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3**********************
4Yocto Project Concepts
5**********************
6
7This chapter provides explanations for Yocto Project concepts that go
8beyond the surface of "how-to" information and reference (or look-up)
9material. Concepts such as components, the :term:`OpenEmbedded Build System`
10workflow,
11cross-development toolchains, shared state cache, and so forth are
12explained.
13
14Yocto Project Components
15========================
16
17The :term:`BitBake` task executor
18together with various types of configuration files form the
19:term:`OpenEmbedded-Core (OE-Core)`. This section
20overviews these components by describing their use and how they
21interact.
22
23BitBake handles the parsing and execution of the data files. The data
24itself is of various types:
25
26- *Recipes:* Provides details about particular pieces of software.
27
28- *Class Data:* Abstracts common build information (e.g. how to build a
29 Linux kernel).
30
31- *Configuration Data:* Defines machine-specific settings, policy
32 decisions, and so forth. Configuration data acts as the glue to bind
33 everything together.
34
35BitBake knows how to combine multiple data sources together and refers
36to each data source as a layer. For information on layers, see the
Andrew Geissler517393d2023-01-13 08:55:19 -060037":ref:`dev-manual/layers:understanding and creating layers`"
Andrew Geisslerc9f78652020-09-18 14:11:35 -050038section of the Yocto Project Development Tasks Manual.
39
40Following are some brief details on these core components. For
41additional information on how these components interact during a build,
42see the
Andrew Geissler09209ee2020-12-13 08:44:15 -060043":ref:`overview-manual/concepts:openembedded build system concepts`"
Andrew Geisslerc9f78652020-09-18 14:11:35 -050044section.
45
Andrew Geisslerc9f78652020-09-18 14:11:35 -050046BitBake
47-------
48
49BitBake is the tool at the heart of the :term:`OpenEmbedded Build System`
50and is responsible
51for parsing the :term:`Metadata`, generating
52a list of tasks from it, and then executing those tasks.
53
54This section briefly introduces BitBake. If you want more information on
55BitBake, see the :doc:`BitBake User Manual <bitbake:index>`.
56
57To see a list of the options BitBake supports, use either of the
Andrew Geisslerc926e172021-05-07 16:11:35 -050058following commands::
Andrew Geisslerc9f78652020-09-18 14:11:35 -050059
60 $ bitbake -h
61 $ bitbake --help
62
63The most common usage for BitBake is ``bitbake recipename``, where
64``recipename`` is the name of the recipe you want to build (referred
65to as the "target"). The target often equates to the first part of a
66recipe's filename (e.g. "foo" for a recipe named ``foo_1.3.0-r0.bb``).
67So, to process the ``matchbox-desktop_1.2.3.bb`` recipe file, you might
Andrew Geisslerc926e172021-05-07 16:11:35 -050068type the following::
Andrew Geisslerc9f78652020-09-18 14:11:35 -050069
70 $ bitbake matchbox-desktop
71
Andrew Geisslerfc113ea2023-03-31 09:59:46 -050072Several different versions of ``matchbox-desktop`` might exist. BitBake chooses
73the one selected by the distribution configuration. You can get more details
74about how BitBake chooses between different target versions and providers in the
75":ref:`bitbake-user-manual/bitbake-user-manual-execution:preferences`" section
Andrew Geisslerc9f78652020-09-18 14:11:35 -050076of the BitBake User Manual.
77
78BitBake also tries to execute any dependent tasks first. So for example,
79before building ``matchbox-desktop``, BitBake would build a cross
80compiler and ``glibc`` if they had not already been built.
81
82A useful BitBake option to consider is the ``-k`` or ``--continue``
83option. This option instructs BitBake to try and continue processing the
84job as long as possible even after encountering an error. When an error
85occurs, the target that failed and those that depend on it cannot be
86remade. However, when you use this option other dependencies can still
87be processed.
88
Andrew Geisslerc9f78652020-09-18 14:11:35 -050089Recipes
90-------
91
92Files that have the ``.bb`` suffix are "recipes" files. In general, a
93recipe contains information about a single piece of software. This
94information includes the location from which to download the unaltered
95source, any source patches to be applied to that source (if needed),
96which special configuration options to apply, how to compile the source
97files, and how to package the compiled output.
98
99The term "package" is sometimes used to refer to recipes. However, since
100the word "package" is used for the packaged output from the OpenEmbedded
101build system (i.e. ``.ipk`` or ``.deb`` files), this document avoids
102using the term "package" when referring to recipes.
103
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500104Classes
105-------
106
107Class files (``.bbclass``) contain information that is useful to share
Andrew Geissler517393d2023-01-13 08:55:19 -0600108between recipes files. An example is the :ref:`ref-classes-autotools` class,
Patrick Williams03907ee2022-05-01 06:28:52 -0500109which contains common settings for any application that is built with
Patrick Williams7784c422022-11-17 07:29:11 -0600110the :wikipedia:`GNU Autotools <GNU_Autotools>`.
Patrick Williams03907ee2022-05-01 06:28:52 -0500111The ":ref:`ref-manual/classes:Classes`" chapter in the Yocto Project
112Reference Manual provides details about classes and how to use them.
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500113
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500114Configurations
115--------------
116
117The configuration files (``.conf``) define various configuration
118variables that govern the OpenEmbedded build process. These files fall
119into several areas that define machine configuration options,
120distribution configuration options, compiler tuning options, general
121common configuration options, and user configuration options in
122``conf/local.conf``, which is found in the :term:`Build Directory`.
123
124
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500125Layers
126======
127
128Layers are repositories that contain related metadata (i.e. sets of
129instructions) that tell the OpenEmbedded build system how to build a
Andrew Geissler3b8a17c2021-04-15 15:55:55 -0500130target. :ref:`overview-manual/yp-intro:the yocto project layer model`
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500131facilitates collaboration, sharing, customization, and reuse within the
132Yocto Project development environment. Layers logically separate
133information for your project. For example, you can use a layer to hold
134all the configurations for a particular piece of hardware. Isolating
135hardware-specific configurations allows you to share other metadata by
136using a different layer where that metadata might be common across
137several pieces of hardware.
138
William A. Kennington IIIac69b482021-06-02 12:28:27 -0700139There are many layers working in the Yocto Project development environment. The
Patrick Williams03907ee2022-05-01 06:28:52 -0500140:yocto_home:`Yocto Project Compatible Layer Index </software-overview/layers/>`
Andrew Geisslerd1e89492021-02-12 15:35:20 -0600141and :oe_layerindex:`OpenEmbedded Layer Index <>` both contain layers from
142which you can use or leverage.
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500143
144By convention, layers in the Yocto Project follow a specific form.
145Conforming to a known structure allows BitBake to make assumptions
146during builds on where to find types of metadata. You can find
147procedures and learn about tools (i.e. ``bitbake-layers``) for creating
148layers suitable for the Yocto Project in the
Andrew Geissler517393d2023-01-13 08:55:19 -0600149":ref:`dev-manual/layers:understanding and creating layers`"
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500150section of the Yocto Project Development Tasks Manual.
151
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500152OpenEmbedded Build System Concepts
153==================================
154
155This section takes a more detailed look inside the build process used by
156the :term:`OpenEmbedded Build System`,
157which is the build
158system specific to the Yocto Project. At the heart of the build system
159is BitBake, the task executor.
160
161The following diagram represents the high-level workflow of a build. The
162remainder of this section expands on the fundamental input, output,
163process, and metadata logical blocks that make up the workflow.
164
165.. image:: figures/YP-flow-diagram.png
Andrew Geisslerd5838332022-05-27 11:33:10 -0500166 :width: 100%
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500167
168In general, the build's workflow consists of several functional areas:
169
170- *User Configuration:* metadata you can use to control the build
171 process.
172
173- *Metadata Layers:* Various layers that provide software, machine, and
174 distro metadata.
175
176- *Source Files:* Upstream releases, local projects, and SCMs.
177
178- *Build System:* Processes under the control of
179 :term:`BitBake`. This block expands
180 on how BitBake fetches source, applies patches, completes
181 compilation, analyzes output for package generation, creates and
182 tests packages, generates images, and generates cross-development
183 tools.
184
185- *Package Feeds:* Directories containing output packages (RPM, DEB or
186 IPK), which are subsequently used in the construction of an image or
187 Software Development Kit (SDK), produced by the build system. These
188 feeds can also be copied and shared using a web server or other means
189 to facilitate extending or updating existing images on devices at
190 runtime if runtime package management is enabled.
191
192- *Images:* Images produced by the workflow.
193
194- *Application Development SDK:* Cross-development tools that are
195 produced along with an image or separately with BitBake.
196
197User Configuration
198------------------
199
200User configuration helps define the build. Through user configuration,
201you can tell BitBake the target architecture for which you are building
202the image, where to store downloaded source, and other build properties.
203
204The following figure shows an expanded representation of the "User
Andrew Geissler3b8a17c2021-04-15 15:55:55 -0500205Configuration" box of the :ref:`general workflow
206figure <overview-manual/concepts:openembedded build system concepts>`:
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500207
208.. image:: figures/user-configuration.png
Andrew Geisslerd5838332022-05-27 11:33:10 -0500209 :width: 100%
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500210
211BitBake needs some basic configuration files in order to complete a
212build. These files are ``*.conf`` files. The minimally necessary ones
213reside as example files in the ``build/conf`` directory of the
214:term:`Source Directory`. For simplicity,
215this section refers to the Source Directory as the "Poky Directory."
216
217When you clone the :term:`Poky` Git repository
218or you download and unpack a Yocto Project release, you can set up the
219Source Directory to be named anything you want. For this discussion, the
220cloned repository uses the default name ``poky``.
221
222.. note::
223
224 The Poky repository is primarily an aggregation of existing
225 repositories. It is not a canonical upstream source.
226
227The ``meta-poky`` layer inside Poky contains a ``conf`` directory that
228has example configuration files. These example files are used as a basis
229for creating actual configuration files when you source
230:ref:`structure-core-script`, which is the
231build environment script.
232
Patrick Williams2390b1b2022-11-03 13:47:49 -0500233Sourcing the build environment script creates a :term:`Build Directory`
234if one does not already exist. BitBake uses the :term:`Build Directory`
235for all its work during builds. The Build Directory has a ``conf`` directory
236that contains default versions of your ``local.conf`` and ``bblayers.conf``
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500237configuration files. These default configuration files are created only
Patrick Williams2390b1b2022-11-03 13:47:49 -0500238if versions do not already exist in the :term:`Build Directory` at the time you
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500239source the build environment setup script.
240
241Because the Poky repository is fundamentally an aggregation of existing
242repositories, some users might be familiar with running the
243:ref:`structure-core-script` script in the context of separate
244:term:`OpenEmbedded-Core (OE-Core)` and BitBake
245repositories rather than a single Poky repository. This discussion
246assumes the script is executed from within a cloned or unpacked version
247of Poky.
248
249Depending on where the script is sourced, different sub-scripts are
Patrick Williams2390b1b2022-11-03 13:47:49 -0500250called to set up the :term:`Build Directory` (Yocto or OpenEmbedded).
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500251Specifically, the script ``scripts/oe-setup-builddir`` inside the poky
Patrick Williams2390b1b2022-11-03 13:47:49 -0500252directory sets up the :term:`Build Directory` and seeds the directory (if
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500253necessary) with configuration files appropriate for the Yocto Project
254development environment.
255
256.. note::
257
258 The
259 scripts/oe-setup-builddir
260 script uses the
261 ``$TEMPLATECONF``
262 variable to determine which sample configuration files to locate.
263
264The ``local.conf`` file provides many basic variables that define a
265build environment. Here is a list of a few. To see the default
266configurations in a ``local.conf`` file created by the build environment
267script, see the
Andrew Geissler87f5cff2022-09-30 13:13:31 -0500268:yocto_git:`local.conf.sample </poky/tree/meta-poky/conf/templates/default/local.conf.sample>`
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500269in the ``meta-poky`` layer:
270
271- *Target Machine Selection:* Controlled by the
272 :term:`MACHINE` variable.
273
274- *Download Directory:* Controlled by the
275 :term:`DL_DIR` variable.
276
277- *Shared State Directory:* Controlled by the
278 :term:`SSTATE_DIR` variable.
279
280- *Build Output:* Controlled by the
281 :term:`TMPDIR` variable.
282
283- *Distribution Policy:* Controlled by the
284 :term:`DISTRO` variable.
285
286- *Packaging Format:* Controlled by the
287 :term:`PACKAGE_CLASSES`
288 variable.
289
290- *SDK Target Architecture:* Controlled by the
291 :term:`SDKMACHINE` variable.
292
293- *Extra Image Packages:* Controlled by the
294 :term:`EXTRA_IMAGE_FEATURES`
295 variable.
296
297.. note::
298
Andrew Geissler9aee5002022-03-30 16:27:02 +0000299 Configurations set in the ``conf/local.conf`` file can also be set
300 in the ``conf/site.conf`` and ``conf/auto.conf`` configuration files.
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500301
302The ``bblayers.conf`` file tells BitBake what layers you want considered
303during the build. By default, the layers listed in this file include
304layers minimally needed by the build system. However, you must manually
305add any custom layers you have created. You can find more information on
306working with the ``bblayers.conf`` file in the
Andrew Geissler517393d2023-01-13 08:55:19 -0600307":ref:`dev-manual/layers:enabling your layer`"
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500308section in the Yocto Project Development Tasks Manual.
309
310The files ``site.conf`` and ``auto.conf`` are not created by the
311environment initialization script. If you want the ``site.conf`` file,
Andrew Geisslerd5838332022-05-27 11:33:10 -0500312you need to create it yourself. The ``auto.conf`` file is typically
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500313created by an autobuilder:
314
315- *site.conf:* You can use the ``conf/site.conf`` configuration
316 file to configure multiple build directories. For example, suppose
317 you had several build environments and they shared some common
318 features. You can set these default build properties here. A good
319 example is perhaps the packaging format to use through the
Andrew Geisslerd5838332022-05-27 11:33:10 -0500320 :term:`PACKAGE_CLASSES` variable.
