Building an Ad Hoc ``.cmxs``
----------------------------

.. TODO(diataxis) howto: Building an Ad Hoc ``.cmxs``

In the model exposed by Dune, a ``.cmxs`` target is created for each
library. However, the ``.cmxs`` format itself is more flexible and is
capable to containing arbitrary ``.cmxa`` and ``.cmx`` files.

For the specific cases where this extra flexibility is needed, one can use
:ref:`variables-for-artifacts` to write explicit rules to build ``.cmxs`` files
not associated to any library.

Below is an example where we build ``my.cmxs`` containing ``foo.cmxa`` and
``d.cmx``. Note how we use a :doc:`/reference/dune/library` stanza to set
up the compilation of ``d.cmx``.

.. code:: dune

    (library
     (name foo)
     (modules a b c))

    (library
     (name dummy)
     (modules d))

    (rule
     (targets my.cmxs)
     (action (run %{ocamlopt} -shared -o %{targets} %{cmxa:foo} %{cmx:d})))