[Haskell-beginners] Re: How to initialize a C struct with FFI

[Maciej Piechotka]

On Sat, 2010-02-06 at 12:28 +0100, Marco De Oliveira wrote:
> Hi,
> 
> I try to initialize a C struct using the FFI API and ghc stays blocked
> when I execute my code (and I got sometimes the error "out of
> memory").
> I do not understand, I do not find any mistake in my code:
> 
> {-# OPTIONS -XTypeSynonymInstances #-}
> 
> import Foreign.C.Types
> import Foreign.Marshal.Alloc
> import Foreign.Ptr
> import Foreign.Storable
> 

It can be written as:
import Foreign
import Foreign.C

> 
> type CStruct = (CULong, CULong)
> 

I guess that you may want to use newtype or data rather then type to
avoid clashes (and have just 98 + FFI):

newtype CStruct = CStruct (CULong, CULong)
data CStruct = CStruct CULong CULong

> instance Storable CStruct where
>   sizeOf _ = 2*sizeOf (undefined::CULong)
>   alignment _ = alignment (undefined::CULong)
>   

To begin with - where's peek and poke?

> test = alloca (\pStruct -> do
>          poke pStruct ( (fromIntegral 1)::CULong, (fromIntegral
> 1)::CULong )
>          )

If you just do this at the beginning you may want to use with (notation
as with newtype/data):

with (CStruct (1, 1)) (\ptr -> ...)


Also is main = test?
> Thanks for your help.
> 
> Marco

Regards