Original bug ID: 7572
Reporter: @sliquister
Status: acknowledged (set by @xavierleroy on 2017-07-18T20:19:38Z)
Resolution: open
Priority: normal
Severity: minor
Version: 4.04.2
Category: runtime system and C interface
Monitored by: @gasche @hcarty @dbuenzli

Bug description

When writing ocaml plugins for C programs, you end with
functions that look like this:

void called_from_the_C_program (char* str) {
  caml_acquire_lock();
  {
    CAMLparam0();
    CAMLlocal1(v, res);
    v = caml_copy_string(str);
    caml_callback(.., v);
    CAMLdrop;
  }
  caml_release_lock();
}

The problem is that this function is most likely broken, because if
the caml function raises, the ocaml runtime will either exit or unwind
the stack past the C program.
We have caml_callback_exn to help with this, but it's not enough
because caml_alloc_string can also raise.

There seems to be no way of catching ocaml exceptions from a block of
C code, short of exporting the C code, writing an ocaml function that
calls it, registering the ocaml function and calling it from C, which
is a bit ridiculous.

There's this caml_alloc_shr_no_raise function, which may be usable
for this purpose but it's undocumented, so I am not sure it's meant
to be used from the outside (and it's probably not very nice to
have to duplicate all the allocation functions with a _no_raise
suffix).

So is there a recommended way to not throw ocaml exceptions in C
bindings? If not, can we create one?