I have two modules. One is a library, installed with Cabal: ------------------------------------------------------------------------- -- Data/Text/IDN/StringPrep.hs {-# LANGUAGE ForeignFunctionInterface #-} module Data.Text.IDN.StringPrep where import Foreign foreign import ccall "stringprep.h &stringprep_nameprep" profileNameprep :: Ptr () ------------------------------------------------------------------------- The other is the main program, either interpreted directly or compiled and run: ------------------------------------------------------------------------- -- TestIDN.hs {-# LANGUAGE ForeignFunctionInterface #-} module Main where import Foreign import Data.Text.IDN.StringPrep foreign import ccall "stringprep.h &stringprep_nameprep" stringprep_nameprep :: Ptr () main = do putStrLn $ "stringprep_nameprep = " ++ show stringprep_nameprep putStrLn $ "profileNameprep = " ++ show profileNameprep ------------------------------------------------------------------------- Now, given that both modules import the exact same C symbol, I would expect that the pointers would be the same when running. This is the case for compiled code: ------------------------------------------------------------------------- $ ghc --make TestIDN [1 of 1] Compiling Main ( TestIDN.hs, TestIDN.o ) Linking TestIDN ... $ ./TestIDN stringprep_nameprep = 0x000000000067e3e0 profileNameprep = 0x000000000067e3e0 ------------------------------------------------------------------------- However, if I use ghci / runhaskell, the pointers are different: ------------------------------------------------------------------------- $ rm TestIDN TestIDN.o TestIDN.hi $ runhaskell TestIDN.hs stringprep_nameprep = 0x00007feebe5fe4e0 profileNameprep = 0x0000000040a908bc ------------------------------------------------------------------------- Specifically, the pointer loaded from the library ('profileNameprep') is invalid -- attempting to pass it to libidn will result in errors, and trying to poke around in it yields segmentation faults. I'm using GHC 6.12.1, on Linux. This problem occurs with both dynamic and static libraries. Does anybody have suggestions about how to solve or work around it?