Raj,

> The issue for me is that I need to be able to modify mutable objects in 
> OCaml (eg. array-modification in-place) from both OCaml and C/Python. 
> However, the OCaml GC moves things around while the execution is in 
> C/Python and this crashes my program.

You strongly should consider malloc()ing data and providing caml
functions to read and write entries from a C array.

In case the data chunk you want to modify from both caml and C holds
non-integer ml values, things become a bit more complicated. Then, you
have to register_global_root() these extra pointers into the ML heap.
Unregistering of global roots then has to be done in the finalizer of
the ML container value that retains a C pointer to malloc()d space.
There, you free() this as well.

Beware the dreaded global root reference loop though!

-- 
best regards,
Thomas Fischbacher
tf@functionality.de