Foreign exported function returns wrong type

I call foo, foreign export ccall foo :: Int -> IO CInt, from C code like this: printf("%d\n", (int)foo((HsInt32)2500)); but when I remove the (int) cast, gcc complains (warnings, not errors) because it thinks that the thing that foo returns is a HsInt32, and not a C int. The type for foo clearly specifies it is a CInt.

The bug is that GHC is probably not generating a CInt, but a HsInt32.

{-# LANGUAGE ForeignFunctionInterface #-}
-- name this file ExportHaskellToCStruct.hs
module ExportHaskellToCStruct(foo) where
import Foreign.C
import Data.List
foreign export ccall foo :: Int -> IO CInt

foo = return . genericLength .  f

f :: Int -> [Int]
f 0 = []
f n = n:(f (n-1))

#include <stdio.h>                                                                                                                                           
#include "/correctpathto/include/HsFFI.h"         /*    FIX PATH for your installation */                                                                                                          
#define __GLASGOW_HASKELL__                                                                                                                                  

#ifdef __GLASGOW_HASKELL__
#include "ExportHaskellToCStruct_stub.h"
#endif

#ifdef __GLASGOW_HASKELL__
extern void __stginit_ExportHaskellToCStruct(void);
#endif

int main(int argc, char *argv[])
{
  int i;

  hs_init(&argc, &argv);
#ifdef __GLASGOW_HASKELL__
  hs_add_root(__stginit_ExportHaskellToCStruct);
#endif

  for (i = 0; i < 5; i++) {
    printf("%d\n", (int)foo((HsInt32)2500)); 
  }

  hs_exit();
  return 0;
}

gcc --version
gcc (Ubuntu 4.3.2-1ubuntu11) 4.3.2

Trac metadata

Trac field	Value
Version	6.10.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (FFI)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed weight to 5

added FFI Tbug Trac import labels

I can't reproduce the problem here - gcc compiles the file without complaint, even with -Wall -Werror, on both x86 and x86_64. I wouldn't expect this to go wrong, because HsInt32 is the same type as int.

changed weight to 3

Oh, I see.

In fact HsInt32 happens to be long on x86, so gcc warns about its use with the %d format in printf, even though on this architecture long is the same size as int. gcc is complaining about a portability problem, but we know there isn't one. In almost every other way, HsInt32 is interchangeable with int.

We could make HsInt32 the same as int, but then the exact same problem would crop up with CLong.

Sigh. It's a pain to make GHC emit a real int here, because GHC doesn't know anything special about the CInt type, as far as GHC is concerned it's just a newtype of Int32 (or whatever).

I'm making this bug low prio, because there are easy workarounds and it's hard to fix.

Trac metadata

Trac field	Value
Priority	normal → low

mentioned in issue #3238

Trac metadata

Trac field	Value
Related	- → #3238

added Plow label