6 Customizing the Extensible SDK

This appendix describes customizations you can apply to the extensible SDK.

6.1 Configuring the Extensible SDK

The extensible SDK primarily consists of a pre-configured copy of the OpenEmbedded build system from which it was produced. Thus, the SDK’s configuration is derived using that build system and the filters shown in the following list. When these filters are present, the OpenEmbedded build system applies them against local.conf and auto.conf:

  • Variables whose values start with “/” are excluded since the assumption is that those values are paths that are likely to be specific to the Build Host.

  • Variables listed in SDK_LOCAL_CONF_BLACKLIST are excluded. These variables are not allowed through from the OpenEmbedded build system configuration into the extensible SDK configuration. Typically, these variables are specific to the machine on which the build system is running and could be problematic as part of the extensible SDK configuration.

    For a list of the variables excluded by default, see the SDK_LOCAL_CONF_BLACKLIST in the glossary of the Yocto Project Reference Manual.

  • Variables listed in SDK_LOCAL_CONF_WHITELIST are included. Including a variable in the value of SDK_LOCAL_CONF_WHITELIST overrides either of the previous two filters. The default value is blank.

  • Classes inherited globally with INHERIT that are listed in SDK_INHERIT_BLACKLIST are disabled. Using SDK_INHERIT_BLACKLIST to disable these classes is the typical method to disable classes that are problematic or unnecessary in the SDK context. The default value blacklists the buildhistory and icecc classes.

Additionally, the contents of conf/sdk-extra.conf, when present, are appended to the end of conf/local.conf within the produced SDK, without any filtering. The sdk-extra.conf file is particularly useful if you want to set a variable value just for the SDK and not the OpenEmbedded build system used to create the SDK.

6.2 Adjusting the Extensible SDK to Suit Your Build Host’s Setup

In most cases, the extensible SDK defaults should work with your Build Host’s setup. However, some cases exist for which you might consider making adjustments:

  • If your SDK configuration inherits additional classes using the INHERIT variable and you do not need or want those classes enabled in the SDK, you can blacklist them by adding them to the SDK_INHERIT_BLACKLIST variable as described in the fourth bullet of the previous section.


    The default value of SDK_INHERIT_BLACKLIST is set using the “?=” operator. Consequently, you will need to either define the entire list by using the “=” operator, or you will need to append a value using either “_append” or the “+=” operator. You can learn more about these operators in the ” Basic Syntax ” section of the BitBake User Manual.


  • If you have classes or recipes that add additional tasks to the standard build flow (i.e. the tasks execute as the recipe builds as opposed to being called explicitly), then you need to do one of the following:

    • After ensuring the tasks are shared state tasks (i.e. the output of the task is saved to and can be restored from the shared state cache) or ensuring the tasks are able to be produced quickly from a task that is a shared state task, add the task name to the value of SDK_RECRDEP_TASKS.

    • Disable the tasks if they are added by a class and you do not need the functionality the class provides in the extensible SDK. To disable the tasks, add the class to the SDK_INHERIT_BLACKLIST variable as described in the previous section.

  • Generally, you want to have a shared state mirror set up so users of the SDK can add additional items to the SDK after installation without needing to build the items from source. See the “Providing Additional Installable Extensible SDK Content” section for information.

  • If you want users of the SDK to be able to easily update the SDK, you need to set the SDK_UPDATE_URL variable. For more information, see the “Providing Updates to the Extensible SDK After Installation” section.

  • If you have adjusted the list of files and directories that appear in COREBASE (other than layers that are enabled through bblayers.conf), then you must list these files in COREBASE_FILES so that the files are copied into the SDK.

  • If your OpenEmbedded build system setup uses a different environment setup script other than oe-init-build-env, then you must set OE_INIT_ENV_SCRIPT to point to the environment setup script you use.


    You must also reflect this change in the value used for the COREBASE_FILES variable as previously described.

6.3 Changing the Extensible SDK Installer Title

You can change the displayed title for the SDK installer by setting the SDK_TITLE variable and then rebuilding the the SDK installer. For information on how to build an SDK installer, see the “Building an SDK Installer” section.

By default, this title is derived from DISTRO_NAME when it is set. If the DISTRO_NAME variable is not set, the title is derived from the DISTRO variable.

The populate_sdk_base class defines the default value of the SDK_TITLE variable as follows:

SDK_TITLE ??= "${@d.getVar('DISTRO_NAME') or d.getVar('DISTRO')} SDK"

While several ways exist to change this variable, an efficient method is to set the variable in your distribution’s configuration file. Doing so creates an SDK installer title that applies across your distribution. As an example, assume you have your own layer for your distribution named “meta-mydistro” and you are using the same type of file hierarchy as does the default “poky” distribution. If so, you could update the SDK_TITLE variable in the ~/meta-mydistro/conf/distro/mydistro.conf file using the following form:

SDK_TITLE = "your_title"

6.4 Providing Updates to the Extensible SDK After Installation

When you make changes to your configuration or to the metadata and if you want those changes to be reflected in installed SDKs, you need to perform additional steps. These steps make it possible for anyone using the installed SDKs to update the installed SDKs by using the devtool sdk-update command:

  1. Create a directory that can be shared over HTTP or HTTPS. You can do this by setting up a web server such as an Apache HTTP Server or Nginx server in the cloud to host the directory. This directory must contain the published SDK.

  2. Set the SDK_UPDATE_URL variable to point to the corresponding HTTP or HTTPS URL. Setting this variable causes any SDK built to default to that URL and thus, the user does not have to pass the URL to the devtool sdk-update command as described in the “Applying Updates to an Installed Extensible SDK” section.

