DefineDLLFunction returning array

Question

I have an external "C" DLL function which returns a pointer to an byte array as well as the number of array elements (bytes)

DLLEXPORT void dllfun(void **a, int *n);

Now I want to use

f=DefineDLLFunction["dllfun","dlllib.dll","void", {"void**","int*}]

to be able to call dllfun via f:

n=0;  
f[a,n]

such that being able to access the array a.

I am aware that if I could define the array a inside Mathematica prior to the call:

n=77;  
a=NETNew["System.Byte[]", n];
f[a,n]

then the dll function could access the array and fill it with some values. The problem is that the dll function uses ist own memory management such that I can not pass memory allocated by Mathematica. If it would help I have the info of the array size n already available in advance to the call.
It seems to me like I would need a version of NETNew for declaring a pointer to an array with known size n without actually reserving the memory for it.

Any hints welcome

Answer 1

ok, after peeking around this is one solution:

(* DLLEXPORT void dllfun(void **a, int *n); // the C language part *)
Needs["NETLink`"];  
(* define f as a dll function with two parameters, 1. pointer to pointer, 2. pointer to int *)
f = DefineDLLFunction["dllfun", "dlllib.dll", "void", {"out IntPtr", "out int"}];  
ptr = NETNew["System.IntPtr"];  (* initialize pointer *)
n = 0; (* initialize length variable *)
f[ptr, n]; (* calling the dll function loads the pointer and the length *)
destination = NETNew["System.Byte[]",n] (* allocate destination *);  
LoadNETType["System.Runtime.InteropServices.Marshal"]; (* need for Marshal`Copy *)  
Marshal`Copy[ptr, destination, 0, n]; (* copy to destination *)   
result = destination//NETObjectToExpression; (* convert to Mathematica Table containing Byte values *)  
ReleaseNETObject[destination]; (* clean up *)  
ReleaseNETObject[ptr]; (* clean up *)