Device Management
A host process can see more than one GPU, and CUDA never guesses which one a call should target — it operates against whichever device is current on the calling thread, a piece of state the program has to set itself. Getting this wrong doesn't usually crash; it silently allocates memory or launches kernels on the wrong GPU, or leaves a multi-threaded program with each thread quietly disagreeing about which device it's using. See Error Handling and Checking for what CUDA_CHECK does with the status these calls return.