# Troubleshooting


## UndefVarError: libcuda not defined

This means that CUDA.jl could not find a suitable CUDA driver. For more information,
re-run with the `JULIA_DEBUG` environment variable set to `CUDA_Driver_jll`.


## UNKNOWN_ERROR(999)

If you encounter this error, there are several known issues that may be causing it:

- a mismatch between the CUDA driver and driver library: on Linux, look for clues in `dmesg`.
- some issue with the forwards-compatible driver library: try running with `JULIA_CUDA_USE_COMPAT=false` (or set the equivalent preference).
- the CUDA driver is in a bad state: this can happen after resume. **Try rebooting**.

Generally though, it's impossible to say what's the reason for the error, but Julia is
likely not to blame. Make sure your set-up works (e.g., try executing `nvidia-smi`, a CUDA C
binary, etc), and if everything looks good file an issue.


## NVML library not found (on Windows)

Check and make sure the `NVSMI` folder is in your `PATH`. By default it may not be. Look in
`C:\Program Files\NVIDIA Corporation` for the `NVSMI` folder - you should see `nvml.dll`
within it. You can add this folder to your `PATH` and check that `nvidia-smi` runs properly.


## The specified module could not be found (on Windows)

Ensure the [Visual C++ Redistributable](https://aka.ms/vs/16/release/vc_redist.x64.exe) is
installed.
