August 2016 - FFI - Haskell.org

Re: [Haskell-cafe] managing embedded structs
by Henning Thielemann 29 Aug '16

29 Aug '16

On Sun, 28 Aug 2016, Alexey Khudyakov wrote: > On 27 August 2016 at 13:23, Henning Thielemann > <lemming(a)henning-thielemann.de> wrote: >> >> I want to write a Haskell interface to a C library that provides two data >> structures A and B, where B is contained in A. That is, if A is freed, then >> B is automatically freed, too. >> > I don't quite understand. Do A and B live in the same buffer and problem in > keeping buffer alive as long as there're pointers to A or B. If so > it's easy problem > and vector solves exactly this problem for storable vectors. Check > implementation > of basicUnsafeSlice. Idea is to share finalizer between ForeignPtr Sharing finalizer between ForeignPtr A and ForeignPtr B might indeed be a solution. It would require dependence on the ghc package (and thus GHC), though. > If they live in different buffers and B should not be collected as > long as A lives... This would be true in another situation, where I create a finalizer by the LLVM-JIT and need to finalize the finalizer's code if it is no longer needed. https://ghc.haskell.org/trac/ghc/ticket/12547

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managing embedded structs
by Henning Thielemann 27 Aug '16

27 Aug '16

I want to write a Haskell interface to a C library that provides two data structures A and B, where B is contained in A. That is, if A is freed, then B is automatically freed, too. There are two ways to obtain a Ptr B. Either construct it separately or as part of A, i.e. b0 :: Ptr B b0 <- createB b1 :: Ptr B b1 <- getBofA =<< createA Now I want to let the garbage collector manage the resources for B in a ForeignPtr. The first case is easy: fb0 :: ForeignPtr B fb0 <- newForeignPtr deleteB =<< createB But how to create an fb1? fa1 :: ForeignPtr A fa1 <- newForeignPtr deleteA =<< createA fb1 :: ForeignPtr B fb1 <- newForeignPtr ??? =<< withForeignPtr fa1 getBofA Obviously, there is nothing I can insert for ???. fb1 must not have its own finalizer, because fa1 already has one. But I have to make sure that fa1 lives at least as long as fb1. How to do that?

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type synonyms for dynamic imports and wrappers
by Henning Thielemann 18 Aug '16

18 Aug '16

The Haskell FFI report says that imports and exports of dynamic functions always have those types: foreign import ccall "dynamic" mkFun :: FunPtr fun -> fun foreign import ccall "wrapper" mkCallback :: fun -> IO (FunPtr fun) where 'fun' is an IO function. Since 'fn' can be a considerably large type expression I usually define type Importer fun = FunPtr fun -> fun type Exporter fun = fun -> IO (FunPtr fun) How about adding these type synonyms to Foreign?

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