PInvoke and char **

I am a complete noob when it comes to c / C ++ and PInvoke, so please tell me about me.

I got this assembly from someone I would like to use in my C # application.

The title is as follows:

int __declspec(dllimport) s2o(WCHAR* filename, char** out, int* len);

I managed to partially process it using:

[DllImport("s2o.dll", EntryPoint = "?skn2obj@@YAHPA_WPAPADPAH@Z", CallingConvention = CallingConvention.Cdecl)]
public static extern int s2o(
    [MarshalAs(UnmanagedType.LPWStr)]
    string filename,
    ref char[] @out,
    ref int len
);

And then calling it like this:

char[] result = null;
int length = 0;
s2o("filepath", ref result, ref length);

This seems to work in part because the “length” actually gets the meaning. Unfortunately, the "result" remains zero.

What should I do to make this work?

Edit:

Well, I managed to work by replacing char [] with IntPtr and then calling "Marshal.PtrToStringAnsi", as Nick suggested:

string result = Marshal.PtrToStringAnsi(ptr);

However, due to comments in the same answer, I'm a little worried about memory usage. There are no other ways in the assembly, so how can I clear them?

Thank!