The ``gcc-crosssdk`` is a transitory
+   compiler and never leaves the build host. Its purpose is to help in
+   the bootstrap process to create the eventual ``gcc-cross-canadian``
+   compiler, which is relocatable. This tool is also a "native" package
+   (i.e. it is designed to run on the build host).
+-  ``gcc-cross-canadian``: The final relocatable cross-compiler. When
+   run on the ```SDKMACHINE`` <&YOCTO_DOCS_REF_URL;#var-SDKMACHINE>`__,
+   this tool produces executable code that runs on the target device.
+   Only one cross-canadian compiler is produced per architecture since
+   they can be targeted at different processor optimizations using
+   configurations passed to the compiler through the compile commands.
+   This circumvents the need for multiple compilers and thus reduces the
+   size of the toolchains.
+.. note::
+   For information on advantages gained when building a
+   cross-development toolchain installer, see the "
+   Building an SDK Installer
+   " appendix in the Yocto Project Application Development and the
+   Extensible Software Development Kit (eSDK) manual.
+Shared State Cache
+==================
+By design, the OpenEmbedded build system builds everything from scratch
+unless `BitBake <&YOCTO_DOCS_REF_URL;#bitbake-term>`__ can determine
+that parts do not need to be rebuilt. Fundamentally, building from
+scratch is attractive as it means all parts are built fresh and no
+possibility of stale data exists that can cause problems. When
+developers hit problems, they typically default back to building from
+scratch so they have a know state from the start.
+Building an image from scratch is both an advantage and a disadvantage
+to the process. As mentioned in the previous paragraph, building from
+scratch ensures that everything is current and starts from a known
+state. However, building from scratch also takes much longer as it
+generally means rebuilding things that do not necessarily need to be
+rebuilt.
+The Yocto Project implements shared state code that supports incremental
+builds. The implementation of the shared state code answers the
+following questions that were fundamental roadblocks within the
+OpenEmbedded incremental build support system:
+-  What pieces of the system have changed and what pieces have not
+   changed?
+-  How are changed pieces of software removed and replaced?
+-  How are pre-built components that do not need to be rebuilt from
+   scratch used when they are available?
+For the first question, the build system detects changes in the "inputs"
+to a given task by creating a checksum (or signature) of the task's
+inputs. If the checksum changes, the system assumes the inputs have
+changed and the task needs to be rerun. For the second question, the
+shared state (sstate) code tracks which tasks add which output to the
+build process. This means the output from a given task can be removed,
+upgraded or otherwise manipulated. The third question is partly
+addressed by the solution for the second question assuming the build
+system can fetch the sstate objects from remote locations and install
+them if they are deemed to be valid.
+.. note::
+   -  The build system does not maintain
+      ```PR`` <&YOCTO_DOCS_REF_URL;#var-PR>`__ information as part of
+      the shared state packages. Consequently, considerations exist that
+      affect maintaining shared state feeds. For information on how the
+      build system works with packages and can track incrementing ``PR``
+      information, see the "`Automatically Incrementing a Binary Package
+      Revision
+      Number <&YOCTO_DOCS_DEV_URL;#automatically-incrementing-a-binary-package-revision-number>`__"
+      section in the Yocto Project Development Tasks Manual.
+   -  The code in the build system that supports incremental builds is
+      not simple code. For techniques that help you work around issues
+      related to shared state code, see the "`Viewing Metadata Used to
+      Create the Input Signature of a Shared State
+      Task <&YOCTO_DOCS_DEV_URL;#dev-viewing-metadata-used-to-create-the-input-signature-of-a-shared-state-task>`__"
+      and "`Invalidating Shared State to Force a Task to
+      Run <&YOCTO_DOCS_DEV_URL;#dev-invalidating-shared-state-to-force-a-task-to-run>`__"
+      sections both in the Yocto Project Development Tasks Manual.
+The rest of this section goes into detail about the overall incremental
+build architecture, the checksums (signatures), and shared state.