  3. Build the extensible SDK normally (i.e., use the bitbake -c populate_sdk_ext imagename command).

  4. Publish the SDK using the following command:

    $ oe-publish-sdk some_path/sdk-installer.sh path_to_shared_http_directory

    You must repeat this step each time you rebuild the SDK with changes that you want to make available through the update mechanism.

Completing the above steps allows users of the existing installed SDKs to simply run devtool sdk-update to retrieve and apply the latest updates. See the “Applying Updates to an Installed Extensible SDK” section for further information.

6.5 Changing the Default SDK Installation Directory

When you build the installer for the Extensible SDK, the default installation directory for the SDK is based on the DISTRO and SDKEXTPATH variables from within the populate_sdk_base class as follows:

SDKEXTPATH ??= "~/${@d.getVar('DISTRO')}_sdk"

You can change this default installation directory by specifically setting the SDKEXTPATH variable.

While a number of ways exist through which you can set this variable, the method that makes the most sense is to set the variable in your distribution’s configuration file. Doing so creates an SDK installer default directory that applies across your distribution. As an example, assume you have your own layer for your distribution named “meta-mydistro” and you are using the same type of file hierarchy as does the default “poky” distribution. If so, you could update the SDKEXTPATH variable in the ~/meta-mydistro/conf/distro/mydistro.conf file using the following form:

SDKEXTPATH = "some_path_for_your_installed_sdk"

After building your installer, running it prompts the user for acceptance of the some_path_for_your_installed_sdk directory as the default location to install the Extensible SDK.

6.6 Providing Additional Installable Extensible SDK Content

If you want the users of an extensible SDK you build to be able to add items to the SDK without requiring the users to build the items from source, you need to do a number of things:

  1. Ensure the additional items you want the user to be able to install are already built:

    • Build the items explicitly. You could use one or more “meta” recipes that depend on lists of other recipes.

    • Build the “world” target and set EXCLUDE_FROM_WORLD_pn-recipename for the recipes you do not want built. See the EXCLUDE_FROM_WORLD variable for additional information.

  2. Expose the sstate-cache directory produced by the build. Typically, you expose this directory by making it available through an Apache HTTP Server or Nginx server.

  3. Set the appropriate configuration so that the produced SDK knows how to find the configuration. The variable you need to set is SSTATE_MIRRORS:

    SSTATE_MIRRORS = "file://.* http://example.com/some_path/sstate-cache/PATH"

    You can set the SSTATE_MIRRORS variable in two different places:

    • If the mirror value you are setting is appropriate to be set for both the OpenEmbedded build system that is actually building the SDK and the SDK itself (i.e. the mirror is accessible in both places or it will fail quickly on the OpenEmbedded build system side, and its contents will not interfere with the build), then you can set the variable in your local.conf or custom distro configuration file. You can then “whitelist” the variable through to the SDK by adding the following:

    • Alternatively, if you just want to set the SSTATE_MIRRORS variable’s value for the SDK alone, create a conf/sdk-extra.conf file either in your Build Directory or within any layer and put your SSTATE_MIRRORS setting within that file.


      This second option is the safest option should you have any doubts as to which method to use when setting SSTATE_MIRRORS .

6.7 Minimizing the Size of the Extensible SDK Installer Download

By default, the extensible SDK bundles the shared state artifacts for everything needed to reconstruct the image for which the SDK was built. This bundling can lead to an SDK installer file that is a Gigabyte or more in size. If the size of this file causes a problem, you can build an SDK that has just enough in it to install and provide access to the devtool command by setting the following in your configuration:

SDK_EXT_TYPE = "minimal"

Setting SDK_EXT_TYPE to “minimal” produces an SDK installer that is around 35 Mbytes in size, which downloads and installs quickly. You need to realize, though, that the minimal installer does not install any libraries or tools out of the box. These libraries and tools must be installed either “on the fly” or through actions you perform using devtool or explicitly with the devtool sdk-install command.

In most cases, when building a minimal SDK you need to also enable bringing in the information on a wider range of packages produced by the system. Requiring this wider range of information is particularly true so that devtool add is able to effectively map dependencies it discovers in a source tree to the appropriate recipes. Additionally, the information enables the devtool search command to return useful results.

To facilitate this wider range of information, you would need to set the following:


See the SDK_INCLUDE_PKGDATA variable for additional information.

Setting the SDK_INCLUDE_PKGDATA variable as shown causes the “world” target to be built so that information for all of the recipes included within it are available. Having these recipes available increases build time significantly and increases the size of the SDK installer by 30-80 Mbytes depending on how many recipes are included in your configuration.

You can use EXCLUDE_FROM_WORLD_pn-recipename for recipes you want to exclude. However, it is assumed that you would need to be building the “world” target if you want to provide additional items to the SDK. Consequently, building for “world” should not represent undue overhead in most cases.


If you set SDK_EXT_TYPE to “minimal”, then providing a shared state mirror is mandatory so that items can be installed as needed. See the ” Providing Additional Installable Extensible SDK Content ” section for more information.

You can explicitly control whether or not to include the toolchain when you build an SDK by setting the SDK_INCLUDE_TOOLCHAIN variable to “1”. In particular, it is useful to include the toolchain when you have set SDK_EXT_TYPE to “minimal”, which by default, excludes the toolchain. Also, it is helpful if you are building a small SDK for use with an IDE or some other tool where you do not want to take extra steps to install a toolchain.