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500321
322- *auto.conf:* The file is usually created and written to by an
323 autobuilder. The settings put into the file are typically the same as
324 you would find in the ``conf/local.conf`` or the ``conf/site.conf``
325 files.
326
327You can edit all configuration files to further define any particular
328build environment. This process is represented by the "User
329Configuration Edits" box in the figure.
330
331When you launch your build with the ``bitbake target`` command, BitBake
332sorts out the configurations to ultimately define your build
333environment. It is important to understand that the
334:term:`OpenEmbedded Build System` reads the
335configuration files in a specific order: ``site.conf``, ``auto.conf``,
336and ``local.conf``. And, the build system applies the normal assignment
337statement rules as described in the
338":doc:`bitbake:bitbake-user-manual/bitbake-user-manual-metadata`" chapter
339of the BitBake User Manual. Because the files are parsed in a specific
340order, variable assignments for the same variable could be affected. For
341example, if the ``auto.conf`` file and the ``local.conf`` set variable1
342to different values, because the build system parses ``local.conf``
343after ``auto.conf``, variable1 is assigned the value from the
344``local.conf`` file.
345
346Metadata, Machine Configuration, and Policy Configuration
347---------------------------------------------------------
348
349The previous section described the user configurations that define
350BitBake's global behavior. This section takes a closer look at the
351layers the build system uses to further control the build. These layers
352provide Metadata for the software, machine, and policies.
353
William A. Kennington IIIac69b482021-06-02 12:28:27 -0700354In general, there are three types of layer input. You can see them below
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500355the "User Configuration" box in the `general workflow
Andrew Geissler3b8a17c2021-04-15 15:55:55 -0500356figure <overview-manual/concepts:openembedded build system concepts>`:
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500357
358- *Metadata (.bb + Patches):* Software layers containing
359 user-supplied recipe files, patches, and append files. A good example
Andrew Geisslerd1e89492021-02-12 15:35:20 -0600360 of a software layer might be the :oe_layer:`meta-qt5 layer </meta-qt5>`
361 from the :oe_layerindex:`OpenEmbedded Layer Index <>`. This layer is for
362 version 5.0 of the popular `Qt <https://wiki.qt.io/About_Qt>`__
363 cross-platform application development framework for desktop, embedded and
364 mobile.
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500365
366- *Machine BSP Configuration:* Board Support Package (BSP) layers (i.e.
367 "BSP Layer" in the following figure) providing machine-specific
368 configurations. This type of information is specific to a particular
Andrew Geissler3b8a17c2021-04-15 15:55:55 -0500369 target architecture. A good example of a BSP layer from the
370 :ref:`overview-manual/yp-intro:reference distribution (poky)` is the
Andrew Geissler09209ee2020-12-13 08:44:15 -0600371 :yocto_git:`meta-yocto-bsp </poky/tree/meta-yocto-bsp>`
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500372 layer.
373
374- *Policy Configuration:* Distribution Layers (i.e. "Distro Layer" in
375 the following figure) providing top-level or general policies for the
376 images or SDKs being built for a particular distribution. For
377 example, in the Poky Reference Distribution the distro layer is the
Andrew Geissler09209ee2020-12-13 08:44:15 -0600378 :yocto_git:`meta-poky </poky/tree/meta-poky>`
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500379 layer. Within the distro layer is a ``conf/distro`` directory that
380 contains distro configuration files (e.g.
Andrew Geissler09209ee2020-12-13 08:44:15 -0600381 :yocto_git:`poky.conf </poky/tree/meta-poky/conf/distro/poky.conf>`
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500382 that contain many policy configurations for the Poky distribution.
383
384The following figure shows an expanded representation of these three
Andrew Geissler3b8a17c2021-04-15 15:55:55 -0500385layers from the :ref:`general workflow figure
386<overview-manual/concepts:openembedded build system concepts>`:
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500387
388.. image:: figures/layer-input.png
389 :align: center
Andrew Geisslerd5838332022-05-27 11:33:10 -0500390 :width: 70%
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500391
392In general, all layers have a similar structure. They all contain a
393licensing file (e.g. ``COPYING.MIT``) if the layer is to be distributed,
394a ``README`` file as good practice and especially if the layer is to be
395distributed, a configuration directory, and recipe directories. You can
396learn about the general structure for layers used with the Yocto Project
397in the
Andrew Geissler517393d2023-01-13 08:55:19 -0600398":ref:`dev-manual/layers:creating your own layer`"
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500399section in the
400Yocto Project Development Tasks Manual. For a general discussion on
401layers and the many layers from which you can draw, see the
Andrew Geissler3b8a17c2021-04-15 15:55:55 -0500402":ref:`overview-manual/concepts:layers`" and
403":ref:`overview-manual/yp-intro:the yocto project layer model`" sections both
404earlier in this manual.
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500405
406If you explored the previous links, you discovered some areas where many
Andrew Geissler09209ee2020-12-13 08:44:15 -0600407layers that work with the Yocto Project exist. The :yocto_git:`Source
408Repositories <>` also shows layers categorized under "Yocto Metadata Layers."
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500409
410.. note::
411
William A. Kennington IIIac69b482021-06-02 12:28:27 -0700412 There are layers in the Yocto Project Source Repositories that cannot be
413 found in the OpenEmbedded Layer Index. Such layers are either
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500414 deprecated or experimental in nature.
415
416BitBake uses the ``conf/bblayers.conf`` file, which is part of the user
417configuration, to find what layers it should be using as part of the
418build.
419
420Distro Layer
421~~~~~~~~~~~~
422
423The distribution layer provides policy configurations for your
424distribution. Best practices dictate that you isolate these types of
425configurations into their own layer. Settings you provide in
426``conf/distro/distro.conf`` override similar settings that BitBake finds
Patrick Williams2390b1b2022-11-03 13:47:49 -0500427in your ``conf/local.conf`` file in the :term:`Build Directory`.
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500428
429The following list provides some explanation and references for what you
430typically find in the distribution layer:
431
432- *classes:* Class files (``.bbclass``) hold common functionality that
433 can be shared among recipes in the distribution. When your recipes
434 inherit a class, they take on the settings and functions for that
435 class. You can read more about class files in the
Andrew Geissler09209ee2020-12-13 08:44:15 -0600436 ":ref:`ref-manual/classes:Classes`" chapter of the Yocto
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500437 Reference Manual.
438
439- *conf:* This area holds configuration files for the layer
440 (``conf/layer.conf``), the distribution
441 (``conf/distro/distro.conf``), and any distribution-wide include
442 files.
443
444- *recipes-*:* Recipes and append files that affect common
445 functionality across the distribution. This area could include
446 recipes and append files to add distribution-specific configuration,
447 initialization scripts, custom image recipes, and so forth. Examples
448 of ``recipes-*`` directories are ``recipes-core`` and
449 ``recipes-extra``. Hierarchy and contents within a ``recipes-*``
450 directory can vary. Generally, these directories contain recipe files
451 (``*.bb``), recipe append files (``*.bbappend``), directories that
452 are distro-specific for configuration files, and so forth.
453
454BSP Layer
455~~~~~~~~~
456
457The BSP Layer provides machine configurations that target specific
458hardware. Everything in this layer is specific to the machine for which
459you are building the image or the SDK. A common structure or form is
460defined for BSP layers. You can learn more about this structure in the
Andrew Geissler09209ee2020-12-13 08:44:15 -0600461:doc:`/bsp-guide/index`.
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500462
463.. note::
464
465 In order for a BSP layer to be considered compliant with the Yocto
466 Project, it must meet some structural requirements.
467
468The BSP Layer's configuration directory contains configuration files for
469the machine (``conf/machine/machine.conf``) and, of course, the layer
470(``conf/layer.conf``).
471
472The remainder of the layer is dedicated to specific recipes by function:
473``recipes-bsp``, ``recipes-core``, ``recipes-graphics``,
William A. Kennington IIIac69b482021-06-02 12:28:27 -0700474``recipes-kernel``, and so forth. There can be metadata for multiple
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500475formfactors, graphics support systems, and so forth.
476
477.. note::
478
479 While the figure shows several
480 recipes-\*
481 directories, not all these directories appear in all BSP layers.
482
483Software Layer
484~~~~~~~~~~~~~~
485
486The software layer provides the Metadata for additional software
487packages used during the build. This layer does not include Metadata
488that is specific to the distribution or the machine, which are found in
489their respective layers.
490
491This layer contains any recipes, append files, and patches, that your
492project needs.
493
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500494Sources
495-------
496
497In order for the OpenEmbedded build system to create an image or any
Andrew Geissler3b8a17c2021-04-15 15:55:55 -0500498target, it must be able to access source files. The :ref:`general workflow
499figure <overview-manual/concepts:openembedded build system concepts>`
500represents source files using the "Upstream Project Releases", "Local
501Projects", and "SCMs (optional)" boxes. The figure represents mirrors,
502which also play a role in locating source files, with the "Source
503Materials" box.
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500504
505The method by which source files are ultimately organized is a function
506of the project. For example, for released software, projects tend to use
507tarballs or other archived files that can capture the state of a release
508guaranteeing that it is statically represented. On the other hand, for a
509project that is more dynamic or experimental in nature, a project might
510keep source files in a repository controlled by a Source Control Manager
511(SCM) such as Git. Pulling source from a repository allows you to
512control the point in the repository (the revision) from which you want
William A. Kennington IIIac69b482021-06-02 12:28:27 -0700513to build software. A combination of the two is also possible.
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500514
515BitBake uses the :term:`SRC_URI`
516variable to point to source files regardless of their location. Each
Andrew Geissler09036742021-06-25 14:25:14 -0500517recipe must have a :term:`SRC_URI` variable that points to the source.
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500518
519Another area that plays a significant role in where source files come
520from is pointed to by the
521:term:`DL_DIR` variable. This area is
522a cache that can hold previously downloaded source. You can also
523instruct the OpenEmbedded build system to create tarballs from Git
524repositories, which is not the default behavior, and store them in the
Andrew Geissler09036742021-06-25 14:25:14 -0500525:term:`DL_DIR` by using the
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500526:term:`BB_GENERATE_MIRROR_TARBALLS`
527variable.
528
Andrew Geissler09036742021-06-25 14:25:14 -0500529Judicious use of a :term:`DL_DIR` directory can save the build system a trip
Patrick Williams2390b1b2022-11-03 13:47:49 -0500530across the Internet when looking for files. A good method for using a download
531directory is to have :term:`DL_DIR` point to an area outside of your
532:term:`Build Directory`. Doing so allows you to safely delete the
533:term:`Build Directory` if needed without fear of removing any downloaded
534source file.
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500535
536The remainder of this section provides a deeper look into the source
537files and the mirrors. Here is a more detailed look at the source file
Andrew Geissler3b8a17c2021-04-15 15:55:55 -0500538area of the :ref:`general workflow figure <overview-manual/concepts:openembedded build system concepts>`:
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500539
540.. image:: figures/source-input.png
541 :align: center
Andrew Geisslerd5838332022-05-27 11:33:10 -0500542 :width: 70%
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500543
544Upstream Project Releases
545~~~~~~~~~~~~~~~~~~~~~~~~~
546
547Upstream project releases exist anywhere in the form of an archived file
548(e.g. tarball or zip file). These files correspond to individual
549recipes. For example, the figure uses specific releases each for
550BusyBox, Qt, and Dbus. An archive file can be for any released product
551that can be built using a recipe.
552
553Local Projects
554~~~~~~~~~~~~~~
555
556Local projects are custom bits of software the user provides. These bits
Andrew Geissler615f2f12022-07-15 14:00:58 -0500557reside somewhere local to a project --- perhaps a directory into which the
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500558user checks in items (e.g. a local directory containing a development
559source tree used by the group).
560
Andrew Geissler517393d2023-01-13 08:55:19 -0600561The canonical method through which to include a local project is to use the
562:ref:`ref-classes-externalsrc` class to include that local project. You use
563either the ``local.conf`` or a recipe's append file to override or set the
564recipe to point to the local directory on your disk to pull in the whole
565source tree.
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500566
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500567Source Control Managers (Optional)
568~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
569
570Another place from which the build system can get source files is with
Andrew Geisslerfc113ea2023-03-31 09:59:46 -0500571:ref:`bitbake-user-manual/bitbake-user-manual-fetching:fetchers` employing
572various Source Control Managers (SCMs) such as Git or Subversion. In such
573cases, a repository is cloned or checked out. The :ref:`ref-tasks-fetch` task
574inside BitBake uses the :term:`SRC_URI` variable and the argument's prefix to
575determine the correct fetcher module.
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500576
577.. note::
578
579 For information on how to have the OpenEmbedded build system generate
Andrew Geissler517393d2023-01-13 08:55:19 -0600580 tarballs for Git repositories and place them in the :term:`DL_DIR`
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500581 directory, see the :term:`BB_GENERATE_MIRROR_TARBALLS`
582 variable in the Yocto Project Reference Manual.
583
584When fetching a repository, BitBake uses the
585:term:`SRCREV` variable to determine
586the specific revision from which to build.
587
588Source Mirror(s)
589~~~~~~~~~~~~~~~~
590
William A. Kennington IIIac69b482021-06-02 12:28:27 -0700591There are two kinds of mirrors: pre-mirrors and regular mirrors. The
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500592:term:`PREMIRRORS` and
593:term:`MIRRORS` variables point to
594these, respectively. BitBake checks pre-mirrors before looking upstream
595for any source files. Pre-mirrors are appropriate when you have a shared
596directory that is not a directory defined by the
597:term:`DL_DIR` variable. A Pre-mirror
598typically points to a shared directory that is local to your
599organization.
