Can the FFI deal with arrays? If so, how?

Question

I'm pretty sure it's possible to send arrays through the FFI, but I can't find any examples. For instance, I have a Haskell array that I send to a int foo(int*) function, or I have a C array int bar[64]; that I send to Haskell.

Ideally I'd want the most efficient way - I don't want any heap allocation or unnecessary copying. Also, it would be nice if I could use Haskell's unboxed arrays in both Haskell and C. So what's the method of doing so?

Answer 1

If you use the Data.Vector library you could use Data.Vector.Storable for your needs. Then you can use functions such as unsafeToForeignPtr or unsafeWith to access the underlying foreign pointer. This allows you to call C-code without any copying or marshaling taking place.

If you want to create a vector from a C-array you can use unsafeFromForeignPtr.

For your examples you can use (assuming c_foo does not modify it's arguments)

import Foreign.Ptr import Foreign.C.Types import System.IO.Unsafe (unsafePerformIO) import qualified Data.Vector.Storable as SV  foreign import ccall unsafe "foo" c_foo :: Ptr CInt -> CInt  haskellFoo :: SV.Vector CInt -> CInt haskellFoo sv = unsafePerformIO $     SV.unsafeWith sv $ \ptr -> return (c_foo ptr) 

This can be be golfed to:

haskellFoo sv = unsafePerformIO $     SV.unsafeWith sv (return . c_foo) 

Note that if your C-function modifies the data, then you shouldn't do this, instead you should make a copy of the data to not break referential transparency.

If you want to use the standard Array-type you could use withStorableArray from Data.Array.Storable in the same way.

Answer 2

The FFI specification is quite readable, so you might want to just sit down and work through the whole thing. However, for this specific question, you can jump to the "Marshalling" section, particularly the Ptr and Storable subsections, which outline what's available for this.