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.. _custom_cmake_toolchains:
Custom CMake Toolchains
To use a custom toolchain defined in an external CMake file, :ref:`set these
environment variables <env_vars>`:
- Set :envvar:`ZEPHYR_TOOLCHAIN_VARIANT` to your toolchain's name
- Set :envvar:`TOOLCHAIN_ROOT` to the path to the directory containing your
toolchain's CMake configuration files.
Zephyr will then include the toolchain cmake files located in the
:file:`TOOLCHAIN_ROOT` directory:
- :file:`cmake/toolchain/<toolchain name>/generic.cmake`: configures the
toolchain for "generic" use, which mostly means running the C preprocessor
on the generated
:ref:`devicetree` file.
- :file:`cmake/toolchain/<toolchain name>/target.cmake`: configures the
toolchain for "target" use, i.e. building Zephyr and your application's
source code.
Here <toolchain name> is the same as the name provided in
See the zephyr files :zephyr_file:`cmake/generic_toolchain.cmake` and
:zephyr_file:`cmake/target_toolchain.cmake` for more details on what your
:file:`generic.cmake` and :file:`target.cmake` files should contain.
You can also set ``ZEPHYR_TOOLCHAIN_VARIANT`` and ``TOOLCHAIN_ROOT`` as CMake
variables when generating a build system for a Zephyr application, like so:
.. code-block:: console
.. code-block:: console
If you do this, ``-C <initial-cache>`` `cmake option`_ may useful. If you save
your :makevar:`ZEPHYR_TOOLCHAIN_VARIANT`, :makevar:`TOOLCHAIN_ROOT`, and other
settings in a file named :file:`my-toolchain.cmake`, you can then invoke cmake
as ``cmake -C my-toolchain.cmake ...`` to save typing.
Zephyr includes :file:`include/toolchain.h` which again includes a toolchain
specific header based on the compiler identifier, such as ``__llvm__`` or
Some custom compilers identify themselves as the compiler on which they are
based, for example ``llvm`` which then gets the :file:`toolchain/llvm.h` included.
This included file may though not be right for the custom toolchain. In order
to solve this, and thus to get the :file:`include/other.h` included instead,
add the set(TOOLCHAIN_USE_CUSTOM 1) cmake line to the generic.cmake and/or
target.cmake files located under
:file:`<TOOLCHAIN_ROOT>/cmake/toolchain/<toolchain name>/`.
When :makevar:`TOOLCHAIN_USE_CUSTOM` is set, the :file:`other.h` must be
available out-of-tree and it must include the correct header for the custom
A good location for the :file:`other.h` header file, would be a
directory under the directory specified in :envvar:`TOOLCHAIN_ROOT` as
To get the toolchain header included in zephyr's build, the
:makevar:`USERINCLUDE` can be set to point to the include directory, as shown
.. code-block:: console
.. _cmake option: