Passing a string as a const char* to C++ library plugin

Hi,

I have a C++ audio library (ZenGarden) that I'm running as a plugin in Unity, this library opens and plays back Pure Data patches.

I'm trying to pass the filepath of a patch to open as a const char* to a function declared in the imported C++ library, but struggling on converting it correctly.

The function is defined as follows in the C++ library:

    ZGGraph *zg_context_new_graph_from_file(ZGContext *context, const char *directory, const char *filename);
 

And in the C# script as follows:

     using UnityEngine;
     using System;
     using System.Runtime.InteropServices;
     using System.IO;
     
     public class ZGUnity : MonoBehaviour {
     
           [DllImport ("__Internal")]
             private static extern IntPtr zg_context_new_graph_from_file(IntPtr context,
                 [MarshalAs(UnmanagedType.LPStr)]String directory,
                 [MarshalAs(UnmanagedType.LPStr)]String filename);
     
         void Awake () {
                 IntPtr graph = zg_context_new_graph_from_file(context, Application.dataPath, "/ball.pd");
           }
 }

When building the app on an iPad it crashes on the splash screen. If I comment out the code in Awake() it runs fine.

I've found a few others having similar issues but none of the suggested solutions have worked for me, for example. I've tried just declaring it as a string, looked at the StringBuilder class and marshalling (as seen in the code I've posted above). I'm very new to C#, if anyone has any pointers I'd much appreciate your input.

Thanks, Joe