I found this post on GitHub: 
https://github.com/ocamllabs/ocaml-ctypes/issues/236
I added the arguments to pass to ld and the mlx_init error disappear 
(now I've an error about strlcpy used by minilibx which is not defined 
on Linux).

So my command is
```
ocamlfind ocamlopt  \
   -linkpkg -package ctypes.foreign \
   -ccopt -I/usr/local/lib \
   -cclib "-Wl,--whole-archive" \
   -cclib -lmlx  \
   -cclib "-Wl,--no-whole-archive" \
   -cclib "-Wl,-E" \
   -cclib -lX11 \
   -cclib -lXext \
   minilibx.cmx \
   hello.ml \
   -o hello.native
```

On 07/31/2016 03:27 PM, Matthew Saffer wrote:
>
> If ld can't find the library, don't you need to change your LD_LOAD_PATH?
>
> On Sun, Jul 31, 2016, 06:35 Danny Willems <contact@danny-willems.be 
> <mailto:contact@danny-willems.be>> wrote:
>
>     If I use a relative path (for example -cclib -L../minilibx) with
>     ocamlopt, it compiles but I have the same issue when running the
>     binary.
>
>     On 07/31/2016 12:22 PM, Danny Willems wrote:
>     > Hello,
>     >
>     > I'm writing a binding OCaml to the C library "minilibx", here a
>     mirror
>     > of the library https://github.com/dannywillems/minilibx.
>     >
>     > The Makefile installs the library file libmlx.a in
>     /usr/local/lib and
>     > you have to add the flags -lmlx -lX11 and -lXext to compile. So you
>     > have something like that to compile a test file written in C:
>     >
>     > ```
>     > gcc  -I /usr/local/include -L/usr/local/lib test.c -lmlx  -lX11
>     -lXext
>     > ```
>     >
>     > I wrote a simple binding to mlx_init, mlx_new_window and mlx_loop in
>     > minilibx.ml <http://minilibx.ml>:
>     >
>     > ```
>     > open Ctypes
>     > open Foreign
>     >
>     > type mlx_ptr = unit ptr
>     > type mlx_win = unit ptr
>     >
>     > let mlx_ptr : mlx_ptr typ = ptr void
>     > let mlx_win : mlx_win typ = ptr void
>     >
>     > let mlx_init =
>     >   foreign "mlx_init" (void @-> returning mlx_ptr)
>     >
>     > let mlx_new_window =
>     >   foreign "mlx_new_window" (mlx_ptr @-> int @-> int @-> string @->
>     > returning mlx_win)
>     >
>     > let mlx_loop =
>     >   foreign "mlx_loop" (mlx_ptr @-> returning void)
>     > ```
>     > with the corresponding interface in minilibx.mli and compile it with
>     >
>     > ```
>     > ocamlfind ocamlc -c -package ctypes.foreign -linkpkg minilibx.mli
>     >
>     > ocamlfind ocamlc -c -package ctypes.foreign -linkpkg minilibx.ml
>     <http://minilibx.ml>
>     > ```
>     >
>     > Now I wrote a test file named hello.ml <http://hello.ml>
>     > ```
>     > let () =
>     >   let i = Minilibx.mlx_init () in
>     >   let w = Minilibx.mlx_new_window i 640 480 "Hello, World" in
>     >   Minilibx.mlx_loop i
>     > ```
>     >
>     > and compile it with
>     >
>     > ```
>     > ocamlfind ocamlc -o hello -ccopt -L/usr/local/lib -cclib -lmlx
>     -cclib
>     > -lX11 -cclib -lXext minilibx.cmo -package ctypes.foreign -linkpkg
>     > hello.ml <http://hello.ml>
>     > ```
>     > and when I execute hello, I have
>     >
>     > ```
>     > Fatal error: exception
>     >
>     Dl.DL_error("/home/dannywillems/.opam/ctypes/lib/stublibs/dllctypes-foreign-base_stubs.so:
>     > undefined symbol: mlx_init")
>     > ```
>     >
>     > So linking seems not working. But I can't find why.
>     >
>     > If I try to compile in native with ocamlopt (cmo -> cmx), linking is
>     > not done at compile time:
>     > ```
>     > /usr/bin/ld: cannot find -lmlx
>     > collect2: error: ld returned 1 exit status
>     > File "caml_startup", line 1:
>     > Error: Error during linking
>     > ```
>     >
>     > Am I using the right option (-ccopt -L/usr/local/lib)? And why
>     ocamlc
>     > doesn't output the error linking message?
>     >
>     > Thank you for you help.
>     >
>     >
>
>
>     --
>     Caml-list mailing list.  Subscription management and archives:
>     https://sympa.inria.fr/sympa/arc/caml-list
>     Beginner's list: http://groups.yahoo.com/group/ocaml_beginners
>     Bug reports: http://caml.inria.fr/bin/caml-bugs
>