c source code in unity project, EntryPointNotFoundException

So I am trying to create a simple (the simplest!) unity project that will run on an iPad using some c code. The code:

//testPlugin.mm
extern "C"
{
    int afunction_c(){
        int i=321;
        return i;
    }
}

And:

//testScript.cs
using UnityEngine;
using System.Collections;
using System.Runtime.InteropServices;

public class test : MonoBehaviour {
	[DllImport ("__Internal")]
	private static extern int afunction_c();
	
	static int afunction_csh(){
		int i=afunction_c();
		return i;
	}
	
	void Update () {
        Debug.Log(afunction_csh());
	}     
}

Assigning the script on a game object and trying to run inside Unity I get:

EntryPointNotFoundException: afunction_c
testScript.afunction_csh () (at Assets/Plugin/iOS/testScript.cs:13)
testScript.Update () (at Assets/Plugin/iOS/testScript.cs:21)

Both files testPlugin.mm and testScript.cs are in Assets/Plugin/iOS/. Anyone has any idea what I am doing wrong here?

Well it seems that adding testPlugin.mm to the project Classes folder in Xcode fixes the problem.