On Sun, Dec 5, 2010 at 10:45 PM, Tyler Pirtle
Hi cafe,
I'm just getting into Foreign.Storable and friends and I'm confused about the class storable. For GHC, there are instances of storable for all kinds of basic types (bool, int, etc) - but I can't find the actual declaration of those instances.
I'm confused that it seems that all Storable instances operate on a Ptr, yet none of these types allow access to an underlying Ptr. I noticed that it's possible via Foreign.Marshal.Utils to call 'new' and get a datatype wrapped by a Ptr, but this isn't memory managed - I'd have to explicitly free it? Is that my only choice?
The Storable class defines how to copy a particular Haskell type to or from a raw memory buffer - specifically represented by the Ptr type. It is most commonly used when interacting with non-Haskell (or 'Foreign') code, which is why a lot of the tools look like they require manual memory management (because foreign-owned resources must often be managed separately anyway). Not all of the means of creating a Ptr type require manual memory management - the 'alloca' family of Haskell functions allocate a buffer and then free it automatically when outside the scope of the passed-in callback (although 'continuation' or 'action' would be the more Haskell-y way to refer to the idea): alloca :: Storable a => (Ptr a -> IO b) -> IO b This can be used to call into C code expecting pointer input or output types to great effect: wrapperAroundForeignCode :: InputType -> IO OutputType wrapperAroundForeignCode in = alloca $ \inPtr -> alloca $ outPtr -> do poke inPtr in c_call inPtr outPtr peek outPtr The functions 'peek' and 'poke' are from the Storable class, and I used the 'alloca' function to allocate temporary storage for the pointers I pass into C-land. Is there a particular problem you're trying to solve? We might be able to offer more specific advice. The Storable and Foreign operations may not even be the best solution to what you're trying to do. Take care, Antoine
Is there a way that given just simply an Int I could obtain a Ptr from it, and then invoke the storable functions on it? Or for that matter, if I go and create some new data type, is there some generic underlying thing (ghc-only or otherwise) that would let me have a Ptr of it?
Thanks,
Tyler
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