On Store_field()

[Thomas Fischbacher <Thomas.Fischbacher@Physik.Uni-Muenchen.DE>]

On Thu, 19 Jan 2006, Damien Doligez wrote:

> >However I strongly recommend instead
> >
> > StoreField(v,n,(value)(void*)p);
> 
> This is the only way.  Always use Store_field, Store_double_field,
> and Store_double_val.

What you *actually* forgot to tell people is that OCaml unfortunately 
lacks a macro with which one can store C pointers to alloc_final 
allocated blocks. If one writes to those using e.g.

  ml_vector = alloc_final(2, finalize_vector, 1, 10);
  Store_field(ml_vector, 1,(value)vector);

with vector being a C pointer to a structure, this may or may not lead to 
random crashes at very unexpected places - because (as I think now) the 
Store_field macro will be over-eager trying to tell the GC about the 
value stored - which it should just ignore in this particular case!

Indeed, this has been discussed before, I think:

http://groups.google.de/group/fa.caml/msg/60ace9405fcf60c0?dmode=source&hl=de

So, I would strongly suggest introducing a macro that behaves like this:

#define Store_c_field(block,offset,x) (Field(block,offset)=(value)x)

so that one could then use

  Store_c_field(ml_vector, 1,vector);

I actually just spent a full week tracking down precisely this issue in a 
not particularly trivial C library interface I am building right now. 
After looking in the weirdest places, ensuring it's not an issue with the 
library wrapped, or the trickier pieces of my own code, I even started 
patching debugging code into the OCaml bytecode's GC and rebuilding...

So, *please* do the world a great favour and tell people about that issue 
in the C interface documentation!

-- 
Thomas Fischbacher
tf@cip.physik.uni-muenchen.de
(Yes, grumpy. Did not sleep over the weekend due to this issue...)