Tutorial “A toolchain example”

This tutorial shows an example using creating an application using a pre-built toolchain. In this example, we will use a pre-built toolchain which (in theory) has been used to prepare a Linux image for an embedded device. With the same toolchain, we can define a releng-tool project that prepares an application that can run on this embedded device.

This example will attempt to prepare a static build of htop that can run on an aarch64 system.

Preparing the host environment


A releng-tool project may create a package to download/setup a toolchain package to use for a build. However, for this example, we will keep the installation/availability of the toolchain outside of the releng-tool project (for simplicity of this tutorial).

First, we will prepare a toolchain on a host system to be used by our releng-tool project. For this tutorial, we will use a toolchain provided by Bootlin. Download the following toolchain package:

Extract and place the toolchain into a desired location. In this example, we will place the toolchain under /opt. However, users can install the toolchain to whatever path they desire:

$ tar -vxf <toolchain-archive>
$ mv aarch64--glibc--stable-2022.08-1 /opt
$ cd /opt/aarch64--glibc--stable-2022.08-1
$ relocate-sdk.sh

Verify the ability to invoke GCC from the toolchain installation:

$ /opt/aarch64--glibc--stable-2022.08-1/bin/aarch64-linux-gcc --version
aarch64-linux-gcc.br_real (Buildroot 2021.11-4428-g6b6741b) 11.3.0
Copyright (C) 2021 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO

Preparing a new releng-tool project

To build htop, this requires two dependencies:

Build a new project with the following structure to support our htop package, dependencies and project configuration:

└── my-tc-project/
    ├── package/
    │   ├── htop/
    │   │   ├── htop
    │   │   └── htop.hash
    │   ├── libnl/
    │   │   ├── libnl
    │   │   └── libnl.hash
    │   └── ncurses/
    │       ├── ncurses
    │       └── ncurses.hash
    └── releng

The configuration

The first changes we will make to our releng-tool project is to define the specifics of the project configuration in releng. Apply the following contents to the configuration:

packages = [

# location of the toolchain to use
MY_TOOLCHAIN_ROOT = '/opt/aarch64--glibc--stable-2022.08-1'
    MY_TOOLCHAIN_ROOT, 'aarch64-buildroot-linux-gnu/sysroot')

# flag buildroot toolchain to be strict with includes

    # configure compiler/linker options
    'CC': f'{MY_TOOLCHAIN_ROOT}/bin/aarch64-linux-gcc',
    'CXX': f'{MY_TOOLCHAIN_ROOT}/bin/aarch64-linux-g++',
    # configure pkg-config to use staging directory
    'PKG_CONFIG_DIR': '',
    'PKG_CONFIG_LIBDIR': '${STAGING_LIB_DIR}/pkgconfig',

    # configure for cross compiling
    '--host': 'aarch64-buildroot-linux',
  • Create a package list which defines htop as a required package to build. Since we plan to make libnl and ncurses dependencies to htop, they do not need to be included.

  • This project will rely on the host system having the toolchain installed. We create some variables to help point to the toolchain’s path and sysroot. Developers can make this flexible by allowing these options to be configured by environment variables or using command line arguments, but this will not be done here to simplify this tutorial.

  • We configure BR_COMPILER_PARANOID_UNSAFE_PATH, an option supported by the generated Bootlin (Buildroot) toolchain to error when using an unsafe path when performing a build (i.e. using the system’s /usr/include path over a releng-tool sysroot path).

  • Prepares two variables MY_TOOLCHAIN_CONF_ENV and MY_TOOLCHAIN_CONF_DEFS which we can later use for the Autotools packages we have. These values help configure toolchain and desired releng-tool paths. Variables set in the root configuration can later be used inside package scripts and post-build scripts. The options specified here are applicable to Autotools packages. Options can vary for other types (e.g. CMake projects typically are configured with a CMake toolchain file).

The releng-tool project’s configuration is ready. Now to define the package definitions/hashes for each package.

The libnl package

Update the libnl package definition (my-tc-project/libnl/libnl) with the following contents:

LIBNL_LICENSE = ['LGPL-2.1-or-later']
LIBNL_SITE = 'https://github.com/thom311/libnl/releases/download/libnl3_7_0/libnl-3.7.0.tar.gz'
LIBNL_TYPE = 'autotools'


    # static lib
    # disable features that are not required
  • The libnl library uses a LGPL-2.1+ license. We configure LIBNL_LICENSE to the equivalent SPDX license identifier, as well as define LIBNL_LICENSE_FILES to point to a copy of the license text. Specifying license information is not required, but can be helpful when generating license data or software bill of materials (SBOM) for a project.

  • This example uses libnl v3.7.0, which we set in the LIBNL_VERSION option. The version value is useful for managing output folders and logging versions of packages.

  • We specify the location to download sources in LIBNL_SITE.

