[Haskell-cafe] advanced cabal configuration

[Kazu Yamamoto (=?iso-2022-jp?B?GyRCOzNLXE9CSScbKEI=?=)]

Hi,

I'm a maintainer of simple-sendfile and need to implement the
following is to rescue 32bit Linux:

- If the COff size is 8, the following FFI is used:

	foreign import ccall unsafe "sendfile64"
	    c_sendfile :: Fd -> Fd -> Ptr COff -> CSize -> IO (#type ssize_t)

- Otherwise, the following FFI is used:

	foreign import ccall unsafe "sendfile"
	    c_sendfile :: Fd -> Fd -> Ptr COff -> CSize -> IO (#type ssize_t)

How can I implement this?

I guess that Setup.hs should evaluate (sizeOf (1 :: COff)) and define
a C macro, say LARGE_OFFSET and write the following code.

        #ifdef LARGE_OFFSET
	foreign import ccall unsafe "sendfile64"
        #else
	foreign import ccall unsafe "sendfile"
        #endif

I have no experience to use Build-Type: other than Simple in cabal
files.  Would someone explain how to implement it concretely or
suggest examples which implement similar things?

For more informaion, please read:
	https://github.com/yesodweb/wai/issues/372

Thanks.

--Kazu