The ctypes port to dune (yallop/ocaml-ctypes#588) is almost working, except for the need to select which subpackage to link based on the -mt predicate in ocamlfind. #96 has some discussion of this, but the specific META file in ctypes is:

package "foreign" (
 version = "0.14.0"
 description = "Dynamic linking of C functions"
 requires(-mt) = "ctypes.foreign.unthreaded"
 requires(mt) = "ctypes.foreign.threaded"

Both the unthreaded and threaded variants have different c stubs, and cannot be linked simultaneously without a module clash. I'm not sure how to make this work without some support for mt in a dune file, as the right package is selected based on whether the executable uses the threads library or not.

Suggestions for workarounds welcome to unblock the ctypes port...