Passing back void * pointer from native plugin to Unity

I was wondering if there was an easy way to pass a void * data type back to Unity without converting it to an invalid data type. A void * pointer can represent a Java ByteBuffer's direct address (obtained via GetDirectBufferAddress), and right now I convert the pointer to an int before passing it back to Unity (this works because I am running 32-bit code; 64-bit would require a long). Here's an example to make things more clear:

C++ example function in native plugin code:
extern "C" int getBufferAddress(){
return (int)bufferAddress; // buffer address is a void * pointer
}

C# side:
[DllImport("theplugin")]
public static extern int getBufferAddress();

The int returned from getBufferAddress can be converted to a (byte *) variable in C# via a cast (in unsafe code).

Anyway, I'm not happy with this approach but it seems work ok for now. Is there a more "correct" approach?

 

I guess I could make the function return a byte pointer and make the C# side do the following:

[DllImport("theplugin")]
public static extern byte * getBufferAddress();

I was wondering if that's the best way, though.

 

I think you should use IntPtr. It represents a platform-specific pointer or handle (meaning - its size is not predetermined as 32-bit).

Your method declaration with [DllImport] should return an IntPtr. This can be used to interact with the returned pointer (are you trying to get a pointer to a ByteBuffer? what would you do with it in C# ?)

 

I can't specify why I want to get a pointer to a ByteBuffer besides the fact that I need access to a shared buffer between C# and an Android plugin (the original code is private/not shareable so I can only ask the question by coming up with similar but fake code). There is a byte buffer that is used in the plugin that the C# code needs to know about too.

I do remember learning about IntPtr a while ago so I feel a bit silly for not remembering that it can be used here. Thank you for the suggestion -- I will definitely try it out. I guess char * (to byte buffer) in C++ will map easily to IntPtr in C#.

 

I have never done such a thing, but if you are trying to share the same buffer between Java and C# code, you should take into consideration that a byte is not the same in C# and Java (read here: http://stackoverflow.com/questions/18428891/java-code-negative-byte-in-byte-array-to-c-sharp)

 

Excellent point. I will make sure that is the case with our usage of Java.nio.ByteBuffer.

 

Hello, this is what I am trying to do as well. I use to have a DLL that fetches an image, does some operation with Opencv and callbacks to Unity. I am trying to convert this into an android plugin but for some reason declaring a stdcall function pointer in the android dynamic library is giving me a syntax error. I thought about passing the pointer of the image back to Unity as a return value but wouldn't the values stored in the buffer gets replaced/destroyed when the C++ function returns?