In fact I got the segfault also with the P.print... So I rewrote the solve function like this withDiffractometer :: Diffractometer -> (Ptr HklEngineList -> IO b) -> IO b withDiffractometer d fun = do let f_engines = difEngineList d withForeignPtr f_engines fun solveTrajPipe' :: Diffractometer -> Pipe Engine Geometry IO () solveTrajPipe' dif = flip evalStateT dif $ forever $ do -- Inside here we are using `StateT Int (Consumer a IO) r` e <- lift await dif <- get let name = engineName e solutions <- lift . lift $ withDiffractometer dif $ \engines -> withCString name $ \cname -> do engine <- c_hkl_engine_list_engine_get_by_name engines cname nullPtr n <- c_hkl_engine_pseudo_axis_names_get engine >>= darrayStringLen solve' engine n e >>= getSolution0 put dif lift $ yield solutions using the evalStateT. It works with million of evaluations :) but now if I remove the get and put lines, I get the segfault. This segfault is located in the solve' method solve' :: Ptr HklEngine -> CSize -> Engine -> IO (ForeignPtr HklGeometryList) solve' engine n (Engine _ ps _) = do let positions = [v | (Parameter _ v _) <- ps] withArray positions $ \values -> c_hkl_engine_pseudo_axis_values_set engine values n unit nullPtr >>= newForeignPtr c_hkl_geometry_list_free so to my opinion I do not manage correctly the life time of my dif object, but I do not understand how I can ensure this life time in until the solve' method proceed. Cheers Frederic