+.. _concepts-overall-architecture:
+Overall Architecture
+--------------------
+When determining what parts of the system need to be built, BitBake
+works on a per-task basis rather than a per-recipe basis. You might
+wonder why using a per-task basis is preferred over a per-recipe basis.
+To help explain, consider having the IPK packaging backend enabled and
+then switching to DEB. In this case, the
+```do_install`` <&YOCTO_DOCS_REF_URL;#ref-tasks-install>`__ and
+```do_package`` <&YOCTO_DOCS_REF_URL;#ref-tasks-package>`__ task outputs
+are still valid. However, with a per-recipe approach, the build would
+not include the ``.deb`` files. Consequently, you would have to
+invalidate the whole build and rerun it. Rerunning everything is not the
+best solution. Also, in this case, the core must be "taught" much about
+specific tasks. This methodology does not scale well and does not allow
+users to easily add new tasks in layers or as external recipes without
+touching the packaged-staging core.
+.. _overview-checksums:
+Checksums (Signatures)
+----------------------
+The shared state code uses a checksum, which is a unique signature of a
+task's inputs, to determine if a task needs to be run again. Because it
+is a change in a task's inputs that triggers a rerun, the process needs
+to detect all the inputs to a given task. For shell tasks, this turns
+out to be fairly easy because the build process generates a "run" shell
+script for each task and it is possible to create a checksum that gives
+you a good idea of when the task's data changes.
+To complicate the problem, there are things that should not be included
+in the checksum. First, there is the actual specific build path of a
+given task - the ```WORKDIR`` <&YOCTO_DOCS_REF_URL;#var-WORKDIR>`__. It
+does not matter if the work directory changes because it should not
+affect the output for target packages. Also, the build process has the
+objective of making native or cross packages relocatable.
+.. note::
+   Both native and cross packages run on the
+   build host
+   . However, cross packages generate output for the target
+   architecture.
+The checksum therefore needs to exclude ``WORKDIR``. The simplistic
+approach for excluding the work directory is to set ``WORKDIR`` to some
+fixed value and create the checksum for the "run" script.
+Another problem results from the "run" scripts containing functions that
+might or might not get called. The incremental build solution contains
+code that figures out dependencies between shell functions. This code is
+used to prune the "run" scripts down to the minimum set, thereby
+alleviating this problem and making the "run" scripts much more readable
+as a bonus.
+So far, solutions for shell scripts exist. What about Python tasks? The
+same approach applies even though these tasks are more difficult. The
+process needs to figure out what variables a Python function accesses
+and what functions it calls. Again, the incremental build solution
+contains code that first figures out the variable and function
+dependencies, and then creates a checksum for the data used as the input
+to the task.
+Like the ``WORKDIR`` case, situations exist where dependencies should be
+ignored. For these situations, you can instruct the build process to
+ignore a dependency by using a line like the following:
+PACKAGE_ARCHS[vardepsexclude] = "MACHINE" This example ensures that the
+```PACKAGE_ARCHS`` <&YOCTO_DOCS_REF_URL;#var-PACKAGE_ARCHS>`__ variable
+does not depend on the value of
+```MACHINE`` <&YOCTO_DOCS_REF_URL;#var-MACHINE>`__, even if it does
+reference it.
+Equally, there are cases where you need to add dependencies BitBake is
+not able to find. You can accomplish this by using a line like the
+following: PACKAGE_ARCHS[vardeps] = "MACHINE" This example explicitly
+adds the ``MACHINE`` variable as a dependency for ``PACKAGE_ARCHS``.
+As an example, consider a case with in-line Python where BitBake is not
+able to figure out dependencies. When running in debug mode (i.e. using
+``-DDD``), BitBake produces output when it discovers something for which
+it cannot figure out dependencies. The Yocto Project team has currently
+not managed to cover those dependencies in detail and is aware of the
+need to fix this situation.
