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Re: Passing C pointers through guile

From: Ludovic Courtès
Subject: Re: Passing C pointers through guile
Date: Wed, 09 Jul 2008 21:35:14 +0200
User-agent: Gnus/5.11 (Gnus v5.11) Emacs/22.2 (gnu/linux)


"Kjetil S. Matheussen" <address@hidden> writes:

> I gave it a try. Unfortunately, I was completely unable to create
> the configure file right now, so the patch is against 1.8.5 (sorry
> if this creats trouble against git repository), and
> it's also untested, since I couldn't build configure.

You have to make sure you are on the `branch_release-1-8', and then
"autoreconf -vfi" should suffice to produce everything.  Autoconf 2.61,
Automake 1.10 and Libtool 1.5.26 is all you need.

> The only thing I'm not too sure about is whether
> the new SCM_I_GSC_T_UINTPTR type in will actually be
> optional or not. I just copied the checking code for the optional
> SCM_I_GSC_T_UINT64 type though:

I think this type shouldn't be optional, because there will always be a
pointer-sized integer type (whereas there could be platform without
64-bit integers).

>> That said, using a Scheme integer to represent a pointer wouldn't be
>> efficient (pointers would likely translate to bignums).
> I think cleaner code would usually be more important in this case,
> but at least there will be a choice.

I'm not sure how much cleaner this is.  Usually, you'll want disjoint
Scheme types, which means you'll end up enclosing the pointer-as-bignum
in a structure or SRFI-9 record or similar.  This leads to even more
overhead.  Conversely, using an opaque field in a Guile struct has the
same effect but with much less overhead.

Another issue is that of memory management.  When using
pointers-as-bignums, all the GC will see is a collection of bignums, not
knowing that these are actually pointers to C objects that should not be
GC'd unless the integer is no longer used---as opposed to "no longer
referenced"!  This actually makes it mandatory to enclose the integer in
a structure or similar, and then to have a guardian on that structure to
allow the C object's destructor to be called when that structure is no
longer referenced.

(Note that it could be a valid approach in some compiler environments.
It just doesn't fit well with Guile's design.)

Anyway, it can't hurt to have the choice.  :-)


PS: You'll have to assign copyright to the FSF so that your code can be
    integrated.  We can discuss it off-line if you want.

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