Calling OCaml from C in dune

2025-02-02

category: tech

These are some notes on putting together a dune file for creating a C executable which calls OCaml code.

The OCaml manual describes how to call OCaml code from C under the section “Advanced example with callbacks” in the chapter “Interfacing C with OCaml”, using the following invocations to ocamlopt:

$ ocamlopt -output-obj -o modcaml.o mod.ml
$ ocamlopt -c modwrap.c
$ cp `ocamlopt -where`/libasmrun.a mod.a && chmod +w mod.a
$ ar r mod.a modcaml.o modwrap.o
$ cc -o prog -I `ocamlopt -where` main.c mod.a -lm

Reading the section in the OCaml manual about ocamlopt command line options carefully, it looks like we can use -output-complete-obj to simplify above commands:

$ ocamlopt -output-complete-obj -o modcaml.o mod.ml
$ ocamlopt -c modwrap.c
$ cc -o prog_native2 -I `ocamlopt -where` main.c modcaml.o modwrap.o -lm

Translating to dune

We first want to recreate the modcaml.o file. Grepping in the dune codebase for -output-complete-obj, we find this code which matches on Executables.Link_mode.t.

The dune docs mention how to specify linking modes for executables, so we’ll try that.

(executables
 (names mod)
 (modes object))

After some trial and error, I’ve figured out that the target corresponding to this rule is mod.exe.o:

$ dune build --verbose mod.exe.o
...
Actual targets:
- _build/default/mod.exe.o
Running[1]: (cd _build/default && /.../.opam/5.2.1/bin/ocamlc.opt -w @1..3@5..28@31..39@43@46..47@49..57@61..62@67@69-40 -strict-sequence -strict-formats -short-paths -keep-locs -g -bin-annot -bin-annot-occurrences -I .mod.eobjs/byte -no-alias-deps -opaque -o .mod.eobjs/byte/dune__exe__Mod.cmi -c -intf mod.mli)
Running[2]: (cd _build/default && /.../.opam/5.2.1/bin/ocamlopt.opt -w @1..3@5..28@31..39@43@46..47@49..57@61..62@67@69-40 -strict-sequence -strict-formats -short-paths -keep-locs -g -I .mod.eobjs/byte -I .mod.eobjs/native -intf-suffix .ml -no-alias-deps -opaque -o .mod.eobjs/native/dune__exe__Mod.cmx -c -impl mod.ml)
Running[3]: (cd _build/default && /.../.opam/5.2.1/bin/ocamlopt.opt -w @1..3@5..28@31..39@43@46..47@49..57@61..62@67@69-40 -strict-sequence -strict-formats -short-paths -keep-locs -g -o mod.exe.o -output-complete-obj .mod.eobjs/native/dune__exe__Mod.cmx)

I’m not exactly sure if there are better ways to do this, but we manually define rules to run the remaining steps:

(rule
 (targets modwrap.o)
 (deps modwrap.c)
 (action
  (run ocamlopt %{deps})))

(rule
 (targets main.exe)
 (deps main.c mod.exe.o modwrap.o)
 (action
  (run %{cc} -o %{targets} -I %{ocaml_where} %{deps} -lm)))

And voila!

$ dune exec ./main.exe
fib(10) = Result is: 89

The full code is available in the calling-ocaml-from-c repository.

index | atom/rss | generated off rev 14870e6