can't receive function parameters from ocaml5 runtime

[tremon015]

I suspect I'm jumping the gun here, but it doesn't appear possible to reliably receive parameter values from the ocaml5 runtime: the sys::Values don't match what is sent from ocaml. Has the ABI changed, perhaps?

First off, every ocaml::func seems to end in a segfault when unpacking the passed parameters, like this one:

Program received signal SIGSEGV, Segmentation fault.
0x00007ffff7f74968 in ocaml::conv::::from_value

Using a native_func gives some more insight into what's happening:

#[ocaml::native_func]
#[ocaml::sig("int -> int -> int -> int -> int")]
pub fn select(sel: Value, fst: Value, snd: Value, ext: Value) -> Value {
   eprintln!("select: sel={:?}, fst={:?}, snd={:?}, ext={:?}", sel, fst, snd, ext);
   let sel = unsafe { sel.int_val() };
   if sel == 0 {
      fst
   }
   else {
      snd
   }
}

   let res = LLc.select 1 2 3 4 in
   traceln "select result: %d" res;

which gives the following output:

select: sel=Raw(5), fst=Raw(9), snd=Root(0x0), ext=Raw(93824995897992)
Program received signal SIGSEGV, Segmentation fault

These values appear consistent over multiple runs, so I don't think it's reading uninitialized memory, just the wrong locations.

I'll supply a gdb backtrace, but I suspect it's of little value given the parameter values above:

Program received signal SIGSEGV, Segmentation fault.
0x00007ffff7f6fce3 in ocaml_boxroot_sys::boxroot_get (br=...)
    at $HOME/.cargo/registry/src/index.crates.io-6f17d22bba15001f/ocaml-boxroot-sys-0.3.0/src/lib.rs:24
24	    *(core::cell::UnsafeCell::raw_get(boxroot_get_ref(br)))
(gdb) bt
#0  0x00007ffff7f6fce3 in ocaml_boxroot_sys::boxroot_get (br=...)
    at $HOME/.cargo/registry/src/index.crates.io-6f17d22bba15001f/ocaml-boxroot-sys-0.3.0/src/lib.rs:24
#1  0x00007ffff7f7146d in ocaml::root::Root::get (self=0x7fffffffe4e8) at src/root.rs:34
#2  0x00007ffff7f6f1c6 in ocaml::value::Value::raw (self=0x7fffffffe4e0)
    at src/value.rs:163
#3  0x00007ffff7f6c0e3 in llc::select (sel=..., fst=..., snd=..., ext=...) at lib.rs:118
#4  <signal handler called>
#5  0x0000555555663793 in camlDune__exe__Main.entry () at src/controller/main.ml:180
#6  0x000055555565f21b in caml_program ()

On the other hand, returning complex values from Rust to OCaml works fine:

      let mut tuple = Value::alloc_tuple(2);
      ocaml::body!(gc: {
         tuple.store_field(gc, 0, &val1);
         tuple.store_field(gc, 1, &val2);
         Value::some(gc, tuple)
      })

This value gets received fine as an (int * int) option on the ocaml side.

This is using

• ocaml-rs from {git = "https://github.com/zshipko/ocaml-rs", branch="ocaml5"}
• rustc 1.70.0 (90c541806 2023-05-31)
• OCaml version 5.0.0

But I get the same results in a clean ocaml 5.1.0 switch.

[tremon015]

nosing around the memory area, I can see the correct values appearing in memory (twice even), but I know too little about either OCaml's or Rust's memory layout to diagnose this issue any further:

0x7fffffffe480:	0x03	0x00	0x00	0x00	0x00	0x00	0x00	0x00
0x7fffffffe488:	0x05	0x00	0x00	0x00	0x00	0x00	0x00	0x00
0x7fffffffe490:	0x03	0x00	0x00	0x00	0x00	0x00	0x00	0x00
0x7fffffffe498:	0x05	0x00	0x00	0x00	0x00	0x00	0x00	0x00
0x7fffffffe4a0:	0x07	0x00	0x00	0x00	0x00	0x00	0x00	0x00
0x7fffffffe4a8:	0x09	0x00	0x00	0x00	0x00	0x00	0x00	0x00
0x7fffffffe4b0:	0x07	0x00	0x00	0x00	0x00	0x00	0x00	0x00
0x7fffffffe4b8:	0x09	0x00	0x00	0x00	0x00	0x00	0x00	0x00

[tremon015]

ok, taking a look at what native_func is supposed to do: I can get it working if I change the Rust function lead-in to this:

pub fn select(sel: ocaml::Raw, fst: ocaml::Raw, snd: ocaml::Raw, ext: ocaml::Raw) -> Value {
   let sel = ocaml::Value::Raw(sel.into());
   let fst = ocaml::Value::Raw(fst.into());
   let snd = ocaml::Value::Raw(snd.into());
   let ext = ocaml::Value::Raw(ext.into());
   eprintln!("select: sel={:?}, fst={:?}, snd={:?}, ext={:?}", sel, fst, snd, ext);

This gives the expected result

select: sel=Raw(3), fst=Raw(5), snd=Raw(7), ext=Raw(9)

And I no longer get a segfault on return. So it seems that the macro no longer properly wraps the function parameters?

[zshipko]

Sorry for the delayed reply - this makes sense to me because ocaml::native_func only works with ocaml::Raw - ocaml::func should be used if you want to be able to use ocaml::Value parameters.