Re: [Caml-list] GC and preventing data relocation

[Markus Mottl <markus@oefai.at>]

On Fri, 21 Mar 2003, Christopher Quinn wrote:
> Markus Mottl wrote:
> >I don't quite understand this solution. How should this prevent the
> >string from being relocated if the string was allocated in the OCaml-heap?
> 
> it wouldn't, but at least the pointer to it is changed also. and the 
> assumption i made was that the glue code knows where it is keeping its 
> pointer to the string.

Well, the glue code can surely know, but unfortunately things are more
complicated, because the C-library can't:

  OCaml
    |
    +--> C-glue code
            |
            +--> C-library
                   |
           +-----> +--> C-glue code (via "callout" function pointer)
           |              |
           |              +--> OCaml (callback)
           |
      problem is here

Once the opaque C-library runs off with a given pointer to a string,
it may generate tons of internal datastructures that depend on this
specific location. There is just no way for me to tell the C-library
to continue elsewhere, because the C-library called me (the glue code)
before the OCaml-callback, not vice versa, and just expects me to return.

Well, I could ask the author of the C-library to add a feature that
adds offsets at each step to dependent pointers to accommodate for
relocations. But he'll probably rather kill me than rewrite 7 KLOCs of
low-level code... ;-)

> >And wouldn't be particulary portable...
> 
> what 3rd party modification to the distributed caml runtime is!?

Indeed...

Regards,
Markus Mottl

--
Markus Mottl                                             markus@oefai.at
Austrian Research Institute
for Artificial Intelligence                  http://www.oefai.at/~markus

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