Re: [Haskell-cafe] Re: Blitting one IOUArray into another

You can't safely convert an IOUArray into a Ptr; Ptr is a raw value which isn't noticed by the garbage collector, so if the data is relocated or GC'd while you have a pointer to it, further access will corrupt memory. Rather, the data inside of an IOUArray is held in a MutableByteArray#. In Data.Array.IO.Internals you can get at the newtype for IOUArray. I have some code that looks like this:

import Foreign.Ptr import Data.Array.Base import Data.Array.IO.Internals import GHC.Exts import Data.Word

foreign import ccall unsafe clear_bitmap :: MutableByteArray# RealWorld -> Word32 -> Word32 -> IO ()

{-# INLINE unsafeGetMutableArray# #-} unsafeGetMutableArray# :: IOUArray Int Word32 -> MutableByteArray# RealWorld unsafeGetMutableArray# (IOUArray (STUArray _ _ array#)) = array#

clearBitmap :: IOUArray Int Word32 -> Word32 -> Word32 -> IO () clearBitmap a1 color size = clear_bitmap (unsafeGetMutableArray# a1) color size

Then the I have a small amount of C code implementing "clear_bitmap": void clear_bitmap(HsWord32* img, HsWord32 color, HsWord32 size) { for(; size; --size, ++img) { *img = color; } } This is OK to do because the "unsafe" ccall guarantees that no GC can happen during the outcall to clear_bitmap, so we can manipulate the pointer directly. If you want to stay entirely in Haskell, there are a bunch of operations on MutableByteArray# in GHC.Exts; see http://www.haskell.org/ghc/docs/6.10-latest/html/libraries/ghc-prim/GHC-Prim... You probably need {-# LANGUAGE MagicHash #-} in order to get these to work; it makes # be a legal symbol in identifiers. It also helps to know the newtype for IO, if you want to write actually usable functions on top of these internal bits.

newtype IO a = IO (State# RealWorld -> (# State# RealWorld, a #))

Of course all of this is GHC-specific, and internal to base and subject to change. But I found it useful. -- ryan On Thu, Jan 8, 2009 at 6:51 AM, Bueno, Denis wrote:

On 01/07/2009 14:36 , "Neal Alexander" wrote:

...

Bueno, Denis wrote:

...

Oh, do you mean by actually calling memcpy via ffi?

http://www.haskell.org/ghc/docs/latest/html/libraries/base/Foreign-Marshal-U... ls.html

Ah, thanks. Is there a way to simply "cast" an IOUArray Int Int64 into something like a Ptr Int64, or will I need to change my code to allocate the arrays differently (using something in Foreign.*)?

I hoogle'd functions "IOUArray a b -> Ptr b", but couldn't find anything. Denis

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