Yes, I had considered this (writing an abstraction layer in C). I have decided initially to try doing it in Haskell, because I thought that Haskell would be a better choice for writing abstraction layers (not as verbose as C). The idea was to write the Haskell-C bindings at as low a level as possible (i.e. directly to the OCI libraries), and then build up from there. Although the bindings may be ugly, I thought it would have been easier to build higher layers in Haskell rather than C. However, if you think it's probably better to build the first layer in C, then I'll do that. Getting back to my question, can I use functions that take twice-deref'ed pointers as args? This is a function that effectively returns some of its work in "out" arguments e.g. a C function like this: int someCFunction(SomeType **ptrptr, int i) (It would create an opaque structure and point *ptrptr at it. If it fails then it would return a negative value.) Is it simply a case of declaring it like this (in ghc):

data SomeType type SomeTypeRef = Ptr SomeType foreign import ccall "blah.h someCFunction" someCFunction :: Ptr SomeTypeRef -> Int -> Int

Or must C functions that are to be called from Haskell return results only in the return value? -----Original Message----- From: Ronald Legere [mailto:rjljr2@yahoo.com] Sent: 31 May 2003 15:25 To: Bayley, Alistair; 'haskell-cafe@haskell.org' Subject: RE: Help: writing ffi bindings to a C library One strategy is to wrap that funtion in another function (in C) that encapsulates the 'typical' call you want to make. THen import this function into haskell. In any event, start with something simpler, to get the hang of it. Try to write a function (in C) that adds two integers,or prints something and import that in to haskell. There are some examples floating around to get you started, but I can't find them at the moment on the CVS tree. Anyone? Also, read "Tackling the awkward squad" (Google it) Ron --- "Bayley, Alistair" wrote:

So...

I'm still trying to write this Oracle OCI ffi binding. Can anyone tell me how to declare the Haskell type for this function:

sword OCIEnvCreate (OCIEnv **envp, ub4 mode, dvoid *ctxp, dvoid *(*malocfp)(dvoid *ctxp, size_t size), dvoid *(*ralocfp)(dvoid *ctxp, dvoid *memptr, size_t newsize), void (*mfreefp)(dvoid *ctxp, dvoid *memptr), size_t xtramem_sz, dvoid **usrmempp);

Note that when I use it, I'm passing 0 (NULL) into almost all of the args, so the usage in C is typically:

rc = OCIEnvCreate(&envhp, OCI_DEFAULT, 0, 0, 0, 0, 0, 0);

i.e. I don't care about most of the args, so that should make the Haskell declaration simpler. Some of the arguments are pointers to functions, for example.

What I can't figure out is how to declare the type of the first arg. Is is Ptr (Ptr OCIEnv) ?

Here's what I have so far (not much, I know):

...

module Main where import Foreign import Foreign.C.Types import Foreign.C.String import Foreign.Ptr

...

data OCIEnv

...

foreign import ccall "oci.h OCIEnvCreate" ociEnvCreate :: Ptr OCIEnv -> Int -> Int -> Int -> Int -> Int -> Int -> Int -> Int

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