"static_wrapper" imports in the FFI

[Iavor Diatchki]

Hi,
I think that one may view a "static_wrapper" import as a pair of an
export like the one you wrote and an import of the address of the
exported function (this is useful because usually it is convenient to
install the handler on the Haskell side).  Perhaps that's a better way
to explain its meaning, rather then the long-winded example that I
wrote. Thanks!

In practice, the foreign export generates slightly less direct code,
because the C code follows 2 indirections: first we enter the closure
for "cbind", and then _that_ code enters the closure for the actual
Haskell function.  Perhaps with sufficient optimization on the C side
this can be avoided although I don't think that it happens at the
moment.

In terms of notation, I like the directness of the "static_wrapper"
declaration (although not so much the "static_wrapper" name!) because
it avoids duplication, thus reducing clutter and potential errors.

-Iavor








On Mon, Mar 15, 2010 at 3:56 PM, Tyson Whitehead <twhitehead at gmail.com> wrote:
> On March 15, 2010 12:48:02 Iavor Diatchki wrote:
>> I have implemented a small extension to the FFI which allows for
>> "static_wrapper" imports.  These are a variation on "wrapper" imports
>> that do not use run-time code generation.  This is important in
>> security sensitive contexts because it avoids executable data.  While
>> "static_wrapper" imports are less general then "wrapper" imports, they
>> can be used to install Haskell handlers in many C libraries where
>> callbacks have an extra "user-data" parameter (e.g., GTK signal
>> handlers).
>
> Hi Iavor,
>
> Would not the following also do what you want
>
>  foreign export ccall "haskellCIntCInt" \
>   cbind :: CInt -> StablePtr (CInt -> IO CInt) -> IO CInt
>
>  cbind :: a -> StablePtr (a-> IO b) -> IO b
>  cbind x f = deRefStablePtr f >>= (\f_ -> f_ x)
>
> On the C side you would then have something like
>
>  register_callback(haskellCIntInt,<wrapped haskell closure>)
>
> where <wrapped haskell closure> would be a stable pointer of type StablePtr
> (CInt -> IO CInt) generated on the haskell side via
>
>  newStablePtr <haskell closure>
>
> and passed to the C code.
>
> Cheers!  -Tyson
>