Question

I am attempting to pass a managed function pointer void (*)(void *) to my unmanaged library. My unmanaged library calls this callback with a pointer to a frame of data protected by a CriticalSection. While the managed callback is running, nothing else can modify the frame of data due to the Critical Section. However, I am getting Access Violations and Heap Corruptions just by entering the callback.

EDIT: I forgot to mention. The StartStreaming() steals the thread it manages. Furthermore, it creates a separate thread for dispatching new data to the given callback. The callback is called in this separate thread.

So far I have done the follow:

//Start Streaming
streaming_thread_ = gcnew Thread(gcnew ThreadStart(&Form1::WorkerThreadFunc));
streaming_thread_->Start();

Where:

extern "C" {
#include "libavcodec\avcodec.h"
#include "libavutil\avutil.h"
}

namespace TEST_OCU {

delegate void myCallbackDelegate(void * usr_data); //Declare a delegate for my unmanaged code

public ref class Form1 : public System::Windows::Forms::Form
{
    public:

    static void WorkerThreadFunc()
    {
        myCallbackDelegate^ del = gcnew myCallbackDelegate(&Form1::frame_callback);

        MessageBox::Show("Starting to Streaming", "Streaming Info");
        if(rtsp_connection_ != NULL)
            rtsp_connection_->StartStreaming();
            //rtsp_connection_->StartStreaming((void (*)(void *)) System::Runtime::InteropServices::Marshal::GetFunctionPointerForDelegate(del).ToPointer() );
        MessageBox::Show("Done Streaming", "Streaming Info");
    }

    static void __cdecl frame_callback(void * frame)
    {
        AVFrame * casted_frame = (AVFrame *)frame;
    }

private: System::Void Form1_Load(System::Object^  sender, System::EventArgs^  e) 
    {
        if(rtsp_connection_ == NULL)
            rtsp_connection_ = new NeyaSystems::RTSPConnection("rtsp://url");
    }

    private: static RTSPConnection * rtsp_connection_ = NULL;
}
}

• I’ve omitted a lot of pointless code…
• StartStreaming defaults to a NULL pointer, in this case I get no corruption
• StartStreaming with the delegated function pointer causes heap corruption
• RTSPConnection is implemented in native C++ and contains C calls as well (libavcodec)
• RTSPConnection contains two threads, communication and frame dispatch thread (calls the managed callback)

Could anyone give me a breadcrumb? Thank you so much in advance.

Answer 1

EDIT: not a problem with cross-thread calls. If the unmanaged caller expects to call a __cdecl function, then you have to decorate the delegate type with an UnmanagedFunctionPointerAttribute attribute.

using namespace System::Runtime::InteropServices;

[UnmanagedFunctionPointerAttribute(CallingConvention::Cdecl)] 
delegate void myCallbackDelegate(void * usr_data);