Editing HSFFIG/Tutorial (section)

==== Bit fields ====

Bit fields (declared as <tt>int somefield:n</tt>) are supported by '''hsffig''', but there are some additional things that need attention.

There is no such thing as ''offsetof'' for bit fields. To implement the <tt>--></tt> and <tt><--</tt> combinators, special inline '''C''' functions are embedded in the hsc code '''hsffig''' generates. This is achieved with the ''#def'' macro that '''hsc2hs''' recognizes.

These functions are placed in a special '''C''' file. This file must be compiled and linked to the final executable.  Consider the following '''C''' header (''bf.h''):

<pre>
union bitfields {
  struct {
    int a:1;
    int b:1;
    int c:1;
    int d:1;
  } x;
  int y;
};
</pre>

and the Haskell program (''bftest.hs''):

<pre>
-- Test for bit fields.

module Main where

import BF_H

main = do
  putStrLn $ "Test of Bit Fields"

  alloca $ \(bfu :: Ptr U_bitfields) -> do
    x <- (bfu --> V_x)
    (x, V_a) <-- fromIntegral 1
    (x, V_b) <-- fromIntegral 0
    (x, V_c) <-- fromIntegral 0
    (x, V_d) <-- fromIntegral 1
    y <- (bfu --> V_y)
    putStrLn $ "y = " ++ (show y)
</pre>

When '''hsffig''' processes ''bf.h'' it creates ''BF_H.hsc''. When '''hsc2hs''' processes ''BF_H.hsc'' it creates ''BF_H.hs'', and additionally ''BF_H_hsc.h'' and ''BF_H_hsc.c''. The latter is recommended to compile with '''ghc''':

<pre>
ghc -c BF_H_hsc.c
</pre>

which provides correct compilation environment, so necessary paths for the runtime header files will be passed to '''gcc''' properly.

After all, the executable may be compiled and linked:

<pre>
ghc -ffi -fglasgow-exts -package HSFFIG --make bftest.hs BF_H_hsc.o -o bftest
</pre>

If ''bftest'' is run, it prints "9", as one might expect (at least on the x86 architecture).