How to Override the Default C Entrypoint With C Stubs

In some cases, it may be necessary to override the default C entry point of an OCaml program. For example, this is the case if you want to let your program handle argument wildcards expansion on Windows.

Let’s consider a trivial “Hello world” program contained in a hello.ml file:

let () = print_endline "Hello, world!"

The default C entry point is a main function, originally defined in runtime/main.c. It can be overridden by defining a main function that will at some point call the OCaml runtime. Let’s write such a minimal example in a main.c file:

#include <stdio.h>

#define CAML_INTERNALS
#include "caml/misc.h"
#include "caml/mlvalues.h"
#include "caml/sys.h"
#include "caml/callback.h"

/* This is the new entry point */
int main(int argc, char_os **argv)
{
    /* Here, we just print a statement */
    printf("Doing stuff before calling the OCaml runtime\n");

    /* Before calling the OCaml runtime */
    caml_main(argv);
    caml_do_exit(0);
    return 0;
}

The foreign_stubs stanza can be leveraged to compile and link our OCaml program with the new C entry point defined in main.c:

(executable
 (name hello)
 (foreign_stubs
  (language c)
  (names main)))

With this dune file, the whole program can be compiled by merely calling dune build. When run, the output shows that it calls the custom entry point we defined:

$ dune build
$ _build/default/hello.exe
Doing stuff before calling the OCaml runtime
Hello, world!