help with python ctypes and nvapi

Question

My end goal is to query NVAPI for gpu usage and other statistics in python. See http://developer.nvidia.com/nvapi

from ctypes import WinDLL
nvapi = WinDLL("nvapi.dll")
print nvapi# <WinDLL 'nvapi.dll', handle 718a0000 at 27c0050>
print nvapi.nvapi_QueryInterface# <_FuncPtr object at 0x026D8E40>
print nvapi.nvapi_QueryInterface()# returns 0
print nvapi.NvAPI_Initialize# AttributeError: function 'NvAPI_Initialize' not found
print nvapi.NvAPI_SYS_GetChipSetInfo# AttributeError: function 'NvAPI_SYS_GetChipSetInfo' not found

Here is a copy of the header file available for download from the link above: http://paste.pound-python.org/show/7337/

At this point, I am just trying to familiarize myself with the api... so what am I doing wrong? I can't figure out how to call any of the functions listed in the header file.

Answer 1

Are you sure it's a WinDLL? From the header file, it looks like a standard C calling convention to me. Have you tried CDLL instead?

EDIT:

I see now. The header you pointed to isn't actually the interface for nvapi.dll--it is a wrapper around it that must be statically linked.

From the docs downloaded from NVIDIA's developer site:

Use a Static Link with Applications

NvAPI cannot be dynamically linked to applications. You must create a static link to the library and then call NvAPI_Initialize(), which loads nvapi.dll dynamically.

If the NVIDIA drivers are not installed on the system or nvapi.dll is not present when the application calls NvAPI_Initialize(), the call just returns an error. The application will still load.

I would guess that the actual calls in nvapi.dll are completely different than the ones exposed in this wrapper library. I can't seem to find any documentation on those though. Perhaps they are internal and change between systems.

If you want to use this interface, I'm not really sure what the best solution is. It's a static library and not a dynamic one, so ctypes wouldn't handle it unless you wrapped it in another DLL. I'm not an expert at native code with Python, so maybe someone else will have an easy fix. Sorry.