18 Optionally Using an External Toolchain

You might want to use an external toolchain as part of your development. If this is the case, the fundamental steps you need to accomplish are as follows:

  • Understand where the installed toolchain resides. For cases where you need to build the external toolchain, you would need to take separate steps to build and install the toolchain.

  • Make sure you add the layer that contains the toolchain to your bblayers.conf file through the BBLAYERS variable.

  • Set the EXTERNAL_TOOLCHAIN variable in your local.conf file to the location in which you installed the toolchain.

The toolchain configuration is very flexible and customizable. It is primarily controlled with the TCMODE variable. This variable controls which tcmode-*.inc file to include from the meta/conf/distro/include directory within the Source Directory.

The default value of TCMODE is “default”, which tells the OpenEmbedded build system to use its internally built toolchain (i.e. tcmode-default.inc). However, other patterns are accepted. In particular, “external-*” refers to external toolchains. One example is the Mentor Graphics Sourcery G++ Toolchain. Support for this toolchain resides in the separate meta-sourcery layer at https://github.com/MentorEmbedded/meta-sourcery/. See its README file for details about how to use this layer.

Another example of external toolchain layer is meta-arm-toolchain supporting GNU toolchains released by ARM.

You can find further information by reading about the TCMODE variable in the Yocto Project Reference Manual’s variable glossary.