600
601Regular mirrors can be any site across the Internet that is used as an
602alternative location for source code should the primary site not be
603functioning for some reason or another.
604
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500605Package Feeds
606-------------
607
608When the OpenEmbedded build system generates an image or an SDK, it gets
609the packages from a package feed area located in the
Andrew Geissler3b8a17c2021-04-15 15:55:55 -0500610:term:`Build Directory`. The :ref:`general workflow figure
611<overview-manual/concepts:openembedded build system concepts>`
612shows this package feeds area in the upper-right corner.
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500613
614This section looks a little closer into the package feeds area used by
615the build system. Here is a more detailed look at the area:
616
617.. image:: figures/package-feeds.png
Andrew Geisslerd5838332022-05-27 11:33:10 -0500618 :width: 100%
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500619
620Package feeds are an intermediary step in the build process. The
621OpenEmbedded build system provides classes to generate different package
622types, and you specify which classes to enable through the
623:term:`PACKAGE_CLASSES`
624variable. Before placing the packages into package feeds, the build
625process validates them with generated output quality assurance checks
Andrew Geissler517393d2023-01-13 08:55:19 -0600626through the :ref:`ref-classes-insane` class.
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500627
Patrick Williams2390b1b2022-11-03 13:47:49 -0500628The package feed area resides in the :term:`Build Directory`. The directory the
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500629build system uses to temporarily store packages is determined by a
630combination of variables and the particular package manager in use. See
631the "Package Feeds" box in the illustration and note the information to
632the right of that area. In particular, the following defines where
633package files are kept:
634
Patrick Williams2390b1b2022-11-03 13:47:49 -0500635- :term:`DEPLOY_DIR`: Defined as ``tmp/deploy`` in the :term:`Build Directory`.
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500636
637- ``DEPLOY_DIR_*``: Depending on the package manager used, the package
638 type sub-folder. Given RPM, IPK, or DEB packaging and tarball
639 creation, the
640 :term:`DEPLOY_DIR_RPM`,
Patrick Williams8e7b46e2023-05-01 14:19:06 -0500641 :term:`DEPLOY_DIR_IPK`, or
642 :term:`DEPLOY_DIR_DEB`
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500643 variables are used, respectively.
644
645- :term:`PACKAGE_ARCH`: Defines
William A. Kennington IIIac69b482021-06-02 12:28:27 -0700646 architecture-specific sub-folders. For example, packages could be
647 available for the i586 or qemux86 architectures.
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500648
649BitBake uses the
650:ref:`do_package_write_* <ref-tasks-package_write_deb>`
651tasks to generate packages and place them into the package holding area
652(e.g. ``do_package_write_ipk`` for IPK packages). See the
653":ref:`ref-tasks-package_write_deb`",
654":ref:`ref-tasks-package_write_ipk`",
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500655and
Patrick Williams8e7b46e2023-05-01 14:19:06 -0500656":ref:`ref-tasks-package_write_rpm`"
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500657sections in the Yocto Project Reference Manual for additional
658information. As an example, consider a scenario where an IPK packaging
William A. Kennington IIIac69b482021-06-02 12:28:27 -0700659manager is being used and there is package architecture support for both
660i586 and qemux86. Packages for the i586 architecture are placed in
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500661``build/tmp/deploy/ipk/i586``, while packages for the qemux86
662architecture are placed in ``build/tmp/deploy/ipk/qemux86``.
663
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500664BitBake Tool
665------------
666
667The OpenEmbedded build system uses
668:term:`BitBake` to produce images and
Andrew Geissler3b8a17c2021-04-15 15:55:55 -0500669Software Development Kits (SDKs). You can see from the :ref:`general workflow
670figure <overview-manual/concepts:openembedded build system concepts>`,
671the BitBake area consists of several functional areas. This section takes a
672closer look at each of those areas.
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500673
674.. note::
675
William A. Kennington IIIac69b482021-06-02 12:28:27 -0700676 Documentation for the BitBake tool is available separately. See the
Andrew Geisslerfc113ea2023-03-31 09:59:46 -0500677 :doc:`BitBake User Manual <bitbake:index>`
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500678 for reference material on BitBake.
679
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500680Source Fetching
681~~~~~~~~~~~~~~~
682
683The first stages of building a recipe are to fetch and unpack the source
684code:
685
686.. image:: figures/source-fetching.png
Andrew Geisslerd5838332022-05-27 11:33:10 -0500687 :width: 100%
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500688
Patrick Williams2390b1b2022-11-03 13:47:49 -0500689The :ref:`ref-tasks-fetch` and :ref:`ref-tasks-unpack` tasks fetch
690the source files and unpack them into the :term:`Build Directory`.
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500691
692.. note::
693
Patrick Williams2194f502022-10-16 14:26:09 -0500694 For every local file (e.g. ``file://``) that is part of a recipe's
695 :term:`SRC_URI` statement, the OpenEmbedded build system takes a
696 checksum of the file for the recipe and inserts the checksum into
697 the signature for the :ref:`ref-tasks-fetch` task. If any local
698 file has been modified, the :ref:`ref-tasks-fetch` task and all
699 tasks that depend on it are re-executed.
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500700
Patrick Williams2390b1b2022-11-03 13:47:49 -0500701By default, everything is accomplished in the :term:`Build Directory`, which has
702a defined structure. For additional general information on the
703:term:`Build Directory`, see the ":ref:`structure-core-build`" section in
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500704the Yocto Project Reference Manual.
705
Patrick Williams2390b1b2022-11-03 13:47:49 -0500706Each recipe has an area in the :term:`Build Directory` where the unpacked
707source code resides. The :term:`S` variable points to this area for a recipe's
708unpacked source code. The name of that directory for any given recipe is
709defined from several different variables. The preceding figure and the
710following list describe the :term:`Build Directory`'s hierarchy:
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500711
712- :term:`TMPDIR`: The base directory
713 where the OpenEmbedded build system performs all its work during the
714 build. The default base directory is the ``tmp`` directory.
715
716- :term:`PACKAGE_ARCH`: The
717 architecture of the built package or packages. Depending on the
718 eventual destination of the package or packages (i.e. machine
719 architecture, :term:`Build Host`, SDK, or
Andrew Geissler09036742021-06-25 14:25:14 -0500720 specific machine), :term:`PACKAGE_ARCH` varies. See the variable's
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500721 description for details.
722
723- :term:`TARGET_OS`: The operating
724 system of the target device. A typical value would be "linux" (e.g.
725 "qemux86-poky-linux").
726
727- :term:`PN`: The name of the recipe used
728 to build the package. This variable can have multiple meanings.
Andrew Geissler09036742021-06-25 14:25:14 -0500729 However, when used in the context of input files, :term:`PN` represents
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500730 the name of the recipe.
731
732- :term:`WORKDIR`: The location
733 where the OpenEmbedded build system builds a recipe (i.e. does the
734 work to create the package).
735
736 - :term:`PV`: The version of the
737 recipe used to build the package.
738
739 - :term:`PR`: The revision of the
740 recipe used to build the package.
741
742- :term:`S`: Contains the unpacked source
743 files for a given recipe.
744
745 - :term:`BPN`: The name of the recipe
Andrew Geissler09036742021-06-25 14:25:14 -0500746 used to build the package. The :term:`BPN` variable is a version of
Andrew Geissler5f350902021-07-23 13:09:54 -0400747 the :term:`PN` variable but with common prefixes and suffixes removed.
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500748
749 - :term:`PV`: The version of the
750 recipe used to build the package.
751
752.. note::
753
William A. Kennington IIIac69b482021-06-02 12:28:27 -0700754 In the previous figure, notice that there are two sample hierarchies:
755 one based on package architecture (i.e. :term:`PACKAGE_ARCH`)
756 and one based on a machine (i.e. :term:`MACHINE`).
757 The underlying structures are identical. The differentiator being
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500758 what the OpenEmbedded build system is using as a build target (e.g.
759 general architecture, a build host, an SDK, or a specific machine).
760
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500761Patching
762~~~~~~~~
763
764Once source code is fetched and unpacked, BitBake locates patch files
765and applies them to the source files:
766
767.. image:: figures/patching.png
Andrew Geisslerd5838332022-05-27 11:33:10 -0500768 :width: 100%
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500769
770The :ref:`ref-tasks-patch` task uses a
771recipe's :term:`SRC_URI` statements
772and the :term:`FILESPATH` variable
773to locate applicable patch files.
774
775Default processing for patch files assumes the files have either
Andrew Geissler09036742021-06-25 14:25:14 -0500776``*.patch`` or ``*.diff`` file types. You can use :term:`SRC_URI` parameters
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500777to change the way the build system recognizes patch files. See the
778:ref:`ref-tasks-patch` task for more
779information.
780
781BitBake finds and applies multiple patches for a single recipe in the
Andrew Geissler09036742021-06-25 14:25:14 -0500782order in which it locates the patches. The :term:`FILESPATH` variable
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500783defines the default set of directories that the build system uses to
784search for patch files. Once found, patches are applied to the recipe's
785source files, which are located in the
786:term:`S` directory.
787
788For more information on how the source directories are created, see the
Andrew Geissler3b8a17c2021-04-15 15:55:55 -0500789":ref:`overview-manual/concepts:source fetching`" section. For
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500790more information on how to create patches and how the build system
791processes patches, see the
Andrew Geissler517393d2023-01-13 08:55:19 -0600792":ref:`dev-manual/new-recipe:patching code`"
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500793section in the
794Yocto Project Development Tasks Manual. You can also see the
Andrew Geissler09209ee2020-12-13 08:44:15 -0600795":ref:`sdk-manual/extensible:use \`\`devtool modify\`\` to modify the source of an existing component`"
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500796section in the Yocto Project Application Development and the Extensible
797Software Development Kit (SDK) manual and the
Andrew Geissler09209ee2020-12-13 08:44:15 -0600798":ref:`kernel-dev/common:using traditional kernel development to patch the kernel`"
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500799section in the Yocto Project Linux Kernel Development Manual.
800
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500801Configuration, Compilation, and Staging
802~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
803
804After source code is patched, BitBake executes tasks that configure and
805compile the source code. Once compilation occurs, the files are copied
806to a holding area (staged) in preparation for packaging:
807
808.. image:: figures/configuration-compile-autoreconf.png
Andrew Geisslerd5838332022-05-27 11:33:10 -0500809 :width: 100%
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500810
811This step in the build process consists of the following tasks:
812
813- :ref:`ref-tasks-prepare_recipe_sysroot`:
814 This task sets up the two sysroots in
815 ``${``\ :term:`WORKDIR`\ ``}``
816 (i.e. ``recipe-sysroot`` and ``recipe-sysroot-native``) so that
817 during the packaging phase the sysroots can contain the contents of
818 the
819 :ref:`ref-tasks-populate_sysroot`
820 tasks of the recipes on which the recipe containing the tasks
821 depends. A sysroot exists for both the target and for the native
822 binaries, which run on the host system.
823
824- *do_configure*: This task configures the source by enabling and
825 disabling any build-time and configuration options for the software
826 being built. Configurations can come from the recipe itself as well
827 as from an inherited class. Additionally, the software itself might
828 configure itself depending on the target for which it is being built.
829
830 The configurations handled by the
831 :ref:`ref-tasks-configure` task
832 are specific to configurations for the source code being built by the
833 recipe.
834
Andrew Geissler517393d2023-01-13 08:55:19 -0600835 If you are using the :ref:`ref-classes-autotools` class,
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500836 you can add additional configuration options by using the
837 :term:`EXTRA_OECONF` or
838 :term:`PACKAGECONFIG_CONFARGS`
839 variables. For information on how this variable works within that
Andrew Geissler517393d2023-01-13 08:55:19 -0600840 class, see the :ref:`ref-classes-autotools` class
Patrick Williams975a06f2022-10-21 14:42:47 -0500841 :yocto_git:`here </poky/tree/meta/classes-recipe/autotools.bbclass>`.
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500842
843- *do_compile*: Once a configuration task has been satisfied,
844 BitBake compiles the source using the
845 :ref:`ref-tasks-compile` task.
846 Compilation occurs in the directory pointed to by the
847 :term:`B` variable. Realize that the
Andrew Geissler09036742021-06-25 14:25:14 -0500848 :term:`B` directory is, by default, the same as the
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500849 :term:`S` directory.
850
851- *do_install*: After compilation completes, BitBake executes the
852 :ref:`ref-tasks-install` task.
Andrew Geissler09036742021-06-25 14:25:14 -0500853 This task copies files from the :term:`B` directory and places them in a
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500854 holding area pointed to by the :term:`D`
855 variable. Packaging occurs later using files from this holding
856 directory.
857
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500858Package Splitting
859~~~~~~~~~~~~~~~~~
860
861After source code is configured, compiled, and staged, the build system
862analyzes the results and splits the output into packages:
863
864.. image:: figures/analysis-for-package-splitting.png
Andrew Geisslerd5838332022-05-27 11:33:10 -0500865 :width: 100%
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500866
867The :ref:`ref-tasks-package` and
868:ref:`ref-tasks-packagedata`
869tasks combine to analyze the files found in the
870:term:`D` directory and split them into
871subsets based on available packages and files. Analysis involves the
872following as well as other items: splitting out debugging symbols,
873looking at shared library dependencies between packages, and looking at
874package relationships.
875
Patrick Williams2194f502022-10-16 14:26:09 -0500876The :ref:`ref-tasks-packagedata` task creates package metadata based on the
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500877analysis such that the build system can generate the final packages. The
878:ref:`ref-tasks-populate_sysroot`
879task stages (copies) a subset of the files installed by the
880:ref:`ref-tasks-install` task into
881the appropriate sysroot. Working, staged, and intermediate results of
882the analysis and package splitting process use several areas:
883
884- :term:`PKGD`: The destination
885 directory (i.e. ``package``) for packages before they are split into
886 individual packages.
