FFI Help

[Simon Peyton-Jones]

Manuel

Would it be worth mentioning or amplifying this point in the FFI spec,
or perhaps in an accompanying Appendix/Commentary of examples and FAQs?
Else someone else is going to trip over it sooner rather than later.

Simon

| -----Original Message-----
| From: haskell-cafe-admin at haskell.org
[mailto:haskell-cafe-admin at haskell.org] On Behalf Of Glynn
| Clements
| Sent: 04 June 2003 08:32
| To: Matthew Donadio
| Cc: haskell-cafe at haskell.org
| Subject: Re: FFI Help
| 
| 
| Matthew Donadio wrote:
| 
| > I am just starting to experiment with FFI, and am running into a
| > problem.  I was to create an FFI to the lgamma(3) found in many of
the
| > newer libm implementations.  My code follows the sig.
| >
| > The lgamma function works.  The gamma function core dumps (I am
using
| > ghc 5.04.3) on me.  gdb reports a SIGSEGV in signgam(), but I'm not
sure
| > why.  I believe that I need to use the monad because signgam is only
| > valid after lgamma returns.
| >
| > Does anyone have an idea what I am doing wrong?
| >
| > Thanks.
| >
| > --
| > Matthew Donadio <m.p.donadio at ieee.org>
| >
| > > module Gamma (gamma, lgamma) where
| >
| > > import System.IO.Unsafe
| >
| > > foreign import ccall "math.h lgamma"  lgammaC  :: Double -> IO
Double
| > > foreign import ccall "math.h signgam" signgamC :: IO Int
| 
| signgam is an "int" variable, but this assumes that it is a function
| of type "int signgam(void)".
| 
| Write a C wrapper "int get_signgam(void) { return signgam; }" and
| import that.
| 
| --
| Glynn Clements <glynn.clements at virgin.net>



| Or alternatively, foreign import the address of the int and read it
| directly with 'peek'.
| 
|     import Foreign
|     ...
|     foreign import ccall "math.h &signgam" signgamC :: Ptr Int32
|     ...
|     gammaIO :: Double -> IO Double
|     gammaIO x = do lg <- lgammaC x
|                    s  <- peek signgamC
|                    return $ fromIntegral s * exp lg
| 
| 
| Regards,
|   Malcolm