# Using ocaml-ctypes with WebAssembly? **URL:** \ **Category:** Ecosystem\ **Tags:** ctypes, yaml, js\_of\_ocaml, ffi\ **Created:** [May 31, 2023, 12:19pm UTC](https://discuss.ocaml.org/t/using-ocaml-ctypes-with-webassembly/12279 "2023-05-31T12:19:02Z")\ **Posts on this page:** 11\ **Page:** 1
**Author:** ![tpetr](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/tpetr/32/4475_2.png) [@tpetr](https://discuss.ocaml.org/u/tpetr)\ **Post date:** [May 31, 2023, 12:19pm UTC](https://discuss.ocaml.org/t/using-ocaml-ctypes-with-webassembly/12279/1 "2023-05-31T12:19:02Z")
Hi there, I’ve been using js\_of\_ocaml to make semgrep run from inside a web browser, and am hitting an interesting issue with one of our dependencies. We use ocaml-yaml for YAML parsing, which uses ocaml-ctypes under the hood to interface with libyaml (a C library). I’ve managed to compile libyaml to WebAssembly and write all the JS glue code to wire everything up. However, the memory offsets calculated by ocaml-ctypes for libyaml struct fields are all slightly off due to the fact that ocaml-ctypes was built on a 64-bit machine (size\_t and void\* are 8 bytes) but WebAssembly is 32-bit (size\_t and void\* are 4 bytes). It looks like sizes and alignments are hardcoded at build time: [ocaml-ctypes/Makefile at 46cc3ffc47119264bf530905fce4a2c3750f4c8c · yallop/ocaml-ctypes · GitHub](https://github.com/yallop/ocaml-ctypes/blob/46cc3ffc47119264bf530905fce4a2c3750f4c8c/Makefile#L129-L131) I did some ugly hacks to work around this with jsoo (see [js: fix libyaml bindings by tpetr · Pull Request #7918 · returntocorp/semgrep · GitHub](https://github.com/returntocorp/semgrep/pull/7918)) because I didn’t think it was feasible to cross-compile or drastically alter ocaml-ctypes, but my hope is that there is a simpler solution that I’m not aware of. Has anyone hit this issue before? Any advice? Thanks! ---
**Author:** ![zeroexcuses](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/zeroexcuses/32/1692_2.png) [@zeroexcuses](https://discuss.ocaml.org/u/zeroexcuses)\ **Post date:** [May 31, 2023, 12:48pm UTC](https://discuss.ocaml.org/t/using-ocaml-ctypes-with-webassembly/12279/2 "2023-05-31T12:48:32Z")
I am not sure if this will be helpful, since I think you are going for a much deeper OCaml / FFI interaction than I did. I have a web app where - OCaml compiled to JS via jsoo - Rust compiled to wasm32 via `cargo build --target=wasm32-unknown-unkonwn` talk to each other. They communicate based on just passing JavaScript objects back & forth. On the rust side, I use [web-sys - The `wasm-bindgen` Guide](https://rustwasm.github.io/wasm-bindgen/web-sys/index.html) . I can Chrome/console.log values from ether side. Two ideas come to mind: 1. are you compiling libyaml (C lib) via emscripten ? If so, is there some emscripten header filer you can include, which will let you read/create javascript objects from libyaml ? 2. I don’t know how’s your Rust. There appears to be a pure Rust YAML parser at [https://crates.io/crates/yaml-rust](https://crates.io/crates/yaml-rust) (never used it) ---
**Author:** ![tpetr](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/tpetr/32/4475_2.png) [@tpetr](https://discuss.ocaml.org/u/tpetr)\ **Post date:** [May 31, 2023, 1:44pm UTC](https://discuss.ocaml.org/t/using-ocaml-ctypes-with-webassembly/12279/3 "2023-05-31T13:44:57Z")
Thanks for the reply and the ideas! You’re correct, this is unfortunately a very deep interaction. Semgrep is tightly integrated with the shape of ocaml-yaml, so I don’t think I can adopt the approach you suggested (we need to be able to support jsoo and native executables at the same time). I just had another idea that I’m investigating: what if we patch ocaml-ctypes’ sizeof and alignment functions? For example: ```ocaml let patched_sizeof : type a. a Ctypes_static.typ -> int = function | Pointer _ -> 4 | Primitive Size_t -> 4 | other -> Ctypes_static.sizeof other ``` If I can manage to override the JS method for `Ctypes_static.sizeof` with `patched_sizeof` then I think things should Just Work ---
