I've added print statements before and after the point where I knew it was dying (by looking at the output), and basically kept adding more closer together until I got to this line. The print before shows up, the print after doesn't.

It did just occur to me that I may not have explicitely disabled buffering on STDOUT, so I'll run it again that way to make sure I'm not just missing some messages that were still in the buffer.

If I replace the packed values with constants then the program won't run. This line gets called successfully hundreds of times already before the program dies, and always with different values in the packet. I can try modifying enough of the callers to pretend I got a successful reply, but that's basically gonna take away all of the socket interaction.