[Caml-list] calling OCaml from C on non-Mac platforms?

[John Nowak]

I'm trying to call OCaml from C. Essentially, I have this for "main.c":

#include <caml/mlvalues.h>
#include <caml/callback.h>

void hello() {
static value *closure = NULL;
if(!closure) closure = caml_named_value("hello");
caml_callback(*closure, Val_unit);
}

int main(int argc, char **argv) {
caml_main(argv);
hello();
return 0;
}

And the following "hello.ml":

let hello () = print_endline "Hello, world!"
let () = Callback.register "hello" hello

I can then compile as follows on OS X 10.5:

ocamlopt -c hello.ml -o hello.cmx
ocamlopt -output-obj hello.cmx -o hello.o
gcc -c main.c -o main.o -I"`ocamlc -where`"
gcc -o hello hello.o main.o -L"`ocamlc -where`" -ldl -lm -lasmrun

This works fine on OS X. However, it does not seem to work on Linux/
AMD64 or FreeBSD/AMD64. I've tried suggestions from all over the web
and I've tried the approach given in the manual and nothing works. In
all non-Mac cases, I get this on the second line:

hello.o: file not recognized: File truncated
Error during linking

One thing I can do is do this instead of the first two lines:

ocamlopt -c -output-obj hello.ml

However, then I get the following:

hello.o: In function `camlHello__hello_58':
(.text+0x8): undefined reference to
`camlPervasives__print_endline_298'
/usr/lib/ocaml/3.10.2/libasmrun.a(startup.o): In function
`caml_main':
(.text+0x25f): undefined reference to `caml_data_segments'
/usr/lib/ocaml/3.10.2/libasmrun.a(startup.o): In function
`caml_main':
(.text+0x273): undefined reference to `caml_code_segments'
...

Does anyone have a correct set of instructions for doing this on non-
Mac platforms? Thanks.

- John

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[Eric Cooper]

On Sun, Nov 15, 2009 at 08:27:49AM -0500, John Nowak wrote:
> I'm trying to call OCaml from C.

> [...]

> ocamlopt -output-obj hello.cmx -o hello.o

This is the main problem. You're clobbering the hello.o that OCaml
produced when compiling hello.ml into native code.

> Does anyone have a correct set of instructions for doing this on
> non-Mac platforms? Thanks.

This worked fine for me:

ocamlopt -output-obj -o temp.o hello.ml
gcc -o hello main.c temp.o -L/usr/lib/ocaml -lasmrun -lm -ldl

[John Nowak]

On Nov 15, 2009, at 9:55 AM, Eric Cooper wrote:

> On Sun, Nov 15, 2009 at 08:27:49AM -0500, John Nowak wrote:
>
>> I'm trying to call OCaml from C.
>> [...]
>> ocamlopt -output-obj hello.cmx -o hello.o
>
> This is the main problem. You're clobbering the hello.o that OCaml
> produced when compiling hello.ml into native code.

Works well now, thanks. Perhaps the documentation could be amended to
make this more clear. Although, in retrospect, I should've realized
what was happening.

- John