Eliot, I am calling something that I *think* simply tells GSL where to find the callbacks and a relevant structure. But I am getting a primitive failure in VMCallbackContext32>>primReturnAs:fromContext:, which I assume means that the library is attempting to call into Pharo. In the debugger's context, ec is set to #'bad number of arguments'. I have looked at the signatures and the blocks, and the argument counts look correct at first glance, albeit toward the end of a long day. Am I being naive? Any better ideas? My next inclination is to set breakpoints in all of the callbacks to seee if any of them get hit. I can't see why they would, but it's possible - especially given other weirdness that I have observed in GSL. It work, but it's a little rough around the (design/elegance) edges at times. Bill