887
888- :term:`PKGDESTWORK`: A
Patrick Williams2194f502022-10-16 14:26:09 -0500889 temporary work area (i.e. ``pkgdata``) used by the :ref:`ref-tasks-package`
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500890 task to save package metadata.
891
892- :term:`PKGDEST`: The parent
893 directory (i.e. ``packages-split``) for packages after they have been
894 split.
895
896- :term:`PKGDATA_DIR`: A shared,
897 global-state directory that holds packaging metadata generated during
898 the packaging process. The packaging process copies metadata from
Andrew Geissler09036742021-06-25 14:25:14 -0500899 :term:`PKGDESTWORK` to the :term:`PKGDATA_DIR` area where it becomes globally
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500900 available.
901
902- :term:`STAGING_DIR_HOST`:
903 The path for the sysroot for the system on which a component is built
904 to run (i.e. ``recipe-sysroot``).
905
906- :term:`STAGING_DIR_NATIVE`:
907 The path for the sysroot used when building components for the build
908 host (i.e. ``recipe-sysroot-native``).
909
910- :term:`STAGING_DIR_TARGET`:
911 The path for the sysroot used when a component that is built to
912 execute on a system and it generates code for yet another machine
Andrew Geissler517393d2023-01-13 08:55:19 -0600913 (e.g. :ref:`ref-classes-cross-canadian` recipes).
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500914
915The :term:`FILES` variable defines the
916files that go into each package in
917:term:`PACKAGES`. If you want
918details on how this is accomplished, you can look at
Patrick Williams975a06f2022-10-21 14:42:47 -0500919:yocto_git:`package.bbclass </poky/tree/meta/classes-global/package.bbclass>`.
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500920
921Depending on the type of packages being created (RPM, DEB, or IPK), the
922:ref:`do_package_write_* <ref-tasks-package_write_deb>`
923task creates the actual packages and places them in the Package Feed
Andrew Geissler3b8a17c2021-04-15 15:55:55 -0500924area, which is ``${TMPDIR}/deploy``. You can see the
925":ref:`overview-manual/concepts:package feeds`" section for more detail on
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500926that part of the build process.
927
928.. note::
929
William A. Kennington IIIac69b482021-06-02 12:28:27 -0700930 Support for creating feeds directly from the ``deploy/*``
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500931 directories does not exist. Creating such feeds usually requires some
932 kind of feed maintenance mechanism that would upload the new packages
933 into an official package feed (e.g. the Ångström distribution). This
934 functionality is highly distribution-specific and thus is not
935 provided out of the box.
936
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500937Image Generation
938~~~~~~~~~~~~~~~~
939
940Once packages are split and stored in the Package Feeds area, the build
941system uses BitBake to generate the root filesystem image:
942
943.. image:: figures/image-generation.png
Andrew Geisslerd5838332022-05-27 11:33:10 -0500944 :width: 100%
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500945
946The image generation process consists of several stages and depends on
947several tasks and variables. The
948:ref:`ref-tasks-rootfs` task creates
949the root filesystem (file and directory structure) for an image. This
950task uses several key variables to help create the list of packages to
951actually install:
952
953- :term:`IMAGE_INSTALL`: Lists
954 out the base set of packages from which to install from the Package
955 Feeds area.
956
957- :term:`PACKAGE_EXCLUDE`:
958 Specifies packages that should not be installed into the image.
959
960- :term:`IMAGE_FEATURES`:
961 Specifies features to include in the image. Most of these features
962 map to additional packages for installation.
963
964- :term:`PACKAGE_CLASSES`:
965 Specifies the package backend (e.g. RPM, DEB, or IPK) to use and
966 consequently helps determine where to locate packages within the
967 Package Feeds area.
968
969- :term:`IMAGE_LINGUAS`:
970 Determines the language(s) for which additional language support
971 packages are installed.
972
973- :term:`PACKAGE_INSTALL`:
974 The final list of packages passed to the package manager for
975 installation into the image.
976
977With :term:`IMAGE_ROOTFS`
978pointing to the location of the filesystem under construction and the
Andrew Geissler09036742021-06-25 14:25:14 -0500979:term:`PACKAGE_INSTALL` variable providing the final list of packages to
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500980install, the root file system is created.
981
982Package installation is under control of the package manager (e.g.
983dnf/rpm, opkg, or apt/dpkg) regardless of whether or not package
984management is enabled for the target. At the end of the process, if
985package management is not enabled for the target, the package manager's
986data files are deleted from the root filesystem. As part of the final
987stage of package installation, post installation scripts that are part
988of the packages are run. Any scripts that fail to run on the build host
989are run on the target when the target system is first booted. If you are
Andrew Geissler87f5cff2022-09-30 13:13:31 -0500990using a
Andrew Geissler517393d2023-01-13 08:55:19 -0600991:ref:`read-only root filesystem <dev-manual/read-only-rootfs:creating a read-only root filesystem>`,
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500992all the post installation scripts must succeed on the build host during
993the package installation phase since the root filesystem on the target
994is read-only.
995
Patrick Williams2194f502022-10-16 14:26:09 -0500996The final stages of the :ref:`ref-tasks-rootfs` task handle post processing. Post
Andrew Geisslerc9f78652020-09-18 14:11:35 -0500997processing includes creation of a manifest file and optimizations.
998
Andrew Geissler517393d2023-01-13 08:55:19 -0600999The manifest file (``.manifest``) resides in the same directory as the root
1000filesystem image. This file lists out, line-by-line, the installed packages.
1001The manifest file is useful for the :ref:`ref-classes-testimage` class,
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001002for example, to determine whether or not to run specific tests. See the
Andrew Geissler517393d2023-01-13 08:55:19 -06001003:term:`IMAGE_MANIFEST` variable for additional information.
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001004
Andrew Geissler9aee5002022-03-30 16:27:02 +00001005Optimizing processes that are run across the image include ``mklibs``
1006and any other post-processing commands as defined by the
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001007:term:`ROOTFS_POSTPROCESS_COMMAND`
Andrew Geissler9aee5002022-03-30 16:27:02 +00001008variable. The ``mklibs`` process optimizes the size of the libraries.
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001009
1010After the root filesystem is built, processing begins on the image
1011through the :ref:`ref-tasks-image`
1012task. The build system runs any pre-processing commands as defined by
1013the
1014:term:`IMAGE_PREPROCESS_COMMAND`
1015variable. This variable specifies a list of functions to call before the
1016build system creates the final image output files.
1017
Patrick Williams2194f502022-10-16 14:26:09 -05001018The build system dynamically creates :ref:`do_image_* <ref-tasks-image>` tasks as needed,
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001019based on the image types specified in the
1020:term:`IMAGE_FSTYPES` variable.
1021The process turns everything into an image file or a set of image files
1022and can compress the root filesystem image to reduce the overall size of
1023the image. The formats used for the root filesystem depend on the
Andrew Geissler09036742021-06-25 14:25:14 -05001024:term:`IMAGE_FSTYPES` variable. Compression depends on whether the formats
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001025support compression.
1026
1027As an example, a dynamically created task when creating a particular
Andrew Geisslerc926e172021-05-07 16:11:35 -05001028image type would take the following form::
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001029
1030 do_image_type
1031
1032So, if the type
Andrew Geissler09036742021-06-25 14:25:14 -05001033as specified by the :term:`IMAGE_FSTYPES` were ``ext4``, the dynamically
Andrew Geisslerc926e172021-05-07 16:11:35 -05001034generated task would be as follows::
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001035
1036 do_image_ext4
1037
1038The final task involved in image creation is the
1039:ref:`do_image_complete <ref-tasks-image-complete>`
1040task. This task completes the image by applying any image post
1041processing as defined through the
1042:term:`IMAGE_POSTPROCESS_COMMAND`
1043variable. The variable specifies a list of functions to call once the
1044build system has created the final image output files.
1045
1046.. note::
1047
1048 The entire image generation process is run under
1049 Pseudo. Running under Pseudo ensures that the files in the root filesystem
1050 have correct ownership.
1051
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001052SDK Generation
1053~~~~~~~~~~~~~~
1054
1055The OpenEmbedded build system uses BitBake to generate the Software
1056Development Kit (SDK) installer scripts for both the standard SDK and
1057the extensible SDK (eSDK):
1058
1059.. image:: figures/sdk-generation.png
Andrew Geisslerd5838332022-05-27 11:33:10 -05001060 :width: 100%
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001061
1062.. note::
1063
1064 For more information on the cross-development toolchain generation,
Andrew Geissler09209ee2020-12-13 08:44:15 -06001065 see the ":ref:`overview-manual/concepts:cross-development toolchain generation`"
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001066 section. For information on advantages gained when building a
Patrick Williams2194f502022-10-16 14:26:09 -05001067 cross-development toolchain using the :ref:`ref-tasks-populate_sdk` task, see the
Andrew Geissler09209ee2020-12-13 08:44:15 -06001068 ":ref:`sdk-manual/appendix-obtain:building an sdk installer`" section in
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001069 the Yocto Project Application Development and the Extensible Software
1070 Development Kit (eSDK) manual.
1071
1072Like image generation, the SDK script process consists of several stages
1073and depends on many variables. The
1074:ref:`ref-tasks-populate_sdk`
1075and
1076:ref:`ref-tasks-populate_sdk_ext`
1077tasks use these key variables to help create the list of packages to
1078actually install. For information on the variables listed in the figure,
Andrew Geissler3b8a17c2021-04-15 15:55:55 -05001079see the ":ref:`overview-manual/concepts:application development sdk`"
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001080section.
1081
Patrick Williams2194f502022-10-16 14:26:09 -05001082The :ref:`ref-tasks-populate_sdk` task helps create the standard SDK and handles
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001083two parts: a target part and a host part. The target part is the part
1084built for the target hardware and includes libraries and headers. The
1085host part is the part of the SDK that runs on the
1086:term:`SDKMACHINE`.
1087
Patrick Williams2194f502022-10-16 14:26:09 -05001088The :ref:`ref-tasks-populate_sdk_ext` task helps create the extensible SDK and
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001089handles host and target parts differently than its counter part does for
1090the standard SDK. For the extensible SDK, the task encapsulates the
1091build system, which includes everything needed (host and target) for the
1092SDK.
1093
1094Regardless of the type of SDK being constructed, the tasks perform some
1095cleanup after which a cross-development environment setup script and any
1096needed configuration files are created. The final output is the
1097Cross-development toolchain installation script (``.sh`` file), which
1098includes the environment setup script.
1099
1100Stamp Files and the Rerunning of Tasks
1101~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1102
1103For each task that completes successfully, BitBake writes a stamp file
1104into the :term:`STAMPS_DIR`
1105directory. The beginning of the stamp file's filename is determined by
1106the :term:`STAMP` variable, and the end
Andrew Geissler3b8a17c2021-04-15 15:55:55 -05001107of the name consists of the task's name and current :ref:`input
1108checksum <overview-manual/concepts:checksums (signatures)>`.
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001109
1110.. note::
1111
Andrew Geissler517393d2023-01-13 08:55:19 -06001112 This naming scheme assumes that :term:`BB_SIGNATURE_HANDLER`
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001113 is "OEBasicHash", which is almost always the case in current
1114 OpenEmbedded.
1115
1116To determine if a task needs to be rerun, BitBake checks if a stamp file
William A. Kennington IIIac69b482021-06-02 12:28:27 -07001117with a matching input checksum exists for the task. In this case,
1118the task's output is assumed to exist and still be valid. Otherwise,
1119the task is rerun.
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001120
1121.. note::
1122
1123 The stamp mechanism is more general than the shared state (sstate)
Andrew Geissler3b8a17c2021-04-15 15:55:55 -05001124 cache mechanism described in the
1125 ":ref:`overview-manual/concepts:setscene tasks and shared state`" section.
1126 BitBake avoids rerunning any task that has a valid stamp file, not just
1127 tasks that can be accelerated through the sstate cache.
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001128
1129 However, you should realize that stamp files only serve as a marker
1130 that some work has been done and that these files do not record task
1131 output. The actual task output would usually be somewhere in
1132 :term:`TMPDIR` (e.g. in some
1133 recipe's :term:`WORKDIR`.) What
1134 the sstate cache mechanism adds is a way to cache task output that
1135 can then be shared between build machines.
1136
Andrew Geissler09036742021-06-25 14:25:14 -05001137Since :term:`STAMPS_DIR` is usually a subdirectory of :term:`TMPDIR`, removing
1138:term:`TMPDIR` will also remove :term:`STAMPS_DIR`, which means tasks will
1139properly be rerun to repopulate :term:`TMPDIR`.
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001140
1141If you want some task to always be considered "out of date", you can
Andrew Geisslerfc113ea2023-03-31 09:59:46 -05001142mark it with the :ref:`nostamp <bitbake-user-manual/bitbake-user-manual-metadata:variable flags>`
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001143varflag. If some other task depends on such a task, then that task will
1144also always be considered out of date, which might not be what you want.
1145
1146For details on how to view information about a task's signature, see the
Andrew Geissler517393d2023-01-13 08:55:19 -06001147":ref:`dev-manual/debugging:viewing task variable dependencies`"
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001148section in the Yocto Project Development Tasks Manual.
1149
1150Setscene Tasks and Shared State
1151~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1152
1153The description of tasks so far assumes that BitBake needs to build
1154everything and no available prebuilt objects exist. BitBake does support
1155skipping tasks if prebuilt objects are available. These objects are
1156usually made available in the form of a shared state (sstate) cache.
