[Haskell-cafe] FFI ccall struct return by value on the stack?

Sylvain Henry sylvain at haskus.fr
Tue Jun 16 14:57:40 UTC 2020

On 16/06/2020 16:44, ☂Josh Chia (謝任中) wrote:
> Based on my limited understanding of Haskell FFI according to the 
> Haskell 2010 Language Report 
> (https://www.haskell.org/onlinereport/haskell2010/haskellch8.html), it 
> is not possible to have a ccall for makeFoo() because Foo is not a 
> marshallable foreign result type. Is my understanding correct?
Yes. Structures are sometimes passed into registers (at least on x86-64) 
depending on their field types. GHC doesn't support this.

> However, I believe I can have a ccall if I change makeFoo() to either 
> of the following:
> Foo* makeFoo(size_t x);
> void makeFoo(Foo* out, size_t x);
> The first involves the C code allocating a Foo and returning a pointer 
> to it (so now there's one more pointer for the C code to deallocate 
> later in another function). The second involves the C code writing a 
> Foo value to a piece of memory allocated in Haskell (possibly using 
> Foreign.Marshall.Alloc.alloca). Both signatures work because Foo* is 
> marshallable but are clumsier to use than the original signature. Is 
> there just no way to return a struct by value on the stack? Is there a 
> cleaner way than the above two?
The second approach seems cleaner to me. With a Storable instance for 
Foo, it's not that much clumsy.


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