+Thus far, this section has limited discussion to the direct inputs into
+a task. Information based on direct inputs is referred to as the
+"basehash" in the code. However, the question of a task's indirect
+inputs still exits - items already built and present in the `Build
+Directory <&YOCTO_DOCS_REF_URL;#build-directory>`__. The checksum (or
+signature) for a particular task needs to add the hashes of all the
+tasks on which the particular task depends. Choosing which dependencies
+to add is a policy decision. However, the effect is to generate a master
+checksum that combines the basehash and the hashes of the task's
+dependencies.
+At the code level, a variety of ways exist by which both the basehash
+and the dependent task hashes can be influenced. Within the BitBake
+configuration file, you can give BitBake some extra information to help
+it construct the basehash. The following statement effectively results
+in a list of global variable dependency excludes (i.e. variables never
+included in any checksum): BB_HASHBASE_WHITELIST ?= "TMPDIR FILE PATH
+PWD BB_TASKHASH BBPATH DL_DIR \\ SSTATE_DIR THISDIR FILESEXTRAPATHS
+FILE_DIRNAME HOME LOGNAME SHELL TERM \\ USER FILESPATH STAGING_DIR_HOST
+STAGING_DIR_TARGET COREBASE PRSERV_HOST \\ PRSERV_DUMPDIR
+PRSERV_DUMPFILE PRSERV_LOCKDOWN PARALLEL_MAKE \\ CCACHE_DIR
+EXTERNAL_TOOLCHAIN CCACHE CCACHE_DISABLE LICENSE_PATH SDKPKGSUFFIX" The
+previous example excludes
+```WORKDIR`` <&YOCTO_DOCS_REF_URL;#var-WORKDIR>`__ since that variable
+is actually constructed as a path within
+```TMPDIR`` <&YOCTO_DOCS_REF_URL;#var-TMPDIR>`__, which is on the
+whitelist.
+The rules for deciding which hashes of dependent tasks to include
+through dependency chains are more complex and are generally
+accomplished with a Python function. The code in
+``meta/lib/oe/sstatesig.py`` shows two examples of this and also
+illustrates how you can insert your own policy into the system if so
+desired. This file defines the two basic signature generators
+`OE-Core <&YOCTO_DOCS_REF_URL;#oe-core>`__ uses: "OEBasic" and
+"OEBasicHash". By default, a dummy "noop" signature handler is enabled
+in BitBake. This means that behavior is unchanged from previous
+versions. OE-Core uses the "OEBasicHash" signature handler by default
+through this setting in the ``bitbake.conf`` file: BB_SIGNATURE_HANDLER
+?= "OEBasicHash" The "OEBasicHash" ``BB_SIGNATURE_HANDLER`` is the same
+as the "OEBasic" version but adds the task hash to the `stamp
+files <#stamp-files-and-the-rerunning-of-tasks>`__. This results in any
+metadata change that changes the task hash, automatically causing the
+task to be run again. This removes the need to bump
+```PR`` <&YOCTO_DOCS_REF_URL;#var-PR>`__ values, and changes to metadata
+automatically ripple across the build.
+It is also worth noting that the end result of these signature
+generators is to make some dependency and hash information available to
+the build. This information includes:
+-  ``BB_BASEHASH_task-``\ taskname: The base hashes for each task in the
+   recipe.
+-  ``BB_BASEHASH_``\ filename\ ``:``\ taskname: The base hashes for each
+   dependent task.
+-  ``BBHASHDEPS_``\ filename\ ``:``\ taskname: The task dependencies for
+   each task.
+-  ``BB_TASKHASH``: The hash of the currently running task.
+Shared State
+------------
+Checksums and dependencies, as discussed in the previous section, solve
+half the problem of supporting a shared state. The other half of the
+problem is being able to use checksum information during the build and
+being able to reuse or rebuild specific components.
+The ```sstate`` <&YOCTO_DOCS_REF_URL;#ref-classes-sstate>`__ class is a
+relatively generic implementation of how to "capture" a snapshot of a
+given task. The idea is that the build process does not care about the
+source of a task's output. Output could be freshly built or it could be
+downloaded and unpacked from somewhere. In other words, the build
+process does not need to worry about its origin.
