1.. _custom_cmake_toolchains:
2
3Custom CMake Toolchains
4#######################
5
6To use a custom toolchain defined in an external CMake file, :ref:`set these
7environment variables <env_vars>`:
8
9- Set :envvar:`ZEPHYR_TOOLCHAIN_VARIANT` to your toolchain's name
10- Set ``TOOLCHAIN_ROOT`` to the path to the directory containing your
11  toolchain's CMake configuration files.
12
13Zephyr will then include the toolchain cmake files located in the
14:file:`TOOLCHAIN_ROOT` directory:
15
16- :file:`cmake/toolchain/<toolchain name>/generic.cmake`: configures the
17  toolchain for "generic" use, which mostly means running the C preprocessor
18  on the generated
19  :ref:`devicetree` file.
20- :file:`cmake/toolchain/<toolchain name>/target.cmake`: configures the
21  toolchain for "target" use, i.e. building Zephyr and your application's
22  source code.
23
24Here <toolchain name> is the same as the name provided in
25:envvar:`ZEPHYR_TOOLCHAIN_VARIANT`
26See the zephyr files :zephyr_file:`cmake/modules/FindHostTools.cmake` and
27:zephyr_file:`cmake/modules/FindTargetTools.cmake` for more details on what your
28:file:`generic.cmake` and :file:`target.cmake` files should contain.
29
30You can also set ``ZEPHYR_TOOLCHAIN_VARIANT`` and ``TOOLCHAIN_ROOT`` as CMake
31variables when generating a build system for a Zephyr application, like so:
32
33.. code-block:: console
34
35   west build ... -- -DZEPHYR_TOOLCHAIN_VARIANT=... -DTOOLCHAIN_ROOT=...
36
37.. code-block:: console
38
39   cmake -DZEPHYR_TOOLCHAIN_VARIANT=... -DTOOLCHAIN_ROOT=...
40
41If you do this, ``-C <initial-cache>`` `cmake option`_ may useful. If you save
42your :makevar:`ZEPHYR_TOOLCHAIN_VARIANT`, :makevar:`TOOLCHAIN_ROOT`, and other
43settings in a file named :file:`my-toolchain.cmake`, you can then invoke cmake
44as ``cmake -C my-toolchain.cmake ...`` to save typing.
45
46Zephyr includes :file:`include/toolchain.h` which again includes a toolchain
47specific header based on the compiler identifier, such as ``__llvm__`` or
48``__GNUC__``.
49Some custom compilers identify themselves as the compiler on which they are
50based, for example ``llvm`` which then gets the :file:`toolchain/llvm.h` included.
51This included file may though not be right for the custom toolchain. In order
52to solve this, and thus to get the :file:`include/other.h` included instead,
53add the set(TOOLCHAIN_USE_CUSTOM 1) cmake line to the generic.cmake and/or
54target.cmake files located under
55:file:`<TOOLCHAIN_ROOT>/cmake/toolchain/<toolchain name>/`.
56
57When :makevar:`TOOLCHAIN_USE_CUSTOM` is set, the :file:`other.h` must be
58available out-of-tree and it must include the correct header for the custom
59toolchain.
60A good location for the :file:`other.h` header file, would be a
61directory under the directory specified in ``TOOLCHAIN_ROOT`` as
62:file:`include/toolchain`.
63To get the toolchain header included in zephyr's build, the
64:makevar:`USERINCLUDE` can be set to point to the include directory, as shown
65here:
66
67.. code-block:: console
68
69   west build -- -DZEPHYR_TOOLCHAIN_VARIANT=... -DTOOLCHAIN_ROOT=... -DUSERINCLUDE=...
70
71.. _cmake option:
72   https://cmake.org/cmake/help/latest/manual/cmake.1.html#options
73