1157
1158.. note::
1159
1160 For information on variables affecting sstate, see the
1161 :term:`SSTATE_DIR`
1162 and
1163 :term:`SSTATE_MIRRORS`
1164 variables.
1165
Andrew Geisslereff27472021-10-29 15:35:00 -05001166The idea of a setscene task (i.e ``do_taskname_setscene``) is a
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001167version of the task where instead of building something, BitBake can
1168skip to the end result and simply place a set of files into specific
1169locations as needed. In some cases, it makes sense to have a setscene
1170task variant (e.g. generating package files in the
1171:ref:`do_package_write_* <ref-tasks-package_write_deb>`
1172task). In other cases, it does not make sense (e.g. a
1173:ref:`ref-tasks-patch` task or a
1174:ref:`ref-tasks-unpack` task) since
1175the work involved would be equal to or greater than the underlying task.
1176
1177In the build system, the common tasks that have setscene variants are
1178:ref:`ref-tasks-package`,
Patrick Williams2194f502022-10-16 14:26:09 -05001179:ref:`do_package_write_* <ref-tasks-package_write_deb>`,
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001180:ref:`ref-tasks-deploy`,
1181:ref:`ref-tasks-packagedata`, and
1182:ref:`ref-tasks-populate_sysroot`.
1183Notice that these tasks represent most of the tasks whose output is an
1184end result.
1185
1186The build system has knowledge of the relationship between these tasks
1187and other preceding tasks. For example, if BitBake runs
1188``do_populate_sysroot_setscene`` for something, it does not make sense
Patrick Williams2194f502022-10-16 14:26:09 -05001189to run any of the :ref:`ref-tasks-fetch`, :ref:`ref-tasks-unpack`, :ref:`ref-tasks-patch`,
1190:ref:`ref-tasks-configure`, :ref:`ref-tasks-compile`, and :ref:`ref-tasks-install` tasks. However, if
1191:ref:`ref-tasks-package` needs to be run, BitBake needs to run those other tasks.
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001192
1193It becomes more complicated if everything can come from an sstate cache
1194because some objects are simply not required at all. For example, you do
William A. Kennington IIIac69b482021-06-02 12:28:27 -07001195not need a compiler or native tools, such as quilt, if there isn't anything
Patrick Williams2194f502022-10-16 14:26:09 -05001196to compile or patch. If the :ref:`do_package_write_* <ref-tasks-package_write_deb>` packages are available
1197from sstate, BitBake does not need the :ref:`ref-tasks-package` task data.
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001198
1199To handle all these complexities, BitBake runs in two phases. The first
1200is the "setscene" stage. During this stage, BitBake first checks the
1201sstate cache for any targets it is planning to build. BitBake does a
William A. Kennington IIIac69b482021-06-02 12:28:27 -07001202fast check to see if the object exists rather than doing a complete download.
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001203If nothing exists, the second phase, which is the setscene stage,
1204completes and the main build proceeds.
1205
1206If objects are found in the sstate cache, the build system works
1207backwards from the end targets specified by the user. For example, if an
1208image is being built, the build system first looks for the packages
1209needed for that image and the tools needed to construct an image. If
1210those are available, the compiler is not needed. Thus, the compiler is
1211not even downloaded. If something was found to be unavailable, or the
1212download or setscene task fails, the build system then tries to install
1213dependencies, such as the compiler, from the cache.
1214
1215The availability of objects in the sstate cache is handled by the
Patrick Williams213cb262021-08-07 19:21:33 -05001216function specified by the :term:`BB_HASHCHECK_FUNCTION`
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001217variable and returns a list of available objects. The function specified
Patrick Williams213cb262021-08-07 19:21:33 -05001218by the :term:`BB_SETSCENE_DEPVALID`
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001219variable is the function that determines whether a given dependency
1220needs to be followed, and whether for any given relationship the
1221function needs to be passed. The function returns a True or False value.
1222
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001223Images
1224------
1225
1226The images produced by the build system are compressed forms of the root
1227filesystem and are ready to boot on a target device. You can see from
Andrew Geissler3b8a17c2021-04-15 15:55:55 -05001228the :ref:`general workflow figure
1229<overview-manual/concepts:openembedded build system concepts>` that BitBake
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001230output, in part, consists of images. This section takes a closer look at
1231this output:
1232
1233.. image:: figures/images.png
1234 :align: center
Andrew Geisslerd5838332022-05-27 11:33:10 -05001235 :width: 75%
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001236
1237.. note::
1238
1239 For a list of example images that the Yocto Project provides, see the
Andrew Geissler09209ee2020-12-13 08:44:15 -06001240 ":doc:`/ref-manual/images`" chapter in the Yocto Project Reference
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001241 Manual.
1242
Patrick Williams2390b1b2022-11-03 13:47:49 -05001243The build process writes images out to the :term:`Build Directory` inside
1244the ``tmp/deploy/images/machine/`` folder as shown in the figure. This
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001245folder contains any files expected to be loaded on the target device.
Patrick Williams2390b1b2022-11-03 13:47:49 -05001246The :term:`DEPLOY_DIR` variable points to the ``deploy`` directory, while the
1247:term:`DEPLOY_DIR_IMAGE` variable points to the appropriate directory
1248containing images for the current configuration.
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001249
1250- kernel-image: A kernel binary file. The
1251 :term:`KERNEL_IMAGETYPE`
1252 variable determines the naming scheme for the kernel image file.
1253 Depending on this variable, the file could begin with a variety of
1254 naming strings. The ``deploy/images/``\ machine directory can contain
1255 multiple image files for the machine.
1256
1257- root-filesystem-image: Root filesystems for the target device (e.g.
1258 ``*.ext3`` or ``*.bz2`` files). The
1259 :term:`IMAGE_FSTYPES`
1260 variable determines the root filesystem image type. The
1261 ``deploy/images/``\ machine directory can contain multiple root
1262 filesystems for the machine.
1263
1264- kernel-modules: Tarballs that contain all the modules built for the
1265 kernel. Kernel module tarballs exist for legacy purposes and can be
1266 suppressed by setting the
1267 :term:`MODULE_TARBALL_DEPLOY`
1268 variable to "0". The ``deploy/images/``\ machine directory can
1269 contain multiple kernel module tarballs for the machine.
1270
1271- bootloaders: If applicable to the target machine, bootloaders
1272 supporting the image. The ``deploy/images/``\ machine directory can
1273 contain multiple bootloaders for the machine.
1274
1275- symlinks: The ``deploy/images/``\ machine folder contains a symbolic
1276 link that points to the most recently built file for each machine.
1277 These links might be useful for external scripts that need to obtain
1278 the latest version of each file.
1279
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001280Application Development SDK
1281---------------------------
1282
Andrew Geissler3b8a17c2021-04-15 15:55:55 -05001283In the :ref:`general workflow figure
1284<overview-manual/concepts:openembedded build system concepts>`, the
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001285output labeled "Application Development SDK" represents an SDK. The SDK
1286generation process differs depending on whether you build an extensible
1287SDK (e.g. ``bitbake -c populate_sdk_ext`` imagename) or a standard SDK
1288(e.g. ``bitbake -c populate_sdk`` imagename). This section takes a
1289closer look at this output:
1290
1291.. image:: figures/sdk.png
Andrew Geisslerd5838332022-05-27 11:33:10 -05001292 :width: 100%
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001293
1294The specific form of this output is a set of files that includes a
1295self-extracting SDK installer (``*.sh``), host and target manifest
1296files, and files used for SDK testing. When the SDK installer file is
1297run, it installs the SDK. The SDK consists of a cross-development
1298toolchain, a set of libraries and headers, and an SDK environment setup
1299script. Running this installer essentially sets up your
1300cross-development environment. You can think of the cross-toolchain as
1301the "host" part because it runs on the SDK machine. You can think of the
1302libraries and headers as the "target" part because they are built for
1303the target hardware. The environment setup script is added so that you
1304can initialize the environment before using the tools.
1305
1306.. note::
1307
1308 - The Yocto Project supports several methods by which you can set up
1309 this cross-development environment. These methods include
1310 downloading pre-built SDK installers or building and installing
1311 your own SDK installer.
1312
1313 - For background information on cross-development toolchains in the
Andrew Geissler3b8a17c2021-04-15 15:55:55 -05001314 Yocto Project development environment, see the
1315 ":ref:`overview-manual/concepts:cross-development toolchain generation`"
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001316 section.
1317
1318 - For information on setting up a cross-development environment, see
Andrew Geissler09209ee2020-12-13 08:44:15 -06001319 the :doc:`/sdk-manual/index` manual.
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001320
1321All the output files for an SDK are written to the ``deploy/sdk`` folder
Patrick Williams2390b1b2022-11-03 13:47:49 -05001322inside the :term:`Build Directory` as shown in the previous figure. Depending
1323on the type of SDK, there are several variables to configure these files.
1324Here are the variables associated with an extensible SDK:
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001325
1326- :term:`DEPLOY_DIR`: Points to
1327 the ``deploy`` directory.
1328
1329- :term:`SDK_EXT_TYPE`:
1330 Controls whether or not shared state artifacts are copied into the
1331 extensible SDK. By default, all required shared state artifacts are
1332 copied into the SDK.
1333
1334- :term:`SDK_INCLUDE_PKGDATA`:
1335 Specifies whether or not packagedata is included in the extensible
1336 SDK for all recipes in the "world" target.
1337
1338- :term:`SDK_INCLUDE_TOOLCHAIN`:
1339 Specifies whether or not the toolchain is included when building the
1340 extensible SDK.
1341
Andrew Geissler9aee5002022-03-30 16:27:02 +00001342- :term:`ESDK_LOCALCONF_ALLOW`:
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001343 A list of variables allowed through from the build system
1344 configuration into the extensible SDK configuration.
1345
Andrew Geissler9aee5002022-03-30 16:27:02 +00001346- :term:`ESDK_LOCALCONF_REMOVE`:
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001347 A list of variables not allowed through from the build system
1348 configuration into the extensible SDK configuration.
1349
Andrew Geissler9aee5002022-03-30 16:27:02 +00001350- :term:`ESDK_CLASS_INHERIT_DISABLE`:
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001351 A list of classes to remove from the
1352 :term:`INHERIT` value globally
1353 within the extensible SDK configuration.
1354
1355This next list, shows the variables associated with a standard SDK:
1356
1357- :term:`DEPLOY_DIR`: Points to
1358 the ``deploy`` directory.
1359
1360- :term:`SDKMACHINE`: Specifies
1361 the architecture of the machine on which the cross-development tools
1362 are run to create packages for the target hardware.
1363
1364- :term:`SDKIMAGE_FEATURES`:
1365 Lists the features to include in the "target" part of the SDK.
1366
1367- :term:`TOOLCHAIN_HOST_TASK`:
1368 Lists packages that make up the host part of the SDK (i.e. the part
Andrew Geissler09036742021-06-25 14:25:14 -05001369 that runs on the :term:`SDKMACHINE`). When you use
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001370 ``bitbake -c populate_sdk imagename`` to create the SDK, a set of
1371 default packages apply. This variable allows you to add more
1372 packages.
1373
1374- :term:`TOOLCHAIN_TARGET_TASK`:
1375 Lists packages that make up the target part of the SDK (i.e. the part
1376 built for the target hardware).
1377
1378- :term:`SDKPATH`: Defines the
1379 default SDK installation path offered by the installation script.
1380
1381- :term:`SDK_HOST_MANIFEST`:
1382 Lists all the installed packages that make up the host part of the
1383 SDK. This variable also plays a minor role for extensible SDK
1384 development as well. However, it is mainly used for the standard SDK.
1385
1386- :term:`SDK_TARGET_MANIFEST`:
1387 Lists all the installed packages that make up the target part of the
1388 SDK. This variable also plays a minor role for extensible SDK
1389 development as well. However, it is mainly used for the standard SDK.
1390
1391Cross-Development Toolchain Generation
1392======================================
1393
1394The Yocto Project does most of the work for you when it comes to
Andrew Geissler09209ee2020-12-13 08:44:15 -06001395creating :ref:`sdk-manual/intro:the cross-development toolchain`. This
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001396section provides some technical background on how cross-development
1397toolchains are created and used. For more information on toolchains, you
Andrew Geissler09209ee2020-12-13 08:44:15 -06001398can also see the :doc:`/sdk-manual/index` manual.
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001399
1400In the Yocto Project development environment, cross-development
1401toolchains are used to build images and applications that run on the
1402target hardware. With just a few commands, the OpenEmbedded build system
1403creates these necessary toolchains for you.
1404
1405The following figure shows a high-level build environment regarding
1406toolchain construction and use.
1407
1408.. image:: figures/cross-development-toolchains.png
Andrew Geisslerd5838332022-05-27 11:33:10 -05001409 :width: 100%
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001410
1411Most of the work occurs on the Build Host. This is the machine used to
Patrick Williams975a06f2022-10-21 14:42:47 -05001412build images and generally work within the Yocto Project
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001413environment. When you run
1414:term:`BitBake` to create an image, the
1415OpenEmbedded build system uses the host ``gcc`` compiler to bootstrap a
1416cross-compiler named ``gcc-cross``. The ``gcc-cross`` compiler is what
1417BitBake uses to compile source files when creating the target image. You
1418can think of ``gcc-cross`` simply as an automatically generated
1419cross-compiler that is used internally within BitBake only.
1420
1421.. note::
1422
Andrew Geissler09036742021-06-25 14:25:14 -05001423 The extensible SDK does not use ``gcc-cross-canadian``
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001424 since this SDK ships a copy of the OpenEmbedded build system and the
Andrew Geissler09036742021-06-25 14:25:14 -05001425 sysroot within it contains ``gcc-cross``.
