Cross-compile an autotools project

Projects built using the autotools plugin can be cross-compiled, allowing software to be developed on one architecture and built for a number of target architectures.

This guide shows how to cross-compile a snap built on the core22 base with the autotools plugin. The example project used in this guide can be found in this repository.

Overview

There are a number of steps to enable cross-compilation for an autotools project:

1. Specify the architectures used.
2. Adjust the options passed to the configure script.
3. Include packages to support cross-compilation.
4. Handle linker warnings.

It is useful to begin with a reminder of the syntax used to specify architectures in snapcraft.yaml files.

The architectures keyword

The architectures keyword is normally used to specify which architectures should be used when building a snap. For example, in the following case the build architecture is the same as the target architecture:

architectures:
  - amd64

It is also possible to specify different architectures for the build process and the target system. This enables cross-compilation to be managed by Snapcraft.

Enable cross-compilation

The architecture keyword can also be used to specify sets of architectures using the build-on and build-for sets. In the simplest case, this can be used to cross-compile on one architecture for a single, different architecture:

architectures:
  - build-on: amd64
    build-for: armhf

In this example, Snapcraft will only build the project on the amd64 architecture, and it will only build it for the armhf architecture.

Adjust the autotools configuration

When building for a particular architecture, Snapcraft will initialise the SNAPCRAFT_ARCH_TRIPLET environment variable in the build environment. This variable describes the platform and architecture that autotools uses to configure cross-compilation.

A project that uses the autotools plugin can adjust the options passed to the configure script for a given part by using the autotools-configure-parameters keyword that the plugin provides. In this case, the cross-compilation-part is configured in this way:

parts:
  cross-compilation-part:
    plugin: autotools
    source: .
    autotools-configure-parameters:
      - --prefix=/usr
      - --host=${SNAPCRAFT_ARCH_TRIPLET}

In this case, the parameters that the plugin passes to configure are overridden to specify the installation prefix and the host architecture to build for. This enables autotools to select the correct toolchain to use for building applications and libraries.

It is also necessary to specify which packages will supply the toolchain and libraries needed for cross-compilation. In this example, the cross-compilation-part also includes these definitions:

    build-packages:
      - on amd64 to armhf:
        - gcc-arm-linux-gnueabihf
        - libc6-dev-armhf-cross
        - libc6-armhf-cross

Advanced grammar is used in the build-packages definition to specify the packages containing the cross-compiler, C library and header files when building on amd64 for armhf platforms.

Build the snap

The snap is built in the usual way, by running snapcraft in the project directory:

$ snapcraft

Because linters are enabled by default for core22 snaps, this will produce warnings like the following:

not a dynamic executable
arm-binfmt-P: Could not open '/lib/ld-linux-armhf.so.3': No such file or directory
Unable to determine library dependencies for '/root/prime/usr/bin/autotools-cross-compile-example'

This is because Snapcraft is unable to resolve the dependencies for the target architecture using the library linter.

Since the /lib/ld-linux-armhf.so.3 library will be present in the base of the target system, these warnings can be suppressed for this case by including a lint section in the project file:

lint:
  ignore:
    - library

Rebuilding the snap should result in a build process without warnings and a snap in the project directory called autotools-cross-compile-example_1.0_armhf.snap or similar. The _armhf component of the file name indicates that the snap has been built for that architecture.

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Last updated 10 months ago.