Getting NVIDIA GPU Usage in C++

Strangely enough, NVAPI has no functions to get GPU usage/load. It turns out that there are some secret functions in nvapi.dll. You can use QueryInterface function to retrieve them by specifying the memory address of the function. Here's the code for Windows.

//
// Getting Nvidia GPU Usage
//
// Reference: Open Hardware Monitor (http://code.google.com/p/open-hardware-monitor)
//

#include <windows.h>
#include <iostream>

// magic numbers, do not change them
#define NVAPI_MAX_PHYSICAL_GPUS   64
#define NVAPI_MAX_USAGES_PER_GPU  34

// function pointer types
typedef int *(*NvAPI_QueryInterface_t)(unsigned int offset);
typedef int (*NvAPI_Initialize_t)();
typedef int (*NvAPI_EnumPhysicalGPUs_t)(int **handles, int *count);
typedef int (*NvAPI_GPU_GetUsages_t)(int *handle, unsigned int *usages);

int main()
{   
    HMODULE hmod = LoadLibraryA("nvapi.dll");
    if (hmod == NULL)
    {
        std::cerr << "Couldn't find nvapi.dll" << std::endl;
        return 1;
    }

    // nvapi.dll internal function pointers
    NvAPI_QueryInterface_t      NvAPI_QueryInterface     = NULL;
    NvAPI_Initialize_t          NvAPI_Initialize         = NULL;
    NvAPI_EnumPhysicalGPUs_t    NvAPI_EnumPhysicalGPUs   = NULL;
    NvAPI_GPU_GetUsages_t       NvAPI_GPU_GetUsages      = NULL;

    // nvapi_QueryInterface is a function used to retrieve other internal functions in nvapi.dll
    NvAPI_QueryInterface = (NvAPI_QueryInterface_t) GetProcAddress(hmod, "nvapi_QueryInterface");

    // some useful internal functions that aren't exported by nvapi.dll
    NvAPI_Initialize = (NvAPI_Initialize_t) (*NvAPI_QueryInterface)(0x0150E828);
    NvAPI_EnumPhysicalGPUs = (NvAPI_EnumPhysicalGPUs_t) (*NvAPI_QueryInterface)(0xE5AC921F);
    NvAPI_GPU_GetUsages = (NvAPI_GPU_GetUsages_t) (*NvAPI_QueryInterface)(0x189A1FDF);

    if (NvAPI_Initialize == NULL || NvAPI_EnumPhysicalGPUs == NULL ||
        NvAPI_EnumPhysicalGPUs == NULL || NvAPI_GPU_GetUsages == NULL)
    {
        std::cerr << "Couldn't get functions in nvapi.dll" << std::endl;
        return 2;
    }

    // initialize NvAPI library, call it once before calling any other NvAPI functions
    (*NvAPI_Initialize)();

    int          gpuCount = 0;
    int         *gpuHandles[NVAPI_MAX_PHYSICAL_GPUS] = { NULL };
    unsigned int gpuUsages[NVAPI_MAX_USAGES_PER_GPU] = { 0 };

    // gpuUsages[0] must be this value, otherwise NvAPI_GPU_GetUsages won't work
    gpuUsages[0] = (NVAPI_MAX_USAGES_PER_GPU * 4) | 0x10000;

    (*NvAPI_EnumPhysicalGPUs)(gpuHandles, &gpuCount);

    // print GPU usage every second
    for (int i = 0; i < 100; i++)
    {
        (*NvAPI_GPU_GetUsages)(gpuHandles[0], gpuUsages);
        int usage = gpuUsages[3];
        std::cout << "GPU Usage: " << usage << std::endl;
        Sleep(1000);
    }

    return 0;
}