Build error when using c stubs and external object file

Learning
1

I’m trying to run some hand written asm code in my ocaml program via c stubs using dune, but i cannot get it to work.

This is my dune file:

(library
 (name c_test)
 (extra_objects test)
 (foreign_stubs (language c) (names file1)))

(rule
 (targets test.o)
 (deps test.asm)
 (action (run nasm -f elf64 %{deps})))

I first tried without the extra_objects stanza, then the build of the library works fine, but it fails when linking the executable with undefined reference to 'ASM_Succ'. When looking at the dune documentation, it seems like the extra_objects stanza is the solution to the problem. But then i get the following error.

/usr/bin/ld: lib/test.o: in function `ASM_Succ':
test.asm:(.text+0x0): multiple definition of `ASM_Succ'; lib/libc_test_stubs.a(test.o):test.asm:(.text+0x0): first defined here

It seems like dune is adding test.o into lib/libc_test_stubs.a and also using test.o directly making my function defined twice. If i remove test.o from libc_test_stubs.a the command that dune runs succeeds.

Am i doing something wrong, or is dune misbehaving? Is there some other stanza i should be using to get this to work?

Any help is highly appreciated!

2

How are you declaring ASM_Succ in file1.c? Are you missing an extern?

Cheers,
Nicolas

3

these are my files:
file1.c

#include "caml/mlvalues.h"
#include "caml/memory.h"

int ASM_Succ(int, int);

CAMLprim value c_succ(value val, value rep) {
  CAMLparam2(val,rep);
  int v = Int_val(val);
  int r = Int_val(rep);
  int ret = ASM_Succ(v,r);

  CAMLreturn (Val_int(ret));
}

test.asm

bits 64
global ASM_Succ

section .text

ASM_Succ:
    inc rdi
    dec rsi
    jnz ASM_Succ
    mov rax, rdi
    ret

test.ml

external succ: int -> int -> int = "c_succ"

So yes, i was missing an extern. but adding it doesn’t seem to help. I still get the same error.

4

Today i found that I can successfully run the asm code if i move the .asm file to the bin folder and move the dune rule to build the asm file and the extra_objects stanza to the executable stanza in the bin folder dune file.

However, this doesn’t seem like a solution to the problem. I would like to be able to keep it in the lib folder. According to the dune docs extra_objects should work for libraries as well.

5

It is hard to give more precise help without all the details (eg the content of all Dune files). Is the code publicly accessible?

Cheers,
Nicolas

6

Created a github repo with the working and not working code.

7

Thanks.
I had a look, this looks like it’s a bug in dune.

It emits ocamlmklib -g -o lib/c_test_stubs lib/file1.o lib/test.o, which will make a cmxa with the following metadata:

Extra C object files: -lc_test_stubs lib/test.o

(making a double link)

Can you open an issue on dune? thanks

8

opened an issue now.

9

For anyone else following the discussion: the bugfix has been merged and will be included in the next release.

Cheers,
Nicolas