[c2hs] #32: #get generated code doesn't work on bitfields

25 Aug 2010

      #32: #get generated code doesn't work on bitfields
--------------------+-------------------------------------------------------
 Reporter:  guest   |        Type:  defect 
   Status:  new     |    Priority:  normal 
Milestone:          |   Component:  general
  Version:  0.16.2  |    Keywords:         
--------------------+-------------------------------------------------------
 Consider the following source files:

 {{{
 /* bitfield.c */
 #include "bitfield.h"

 static testStruct makeItFrom;

 testStruct* makeIt() {
     makeItFrom.a = 0;
     makeItFrom.b = 1;
     return &makeItFrom;
 }
 }}}

 {{{
 /* bitfield.h */
 typedef struct testStruct_ testStruct;
 struct testStruct_
 {
     unsigned       a : 31;
     unsigned       b :  1;
 };
 testStruct* makeIt();
 }}}

 {{{
 {-# LANGUAGE ForeignFunctionInterface #-}

 #include "bitfield.h"

 import C2HS

 {#pointer *testStruct as TestStructPtr #}

 main = do
     x <- {#call makeIt #}
     print =<< ({#get testStruct->b #} x)
 }}}

 Compile as follows:

 {{{
 ezyang@javelin:~/Dev/haskell/c2hs-bitfield$ gcc -c -o bitfield.o
 bitfield.c
 ezyang@javelin:~/Dev/haskell/c2hs-bitfield$ c2hs Bitfield.chs
 ezyang@javelin:~/Dev/haskell/c2hs-bitfield$ ghc --make Bitfield.hs
 bitfield.o
 [2 of 2] Compiling Main             ( Bitfield.hs, Bitfield.o )
 Linking Bitfield ...
 }}}

 When you run the resulting executable, the expected output is 1, but the
 actual output is 0.

 Looking at the generated HS:

 {{{
 main = do
     x <- makeIt
 {-# LINE 10 "Bitfield.chs" #-}
     print =<< ((\ptr -> do {val <- peekByteOff ptr 4 ::IO CUInt{-:1-};
 return $ (val `shiftL` (32 - 1)) `shiftR` (32 - 1)}) x)
 }}}

 The byte offset is obviously bogus (the important information must be in
 offsets 0, 1, 2 or 3). Less obvious is what the correct behavior in all
 cases is: the bitfield arrangement appears to be compiler dependent. Maybe
 C2HS should just bug out and say that bitfields are not supported.

-- 
Ticket URL: http://hackage.haskell.org/trac/c2hs/ticket/32
c2hs http://www.cse.unsw.edu.au/~chak/haskell/c2hs/
C->Haskell, An Interface Generator for Haskell