Re: Need for scm_remember_upto_here_* in guile-2.0

[Chris Vine <chris@cvine.freeserve.co.uk>]

On Wed, 23 Sep 2015 09:26:27 +0200
David Kastrup <dak@gnu.org> wrote:
> Chris Vine <chris@cvine.freeserve.co.uk> writes:
> 
> > On Wed, 23 Sep 2015 00:35:09 +0100
> > Chris Vine <chris@cvine.freeserve.co.uk> wrote:
> >> I think you were trying to answer my question, which was: "... is
> >> the point that the scm_remember_upto_here_* functions are in
> >> practice only needed for smobs which provide their own free
> >> function?" (that is, which have no finalisers). I think you were
> >> giving the answer "Yes".
> >
> > Err, "... which have finalisers".
> >
> > Assuming this is the case, the guile manual example is rather
> > unhelpful.
> 
> There are other forms of finalization than giving back memory, like
> closing file descriptors, relinquishing a lock or semaphore and so on.
> Also, a pointer to an array (rather than something more opaque like
> SCM) is much more likely to be subject to strength reduction and
> address arithmetic by the compiler, leading to a situation where
> looping through some array does not leave a live pointer in
> registers and stack frame in a form recognizable by libgc.

I am not sure if we are on the same page.  I realise that finalisers are
not only concerned with memory.  In fact, because there is proper
garbage collection in guile-2.0, in a well designed smob memory is
likely to be the least common thing they manage.  It so happens that
you set the smob's finalisers (whatever they happen to do) with
scm_set_smob_free() (the "free function" that I referred to).

However, I haven't understood your point about arrays.  If they are
allocated with malloc() you need a finaliser and you may need to call
scm_remember_upto_here_1() to prevent the smob's finaliser from firing
while they are still in use.  If they are allocated with scm_gc_malloc()
you don't, as I understand it.  Were you making a point in addition to
that, and if so could you expand on it for me?

Chris