Thinking to take advantage of fortuitous heap layout of some Haskell
values for interfacing with C, I've written the following function:
addressOf :: a -> Ptr ()
addressOf x = x `seq` unsafeCoerce# (Box x)
data Box x = Box x
For example,
data A = A {-# UNPACK #-} !(Ptr Word8) {-# UNPACK #-} !CInt
main = let a = A nullPtr 12
p = addressOf a `plusPtr` 4
in do x <- peek p :: IO Int
y <- peek p :: IO Int
print (x, y)
prints
(0, 12)
One thing I don't understand is that this fails if I use Just
rather than inventing my box type. I suppose the info table for
Just is set up to support a vectored return for pattern matching
on Maybe? (the commentary isn't very clear here. The section
http://hackage.haskell.org/trac/ghc/wiki/Commentary/Rts/HaskellExecution#ReturnConvention
says, in full:
"Return Convention
Direct Returns
Vectored Returns"
)
The reason I'm messing about with this stuff is that I'm pretty sure
passing p to C code would give a usable pointer to
struct a {char *; int;};
Obviously my plot will be spoiled if the GC comes along and relocates
the value while C code is trying to use it.
Are there any other pitfalls with this approach?
A different and in all likelihood saner approach is building up more
tools for manipulating pointers to C data from Haskell, perhaps along
the lines of cmucl's support for "Alien Objects".
http://www.math.sunysb.edu/~sorin/online-docs/cmucl/aliens.html
The main reason to even think about touching the heap representation of
Haskell objects is so that the values can be manipulated by pure code,
pattern matched, etc.
Brandon
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