+Two types of output exist. One type is just about creating a directory
+in ```WORKDIR`` <&YOCTO_DOCS_REF_URL;#var-WORKDIR>`__. A good example is
+the output of either
+```do_install`` <&YOCTO_DOCS_REF_URL;#ref-tasks-install>`__ or
+```do_package`` <&YOCTO_DOCS_REF_URL;#ref-tasks-package>`__. The other
+type of output occurs when a set of data is merged into a shared
+directory tree such as the sysroot.
+The Yocto Project team has tried to keep the details of the
+implementation hidden in ``sstate`` class. From a user's perspective,
+adding shared state wrapping to a task is as simple as this
+```do_deploy`` <&YOCTO_DOCS_REF_URL;#ref-tasks-deploy>`__ example taken
+from the ```deploy`` <&YOCTO_DOCS_REF_URL;#ref-classes-deploy>`__ class:
+DEPLOYDIR = "${WORKDIR}/deploy-${PN}" SSTATETASKS += "do_deploy"
+do_deploy[sstate-inputdirs] = "${DEPLOYDIR}"
+do_deploy[sstate-outputdirs] = "${DEPLOY_DIR_IMAGE}" python
+do_deploy_setscene () { sstate_setscene(d) } addtask do_deploy_setscene
+do_deploy[dirs] = "${DEPLOYDIR} ${B}" do_deploy[stamp-extra-info] =
+"${MACHINE_ARCH}" The following list explains the previous example:
+-  Adding "do_deploy" to ``SSTATETASKS`` adds some required
+   sstate-related processing, which is implemented in the
+   ```sstate`` <&YOCTO_DOCS_REF_URL;#ref-classes-sstate>`__ class, to
+   before and after the
+   ```do_deploy`` <&YOCTO_DOCS_REF_URL;#ref-tasks-deploy>`__ task.
+-  The ``do_deploy[sstate-inputdirs] = "${DEPLOYDIR}"`` declares that
+   ``do_deploy`` places its output in ``${DEPLOYDIR}`` when run normally
+   (i.e. when not using the sstate cache). This output becomes the input
+   to the shared state cache.
+-  The ``do_deploy[sstate-outputdirs] = "${DEPLOY_DIR_IMAGE}"`` line
+   causes the contents of the shared state cache to be copied to
+   ``${DEPLOY_DIR_IMAGE}``.
+   .. note::
+      If
+      do_deploy
+      is not already in the shared state cache or if its input checksum
+      (signature) has changed from when the output was cached, the task
+      runs to populate the shared state cache, after which the contents
+      of the shared state cache is copied to
+      ${DEPLOY_DIR_IMAGE}
+      . If
+      do_deploy
+      is in the shared state cache and its signature indicates that the
+      cached output is still valid (i.e. if no relevant task inputs have
+      changed), then the contents of the shared state cache copies
+      directly to
+      ${DEPLOY_DIR_IMAGE}
+      by the
+      do_deploy_setscene
+      task instead, skipping the
+      do_deploy
+      task.
+-  The following task definition is glue logic needed to make the
+   previous settings effective: python do_deploy_setscene () {
+   sstate_setscene(d) } addtask do_deploy_setscene ``sstate_setscene()``
+   takes the flags above as input and accelerates the ``do_deploy`` task
+   through the shared state cache if possible. If the task was
+   accelerated, ``sstate_setscene()`` returns True. Otherwise, it
+   returns False, and the normal ``do_deploy`` task runs. For more
+   information, see the "`setscene <&YOCTO_DOCS_BB_URL;#setscene>`__"
+   section in the BitBake User Manual.
+-  The ``do_deploy[dirs] = "${DEPLOYDIR} ${B}"`` line creates
+   ``${DEPLOYDIR}`` and ``${B}`` before the ``do_deploy`` task runs, and
+   also sets the current working directory of ``do_deploy`` to ``${B}``.
