No way to create a foreign pointer for input and have it as an out parameter about c2hs HOT 4 CLOSED

I don't know who originally created this ticket in the old C2HS Trac, but here's my understanding of what's wanted here. Suppose we have a C header file issue46.h with contents:

typedef struct {
  int a;
  float b;
} oid;

void func(oid *obj, int aval, float bval);

then we'd like for the following C2HS code (the + syntax in the parameter list in the fun hook is new):

module Main where

#include "issue46.h"

{#pointer *oid as Oid foreign newtype#}

{#fun func as ^ {+, `Int', `Float'} -> `Oid'#}

to produce Haskell code like this:

module Main where

newtype Oid = Oid (ForeignPtr Oid)
withOid :: Oid -> (Ptr Oid -> IO b) -> IO b
withOid (Oid fptr) = withForeignPtr fptr

func :: Int -> Float -> IO Oid
func a2 a3 =
  mallocForeignPtrBytes 8 $ \oidfptr ->
  withForeignPtr oidfptr $ \oidptr ->
    let {a2' = fromIntegral a2} in
    let {a3' = realToFrac a3} in
    func'_ oidptr a2' a3' >>
    return $ Oid oidfptr

foreign import ccall safe "Issue46.chs.h func"
  func'_ :: Ptr (Oid) -> CInt -> CFloat -> IO ()

(some imports are missing).

Here, the + as a parameter in the fun hook marks that the argument to the corresponding C function is going to be used to do this sort of fast allocation and return.

Does this seem like a reasonable approach? From what I can tell, it ought to cover the requirements of the original ticket. It would be useful to know which C library the original writer of the ticket was working with, to help get some better ideas of how this might work.

from c2hs.

I may have written the original ticket, but I'm not convinced. Doesn't seem like my writing style. But it is something I've wanted before, and I think the proposed solution is pretty reasonable.

from c2hs.

@JohnLato Thanks for the comment, John. No-one else has offered an opinion, so I think I'll just go ahead and do things the way I'd planned. It seems like it should be a useful capability whoever originally asked for it.

from c2hs.

I've implemented this pretty much as outlined above. It works for a simple test case, but there's a strong chance that it will be possible to break things by some combination of the new "+" parameters and other C2HS features. I'll close the issue for now anyway.

from c2hs.