Hi, I tried googling and searching the haskellwiki about this but wasn't lucky enough. My question is: is there a way to send struct arguments to C functions via the FFI or do I need to create a C wrapper? I guess there isn't, and while I can live without it, I'd like to leave no doubt. Details: I have something like ==================== typedef struct vect { float x,y; } vect; void func(vect v); ===================== on the C side and ==================== -- Please disregard float /= Float, just an example :) data Vector = Vector Float Float instance Storable Vector where ... ==================== on the Haskell side, and I want to call func with Vector as argument. Now, Vector isn't a basic FFI type, although it implements Storable. So does that mean that I need to create something like ==================== void funcWrapper(vect *v) { func(*v); } ==================== and then allocate some temporary memory on the Haskell side to use func? Cheers! -- Felipe.
Felipe Lessa wrote:
Hi,
I tried googling and searching the haskellwiki about this but wasn't lucky enough. My question is: is there a way to send struct arguments to C functions via the FFI or do I need to create a C wrapper? I guess there isn't, and while I can live without it, I'd like to leave no doubt.
Sometimes there is such a way. See below.
Details:
I have something like
==================== typedef struct vect { float x,y; } vect;
void func(vect v);
For most architectures stack layout of void func(vect v); and void func(float x, float y); is exactly the same, so for FFI purposes this 'func' can be declared as something like: foreign import ccall unsafe :: Float -> Float -> IO () Cheers, Kyra
On Wed, 2008-07-16 at 22:45 -0300, Felipe Lessa wrote:
Hi,
I tried googling and searching the haskellwiki about this but wasn't lucky enough. My question is: is there a way to send struct arguments to C functions via the FFI or do I need to create a C wrapper? I guess there isn't, and while I can live without it, I'd like to leave no doubt.
Correct. The FFI spec does not support C structs as C function parameters or results. You'll need a wrapper. If the struct is passed by reference of course then you're fine, but if it's by value then you need a C wrapper. The reason is because it's beyond the scope of the FFI to know the structure layout and how to map that to haskell types. That's the domain of FFI pre-processors. However I don't know of any FFI pre-processors that help in this case. Passing C structs by value seems to be pretty rare in exported C APIs. Duncan
If the struct is passed by reference of course then you're fine, but if it's by value then you need a C wrapper. The reason is because it's beyond the scope of the FFI to know the structure layout and how to map that to haskell types. That's the domain of FFI pre-processors. However I don't know of any FFI pre-processors that help in this case. Passing C structs by value seems to be pretty rare in exported C APIs.
Yes, but programmer *knows* the structure layout, so she usually can emulate it with a sequence of primary ffi type arguments. It's pretty trivial for the original example (see my previous post on this subj) and can be extended further. For example, in my homebrew COM library I pretty successfully marshall 16-byte Variants *by value* by means of two consecutive "legal" Word64 arguments. Cheers, Kyra
On Thu, Jul 17, 2008 at 12:08 PM, kyra
Yes, but programmer *knows* the structure layout, so she usually can emulate it with a sequence of primary ffi type arguments. It's pretty trivial for the original example (see my previous post on this subj) and can be extended further. For example, in my homebrew COM library I pretty successfully marshall 16-byte Variants *by value* by means of two consecutive "legal" Word64 arguments.
I am concerned, however, with the portability of the library. I mean, is the calling convention for both void func(vect v); void func(float x, float y); the same on x86? On x86-64? On Windows? On Linux? I guess it would be a lot faster to pass the arguments on the stack than alloca'ting, copying to the new area and then copying from the area to the stack, but I don't want to sacrifice the portability. Thanks, -- Felipe.
From: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Duncan Coutts Sent: 17 July 2008 12:46
On Wed, 2008-07-16 at 22:45 -0300, Felipe Lessa wrote:
Hi,
I tried googling and searching the haskellwiki about this but wasn't lucky enough. My question is: is there a way to send struct arguments to C functions via the FFI or do I need to create a C wrapper? I guess there isn't, and while I can live without it, I'd like to leave no doubt.
If the struct is passed by reference of course then you're fine, but if it's by value then you need a C wrapper. The reason is because it's beyond the scope of the FFI to know the structure layout and how to map that to haskell types. That's the domain of FFI pre-processors. However I don't know of any FFI pre-processors that help in this case. Passing C structs by value seems to be pretty rare in exported C APIs.
hsc2hs can help a bit (I haven't used the other FFI tools, so don't take
this as an endorsement of hsc2hs over them). You could create a wrapper
that marshals your Vector to a vector struct like this:
---- .hsc file:
#include
On Thu, Jul 17, 2008 at 12:37 PM, Bayley, Alistair
-- inline your *vect->vect wrapper (ends up in generated .c file) #def void funcWrapper(vect *v) { func(*v); }
foreign import stdcall funcWr unsafe "funcWrapper" :: VectorC -> IO ()
I am using hsc2hs currently, but googling about #def with Cabal I found out that some people were having trouble to make Cabal discover that hsc2hs had created a new C file. Specifically, bug #245 [1] which says that the milestone is undefined. So for now I'm creating a wrapper.[ch] myself. [1] http://hackage.haskell.org/trac/hackage/ticket/245 -- Felipe.
On Thu, 2008-07-17 at 13:36 -0300, Felipe Lessa wrote:
I am using hsc2hs currently, but googling about #def with Cabal I found out that some people were having trouble to make Cabal discover that hsc2hs had created a new C file. Specifically, bug #245 [1] which says that the milestone is undefined.
Note that a milestone of _|_ doesn't mean we don't want a fix, just that nobody has said they're going to do it. We would welcome a fix for this if someone wants to contribute. (Of course we'd like contributions to fix all our bugs!) Duncan
Felipe Lessa wrote:
Hi,
I tried googling and searching the haskellwiki about this but wasn't lucky enough. My question is: is there a way to send struct arguments to C functions via the FFI or do I need to create a C wrapper? I guess there isn't, and while I can live without it, I'd like to leave no doubt.
You might find this old post of mine useful. http://therning.org/magnus/archives/315
Details:
I have something like
==================== typedef struct vect { float x,y; } vect;
void func(vect v); =====================
on the C side and
==================== -- Please disregard float /= Float, just an example :) data Vector = Vector Float Float
instance Storable Vector where ... ====================
on the Haskell side, and I want to call func with Vector as argument. Now, Vector isn't a basic FFI type, although it implements Storable. So does that mean that I need to create something like
==================== void funcWrapper(vect *v) { func(*v); } ====================
and then allocate some temporary memory on the Haskell side to use func?
Cheers!
-- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus＠therning．org http://therning.org/magnus Haskell is an even 'redder' pill than Lisp or Scheme. -- PaulPotts
participants (5)
-
Bayley, Alistair -
Duncan Coutts -
Felipe Lessa -
kyra -
Magnus Therning