+   For more information, see the "`Variable
+   Flags <&YOCTO_DOCS_BB_URL;#variable-flags>`__" section in the BitBake
+   User Manual.
+   .. note::
+      In cases where
+      sstate-inputdirs
+      and
+      sstate-outputdirs
+      would be the same, you can use
+      sstate-plaindirs
+      . For example, to preserve the
+      ${PKGD}
+      and
+      ${PKGDEST}
+      output from the
+      do_package
+      task, use the following:
+      ::
+              do_package[sstate-plaindirs] = "${PKGD} ${PKGDEST}"
+                                     
+-  The ``do_deploy[stamp-extra-info] = "${MACHINE_ARCH}"`` line appends
+   extra metadata to the `stamp
+   file <#stamp-files-and-the-rerunning-of-tasks>`__. In this case, the
+   metadata makes the task specific to a machine's architecture. See
+   "`The Task List <&YOCTO_DOCS_BB_URL;#ref-bitbake-tasklist>`__"
+   section in the BitBake User Manual for more information on the
+   ``stamp-extra-info`` flag.
+-  ``sstate-inputdirs`` and ``sstate-outputdirs`` can also be used with
+   multiple directories. For example, the following declares
+   ``PKGDESTWORK`` and ``SHLIBWORK`` as shared state input directories,
+   which populates the shared state cache, and ``PKGDATA_DIR`` and
+   ``SHLIBSDIR`` as the corresponding shared state output directories:
+   do_package[sstate-inputdirs] = "${PKGDESTWORK} ${SHLIBSWORKDIR}"
+   do_package[sstate-outputdirs] = "${PKGDATA_DIR} ${SHLIBSDIR}"
+-  These methods also include the ability to take a lockfile when
+   manipulating shared state directory structures, for cases where file
+   additions or removals are sensitive: do_package[sstate-lockfile] =
+   "${PACKAGELOCK}"
+Behind the scenes, the shared state code works by looking in
+```SSTATE_DIR`` <&YOCTO_DOCS_REF_URL;#var-SSTATE_DIR>`__ and
+```SSTATE_MIRRORS`` <&YOCTO_DOCS_REF_URL;#var-SSTATE_MIRRORS>`__ for
+shared state files. Here is an example: SSTATE_MIRRORS ?= "\\ file://.\*
+http://someserver.tld/share/sstate/PATH;downloadfilename=PATH \\n \\
+file://.\* file:///some/local/dir/sstate/PATH"
+.. note::
+   The shared state directory (
+   SSTATE_DIR
+   ) is organized into two-character subdirectories, where the
+   subdirectory names are based on the first two characters of the hash.
+   If the shared state directory structure for a mirror has the same
+   structure as
+   SSTATE_DIR
+   , you must specify "PATH" as part of the URI to enable the build
+   system to map to the appropriate subdirectory.
+The shared state package validity can be detected just by looking at the
+filename since the filename contains the task checksum (or signature) as
+described earlier in this section. If a valid shared state package is
+found, the build process downloads it and uses it to accelerate the
+task.
+The build processes use the ``*_setscene`` tasks for the task
+acceleration phase. BitBake goes through this phase before the main
+execution code and tries to accelerate any tasks for which it can find
+shared state packages. If a shared state package for a task is
+available, the shared state package is used. This means the task and any
+tasks on which it is dependent are not executed.
+As a real world example, the aim is when building an IPK-based image,
+only the
+```do_package_write_ipk`` <&YOCTO_DOCS_REF_URL;#ref-tasks-package_write_ipk>`__
+tasks would have their shared state packages fetched and extracted.
+Since the sysroot is not used, it would never get extracted. This is
+another reason why a task-based approach is preferred over a
+recipe-based approach, which would have to install the output from every
+task.