  • The libnl library uses Autotools. This means we can use LIBNL_TYPE to configure the helper type and avoid the need to create custom configure/build scripts to run ./configure, etc. (since releng-tool will handle this for us).

  • We configure LIBNL_CONF_ENV and LIBNL_CONF_DEFS to use the configuration options we prepared in the root configuration. This allows this Autotools package to use the desired toolchain.

  • This package is configured to install (LIBNL_INSTALL_TYPE) into the staging area (instead of, by default, into the target directory). Since we are creating a static library for htop to link against, there is no need to place any generated content from the libnl package into the target area.

  • Finally, we configure LIBNL_CONF_OPTS to tweak other options supported by this package. We only need a static library, so we explicitly indicate to enable static builds and disable shared builds. We also disable a series of other features not needed for this example.

    Using configuration options can be useful for disabling certain project options such as disabling unit tests (which may not be desired for these types of builds). It is always good to explicitly define configuration entries when possible, just in case default values change for an option.

The above libnl package specifies a remote URL to download library sources. These sources should be validated to ensure data is not corrupted or manipulated. To do this, create a hash file alongside the package definition called libnl.hash with the contents:

# https://github.com/thom311/libnl/releases/download/libnl3_7_0/libnl-3.7.0.tar.gz.sha256sum
sha256 9fe43ccbeeea72c653bdcf8c93332583135cda46a79507bfd0a483bb57f65939 libnl-3.7.0.tar.gz
# locally computed
sha256 dc626520dcd53a22f727af3ee42c770e56c97a64fe3adb063799d8ab032fe551 COPYING

In this hash file, expected hashes for resources can be configured and checked when releng-tool attempts to fetch resources from remote sources. Ideally, hashes provided from a third-party package release can be directly added into these files (<hash-type> <hash> <file>). In this example, libnl provides expected hashes for archives when they make a release. We copy the hash contents into our local hash file (with a helpful comment to indicate where it came from).

In addition, we also provide a hash of the license document. While not required, this can be useful in detecting if the license of a package changes between versions.

The ncurses package

Update the ncurses package definition (my-tc-project/ncurses/ncurses) with the following contents:

NCURSES_SITE = 'https://invisible-mirror.net/archives/ncurses/ncurses-${NCURSES_VERSION}.tar.gz'
NCURSES_TYPE = 'autotools'


    # static lib
    # disable features that are not required

The above almost mimics the previous libnl package with the exception of this package having a different license and different configuration options available.

Since ncurses package specifies a remote URL to download library sources, we also want to update ncurses.hash with the contents:

# locally computed after verification with:
#  https://invisible-island.net/archives/ncurses/ncurses-6.4.tar.gz.asc
#  19882D92DDA4C400C22C0D56CC2AF4472167BE03
sha256 6931283d9ac87c5073f30b6290c4c75f21632bb4fc3603ac8100812bed248159 ncurses-6.4.tar.gz
# locally computed
sha256 63de87399e9fc8860236082b6b0520e068e9eb1fad0ebd30202aa30bb6f690ac COPYING

In the previous libnl package, maintainers provided an official SHA-256 hash to use for our local hash file. For ncurses, maintainers provide a GPG signature of their archives instead. To handle this, we manually download the archive and signature file to verify its contents. Once verified, we generated our own SHA-256 sum and place it into this hash file (with a helpful comment). And as done in the previous package, we also provide a hash of the license document.

The htop package

Update the htop package definition (my-tc-project/htop/htop) with the following contents:


HTOP_LICENSE = ['GPL-2.0-or-later']
HTOP_SITE = 'https://github.com/htop-dev/htop/releases/download/${HTOP_VERSION}/htop-${HTOP_VERSION}.tar.xz'
HTOP_TYPE = 'autotools'
HTOP_VERSION = '3.2.2'


    # disable features that are not required
  • This package defines a list of package dependencies for htop. Specifically, we list libnl and ncurses, which will force releng-tool to configure/build these packages before processing the htop package.

Since htop package specifies a remote URL to download library sources, we also want to update htop.hash with the contents:

# https://github.com/htop-dev/htop/releases/download/3.2.2/htop-3.2.2.tar.xz.sha256
sha256 bac9e9ab7198256b8802d2e3b327a54804dc2a19b77a5f103645b11c12473dc8 htop-3.2.2.tar.xz
# locally computed
sha256 8177f97513213526df2cf6184d8ff986c675afb514d4e68a404010521b880643 COPYING

Performing a build

With a configuration and packages prepared, the project should be ready to be built. While in the my-tc-project folder, invoke releng-tool:

$ releng-tool
fetching libnl...
configuring libnl...
building libnl...
building htop...
installing htop...
generating sbom information...
generating license information...
(success) completed (0:01:15)

Once completed, the target directory will have a compiled htop executable that can be copied over to and run on an aarch64-running target.

This concludes this tutorial.