Question

We have an out of process COM application that was written in C++ that implements are networking protocol. When a packet of data is received a callback is invoked into the application that registered for the packet.

The callback interface is defined as such:

[helpstring("method MessageHandler")] HRESULT MessageHandler([in,size_is(nSize)] char * szBuf, int nSize, DWORD dwTransCode, DWORD dwSenderID, BSTR bstrFromIP);

Using this in C++ has not been an issue. We now have a case where we have a C++/CLI application that needs to receive callbacks. After hacking away until the compiler was happy, I arrived at the following implementation:

ref class MessageHandlerClass : public MessageRouterCallback
    {
    public:
        virtual void MessageHandler(signed char %buffer, int size, unsigned int msgId, unsigned int fromId, System::String^ fromIp)
        {
            switch (msgId)
            {
            case MaintenanceMsgs::maintenance_event_message::ID:
                {
                    SomeStructure msg;
                    myHandler->HandleMaintenanceEvent(&msg);
                }
            }
        }

This is my first foray into C++/CLI.

First Question: What they heck does ‘%’ mean in ‘signed char %buffer’?

Second Question: I can place a breakpoint and see that the callback is getting called. I can look at the ‘buffer’ argument in the memory debugger and it contains the data I expect. I have been VERY unsuccessful at pulling that data out and placing it into the variable ‘msg’. I know I can’t do a cast like in C++, but every example I’ve been tried (Mostly InteropServices::Marshal and some pin_ptr stuff) doesn’t seem to get me anywhere.

SomeStructure is declared in a header file and is included by both the C++ and the C++/CLI application. SomeStructure contains 2 unsigned shorts followed by three character arrays.

Any direction on where to go from here would be greatly appreciated.

Answer 1

Ok. It just ‘clicked’… So I’ll answer my own question here.

First Question: What they heck does ‘%’ mean in ‘signed char %buffer’?

The ‘%’ just means its ‘tracked’ and will be garbage collected, or at least that’s what I think it means 🙂

Second Question: How to marshal.

First I had to get to the ‘internal pointer’ and C++/CLI provides the & operator for that. Then I was able to simply memcpy the data over.

pin_ptr<signed char> p = &buffer;

MaintenanceMsgs::maintenance_event_message msg;
memcpy((void*)&msg, (void*)p, sizeof(msg));

myHandler->HandleMaintenanceEvent(&msg);

Is this safe?