Re: dynamic foreign function interface

[ludo@gnu.org (Ludovic Courtès)]

Hello!

Andy Wingo <wingo@pobox.com> writes:

> I just finished up some work to make a dynamic foreign function
> interface. This is on the wip-ffi branch.

Nice!

> By "dynamic", I mean that you don't have to write C and compile it; you
> can do everything at runtime from Scheme. You use dynamic-func and
> dynamic-link to get the raw function pointer, and make-foreign-function
> to turn that function pointer into a Scheme procedure.
>
> The interface is very low-level. Obviously declaring that an arbitrary
> symbol resolved via `dlsym' is of a certain function type is an unsafe
> operation that can lead to crashes.

If it’s in a module of its own, then users will (in theory) be able to
prevent its use by “untrusted” code, which would be fine.

> Apart from that typing problem, you have pointer and struct types. If
> you say that the function takes an int8, Guile will ensure that it can
> make an int8; but if you say that the function takes a pointer or a
> by-value struct, Guile will only ensure that the arg is a "foreign"
> (from foreign.[ch]) pointer, only checking lengths in the case that it's
> a struct of known length.
>
> The intention is to provide an expressive Scheme layer, on top of which
> any safety constructs can be built as needed.

Cool.

One thing that would be neat is to integrate nicely with GNU ld’s symbol
versioning (it would work around some of the safety lost by not
compiling actual C code.)  For instance, one would be able to say:

  (dynamic-func "foo" lib "FOO_0.2")

That would use dlvsym() on GNU and ignore the last argument on other
systems.  (Though ideally ltdl would provide a wrapper for dlvsym).

> I would merge it now, except for the fact that it depends on libffi.
> Libffi is very portable, and probably exists for all of Guile's
> architectures, but it is an extra dependency. Should we require libffi
> in Guile 1.9.8? Or should we build the necessary pieces conditionally?

Well, it’s always annoying to add a dependency, but OTOH it was bound to
happen.  So... let’s go?

Thanks,
Ludo’.