+Automatically Added Runtime Dependencies
+========================================
+The OpenEmbedded build system automatically adds common types of runtime
+dependencies between packages, which means that you do not need to
+explicitly declare the packages using
+```RDEPENDS`` <&YOCTO_DOCS_REF_URL;#var-RDEPENDS>`__. Three automatic
+mechanisms exist (``shlibdeps``, ``pcdeps``, and ``depchains``) that
+handle shared libraries, package configuration (pkg-config) modules, and
+``-dev`` and ``-dbg`` packages, respectively. For other types of runtime
+dependencies, you must manually declare the dependencies.
+-  ``shlibdeps``: During the
+   ```do_package`` <&YOCTO_DOCS_REF_URL;#ref-tasks-package>`__ task of
+   each recipe, all shared libraries installed by the recipe are
+   located. For each shared library, the package that contains the
+   shared library is registered as providing the shared library. More
+   specifically, the package is registered as providing the
+   `soname <https://en.wikipedia.org/wiki/Soname>`__ of the library. The
+   resulting shared-library-to-package mapping is saved globally in
+   ```PKGDATA_DIR`` <&YOCTO_DOCS_REF_URL;#var-PKGDATA_DIR>`__ by the
+   ```do_packagedata`` <&YOCTO_DOCS_REF_URL;#ref-tasks-packagedata>`__
+   task.
+   Simultaneously, all executables and shared libraries installed by the
+   recipe are inspected to see what shared libraries they link against.
+   For each shared library dependency that is found, ``PKGDATA_DIR`` is
+   queried to see if some package (likely from a different recipe)
+   contains the shared library. If such a package is found, a runtime
+   dependency is added from the package that depends on the shared
+   library to the package that contains the library.
+   The automatically added runtime dependency also includes a version
+   restriction. This version restriction specifies that at least the
+   current version of the package that provides the shared library must
+   be used, as if "package (>= version)" had been added to ``RDEPENDS``.
+   This forces an upgrade of the package containing the shared library
+   when installing the package that depends on the library, if needed.
+   If you want to avoid a package being registered as providing a
+   particular shared library (e.g. because the library is for internal
+   use only), then add the library to
+   ```PRIVATE_LIBS`` <&YOCTO_DOCS_REF_URL;#var-PRIVATE_LIBS>`__ inside
+   the package's recipe.
+-  ``pcdeps``: During the ``do_package`` task of each recipe, all
+   pkg-config modules (``*.pc`` files) installed by the recipe are
+   located. For each module, the package that contains the module is
+   registered as providing the module. The resulting module-to-package
+   mapping is saved globally in ``PKGDATA_DIR`` by the
+   ``do_packagedata`` task.
+   Simultaneously, all pkg-config modules installed by the recipe are
+   inspected to see what other pkg-config modules they depend on. A
+   module is seen as depending on another module if it contains a
+   "Requires:" line that specifies the other module. For each module
+   dependency, ``PKGDATA_DIR`` is queried to see if some package
+   contains the module. If such a package is found, a runtime dependency
+   is added from the package that depends on the module to the package
+   that contains the module.
+   .. note::
+      The
+      pcdeps
+      mechanism most often infers dependencies between
+      -dev
+      packages.
+-  ``depchains``: If a package ``foo`` depends on a package ``bar``,
+   then ``foo-dev`` and ``foo-dbg`` are also made to depend on
+   ``bar-dev`` and ``bar-dbg``, respectively. Taking the ``-dev``
+   packages as an example, the ``bar-dev`` package might provide headers
+   and shared library symlinks needed by ``foo-dev``, which shows the
+   need for a dependency between the packages.
+   The dependencies added by ``depchains`` are in the form of
+   ```RRECOMMENDS`` <&YOCTO_DOCS_REF_URL;#var-RRECOMMENDS>`__.
+   .. note::
+      By default,
+      foo-dev
+      also has an
+      RDEPENDS
+      -style dependency on
+      foo
+      , because the default value of
+      RDEPENDS_${PN}-dev
+      (set in
+      bitbake.conf
+      ) includes "${PN}".
