Hello and thank you for this precise (and impressive) answer,
(I'm working together with Julien)

But actually, we need also to get a bigarray from string. So the writer 
could get a string as usual, copy it once in a shared bigarray, and then 
let the readers read it, without any more copies.

Anyway, I'm also quite afraid of including suck black magic in our 
project ;) We're OK for writing some C, we already did. Maybe it would 
be easier than Obj.Magic stuff.

Avoiding copies is our goal, but it seems impossible to get a string of 
which Caml doesn't own the blocks, in order to make a mmaped mem area 
corresponding to many strings in the writer/readers processes.

--
David