[questions such as this one should go to cafe] On Wednesday 05 April 2006 20:41, minh thu wrote:
1/ i want to read some binary file (e.g. targa file format : *.tga).
-- first way : via IOUArray showInfoHeader1 handle = do a <- newArray_ (1,8) :: IO (IOUArray Int Word8) hGetArray handle a 8 idLength <- readArray a 1 -- or getElems... putStrLn ("id length : " ++ show idLength) return ()
-- second way : via c-like array showInfoHeader2 handle = do b <- mallocArray 8 :: IO (Ptr Word8) hGetBuf handle b 8 [idLength] <- peekArray 1 b -- or peakArray 8 b
The index should be 0 if you want to read the first byte. Also, if you are only interested in the first byte, you could simply idLength <- peek b or if it is not the first byte, then idLength <- peekByteOff b i However, it is better to use arrays, than pointers.
putStrLn ("id length : " ++ show idLength) free b return ()
so, briefly, i have to read some content into some kind of buffer (IOUArray Int Word8 or Ptr Word8), then get one (or more) elements from the buffor into a standard haskell variable (is it the correct word ?) (or list).
in the second case, i also have to free the buffer.
Or use alloca or allocaBytes, which are both a lot faster than malloc and free.
in some case, when the data is more than one Word8 long, i have to 'reconstruct' it, i.e.:
[x1,x2] <- getElems a
This will give you a run-time error, because you array is 8 elements long, not 2. You can do x1:x2:_ <- ... or better still x1 <- readArray b 1 x2 <- readArray b 2 Still better: Use one of the available binary serialisation libraries. They are already tuned for efficiency and give you a much nicer high-level API.
let x = fromIntegral x1 + fromIntegral x2 * 256 :: Int
is it the correct way to read binary files ?
Depends on the byte order that is used in your file format. If it is big-endian then correct, else not correct. (I hope I did get this right; I always tend to confuse big- and little-endian.)
2/ haskell is (i heard that once ... :-) a high level language, so it has (must have) good support for abstraction...
Sure. See abve mentioned libraries.
but in 1/, i have to choose between different kind of array representation (and i dont know which one is better) and it seems to me that the resulting code (compiled) would have to be the same.
I strongly recommend using some Array type (IOU or whatever). Ptr is really just a raw pointer into memory: no protection from out-of-bounds access, etc. much like in C. Ptr has been invented for interfacing with C routines, not for regular Haskell programming. HTH, Ben