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001426
Andrew Geisslerc926e172021-05-07 16:11:35 -05001427The chain of events that occurs when the standard toolchain is bootstrapped::
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001428
Andrew Geissler6ce62a22020-11-30 19:58:47 -06001429 binutils-cross -> linux-libc-headers -> gcc-cross -> libgcc-initial -> glibc -> libgcc -> gcc-runtime
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001430
Andrew Geissler6ce62a22020-11-30 19:58:47 -06001431- ``gcc``: The compiler, GNU Compiler Collection (GCC).
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001432
Andrew Geissler6ce62a22020-11-30 19:58:47 -06001433- ``binutils-cross``: The binary utilities needed in order
1434 to run the ``gcc-cross`` phase of the bootstrap operation and build the
1435 headers for the C library.
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001436
Andrew Geissler6ce62a22020-11-30 19:58:47 -06001437- ``linux-libc-headers``: Headers needed for the cross-compiler and C library build.
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001438
Andrew Geissler6ce62a22020-11-30 19:58:47 -06001439- ``libgcc-initial``: An initial version of the gcc support library needed
1440 to bootstrap ``glibc``.
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001441
Andrew Geissler6ce62a22020-11-30 19:58:47 -06001442- ``libgcc``: The final version of the gcc support library which
1443 can only be built once there is a C library to link against.
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001444
1445- ``glibc``: The GNU C Library.
1446
1447- ``gcc-cross``: The final stage of the bootstrap process for the
1448 cross-compiler. This stage results in the actual cross-compiler that
1449 BitBake uses when it builds an image for a targeted device.
1450
Andrew Geissler6ce62a22020-11-30 19:58:47 -06001451 This tool is a "native" tool (i.e. it is designed to run on
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001452 the build host).
1453
1454- ``gcc-runtime``: Runtime libraries resulting from the toolchain
1455 bootstrapping process. This tool produces a binary that consists of
1456 the runtime libraries need for the targeted device.
1457
1458You can use the OpenEmbedded build system to build an installer for the
1459relocatable SDK used to develop applications. When you run the
1460installer, it installs the toolchain, which contains the development
1461tools (e.g., ``gcc-cross-canadian``, ``binutils-cross-canadian``, and
1462other ``nativesdk-*`` tools), which are tools native to the SDK (i.e.
Patrick Williams2390b1b2022-11-03 13:47:49 -05001463native to :term:`SDK_ARCH`), you need to cross-compile and test your
1464software. The figure shows the commands you use to easily build out
1465this toolchain. This cross-development toolchain is built to execute on the
1466:term:`SDKMACHINE`, which might or might not be the same machine as
1467the Build Host.
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001468
1469.. note::
1470
1471 If your target architecture is supported by the Yocto Project, you
1472 can take advantage of pre-built images that ship with the Yocto
1473 Project and already contain cross-development toolchain installers.
1474
Andrew Geisslerc926e172021-05-07 16:11:35 -05001475Here is the bootstrap process for the relocatable toolchain::
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001476
1477 gcc -> binutils-crosssdk -> gcc-crosssdk-initial -> linux-libc-headers -> glibc-initial -> nativesdk-glibc -> gcc-crosssdk -> gcc-cross-canadian
1478
1479- ``gcc``: The build host's GNU Compiler Collection (GCC).
1480
1481- ``binutils-crosssdk``: The bare minimum binary utilities needed in
1482 order to run the ``gcc-crosssdk-initial`` phase of the bootstrap
1483 operation.
1484
1485- ``gcc-crosssdk-initial``: An early stage of the bootstrap process for
1486 creating the cross-compiler. This stage builds enough of the
1487 ``gcc-crosssdk`` and supporting pieces so that the final stage of the
1488 bootstrap process can produce the finished cross-compiler. This tool
1489 is a "native" binary that runs on the build host.
1490
1491- ``linux-libc-headers``: Headers needed for the cross-compiler.
1492
1493- ``glibc-initial``: An initial version of the Embedded GLIBC needed to
1494 bootstrap ``nativesdk-glibc``.
1495
1496- ``nativesdk-glibc``: The Embedded GLIBC needed to bootstrap the
1497 ``gcc-crosssdk``.
1498
1499- ``gcc-crosssdk``: The final stage of the bootstrap process for the
1500 relocatable cross-compiler. The ``gcc-crosssdk`` is a transitory
1501 compiler and never leaves the build host. Its purpose is to help in
1502 the bootstrap process to create the eventual ``gcc-cross-canadian``
1503 compiler, which is relocatable. This tool is also a "native" package
1504 (i.e. it is designed to run on the build host).
1505
1506- ``gcc-cross-canadian``: The final relocatable cross-compiler. When
1507 run on the :term:`SDKMACHINE`,
1508 this tool produces executable code that runs on the target device.
1509 Only one cross-canadian compiler is produced per architecture since
1510 they can be targeted at different processor optimizations using
1511 configurations passed to the compiler through the compile commands.
1512 This circumvents the need for multiple compilers and thus reduces the
1513 size of the toolchains.
1514
1515.. note::
1516
1517 For information on advantages gained when building a
1518 cross-development toolchain installer, see the
Andrew Geissler09209ee2020-12-13 08:44:15 -06001519 ":ref:`sdk-manual/appendix-obtain:building an sdk installer`" appendix
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001520 in the Yocto Project Application Development and the
1521 Extensible Software Development Kit (eSDK) manual.
1522
1523Shared State Cache
1524==================
1525
1526By design, the OpenEmbedded build system builds everything from scratch
1527unless :term:`BitBake` can determine
1528that parts do not need to be rebuilt. Fundamentally, building from
William A. Kennington IIIac69b482021-06-02 12:28:27 -07001529scratch is attractive as it means all parts are built fresh and there is
1530no possibility of stale data that can cause problems. When
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001531developers hit problems, they typically default back to building from
Andrew Geissler595f6302022-01-24 19:11:47 +00001532scratch so they have a known state from the start.
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001533
1534Building an image from scratch is both an advantage and a disadvantage
1535to the process. As mentioned in the previous paragraph, building from
1536scratch ensures that everything is current and starts from a known
1537state. However, building from scratch also takes much longer as it
1538generally means rebuilding things that do not necessarily need to be
1539rebuilt.
1540
1541The Yocto Project implements shared state code that supports incremental
1542builds. The implementation of the shared state code answers the
1543following questions that were fundamental roadblocks within the
1544OpenEmbedded incremental build support system:
1545
1546- What pieces of the system have changed and what pieces have not
1547 changed?
1548
1549- How are changed pieces of software removed and replaced?
1550
1551- How are pre-built components that do not need to be rebuilt from
1552 scratch used when they are available?
1553
1554For the first question, the build system detects changes in the "inputs"
1555to a given task by creating a checksum (or signature) of the task's
1556inputs. If the checksum changes, the system assumes the inputs have
1557changed and the task needs to be rerun. For the second question, the
1558shared state (sstate) code tracks which tasks add which output to the
1559build process. This means the output from a given task can be removed,
1560upgraded or otherwise manipulated. The third question is partly
1561addressed by the solution for the second question assuming the build
1562system can fetch the sstate objects from remote locations and install
1563them if they are deemed to be valid.
1564
1565.. note::
1566
1567 - The build system does not maintain
1568 :term:`PR` information as part of
William A. Kennington IIIac69b482021-06-02 12:28:27 -07001569 the shared state packages. Consequently, there are considerations that
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001570 affect maintaining shared state feeds. For information on how the
Andrew Geissler09036742021-06-25 14:25:14 -05001571 build system works with packages and can track incrementing :term:`PR`
Andrew Geissler517393d2023-01-13 08:55:19 -06001572 information, see the ":ref:`dev-manual/packages:automatically incrementing a package version number`"
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001573 section in the Yocto Project Development Tasks Manual.
1574
1575 - The code in the build system that supports incremental builds is
Andrew Geisslereff27472021-10-29 15:35:00 -05001576 complex. For techniques that help you work around issues
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001577 related to shared state code, see the
Andrew Geissler517393d2023-01-13 08:55:19 -06001578 ":ref:`dev-manual/debugging:viewing metadata used to create the input signature of a shared state task`"
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001579 and
Andrew Geissler517393d2023-01-13 08:55:19 -06001580 ":ref:`dev-manual/debugging:invalidating shared state to force a task to run`"
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001581 sections both in the Yocto Project Development Tasks Manual.
1582
1583The rest of this section goes into detail about the overall incremental
1584build architecture, the checksums (signatures), and shared state.
1585
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001586Overall Architecture
1587--------------------
1588
1589When determining what parts of the system need to be built, BitBake
1590works on a per-task basis rather than a per-recipe basis. You might
1591wonder why using a per-task basis is preferred over a per-recipe basis.
1592To help explain, consider having the IPK packaging backend enabled and
1593then switching to DEB. In this case, the
1594:ref:`ref-tasks-install` and
1595:ref:`ref-tasks-package` task outputs
1596are still valid. However, with a per-recipe approach, the build would
1597not include the ``.deb`` files. Consequently, you would have to
1598invalidate the whole build and rerun it. Rerunning everything is not the
1599best solution. Also, in this case, the core must be "taught" much about
1600specific tasks. This methodology does not scale well and does not allow
1601users to easily add new tasks in layers or as external recipes without
1602touching the packaged-staging core.
1603
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001604Checksums (Signatures)
1605----------------------
1606
1607The shared state code uses a checksum, which is a unique signature of a
1608task's inputs, to determine if a task needs to be run again. Because it
1609is a change in a task's inputs that triggers a rerun, the process needs
1610to detect all the inputs to a given task. For shell tasks, this turns
1611out to be fairly easy because the build process generates a "run" shell
1612script for each task and it is possible to create a checksum that gives
1613you a good idea of when the task's data changes.
1614
1615To complicate the problem, there are things that should not be included
1616in the checksum. First, there is the actual specific build path of a
Andrew Geissler615f2f12022-07-15 14:00:58 -05001617given task --- the :term:`WORKDIR`. It
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001618does not matter if the work directory changes because it should not
1619affect the output for target packages. Also, the build process has the
1620objective of making native or cross packages relocatable.
1621
1622.. note::
1623
1624 Both native and cross packages run on the
1625 build host. However, cross packages generate output for the target
1626 architecture.
1627
Andrew Geissler09036742021-06-25 14:25:14 -05001628The checksum therefore needs to exclude :term:`WORKDIR`. The simplistic
1629approach for excluding the work directory is to set :term:`WORKDIR` to some
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001630fixed value and create the checksum for the "run" script.
1631
1632Another problem results from the "run" scripts containing functions that
1633might or might not get called. The incremental build solution contains
1634code that figures out dependencies between shell functions. This code is
1635used to prune the "run" scripts down to the minimum set, thereby
1636alleviating this problem and making the "run" scripts much more readable
1637as a bonus.
1638
William A. Kennington IIIac69b482021-06-02 12:28:27 -07001639So far, there are solutions for shell scripts. What about Python tasks? The
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001640same approach applies even though these tasks are more difficult. The
1641process needs to figure out what variables a Python function accesses
1642and what functions it calls. Again, the incremental build solution
1643contains code that first figures out the variable and function
1644dependencies, and then creates a checksum for the data used as the input
1645to the task.
1646
Andrew Geissler09036742021-06-25 14:25:14 -05001647Like the :term:`WORKDIR` case, there can be situations where dependencies should be
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001648ignored. For these situations, you can instruct the build process to
Andrew Geisslerc926e172021-05-07 16:11:35 -05001649ignore a dependency by using a line like the following::
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001650
1651 PACKAGE_ARCHS[vardepsexclude] = "MACHINE"
1652
1653This example ensures that the :term:`PACKAGE_ARCHS` variable
1654does not depend on the value of :term:`MACHINE`, even if it does
1655reference it.
1656
1657Equally, there are cases where you need to add dependencies BitBake is
1658not able to find. You can accomplish this by using a line like the
Andrew Geisslerc926e172021-05-07 16:11:35 -05001659following::
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001660
1661 PACKAGE_ARCHS[vardeps] = "MACHINE"
1662
1663This example explicitly
Andrew Geissler09036742021-06-25 14:25:14 -05001664adds the :term:`MACHINE` variable as a dependency for :term:`PACKAGE_ARCHS`.
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001665
1666As an example, consider a case with in-line Python where BitBake is not
1667able to figure out dependencies. When running in debug mode (i.e. using
1668``-DDD``), BitBake produces output when it discovers something for which
1669it cannot figure out dependencies. The Yocto Project team has currently
1670not managed to cover those dependencies in detail and is aware of the
1671need to fix this situation.
1672
1673Thus far, this section has limited discussion to the direct inputs into
1674a task. Information based on direct inputs is referred to as the
1675"basehash" in the code. However, the question of a task's indirect
Andrew Geissler615f2f12022-07-15 14:00:58 -05001676inputs still exits --- items already built and present in the
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001677:term:`Build Directory`. The checksum (or
1678signature) for a particular task needs to add the hashes of all the
1679tasks on which the particular task depends. Choosing which dependencies
Andrew Geissler595f6302022-01-24 19:11:47 +00001680to add is a policy decision. However, the effect is to generate a
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001681checksum that combines the basehash and the hashes of the task's
1682dependencies.
