[Caml-list] Implementing co-op threads in ocaml

[Jonathan Roewen <jonathan.roewen@gmail.com>]

Hi,

I have a few questions about how to allocate some C-side values in an
ocaml type, for the purposes of implementing custom threading in a
native app.

After reading through some systhreads code, I've come up with the
notion that the following 5 C-land values are probably key to making
this work:

  char * bottom_of_stack;       /* Saved value of caml_bottom_of_stack */
  unsigned long last_retaddr;   /* Saved value of caml_last_return_address */
  value * gc_regs;              /* Saved value of caml_gc_regs */
  char * exception_pointer;     /* Saved value of caml_exception_pointer */
  struct caml__roots_block * local_roots; /* Saved value of local_roots */

I've been trying to make sense of all the rules and things in the
ocaml manuals for storing C data in an ocaml value, but am having some
trouble. Especially with gc_regs, which I assume points to a value
inside the ocaml heap.

For the purposes of the rules that everything being on a 32-bit
boundary, all the pointers would be fine, and the fact that
last_retaddr is a 32bit value, all this would be fine.

Since I'm doing co-op threading primarily in OCaml, I'm also assuming
I don't need to create custom stacks for each thread (such as in C),
and having to swap stacks; therefore, access to ocaml global
values/functions should be fine. Is this assumption correct?

Am I also correct to think that the only real trick to this would be
making sure the GC scans all thread roots? If say my threads are in a
list, I could have an external function that scans the thread, and
just iter over this list?

I'm especially interested in how to correctly handle the C-specifics
for storing the pointers and things, but other feedback is also
welcome =)

Jonathan