I've got native OCaml code running! It seems to pass all the runtime tests, and as I fixed some GC segfaults I'm now pretty confident in the backend code. It's not optimized at all, but you can now experiment with OCaml on ESP32 MCUs.

# OCaml cross-compiler installation

Switch opam to OCaml version 4.06 or a 4.07 as only these two versions are supported, using the opam switch command.

Add my cross-compilation repository in opam, with:

opam repo add cross-esp32 https://github.com/TheLortex/opam-cross-esp32.git

After that, you'll be able to install the cross-compiler, with:

opam install ocaml-esp32

You now have an OCaml cross-compiler for esp32 installed in ~/.opam/<switch>/esp32-sysroot/.

# Cross-compiling OCaml libraries

Your favorite build system can already cross-compile code. You can either use

• jbuilder build -x esp32 ...
• ocamlfind -toolchain esp32 ...
• env OCAMLFIND_TOOLCHAIN=esp32 ocamlbuild ... to build libraries.

The libraries used by these build systems are located in ~/.opam/<switch>/esp32-sysroot/lib/. A good thing to know is that jbuilder automatically makes the difference between host code and target code, hence enabling the use of ppx tools.

# Building an ESP32 application

To build an esp32 application, you'll need to link native code with the ESP-Iot Development Framework which contains libraries, bootloader code and linker scripts.

Hello caml is an example of project structure using jbuilder that builds a flashable binary for esp32.

Basically, it's not complicated:

• Build and ship OCaml code into a single object:
  • using the -output-complete-obj option on the native compiler ocamlopt.
  • using jbuilder build -x esp32 _build/default.esp32/main.exe.o that will automatically use the option.
• This object file's entry point is caml_main. So the next thing to do is create a startup-c.c file that defines a function app_main which calls caml_main.
• Then everything needs to be linked together, and the ESP32 framework takes the lead to create the final binary.