JNI C++ android app crashes when call function

Question

I have a .cpp file to use with java on android:

#include<iostream>
#include<jni.h>

jint Java_com_example_gatsj_tutorjatek_MainActivity_Sum(JNIEnv* env, jobject obj)
{
    return 5;
}

I use it here:

package com.example.gatsj.tutorjatek;

import android.app.Activity;
import android.os.Bundle;

public class MainActivity extends Activity
{
    public native int Sum();

    static
    {
        System.loadLibrary("TestCPP");
    }

    @Override
    protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);

        int x = Sum();//IF I REMOVE THIS LINE THE APP DOESN'T CRASH
    }
}

I build it in Android Studio using Gradle and this CMakeLists.txt:

cmake_minimum_required(VERSION 3.4.1)

add_library( # Specifies the name of the library.
             TestCPP

             # Sets the library as a shared library.
             SHARED

             # Provides a relative path to your source file(s).
             src/main/cpp/TestCPP.cpp )

When I start the app on my phone it crashes. But if I remove the "int x = Sum();" line, the app can start.

The "loadLibrary" and "native" method part is still in the code but without the "int x = Sum();" line, the app doesn't crash.

How can I use the Sum() method? What causes the problem?

Answer 1

Since C++ is being used instead of C, you should wrap the defination of your native methods inside extern "C" in your cpp file.

extern "C" {
    // your native method definations.
}