> Is the OCaml runtime Boehm-safe? That is, can it be run with Boehm
> turned on and traversing OCaml's heap? (So that the OCaml heap can
> provide roots to Boehm.)

I conjecture the answer is "yes", although it's hard to tell for sure
without a precise specification of what is/is not OK with the
Boehm-Demers-Weiser collector.

>From the standpoint of this collector, OCaml's heap is just a set of
large-ish blocks allocated with malloc()  (*) and containing a zillion
pointers within those blocks.  OCaml doesn't play any dirty tricks
with pointers: no xoring of two pointers, no pointers represented as
offsets from a base, no pointers one below or one above a malloc-ed
block.  Most pointers are word-aligned but we sometimes play tricks
with the low 2 bits.

Of course, almost all Caml pointers point inside those malloc-ed
blocks, not to the beginning, but I'm confident that the B-D-W collector
can handle this, otherwise it would fail on pretty much any existing C
code.

This said, I agree with Basile that what you're trying to achieve
(coexistence between several GCs) is risky, and that a design based on
message passing and separated memory spaces would be more robust, if
feasible.

- Xavier Leroy


(*) In 3.10 and earlier releases, OCaml sometimes used mmap() instead
of malloc() to obtain these blocks.  Starting from 3.11, malloc() is
the only interface OCaml uses to obtain memory from the OS.