Re: [Caml-list] CamlDL/Abstract pointers problem

[skaller <skaller@tpg.com.au>]

On Fri, 2004-01-30 at 02:02, Richard Jones wrote:
> On Thu, Jan 29, 2004 at 01:06:39PM +0000, ronniec95@lineone.net wrote:
> > All I want to do is just give Ocaml a pointer to something I've created
> > in C and pass it back to other C functions later; don't want it to do anything
> > with it (including moving it around the ocaml heap).\
> 
> It's my understanding that you can't just pass a pointer back to OCaml
> and cast it to a value.  OCaml will think that the pointer points to
> an OCaml heap object, and hence try to examine / move / delete it in
> the garbage collector. 

Nope: RTFM:

                                  18.2
                                  The value type
All Caml objects are represented by the C type value, defined in the
include file caml/mlvalues.h, along with macros to manipulate values of
that type. An object of type value is either: 
      * an unboxed integer;
      * a pointer to a block inside the heap (such as the blocks
        allocated through one of the alloc_* functions below);
      * a pointer to an object outside the heap (e.g., a pointer to a
        block allocated by malloc, or to a C variable).

18.2.3 Pointers outside the heap

Any word-aligned pointer to an address outside the heap can be safely
cast to and from the type value. This includes pointers returned by
malloc, and pointers to C variables (of size at least one word) obtained
with the & operator.

[Note the manual is wrong here in that a C variable of word
size need not be word aligned .. it isn't necessary on x86 machines,
x86 doesn't require integers or addresses to be aligned..however
most C compilers do the alignment anyhow because it 
improves performance. I think floats do have to be aligned though,
and thus malloc must return an even address] 

-- 
John Max Skaller, mailto:skaller@tpg.com.au
snail:25/85c Wigram Rd, Glebe, NSW 2037, Australia.
voice:61-2-9660-0850. Checkout Felix: http://felix.sf.net




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