"Simon Peyton-Jones" <simonpj@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@haskell.org [mailto:haskell-cafe-admin@haskell.org] On Behalf Of Glynn | Clements | Sent: 04 June 2003 08:32 | To: Matthew Donadio | Cc: haskell-cafe@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@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@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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