[GHC] #15933: Wrong size for int for callbacks into Haskell from foreign code

22 Nov 2018

      #15933: Wrong size for int for callbacks into Haskell from foreign code
-------------------------------------+-------------------------------------
           Reporter:  iliastsi       |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.3
          Component:  Compiler       |           Version:  8.4.3
  (FFI)                              |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Incorrect result
  Unknown/Multiple                   |  at runtime
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 The following code produces different results on amd64 (64-bit little
 endian) and s390x (64-bit big endian).

 `foo.h:`
 {{{#!c
 typedef void(*hs_callback)(int x);
 extern void function_in_c(hs_callback cb);
 }}}
 `foo.c:`
 {{{#!c
 #include "foo.h"

 void function_in_c(hs_callback cb)
 {
     int x = 10;
     cb(x);
 }
 }}}
 `Foo.hs:`
 {{{#!hs
 module Main(main) where

 import Foreign
 import Foreign.C

 type HsCallback = CInt -> IO ()

 foreign import ccall "foo.h function_in_c"
   functionInC :: FunPtr HsCallback -> IO ()

 foreign import ccall "wrapper"
   wrap :: HsCallback -> IO (FunPtr HsCallback)

 main = do
   f <- wrap $ \x -> print x
   functionInC f
   freeHaskellFunPtr f
 }}}

 On amd64 the output is 10, but on s390x is 0. On both machines, `sizeOf
 (undefined :: CInt) == sizeof(int) == 4`. When changing `HsCallback` to:
 {{{#!hs
 type HsCallback = Int -> IO ()
 }}}
 both produce the same, correct result, but the above seems wrong, as
 `sizeOf (undefined :: Int) == 8`.

 This has been reproduced with both ghc-8.4 and ghc-8.2, and causes dbus to
 fail to build on s390x (https://github.com/rblaze/haskell-dbus/issues/26),
 as it relies on libxml-sax which contains the above (simplified) code.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15933
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