+   To ensure that the dependency chain is never broken, ``-dev`` and
+   ``-dbg`` packages are always generated by default, even if the
+   packages turn out to be empty. See the
+   ```ALLOW_EMPTY`` <&YOCTO_DOCS_REF_URL;#var-ALLOW_EMPTY>`__ variable
+   for more information.
+The ``do_package`` task depends on the ``do_packagedata`` task of each
+recipe in ```DEPENDS`` <&YOCTO_DOCS_REF_URL;#var-DEPENDS>`__ through use
+of a ``[``\ ```deptask`` <&YOCTO_DOCS_BB_URL;#variable-flags>`__\ ``]``
+declaration, which guarantees that the required
+shared-library/module-to-package mapping information will be available
+when needed as long as ``DEPENDS`` has been correctly set.
+Fakeroot and Pseudo
+===================
+Some tasks are easier to implement when allowed to perform certain
+operations that are normally reserved for the root user (e.g.
+```do_install`` <&YOCTO_DOCS_REF_URL;#ref-tasks-install>`__,
+```do_package_write*`` <&YOCTO_DOCS_REF_URL;#ref-tasks-package_write_deb>`__,
+```do_rootfs`` <&YOCTO_DOCS_REF_URL;#ref-tasks-rootfs>`__, and
+```do_image*`` <&YOCTO_DOCS_REF_URL;#ref-tasks-image>`__). For example,
+the ``do_install`` task benefits from being able to set the UID and GID
+of installed files to arbitrary values.
+One approach to allowing tasks to perform root-only operations would be
+to require `BitBake <&YOCTO_DOCS_REF_URL;#bitbake-term>`__ to run as
+root. However, this method is cumbersome and has security issues. The
+approach that is actually used is to run tasks that benefit from root
+privileges in a "fake" root environment. Within this environment, the
+task and its child processes believe that they are running as the root
+user, and see an internally consistent view of the filesystem. As long
+as generating the final output (e.g. a package or an image) does not
+require root privileges, the fact that some earlier steps ran in a fake
+root environment does not cause problems.
+The capability to run tasks in a fake root environment is known as
+"`fakeroot <http://man.he.net/man1/fakeroot>`__", which is derived from
+the BitBake keyword/variable flag that requests a fake root environment
+for a task.
+In the `OpenEmbedded build
+system <&YOCTO_DOCS_REF_URL;#build-system-term>`__, the program that
+implements fakeroot is known as
+`Pseudo <https://www.yoctoproject.org/software-item/pseudo/>`__. Pseudo
+overrides system calls by using the environment variable ``LD_PRELOAD``,
+which results in the illusion of running as root. To keep track of
+"fake" file ownership and permissions resulting from operations that
+require root permissions, Pseudo uses an SQLite 3 database. This
+database is stored in
+``${``\ ```WORKDIR`` <&YOCTO_DOCS_REF_URL;#var-WORKDIR>`__\ ``}/pseudo/files.db``
+for individual recipes. Storing the database in a file as opposed to in
+memory gives persistence between tasks and builds, which is not
+accomplished using fakeroot.
+.. note::
+   If you add your own task that manipulates the same files or
+   directories as a fakeroot task, then that task also needs to run
+   under fakeroot. Otherwise, the task cannot run root-only operations,
+   and cannot see the fake file ownership and permissions set by the
+   other task. You need to also add a dependency on
+   virtual/fakeroot-native:do_populate_sysroot
+   , giving the following:
+   ::
+             fakeroot do_mytask () {
+                 ...
+             }
+             do_mytask[depends] += "virtual/fakeroot-native:do_populate_sysroot"
+                      
+For more information, see the
+```FAKEROOT*`` <&YOCTO_DOCS_BB_URL;#var-FAKEROOT>`__ variables in the
+BitBake User Manual. You can also reference the "`Why Not
+Fakeroot? <https://github.com/wrpseudo/pseudo/wiki/WhyNotFakeroot>`__"
+article for background information on Fakeroot and Pseudo.