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

6 Feb 2010

      Hi Maciej,

Thanks to have take some time to read this code.

I rewrite using your comments:

import Foreign
import Foreign.C

data CStruct = CStruct (CULong, CULong)

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

test = with (CStruct (1,1)) (\ptr -> return ())

The class Storable does not need to make your own peek and poke method (the
class provide a default implementation).
But with, i have still the behavior when I try this code in ghci.

Please, can you test this code and tell me why ghci stay blocked after
executing the method test.

BR

Marco

2010/2/6 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
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