I recently had some trouble getting ctypes working, so I thought I’d share my solution.

I found the Real World OCaml chapter on FFIs, and I tried following their advice first. They suggest:

brew install libffi
opam install ctypes

But it doesn’t quite work. For one, you also need to install ctypes-foreign. For two, the brew installation of libffi doesn’t automatically install itself globally, so ctypes can’t find it.

Here’s the right incantation:

brew install libffi
cd /usr/lib/
sudo mv libffi.dylib libffi.dylib.orig
sudo ln -s /usr/local/opt/libffi/lib/libffi.dylib
LIBFFI_CFLAGS=-I/usr/include/ffi CFLAGS=-I/usr/include/ffi opam install ctypes ctypes-foreign

Now, this is merely a good start. Once I had this much working, I still couldn’t get OCaml to call C functions! It came down to a bear of a linking issue… no combination of LD_PATH and LD_LIBRARY_PATH and LD_PRELOAD would help, nor would -lflags -cclib,-L. I just couldn’t get ctypes to see my shared library! I finally found an option to ld that does the trick: -force_load. Here’s my build line:

ocamlbuild -no-hygiene -pkg ctypes.foreign -lflags "-cclib -force_load /path/to/libfoo.a" bar.native

I imagine you can get away without -no-hygiene; I couldn’t get corebuild to work at all.

In any case, special thanks to Arjun Narayan for helping me debug this, and to Spiros Eliopolous for mockery absolutely nothing.