FFI Help

[Manuel M T Chakravarty]

"Simon Peyton-Jones" <simonpj at microsoft.com> wrote,

> 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.

Good point.  I think this is central enough to include it
into the spec itself.  Hence, I have added the following
text to the end of 4.1.1 (where the import declarations for C
are described):

 It should be noted that a static foreign declaration that does not import an
 address (i.e., where \gterm\& is not used in the specification of the external
 entity) always refers to a C function, even if the Haskell type is
 non-functional.  For example, 
 %
 \begin{quote}
 \begin{verbatim}
 foreign import ccall foo :: CInt
 \end{verbatim}
 \end{quote}
 %
 refers to a pure C function \code{foo} with no arguments that returns an
 integer value.  Similarly, if the type is \code{IO CInt}, the declaration
 refers to an impure nullary function.  If a Haskell program needs to access a
 C variable \code{bar} of integer type,
 %
 \begin{quote}
 \begin{verbatim}
 foreign import ccall "&" bar :: Ptr CInt
 \end{verbatim}
 \end{quote}
 %
 must be used to obtain a pointer referring to the variable.  The variable can
 be read and updated using the routines provided by the module \code{Storable}
 (cf.\ Section~\ref{sec:Storable}).

I hope that this clarifies the point.

Thanks,
Manuel

> | -----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
> 
> 
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