BuildElement - Abstract class for build elements

The BuildElement class is a convenience element one can derive from for implementing the most common case of element.

Built-in functionality

The BuildElement base class provides built in functionality that could be overridden by the individual plugins.

This section will give a brief summary of how some of the common features work, some of them or the variables they use will be further detailed in the following sections.

The strip-binaries variable

The strip-binaries variable is by default empty. You need to use the appropiate commands depending of the system you are building. If you are targetting Linux, ones known to work are the ones used by the freedesktop-sdk, you can take a look to them in their project.conf

Location for staging dependencies

The BuildElement supports the “location” dependency configuration, which means you can use this configuration for any BuildElement class.

The “location” configuration defines where the dependency will be staged in the build sandbox.


Here is an example of how one might stage some dependencies into an alternative location while staging some elements in the sandbox root.

# Stage these build dependencies in /opt
- baseproject.bst:opt-dependencies.bst
    location: /opt

# Stage these tools in "/" and require them as
# runtime dependencies.
- baseproject.bst:base-tools.bst


The order of dependencies specified is not significant.

The staging locations will be sorted so that elements are staged in parent directories before subdirectories.

Location for running commands

The command-subdir variable sets where commands will be executed, and the directory will be created automatically if it does not exist.

The command-subdir is a relative path from %{build-root}, and cannot be a parent or adjacent directory, it must expand to a subdirectory of ${build-root}.

Location for configuring the project

The conf-root is the location that specific build elements can use to look for build configuration files. This is used by elements such as autotools, cmake, meson, setuptools and pip.

The default value of conf-root is defined by default as .. This means that if the conf-root is not explicitly set to another directory, the configuration files are expected to be found in command-subdir.

Separating source and build directories

A typical example of using conf-root is when performing autotools builds where your source directory is separate from your build directory.

This can be achieved in build elements which use conf-root as follows:

  # Specify that build configuration scripts are found in %{build-root}
  conf-root: "%{build-root}"

  # The build will run in the `_build` subdirectory
  command-subdir: _build

Install Location

Build elements must install the build output to the directory defined by install-root.

You need not set or change the install-root variable as it will be defined automatically on your behalf, and it is used to collect build output when creating the resulting artifacts.

It is important to know about install-root in order to write your own custom install instructions, for example the cmake element will use it to specify the DESTDIR.

Abstract method implementations


In Element.configure_sandbox(), the BuildElement will ensure that the sandbox locations described by the %{build-root} and %{install-root} variables are marked and will be mounted read-write for the assemble phase.

The working directory for the sandbox will be configured to be the %{build-root}, unless the %{command-subdir} variable is specified for the element in question, in which case the working directory will be configured as %{build-root}/%{command-subdir}.


In Element.stage(), the BuildElement will do the following operations:

  • Stage all of the build dependencies into the sandbox root.

  • Run the integration commands for all staged dependencies using Element.integrate()

  • Stage any Source on the given element to the %{build-root} location inside the sandbox, using Element.stage_sources()


In Element.assemble(), the BuildElement will proceed to run sandboxed commands which are expected to be found in the element configuration.

Commands are run in the following order:

  • configure-commands: Commands to configure the build scripts

  • build-commands: Commands to build the element

  • install-commands: Commands to install the results into %{install-root}

  • strip-commands: Commands to strip debugging symbols installed binaries

The result of the build is expected to end up in %{install-root}, and as such; Element.assemble() method will return the %{install-root} for artifact collection purposes.


In the case that the element is currently workspaced, the configure-commands will only be run in subsequent builds until they succeed at least once, unless bst workspace reset –soft is called on the workspace to explicitly avoid an incremental build.

class BuildElement(context: Context, project: Project, load_element: LoadElement, plugin_conf: str | None, *, artifact_key: str = None)

Bases: Element


Configure the Plugin from loaded configuration data


node – The loaded configuration dictionary

  • .SourceError – If it’s a Source implementation

  • .ElementError – If it’s an Element implementation

Plugin implementors should implement this method to read configuration data and store it.

The MappingNode.validate_keys() method should be used to ensure that the user has not specified keys in node which are unsupported by the plugin.


Configure the Element with regards to it’s build dependencies

Elements can use this method to parse custom configuration which define their relationship to their build dependencies.

If this method is implemented, then it will be called with all direct build dependencies specified in their element declaration in a list.

If the dependency was declared with custom configuration, it will be provided along with the dependency element, otherwise None will be passed with dependencies which do not have any additional configuration.

If the user has specified the same build dependency multiple times with differing configurations, then those build dependencies will be provided multiple times in the dependencies list.


dependencies (list) – A list of DependencyConfiguration objects


.ElementError – When the element raises an error

The format of the MappingNode provided as DependencyConfiguration.config method should be called by the implementing plugin in order to validate it.


It is unnecessary to implement this method if the plugin does not support any custom dependency configuration.


Preflight Check

  • .SourceError – If it’s a Source implementation

  • .ElementError – If it’s an Element implementation

This method is run after Plugin.configure() and after the pipeline is fully constructed.

Implementors should simply raise SourceError or ElementError with an informative message in the case that the host environment is unsuitable for operation.

Plugins which require host tools (only sources usually) should obtain them with utils.get_host_tool() which will raise an error automatically informing the user that a host tool is needed.


Return something which uniquely identifies the plugin input


A string, list or dictionary which uniquely identifies the input

This is used to construct unique cache keys for elements and sources, sources should return something which uniquely identifies the payload, such as an sha256 sum of a tarball content.

Elements and Sources should implement this by collecting any configurations which could possibly affect the output and return a dictionary of these settings.

For Sources, this is guaranteed to only be called if Source.is_resolved() has returned True which is to say that the Source is expected to have an exact source ref indicating exactly what source is going to be staged.


If your plugin is concerned with API stability, then future extensions of your plugin YAML configuration which affect the unique key returned here should be added to this key with care.

A good rule of thumb is to only compute the new value in the returned key if the value of the newly added YAML key is not equal to it’s default value.


Configures the the sandbox for execution


sandbox – The build sandbox

:raises (ElementError): When the element raises an error

Elements must implement this method to configure the sandbox object for execution.


Stage inputs into the sandbox directories


sandbox – The build sandbox

:raises (ElementError): When the element raises an error

Elements must implement this method to populate the sandbox directory with data. This is done either by staging Source objects, by staging the artifacts of the elements this element depends on, or both.


Assemble the output artifact


sandbox – The build sandbox


An absolute path within the sandbox to collect the artifact from

:raises (ElementError): When the element raises an error

Elements must implement this method to create an output artifact from its sources and dependencies.


Generate a build (sh) script to build this element


A string containing the shell commands required to build the element

BuildStream guarantees the following environment when the generated script is run:

  • All element variables have been exported.

  • The cwd is self.get_variable(‘build-root’)/self.normal_name.

  • $PREFIX is set to self.get_variable(‘install-root’).

  • The directory indicated by $PREFIX is an empty directory.

Files are expected to be installed to $PREFIX.

If the script fails, it is expected to return with an exit code != 0.