Re: [Caml-list] Interfacing with C: bad practice

["Richard W.M. Jones" <rich@annexia.org>]

On Tue, Aug 16, 2011 at 05:51:38PM +0200, rixed@happyleptic.org wrote:
> -[ Tue, Aug 16, 2011 at 04:25:50PM +0100, Richard W.M. Jones ]----
> > I think this must be a bug in your C compiler.  The address of list is
> > stashed in the roots struct, so the C compiler should know that list
> > can be changed by the call to caml_copy_string.
> 
> Are you certain that the C abstract machine allow for any value stored
> within the frameset of a function to be changed by a function call when
> the address of the variable at hand is not passed to this function? And
> mandate the C compiler to handle this scenario? In other words, mandate
> the C compiler to reload from the stack all values between any function
> call?
> 
> I don't think so ; or more likely I have not understood your view on
> this matter?

Well it would certainly help if we had a piece of runnable code to
look at.  The code supplied in the original email contains an infinite
loop.

Nevertheless, the C compiler isn't allowed to just push 'list' blindly
onto the stack and assume it doesn't change across the call to
'caml_copy_string'.  Calls in C are sequence points, and the compiler
cannot possibly assume that 'list' won't change, because the address
of 'list' has been taken *and* is directly reachable in two steps from
caml__local_roots, which is a global variable.

If the compiler is doing this (and we don't really have proof of that
because there is no runnable code nor is a compiler + version
specified), then the compiler has a bug.

Rich.

-- 
Richard Jones
Red Hat