1683
William A. Kennington IIIac69b482021-06-02 12:28:27 -07001684At the code level, there are multiple ways by which both the basehash
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001685and the dependent task hashes can be influenced. Within the BitBake
1686configuration file, you can give BitBake some extra information to help
1687it construct the basehash. The following statement effectively results
1688in a list of global variable dependency excludes (i.e. variables never
Andrew Geisslerc926e172021-05-07 16:11:35 -05001689included in any checksum)::
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001690
Andrew Geissler9aee5002022-03-30 16:27:02 +00001691 BB_BASEHASH_IGNORE_VARS ?= "TMPDIR FILE PATH PWD BB_TASKHASH BBPATH DL_DIR \\
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001692 SSTATE_DIR THISDIR FILESEXTRAPATHS FILE_DIRNAME HOME LOGNAME SHELL TERM \\
1693 USER FILESPATH STAGING_DIR_HOST STAGING_DIR_TARGET COREBASE PRSERV_HOST \\
1694 PRSERV_DUMPDIR PRSERV_DUMPFILE PRSERV_LOCKDOWN PARALLEL_MAKE \\
1695 CCACHE_DIR EXTERNAL_TOOLCHAIN CCACHE CCACHE_DISABLE LICENSE_PATH SDKPKGSUFFIX"
1696
Andrew Geissler595f6302022-01-24 19:11:47 +00001697The previous example does not include :term:`WORKDIR` since that variable is
1698actually constructed as a path within :term:`TMPDIR`, which is included above.
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001699
1700The rules for deciding which hashes of dependent tasks to include
1701through dependency chains are more complex and are generally
1702accomplished with a Python function. The code in
1703``meta/lib/oe/sstatesig.py`` shows two examples of this and also
1704illustrates how you can insert your own policy into the system if so
1705desired. This file defines the two basic signature generators
1706:term:`OpenEmbedded-Core (OE-Core)` uses: "OEBasic" and
1707"OEBasicHash". By default, a dummy "noop" signature handler is enabled
1708in BitBake. This means that behavior is unchanged from previous
1709versions. OE-Core uses the "OEBasicHash" signature handler by default
Andrew Geisslerc926e172021-05-07 16:11:35 -05001710through this setting in the ``bitbake.conf`` file::
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001711
1712 BB_SIGNATURE_HANDLER ?= "OEBasicHash"
1713
Andrew Geissler09036742021-06-25 14:25:14 -05001714The "OEBasicHash" :term:`BB_SIGNATURE_HANDLER` is the same
Andrew Geissler3b8a17c2021-04-15 15:55:55 -05001715as the "OEBasic" version but adds the task hash to the :ref:`stamp
1716files <overview-manual/concepts:stamp files and the rerunning of tasks>`. This
1717results in any metadata change that changes the task hash, automatically causing
1718the task to be run again. This removes the need to bump
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001719:term:`PR` values, and changes to metadata
1720automatically ripple across the build.
1721
1722It is also worth noting that the end result of these signature
1723generators is to make some dependency and hash information available to
1724the build. This information includes:
1725
Patrick Williams0ca19cc2021-08-16 14:03:13 -05001726- ``BB_BASEHASH:task-``\ taskname: The base hashes for each task in the
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001727 recipe.
1728
1729- ``BB_BASEHASH_``\ filename\ ``:``\ taskname: The base hashes for each
1730 dependent task.
1731
Andrew Geissler09036742021-06-25 14:25:14 -05001732- :term:`BB_TASKHASH`: The hash of the currently running task.
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001733
1734Shared State
1735------------
1736
1737Checksums and dependencies, as discussed in the previous section, solve
1738half the problem of supporting a shared state. The other half of the
1739problem is being able to use checksum information during the build and
1740being able to reuse or rebuild specific components.
1741
Andrew Geissler517393d2023-01-13 08:55:19 -06001742The :ref:`ref-classes-sstate` class is a relatively generic implementation of
1743how to "capture" a snapshot of a given task. The idea is that the build process
1744does not care about the source of a task's output. Output could be freshly
1745built or it could be downloaded and unpacked from somewhere. In other words,
1746the build process does not need to worry about its origin.
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001747
1748Two types of output exist. One type is just about creating a directory
1749in :term:`WORKDIR`. A good example is
1750the output of either
1751:ref:`ref-tasks-install` or
1752:ref:`ref-tasks-package`. The other
1753type of output occurs when a set of data is merged into a shared
1754directory tree such as the sysroot.
1755
1756The Yocto Project team has tried to keep the details of the
Andrew Geissler517393d2023-01-13 08:55:19 -06001757implementation hidden in the :ref:`ref-classes-sstate` class. From a user's perspective,
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001758adding shared state wrapping to a task is as simple as this
Andrew Geissler517393d2023-01-13 08:55:19 -06001759:ref:`ref-tasks-deploy` example taken from the :ref:`ref-classes-deploy` class::
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001760
1761 DEPLOYDIR = "${WORKDIR}/deploy-${PN}"
1762 SSTATETASKS += "do_deploy"
1763 do_deploy[sstate-inputdirs] = "${DEPLOYDIR}"
1764 do_deploy[sstate-outputdirs] = "${DEPLOY_DIR_IMAGE}"
1765
1766 python do_deploy_setscene () {
1767 sstate_setscene(d)
1768 }
1769 addtask do_deploy_setscene
1770 do_deploy[dirs] = "${DEPLOYDIR} ${B}"
1771 do_deploy[stamp-extra-info] = "${MACHINE_ARCH}"
1772
1773The following list explains the previous example:
1774
Andrew Geissler517393d2023-01-13 08:55:19 -06001775- Adding ``do_deploy`` to ``SSTATETASKS`` adds some required sstate-related
1776 processing, which is implemented in the :ref:`ref-classes-sstate` class, to
1777 before and after the :ref:`ref-tasks-deploy` task.
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001778
1779- The ``do_deploy[sstate-inputdirs] = "${DEPLOYDIR}"`` declares that
Patrick Williams2194f502022-10-16 14:26:09 -05001780 :ref:`ref-tasks-deploy` places its output in ``${DEPLOYDIR}`` when run normally
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001781 (i.e. when not using the sstate cache). This output becomes the input
1782 to the shared state cache.
1783
1784- The ``do_deploy[sstate-outputdirs] = "${DEPLOY_DIR_IMAGE}"`` line
1785 causes the contents of the shared state cache to be copied to
1786 ``${DEPLOY_DIR_IMAGE}``.
1787
1788 .. note::
1789
Patrick Williams2194f502022-10-16 14:26:09 -05001790 If :ref:`ref-tasks-deploy` is not already in the shared state cache or if its input
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001791 checksum (signature) has changed from when the output was cached, the task
1792 runs to populate the shared state cache, after which the contents of the
1793 shared state cache is copied to ${:term:`DEPLOY_DIR_IMAGE`}. If
Patrick Williams2194f502022-10-16 14:26:09 -05001794 :ref:`ref-tasks-deploy` is in the shared state cache and its signature indicates
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001795 that the cached output is still valid (i.e. if no relevant task inputs
1796 have changed), then the contents of the shared state cache copies
Andrew Geissler09036742021-06-25 14:25:14 -05001797 directly to ${:term:`DEPLOY_DIR_IMAGE`} by the ``do_deploy_setscene`` task
Patrick Williams2194f502022-10-16 14:26:09 -05001798 instead, skipping the :ref:`ref-tasks-deploy` task.
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001799
1800- The following task definition is glue logic needed to make the
Andrew Geisslerc926e172021-05-07 16:11:35 -05001801 previous settings effective::
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001802
1803 python do_deploy_setscene () {
1804 sstate_setscene(d)
1805 }
1806 addtask do_deploy_setscene
1807
Andrew Geisslerfc113ea2023-03-31 09:59:46 -05001808 ``sstate_setscene()`` takes the flags above as input and accelerates the
1809 :ref:`ref-tasks-deploy` task through the shared state cache if possible. If
1810 the task was accelerated, ``sstate_setscene()`` returns True. Otherwise, it
1811 returns False, and the normal :ref:`ref-tasks-deploy` task runs. For more
1812 information, see the ":ref:`bitbake-user-manual/bitbake-user-manual-execution:setscene`"
1813 section in the BitBake User Manual.
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001814
Andrew Geisslerfc113ea2023-03-31 09:59:46 -05001815- The ``do_deploy[dirs] = "${DEPLOYDIR} ${B}"`` line creates ``${DEPLOYDIR}``
1816 and ``${B}`` before the :ref:`ref-tasks-deploy` task runs, and also sets the
1817 current working directory of :ref:`ref-tasks-deploy` to ``${B}``. For more
1818 information, see the ":ref:`bitbake-user-manual/bitbake-user-manual-metadata:variable flags`"
1819 section in the BitBake User Manual.
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001820
1821 .. note::
1822
1823 In cases where ``sstate-inputdirs`` and ``sstate-outputdirs`` would be
1824 the same, you can use ``sstate-plaindirs``. For example, to preserve the
Patrick Williams2194f502022-10-16 14:26:09 -05001825 ${:term:`PKGD`} and ${:term:`PKGDEST`} output from the :ref:`ref-tasks-package`
Andrew Geisslerc926e172021-05-07 16:11:35 -05001826 task, use the following::
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001827
1828 do_package[sstate-plaindirs] = "${PKGD} ${PKGDEST}"
1829
1830
Andrew Geisslerfc113ea2023-03-31 09:59:46 -05001831- The ``do_deploy[stamp-extra-info] = "${MACHINE_ARCH}"`` line appends extra
1832 metadata to the :ref:`stamp file <overview-manual/concepts:stamp files and the rerunning of tasks>`.
1833 In this case, the metadata makes the task specific to a machine's architecture.
1834 See the ":ref:`bitbake-user-manual/bitbake-user-manual-execution:the task list`"
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001835 section in the BitBake User Manual for more information on the
1836 ``stamp-extra-info`` flag.
1837
1838- ``sstate-inputdirs`` and ``sstate-outputdirs`` can also be used with
1839 multiple directories. For example, the following declares
Andrew Geissler09036742021-06-25 14:25:14 -05001840 :term:`PKGDESTWORK` and ``SHLIBWORK`` as shared state input directories,
1841 which populates the shared state cache, and :term:`PKGDATA_DIR` and
Andrew Geisslerc926e172021-05-07 16:11:35 -05001842 ``SHLIBSDIR`` as the corresponding shared state output directories::
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001843
1844 do_package[sstate-inputdirs] = "${PKGDESTWORK} ${SHLIBSWORKDIR}"
1845 do_package[sstate-outputdirs] = "${PKGDATA_DIR} ${SHLIBSDIR}"
1846
1847- These methods also include the ability to take a lockfile when
1848 manipulating shared state directory structures, for cases where file
Andrew Geisslerc926e172021-05-07 16:11:35 -05001849 additions or removals are sensitive::
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001850
1851 do_package[sstate-lockfile] = "${PACKAGELOCK}"
1852
1853Behind the scenes, the shared state code works by looking in
1854:term:`SSTATE_DIR` and
1855:term:`SSTATE_MIRRORS` for
Andrew Geisslerc926e172021-05-07 16:11:35 -05001856shared state files. Here is an example::
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001857
1858 SSTATE_MIRRORS ?= "\
Andrew Geissler7e0e3c02022-02-25 20:34:39 +00001859 file://.* https://someserver.tld/share/sstate/PATH;downloadfilename=PATH \
Patrick Williams93c203f2021-10-06 16:15:23 -05001860 file://.* file:///some/local/dir/sstate/PATH"
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001861
1862.. note::
1863
Andrew Geissler5f350902021-07-23 13:09:54 -04001864 The shared state directory (:term:`SSTATE_DIR`) is organized into two-character
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001865 subdirectories, where the subdirectory names are based on the first two
1866 characters of the hash.
1867 If the shared state directory structure for a mirror has the same structure
Andrew Geissler09036742021-06-25 14:25:14 -05001868 as :term:`SSTATE_DIR`, you must specify "PATH" as part of the URI to enable the build
Andrew Geisslerc9f78652020-09-18 14:11:35 -05001869 system to map to the appropriate subdirectory.
1870
1871The shared state package validity can be detected just by looking at the
1872filename since the filename contains the task checksum (or signature) as
1873described earlier in this section. If a valid shared state package is
1874found, the build process downloads it and uses it to accelerate the
1875task.
1876
1877The build processes use the ``*_setscene`` tasks for the task
1878acceleration phase. BitBake goes through this phase before the main
1879execution code and tries to accelerate any tasks for which it can find
1880shared state packages. If a shared state package for a task is
1881available, the shared state package is used. This means the task and any
1882tasks on which it is dependent are not executed.
1883
1884As a real world example, the aim is when building an IPK-based image,
1885only the
1886:ref:`ref-tasks-package_write_ipk`
1887tasks would have their shared state packages fetched and extracted.
1888Since the sysroot is not used, it would never get extracted. This is
1889another reason why a task-based approach is preferred over a
1890recipe-based approach, which would have to install the output from every
1891task.
1892
Andrew Geissler595f6302022-01-24 19:11:47 +00001893Hash Equivalence
1894----------------
1895
1896The above section explained how BitBake skips the execution of tasks
1897whose output can already be found in the Shared State cache.
1898
1899During a build, it may often be the case that the output / result of a task might
1900be unchanged despite changes in the task's input values. An example might be
1901whitespace changes in some input C code. In project terms, this is what we define
1902as "equivalence".
1903
1904To keep track of such equivalence, BitBake has to manage three hashes
1905for each task:
1906
1907- The *task hash* explained earlier: computed from the recipe metadata,
1908 the task code and the task hash values from its dependencies.
1909 When changes are made, these task hashes are therefore modified,
1910 causing the task to re-execute. The task hashes of tasks depending on this
1911 task are therefore modified too, causing the whole dependency
1912 chain to re-execute.
1913
1914- The *output hash*, a new hash computed from the output of Shared State tasks,
1915 tasks that save their resulting output to a Shared State tarball.
1916 The mapping between the task hash and its output hash is reported
1917 to a new *Hash Equivalence* server. This mapping is stored in a database
1918 by the server for future reference.
1919
1920- The *unihash*, a new hash, initially set to the task hash for the task.
1921 This is used to track the *unicity* of task output, and we will explain
1922 how its value is maintained.
