Wrong size for Int in arguments to functions wrapped to FunPtr by FFI wrappers (on 64-bit machine)

[shahn]

An operation

  foo :: Int -> IO ()

is wrapped to a function pointer ptr :: FunPtr (Int -> IO ()) (via an FFI wrapper). If you pass this pointer to a C function, that calls it with a negative parameter (e.g. -1), the operation is executed with a positive parameter (something near 4294967295 ( == 2 ^ 32 - 1)).

This works correctly on x86 (32-bit).

See attachment for an example.

[duncan]

I think this is an incorrect use of the FFI. The example uses Haskell Int and passes it to C code which uses it at C's type int. These are not the same types, though they do happen to coincide on x86 32bit. The example should be using CInt on the Haskell side. Alternatively, if the intention really is to use the Haskell Int type then the C side code must use HsInt (which is defined in HsFFI.h).

The reason we get the observed result is because on x86-64, GHC uses a 64-bit Int while C's int remains 32bit so -1 in a 32bit int is all 1 bits but that bit pattern as a larger int is 2^32-1.

So I'm closing the ticket as invalid. If anyone thinks this is incorrect then please do re-open it.

[shahn]

Replying to duncan:

I was unsure about the correct use of the FFI.

However, the described behavior is also shown when using CInt instead of Int.

I am re-opening the bug...

[PaulLiu]

This bug affects 32-bit machines too if the callback function is made of type: CChar -> IO (), and when C function calls it with -1, it would receive 255 instead.

Functions like rts_mkInt8, rts_mkInt16, etc. in RtsAPI.c would cast values into StgInt after applying a mask. This mask would definitely change the sign of negative numbers. So I wonder why the actual callback into Haskell function would ignore such mask and treat it just like an ordinary StgInt casted back into Int8, Int16, etc.

[simonmar]

Fixed:

Tue Sep 30 04:56:11 PDT 2008  Simon Marlow <marlowsd@gmail.com>
  * fix #2594: we were erroneously applying masks, as the reporter suggested

[igloo]

Merged