Unable to create pipeline in C++ Dll called by C#

harris66

Hello everyone,

I am pretty new to DepthAI and c++ dll. I am trying to build a application to pass the oak camera's image frame to C# environment. I am using P/Invoke and c++ DLL. The loadRGBImg function can be successfully built and called by C# while comes into the error:
Fatal error. System.AccessViolationException: Attempted to read or write protected memory. This is often an indication that other memory is corrupt.

Repeat 2 times:

--------------------------------

at Program.loadRGBImg(IntPtr ByRef, Int32 ByRef, Int32 ByRef, Int32 ByRef)

--------------------------------

at Program.Main(System.String[])

Here's the function code in cpp

bool loadRGBImg(unsigned char** imageData, int* width, int* height, int* channels) {

// Create pipeline

dai::Pipeline pipeline;

auto colorCam = pipeline.create<dai::node::ColorCamera>();

auto xlinkOut = pipeline.create<dai::node::XLinkOut>();

xlinkOut->setStreamName("preview");

colorCam->setInterleaved(true);

colorCam->preview.link(xlinkOut->input);

std::cout << "pipeline built" << std::endl;

try {

dai::Device device(pipeline);

// Get output queue

auto preview = device.getOutputQueue("preview");

// Receive 'preview' frame from device

auto imgFrame = preview->get<dai::ImgFrame>();

// Show the received 'preview' frame

cv::Mat frame(imgFrame->getHeight(), imgFrame->getWidth(), CV_8UC3, imgFrame->getData().data());

std::cout << "frame getted" << std::endl;

*width = frame.cols;

*height = frame.rows;

*channels = frame.channels();

int dataSize = frame.total() * frame.elemSize();

*imageData = (unsigned char*)CoTaskMemAlloc(dataSize);

if (*imageData == nullptr) {

std::cout << "Memory allocation failed" << std::endl;

return false;

}

memcpy(*imageData, frame.data, dataSize);

std::cout << "Image data copied successfully" << std::endl;

return true;

}

catch (const std::runtime_error& err) {

std::cout << err.what() << std::endl;

return false;

}

It supposed to be some memory relative issues based on my understanding, if I cannot do it in this way, is there any other way to achieve this ingestion? Thanks for the support in advanced!

jakaskerl

Hi @harris66

If you haven't already, I'd suggest trying GPT for this since it not directly supported by Luxonis.

It appears that you are attempting to pass an image frame from a DepthAI device (OAK camera) to a C# application using P/Invoke and a C++ DLL. The error System.AccessViolationException indicates that there is an issue with memory access, likely due to incorrect memory management between the managed (C#) and unmanaged (C++) environments.

Let's first ensure that the C++ function is correctly allocating and managing memory, and then ensure that the C# side correctly handles the memory allocated by the C++ code. Below is a revised version of your C++ function and a sample C# code snippet to call it.

Revised C++ Function

#include <depthai/depthai.hpp>
#include <opencv2/opencv.hpp>
#include <iostream>
#include <windows.h> // For CoTaskMemAlloc and CoTaskMemFree

extern "C" __declspec(dllexport) bool loadRGBImg(unsigned char** imageData, int* width, int* height, int* channels) {
    // Create pipeline
    dai::Pipeline pipeline;

    auto colorCam = pipeline.create<dai::node::ColorCamera>();
    auto xlinkOut = pipeline.create<dai::node::XLinkOut>();

    xlinkOut->setStreamName("preview");

    colorCam->setInterleaved(true);
    colorCam->preview.link(xlinkOut->input);

    std::cout << "pipeline built" << std::endl;

    try {
        dai::Device device(pipeline);

        // Get output queue
        auto preview = device.getOutputQueue("preview");

        // Receive 'preview' frame from device
        auto imgFrame = preview->get<dai::ImgFrame>();

        // Show the received 'preview' frame
        cv::Mat frame(imgFrame->getHeight(), imgFrame->getWidth(), CV_8UC3, imgFrame->getData().data());

        std::cout << "frame getted" << std::endl;

        *width = frame.cols;
        *height = frame.rows;
        *channels = frame.channels();

        int dataSize = frame.total() * frame.elemSize();
        *imageData = (unsigned char*)CoTaskMemAlloc(dataSize);

        if (*imageData == nullptr) {
            std::cout << "Memory allocation failed" << std::endl;
            return false;
        }

        memcpy(*imageData, frame.data, dataSize);

        std::cout << "Image data copied successfully" << std::endl;

        return true;
    } catch (const std::runtime_error& err) {
        std::cout << err.what() << std::endl;
        return false;
    }
}

C# Code to Call the C++ Function

Ensure that you handle the memory allocated by the C++ code properly. You will need to free the allocated memory after you are done using it.

using System;
using System.Runtime.InteropServices;

class Program
{
    // Define the P/Invoke signature for the C++ function
    [DllImport("YourCppDllName.dll", CallingConvention = CallingConvention.Cdecl)]
    private static extern bool loadRGBImg(out IntPtr imageData, out int width, out int height, out int channels);

    static void Main(string[] args)
    {
        IntPtr imageData = IntPtr.Zero;
        int width = 0, height = 0, channels = 0;

        if (loadRGBImg(out imageData, out width, out height, out channels))
        {
            Console.WriteLine($"Image received: {width}x{height}, Channels: {channels}");

            // Convert the image data to a byte array
            byte[] managedArray = new byte[width * height * channels];
            Marshal.Copy(imageData, managedArray, 0, managedArray.Length);

            // Use the image data (for example, display it using OpenCV in C#)
            // ...

            // Free the allocated memory
            Marshal.FreeCoTaskMem(imageData);
        }
        else
        {
            Console.WriteLine("Failed to load image.");
        }
    }
}

Explanation

C++ Function:
- Allocates memory for the image data using CoTaskMemAlloc, which is compatible with Marshal.FreeCoTaskMem in C#.
- Copies the image data from the OpenCV cv::Mat object to the allocated memory.
- Returns true if successful, or false if there is an error.
C# Code:
- Uses DllImport to declare the external C++ function.
- Calls the C++ function and retrieves the image data, width, height, and channels.
- Copies the unmanaged memory to a managed byte array using Marshal.Copy.
- Frees the unmanaged memory using Marshal.FreeCoTaskMem to avoid memory leaks.

Make sure to replace "YourCppDllName.dll" with the actual name of your compiled DLL. This setup should help avoid the System.AccessViolationException and ensure proper memory management between C++ and C#.

Thanks,
Jaka