I am using the call_external command to run a little program that is written in CUDA (there is a C wrapper, which is what IDL calls). my problem is, it only works sometimes. Using the same shared object file, and the same instance of IDL, sometimes it gets all zeros back. I have done a bunch of debugging, and it seems the problem is with the loading of the SO file from IDL. Any idea what could cause this? Let me know if you need more details. EDIT to add more info: I am running UBUNTU, 64 bit. It seems to have fixed itself. After it doing this for weeks...it just doesn't anymore. Problems that resolve themselves suck, so I would still appreciate ideas on what might cause this so I can make the program stable.