#ocaml #ffi #rust

sys ocaml-sys

Low-level OCaml bindings for Rust

23 releases (13 breaking)

Uses new Rust 2021

0.23.0 Jun 6, 2022
0.22.1 Mar 23, 2022
0.22.0 May 16, 2021
0.20.1 Mar 16, 2021
0.14.0 Jul 3, 2020

#165 in Development tools

Download history 6715/week @ 2022-06-03 3943/week @ 2022-06-10 5521/week @ 2022-06-17 5935/week @ 2022-06-24 3412/week @ 2022-07-01 3221/week @ 2022-07-08 4893/week @ 2022-07-15 4581/week @ 2022-07-22 5135/week @ 2022-07-29 4309/week @ 2022-08-05 2453/week @ 2022-08-12 4471/week @ 2022-08-19 6605/week @ 2022-08-26 3632/week @ 2022-09-02 4386/week @ 2022-09-09 3244/week @ 2022-09-16

18,905 downloads per month
Used in 3 crates (2 directly)

ISC license


ocaml-rs - OCaml extensions in Rust

ocaml-rs allows for OCaml extensions to be written directly in Rust with no C stubs. It was originally forked from raml, but has been almost entirely re-written thanks to support from the OCaml Software Foundation.

Works with OCaml versions 4.07.0 and up

Please report any issues on github

NOTE: While ocaml-rs can be used safely, it does not prevent a wide range of potential errors or mistakes. It should be thought of as a Rust implementation of the existing C API. ocaml-interop can be used to perform safe OCaml/Rust interop. ocaml-rs uses ocaml-interop behind the scenes to interact with the garbage collector. ocaml-rs also exports an interop module, which is an alias for ocaml_interop and the two interfaces can be combined if desired.


Getting started

ocaml-rust-starter is a basic example to help get started with ocaml-rs.

On the Rust side, you will need to add the following to your Cargo.toml:

ocaml = "*"


ocaml = {git = "https://github.com/zshipko/ocaml-rs"}

For macOS you will need also to add the following to your project's .cargo/config file:

rustflags = ["-C", "link-args=-Wl,-undefined,dynamic_lookup"]

This is because macOS doesn't allow undefined symbols in dynamic libraries by default.

Additionally, if you plan on releasing to opam, you will need to vendor your Rust dependencies to avoid making network requests during the build phase, since reaching out to crates.io/github will be blocked by the opam sandbox. To do this you should run:

cargo vendor

then follow the instructions for editing .cargo/config

Build options

By default, building ocaml-sys will invoke the ocamlopt command to figure out the version and location of the OCaml compiler. There are a few environment variables to control this.

  • OCAMLOPT (default: ocamlopt) is the command that will invoke ocamlopt
  • OCAML_VERSION (default: result of $OCAMLOPT -version) is the target runtime OCaml version.
  • OCAML_WHERE_PATH (default: result of $OCAMLOPT -where) is the path of the OCaml standard library.
  • OCAML_INTEROP_NO_CAML_STARTUP (default: unset) can be set when loading an ocaml-rs library into an OCaml bytecode runtime (such as utop) to avoid linking issues with caml_startup

If both OCAML_VERSION and OCAML_WHERE_PATH are present, their values are used without invoking ocamlopt. If any of those two env variables is undefined, then ocamlopt will be invoked to obtain both values.

Defining the OCAML_VERSION and OCAML_WHERE_PATH variables is useful for saving time in CI environments where an OCaml install is not really required (to run clippy for example).


  • derive
    • enabled by default, adds #[ocaml::func] and friends and derive implementations for FromValue and ToValue
  • link
    • link the native OCaml runtime, this should only be used when no OCaml code will be linked statically
  • no-std
    • Allows ocaml to be used in #![no_std] environments like MirageOS