IntPtr does not contain native value

Question

I have a native method that has to deliver a byte array to a .NET wrapper. The natove method looks like:

__declspec(dllexport) int WaitForData(unsigned char* pBuffer)
{
    return GetData(pBuffer);
}

GetData allocates a memory region using malloc and copies some data (a byte stream) into it. This byte stream was received via a socket connection. The return value is the length of pBuffer.

This method has to be called from .NET. The import declaration looks as follows:

[DllImport("CommunicationProxy.dll")]
public static extern int WaitForData(IntPtr buffer);

[EDIT]

The the P/Invoke Interop Assistant, that dasblinkenlight advised, translates the prototype to the following import signature:

public static extern  int WaitForData(System.IntPtr pBuffer)

The result is the same: ptr is 0 after calling the method.

[/EDIT]

Atfer the method was called, the result is extracted:

IntPtr ptr = new IntPtr();
int length = Wrapper.WaitForData(ref ptr);

byte[] buffer = new byte[length];
for(int i = 0;i<length;i++)
{
    buffer[i] = System.Runtime.InteropServices.Marshal.ReadByte(ptr, i);
}
Wrapper.FreeMemory(ptr);

The problem is, that the managed variable ptr doesn't contain the value that the native varible pBuffer contains. ptr is always 0 when Wrapper.WaitForData returns although pBuffer pointed to an allocated memory area.

Is there a mistake in the prototype? How does a pointer to a byte array need to be marshalled?

Answer 1

you need to pass a reference to a pointer or 'double pointer' like that

__declspec(dllexport) int WaitForData(unsigned char** pBuffer)

and then change the value of the pointer(because it's passed by value)

*pBuffer = 'something'

other option - return the pointer(then you'll have to handle the int/length some other way)

btw that's why your automatically generated prototype looks like this(doesn't have out, ref modifiers)