I cannot use Native Plugins using C on iOS

I use Unity5 now. I'm trying to use native plugins as test. My problem is the title, so I cannot use Native Plugins. (Xcode raised a BAD_ACCESS error.)

Flow:

1. I created an Unity project.

2. I wrote C# code for using Native code.

3. I added a C code into a Plugins folder.

   ---

Codes are below:

C is

 float VersionNumber() {
     return 1.0;
 }

C# is

 using UnityEngine;
 using System.Collections;
 using System.Runtime.InteropServices;
 
 public class NativeTest : MonoBehaviour {
     [DllImport("__Internal")]
     private static extern int VersionNumber();
 
     // Use this for initialization
     void Start () {
         Debug.Log("Version number is " + VersionNumber());
     }
 }
 

These are correct to build, but It will raise a BAD_ACCESS error. WHY?