1923
1924When Hash Equivalence is enabled, BitBake computes the task hash
1925for each task by using the unihash of its dependencies, instead
1926of their task hash.
1927
1928Now, imagine that a Shared State task is modified because of a change in
1929its code or metadata, or because of a change in its dependencies.
1930Since this modifies its task hash, this task will need re-executing.
1931Its output hash will therefore be computed again.
1932
1933Then, the new mapping between the new task hash and its output hash
1934will be reported to the Hash Equivalence server. The server will
1935let BitBake know whether this output hash is the same as a previously
1936reported output hash, for a different task hash.
1937
1938If the output hash is already known, BitBake will update the task's
1939unihash to match the original task hash that generated that output.
1940Thanks to this, the depending tasks will keep a previously recorded
1941task hash, and BitBake will be able to retrieve their output from
1942the Shared State cache, instead of re-executing them. Similarly, the
1943output of further downstream tasks can also be retrieved from Shared
Patrick Williamsb542dec2023-06-09 01:26:37 -05001944State.
Andrew Geissler595f6302022-01-24 19:11:47 +00001945
1946If the output hash is unknown, a new entry will be created on the Hash
1947Equivalence server, matching the task hash to that output.
1948The depending tasks, still having a new task hash because of the
1949change, will need to re-execute as expected. The change propagates
1950to the depending tasks.
1951
1952To summarize, when Hash Equivalence is enabled, a change in one of the
1953tasks in BitBake's run queue doesn't have to propagate to all the
1954downstream tasks that depend on the output of this task, causing a
1955full rebuild of such tasks, and so on with the next depending tasks.
1956Instead, when the output of this task remains identical to previously
1957recorded output, BitBake can safely retrieve all the downstream
1958task output from the Shared State cache.
1959
1960.. note::
1961
1962 Having :doc:`/test-manual/reproducible-builds` is a key ingredient for
1963 the stability of the task's output hash. Therefore, the effectiveness
1964 of Hash Equivalence strongly depends on it.
1965
1966This applies to multiple scenarios:
1967
1968- A "trivial" change to a recipe that doesn't impact its generated output,
1969 such as whitespace changes, modifications to unused code paths or
1970 in the ordering of variables.
1971
1972- Shared library updates, for example to fix a security vulnerability.
1973 For sure, the programs using such a library should be rebuilt, but
1974 their new binaries should remain identical. The corresponding tasks should
1975 have a different output hash because of the change in the hash of their
1976 library dependency, but thanks to their output being identical, Hash
1977 Equivalence will stop the propagation down the dependency chain.
1978
1979- Native tool updates. Though the depending tasks should be rebuilt,
1980 it's likely that they will generate the same output and be marked
1981 as equivalent.
1982
1983This mechanism is enabled by default in Poky, and is controlled by three
1984variables:
1985
1986- :term:`bitbake:BB_HASHSERVE`, specifying a local or remote Hash
1987 Equivalence server to use.
1988
1989- :term:`BB_HASHSERVE_UPSTREAM`, when ``BB_HASHSERVE = "auto"``,
1990 allowing to connect the local server to an upstream one.
1991
Patrick Williams520786c2023-06-25 16:20:36 -05001992- :term:`bitbake:BB_SIGNATURE_HANDLER`, which must be set to ``OEEquivHash``.
Andrew Geissler595f6302022-01-24 19:11:47 +00001993
1994Therefore, the default configuration in Poky corresponds to the
1995below settings::
1996
1997 BB_HASHSERVE = "auto"
1998 BB_SIGNATURE_HANDLER = "OEEquivHash"
1999
2000Rather than starting a local server, another possibility is to rely
2001on a Hash Equivalence server on a network, by setting::
2002
2003 BB_HASHSERVE = "<HOSTNAME>:<PORT>"
2004
2005.. note::
2006
2007 The shared Hash Equivalence server needs to be maintained together with the
2008 Shared State cache. Otherwise, the server could report Shared State hashes
2009 that only exist on specific clients.
2010
2011 We therefore recommend that one Hash Equivalence server be set up to
2012 correspond with a given Shared State cache, and to start this server
2013 in *read-only mode*, so that it doesn't store equivalences for
2014 Shared State caches that are local to clients.
2015
2016 See the :term:`BB_HASHSERVE` reference for details about starting
2017 a Hash Equivalence server.
2018
2019See the `video <https://www.youtube.com/watch?v=zXEdqGS62Wc>`__
2020of Joshua Watt's `Hash Equivalence and Reproducible Builds
2021<https://elinux.org/images/3/37/Hash_Equivalence_and_Reproducible_Builds.pdf>`__
2022presentation at ELC 2020 for a very synthetic introduction to the
2023Hash Equivalence implementation in the Yocto Project.
2024
Andrew Geisslerc9f78652020-09-18 14:11:35 -05002025Automatically Added Runtime Dependencies
2026========================================
2027
2028The OpenEmbedded build system automatically adds common types of runtime
2029dependencies between packages, which means that you do not need to
2030explicitly declare the packages using
William A. Kennington IIIac69b482021-06-02 12:28:27 -07002031:term:`RDEPENDS`. There are three automatic
2032mechanisms (``shlibdeps``, ``pcdeps``, and ``depchains``) that
Andrew Geisslerc9f78652020-09-18 14:11:35 -05002033handle shared libraries, package configuration (pkg-config) modules, and
2034``-dev`` and ``-dbg`` packages, respectively. For other types of runtime
2035dependencies, you must manually declare the dependencies.
2036
2037- ``shlibdeps``: During the
2038 :ref:`ref-tasks-package` task of
2039 each recipe, all shared libraries installed by the recipe are
2040 located. For each shared library, the package that contains the
2041 shared library is registered as providing the shared library. More
2042 specifically, the package is registered as providing the
Patrick Williams7784c422022-11-17 07:29:11 -06002043 :wikipedia:`soname <Soname>` of the library. The
Andrew Geisslerc9f78652020-09-18 14:11:35 -05002044 resulting shared-library-to-package mapping is saved globally in
2045 :term:`PKGDATA_DIR` by the
2046 :ref:`ref-tasks-packagedata`
2047 task.
2048
2049 Simultaneously, all executables and shared libraries installed by the
2050 recipe are inspected to see what shared libraries they link against.
Andrew Geissler09036742021-06-25 14:25:14 -05002051 For each shared library dependency that is found, :term:`PKGDATA_DIR` is
Andrew Geisslerc9f78652020-09-18 14:11:35 -05002052 queried to see if some package (likely from a different recipe)
2053 contains the shared library. If such a package is found, a runtime
2054 dependency is added from the package that depends on the shared
2055 library to the package that contains the library.
2056
2057 The automatically added runtime dependency also includes a version
2058 restriction. This version restriction specifies that at least the
2059 current version of the package that provides the shared library must
Andrew Geissler09036742021-06-25 14:25:14 -05002060 be used, as if "package (>= version)" had been added to :term:`RDEPENDS`.
Andrew Geisslerc9f78652020-09-18 14:11:35 -05002061 This forces an upgrade of the package containing the shared library
2062 when installing the package that depends on the library, if needed.
2063
2064 If you want to avoid a package being registered as providing a
2065 particular shared library (e.g. because the library is for internal
2066 use only), then add the library to
2067 :term:`PRIVATE_LIBS` inside
2068 the package's recipe.
2069
Patrick Williams2194f502022-10-16 14:26:09 -05002070- ``pcdeps``: During the :ref:`ref-tasks-package` task of each recipe, all
Andrew Geisslerc9f78652020-09-18 14:11:35 -05002071 pkg-config modules (``*.pc`` files) installed by the recipe are
2072 located. For each module, the package that contains the module is
2073 registered as providing the module. The resulting module-to-package
Andrew Geissler09036742021-06-25 14:25:14 -05002074 mapping is saved globally in :term:`PKGDATA_DIR` by the
Patrick Williams2194f502022-10-16 14:26:09 -05002075 :ref:`ref-tasks-packagedata` task.
Andrew Geisslerc9f78652020-09-18 14:11:35 -05002076
2077 Simultaneously, all pkg-config modules installed by the recipe are
2078 inspected to see what other pkg-config modules they depend on. A
2079 module is seen as depending on another module if it contains a
2080 "Requires:" line that specifies the other module. For each module
Andrew Geissler09036742021-06-25 14:25:14 -05002081 dependency, :term:`PKGDATA_DIR` is queried to see if some package
Andrew Geisslerc9f78652020-09-18 14:11:35 -05002082 contains the module. If such a package is found, a runtime dependency
2083 is added from the package that depends on the module to the package
2084 that contains the module.
2085
2086 .. note::
2087
2088 The
2089 pcdeps
2090 mechanism most often infers dependencies between
2091 -dev
2092 packages.
2093
2094- ``depchains``: If a package ``foo`` depends on a package ``bar``,
2095 then ``foo-dev`` and ``foo-dbg`` are also made to depend on
2096 ``bar-dev`` and ``bar-dbg``, respectively. Taking the ``-dev``
2097 packages as an example, the ``bar-dev`` package might provide headers
2098 and shared library symlinks needed by ``foo-dev``, which shows the
2099 need for a dependency between the packages.
2100
2101 The dependencies added by ``depchains`` are in the form of
2102 :term:`RRECOMMENDS`.
2103
2104 .. note::
2105
Andrew Geissler5f350902021-07-23 13:09:54 -04002106 By default, ``foo-dev`` also has an :term:`RDEPENDS`-style dependency on
Patrick Williams0ca19cc2021-08-16 14:03:13 -05002107 ``foo``, because the default value of ``RDEPENDS:${PN}-dev`` (set in
Andrew Geisslerd5838332022-05-27 11:33:10 -05002108 ``bitbake.conf``) includes "${PN}".
Andrew Geisslerc9f78652020-09-18 14:11:35 -05002109
2110 To ensure that the dependency chain is never broken, ``-dev`` and
2111 ``-dbg`` packages are always generated by default, even if the
2112 packages turn out to be empty. See the
2113 :term:`ALLOW_EMPTY` variable
2114 for more information.
2115
Andrew Geisslerfc113ea2023-03-31 09:59:46 -05002116The :ref:`ref-tasks-package` task depends on the :ref:`ref-tasks-packagedata`
2117task of each recipe in :term:`DEPENDS` through use of a
2118``[``\ :ref:`deptask <bitbake-user-manual/bitbake-user-manual-metadata:variable flags>`\ ``]``
2119declaration, which guarantees that the required shared-library /
2120module-to-package mapping information will be available when needed as long as
2121:term:`DEPENDS` has been correctly set.
Andrew Geisslerc9f78652020-09-18 14:11:35 -05002122
2123Fakeroot and Pseudo
2124===================
2125
2126Some tasks are easier to implement when allowed to perform certain
2127operations that are normally reserved for the root user (e.g.
2128:ref:`ref-tasks-install`,
2129:ref:`do_package_write* <ref-tasks-package_write_deb>`,
2130:ref:`ref-tasks-rootfs`, and
Patrick Williams2194f502022-10-16 14:26:09 -05002131:ref:`do_image_* <ref-tasks-image>`). For example,
2132the :ref:`ref-tasks-install` task benefits from being able to set the UID and GID
Andrew Geisslerc9f78652020-09-18 14:11:35 -05002133of installed files to arbitrary values.
2134
2135One approach to allowing tasks to perform root-only operations would be
2136to require :term:`BitBake` to run as
2137root. However, this method is cumbersome and has security issues. The
2138approach that is actually used is to run tasks that benefit from root
2139privileges in a "fake" root environment. Within this environment, the
2140task and its child processes believe that they are running as the root
2141user, and see an internally consistent view of the filesystem. As long
2142as generating the final output (e.g. a package or an image) does not
2143require root privileges, the fact that some earlier steps ran in a fake
2144root environment does not cause problems.
2145
2146The capability to run tasks in a fake root environment is known as
2147"`fakeroot <http://man.he.net/man1/fakeroot>`__", which is derived from
2148the BitBake keyword/variable flag that requests a fake root environment
2149for a task.
2150
Andrew Geisslerd1e89492021-02-12 15:35:20 -06002151In the :term:`OpenEmbedded Build System`, the program that implements
2152fakeroot is known as :yocto_home:`Pseudo </software-item/pseudo/>`. Pseudo
Andrew Geisslerc9f78652020-09-18 14:11:35 -05002153overrides system calls by using the environment variable ``LD_PRELOAD``,
2154which results in the illusion of running as root. To keep track of
2155"fake" file ownership and permissions resulting from operations that
2156require root permissions, Pseudo uses an SQLite 3 database. This
2157database is stored in
2158``${``\ :term:`WORKDIR`\ ``}/pseudo/files.db``
2159for individual recipes. Storing the database in a file as opposed to in
2160memory gives persistence between tasks and builds, which is not
2161accomplished using fakeroot.
2162
2163.. note::
2164
2165 If you add your own task that manipulates the same files or
2166 directories as a fakeroot task, then that task also needs to run
2167 under fakeroot. Otherwise, the task cannot run root-only operations,
2168 and cannot see the fake file ownership and permissions set by the
2169 other task. You need to also add a dependency on
Andrew Geisslerc926e172021-05-07 16:11:35 -05002170 ``virtual/fakeroot-native:do_populate_sysroot``, giving the following::
Andrew Geisslerc9f78652020-09-18 14:11:35 -05002171
2172 fakeroot do_mytask () {
2173 ...
2174 }
2175 do_mytask[depends] += "virtual/fakeroot-native:do_populate_sysroot"
2176
2177
2178For more information, see the
2179:term:`FAKEROOT* <bitbake:FAKEROOT>` variables in the
2180BitBake User Manual. You can also reference the "`Why Not
2181Fakeroot? <https://github.com/wrpseudo/pseudo/wiki/WhyNotFakeroot>`__"
2182article for background information on Fakeroot and Pseudo.