Re: [Caml-list] Scanning objects outside the OCaml heap

[Pierre Chambart <pierre.chambart@ocamlpro.com>]

On 07/07/2015 00:21, Michael Hicks wrote:
>
> I'm interested in implementing a custom memory management strategy
> that I can prove is safe in a language I will compile to OCaml. I'm
> thinking I'd like to allocate freeable memory outside the OCaml heap,
> but as OCaml values. Since these values can point to OCaml
> heap-resident data, they need to be scanned. But I don't want to have
> to explicitly register/deregister them as roots to the GC, or track
> all mutations to the values, if I can help it. Rather, having a single
> "persistent root" to my memory area, for purposes of scanning, would
> be ideal.
>
> Thanks in advance for any help,

There is an undocumented and unexported way to do something like that:
the caml_scan_roots_hook.
You can register a callback in this global variable that takes a
scanning action (another callback) to call
on every of your roots.

See https://github.com/ocaml/ocaml/blob/trunk/byterun/caml/roots.h#L34
for the definition
and https://github.com/ocaml/ocaml/blob/trunk/asmrun/roots.c#L270 as an
example of how the scanning
action should be called.

Since those roots are scanned only at the end of a gc major cycle, you
must handle mutation specificaly (like
mutations in the major heap). You will have to track them as you must
not have unnoticed pointers from outside
of the minor heap to the minor heap. You will have to addapt caml_modify
as it suppose that values not in the minor
are in the major heap. Since it is declared using a weak symbol, this
can be redefined without changing the
runtime. See http://caml.inria.fr/mantis/view.php?id=6084 .

Take care, as this is not exported in the official interface, this could
break at any version change of the runtime.
Have fun with your potential hard to debug segfaults ;)
-- 
Pierre