**Author:** ![zeroexcuses](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/zeroexcuses/32/1692_2.png) [@zeroexcuses](https://discuss.ocaml.org/u/zeroexcuses)\ **Post date:** [May 31, 2023, 3:17pm UTC](https://discuss.ocaml.org/t/using-ocaml-ctypes-with-webassembly/12279/4 "2023-05-31T15:17:35Z")
I agree; for things that are deeply intertwined, it probably makes more sense to have ocaml-ctypes detect whether it is interacting with js/wasm and act accordingly. ---
**Author:** ![zeroexcuses](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/zeroexcuses/32/1692_2.png) [@zeroexcuses](https://discuss.ocaml.org/u/zeroexcuses)\ **Post date:** [May 31, 2023, 3:19pm UTC](https://discuss.ocaml.org/t/using-ocaml-ctypes-with-webassembly/12279/5 "2023-05-31T15:19:01Z")
PS, I wonder, for a temporary hack, if you could modify it to … read the ptr size as an Environment variable? 🙂 ---
**Author:** ![tpetr](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/tpetr/32/4475_2.png) [@tpetr](https://discuss.ocaml.org/u/tpetr)\ **Post date:** [May 31, 2023, 3:39pm UTC](https://discuss.ocaml.org/t/using-ocaml-ctypes-with-webassembly/12279/6 "2023-05-31T15:39:32Z")
Yeah that could be a good approach too. 👍 Newbie question: Let’s say that I have a branch of ocaml-ctypes with this functionality. I’m assuming that getting my PR merged and an official ocaml-ctypes release cut could take awhile… do you know how hard it would be to get my opam project to depend on this branch? I know I can run `opam pin` on the command line, but it’s unclear if I can do the same in a .opam file. ---
**Author:** ![yallop](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/yallop/32/517_2.png) [@yallop](https://discuss.ocaml.org/u/yallop)\ **Post date:** [May 31, 2023, 6:27pm UTC](https://discuss.ocaml.org/t/using-ocaml-ctypes-with-webassembly/12279/7 "2023-05-31T18:27:04Z")
ctypes supports cross-compilation. Details here: [Don't run compiled binaries while cross-compiling by whitequark · Pull Request #383 · yallop/ocaml-ctypes · GitHub](https://github.com/yallop/ocaml-ctypes/pull/383) ---
**Author:** ![tpetr](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/tpetr/32/4475_2.png) [@tpetr](https://discuss.ocaml.org/u/tpetr)\ **Post date:** [May 31, 2023, 9:39pm UTC](https://discuss.ocaml.org/t/using-ocaml-ctypes-with-webassembly/12279/8 "2023-05-31T21:39:32Z")
Interesting, thanks. I’ve been doing some more digging (and learning) and I realize now that the memory offsets are baked into libyaml at build time (because it’s using the stubgen functionality), so it really does look like cross-compilation is needed here. Is there a good primer on how to set up 32-bit cross-compilation in dune? I’ve been googling and it seems like the latest 32-bit opam variant is fairly old (4.11.2, semgrep wants 4.13, but I’m not sure how hard of a requirement that is). ---
**Author:** ![yallop](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/yallop/32/517_2.png) [@yallop](https://discuss.ocaml.org/u/yallop)\ **Post date:** [May 31, 2023, 10:01pm UTC](https://discuss.ocaml.org/t/using-ocaml-ctypes-with-webassembly/12279/9 "2023-05-31T22:01:46Z")
> [@tpetr](#): > > I’ve been googling and it seems like the latest 32-bit opam variant is fairly old (4.11.2, semgrep wants 4.13, but I’m not sure how hard of a requirement that is). You can install 32-bit versions of recent compilers using opam’s `ocaml-option-*` machinery. For example, the following command installs a 32-bit OCaml 4.13 switch: ```bash opam switch create 4.13+32-bit --packages=ocaml-variants.4.13.0+options,ocaml-option-32bit --repos=default,beta ``` ---
**Author:** ![tpetr](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/tpetr/32/4475_2.png) [@tpetr](https://discuss.ocaml.org/u/tpetr)\ **Post date:** [June 1, 2023, 1:27pm UTC](https://discuss.ocaml.org/t/using-ocaml-ctypes-with-webassembly/12279/10 "2023-06-01T13:27:26Z")
Excellent, thank you for the pointer! It looks like the 32-bit compilers are only built for x86 (I’m on an M1 MBP), so I just need to figure out how to get a compiler that works for me and then I think we should be golden. ---
**Author:** ![tpetr](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/tpetr/32/4475_2.png) [@tpetr](https://discuss.ocaml.org/u/tpetr)\ **Post date:** [June 5, 2023, 2:14pm UTC](https://discuss.ocaml.org/t/using-ocaml-ctypes-with-webassembly/12279/11 "2023-06-05T14:14:15Z")
I’m going to mark @yallop’s suggestion to cross-compile as the answer. Thanks, all!