SIGABORT issued when calling Android Java JNI GetDoubleField()

Question

I am getting a SIGABORT when calling GetDoubleField().

I have written a small sample program to show the issue...

MainActivity.java

package net.directionalsystems.jnitest;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.util.Log;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {

// Used to load the 'native-lib' library on application startup.
static {
    System.loadLibrary("native-lib");
}

private static final String TAG = "Mainactivity";

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    // Example of a call to a native method
    TextView tv = findViewById(R.id.sample_text);

    JniComms jc = new JniComms();
    jc.latitude = 10;
    jc.logitude = 20;
    jc.txtLocation = "Hello";

    try {
        tv.setText(stringFromJNI(jc));
    }
    catch (Exception e)
    {

        Log.e(TAG, "onCreate: ", e);
    }
}

/**
 * A native method that is implemented by the 'native-lib' native library,
 * which is packaged with this application.
 */
public native String stringFromJNI(JniComms jc);

}

JniComms.java

package net.directionalsystems.jnitest;

import java.util.ArrayList;

public class JniComms
{
    public double latitude;
    public double logitude;
    public String txtLocation;
    //public ArrayList<BleStatus> bleStatusArray;

}

native-lib.cpp

#include <jni.h>
#include <string>

extern "C" JNIEXPORT jstring JNICALL
Java_net_directionalsystems_jnitest_MainActivity_stringFromJNI(
        JNIEnv *env,
        jobject jo,
        jobject jc)
        {

            jclass jcClass = env->GetObjectClass(jc);
            jfieldID iId = env->GetFieldID(jcClass, "latitude", "D");

            // Executing the next line causes SIGABORT
            jdouble latitude = env->GetDoubleField(jcClass, iId);

            std::string hello = "Hello world";
    return env->NewStringUTF(hello.c_str());
}

The build.gradle file follows...

apply plugin: 'com.android.application'

android {
    compileSdkVersion 29
    buildToolsVersion "29.0.2"
    defaultConfig {
        applicationId "net.directionalsystems.jnitest"
        minSdkVersion 23
        targetSdkVersion 29
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
        externalNativeBuild {
            cmake {
                cppFlags ""
            }
        }
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
        }
        customDebugType {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
            debuggable true
        }
    }
    externalNativeBuild {
        cmake {
            path "src/main/cpp/CMakeLists.txt"
            version "3.10.2"
        }
    }
}

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation 'androidx.appcompat:appcompat:1.0.2'
    implementation 'androidx.constraintlayout:constraintlayout:1.1.3'
    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'androidx.test.ext:junit:1.1.0'
    androidTestImplementation 'androidx.test.espresso:espresso-core:3.1.1'
}

This has got to be something simple and stupid that I am doing.

I would like to debug into the NDK code but all I get is the jni.h file displayed and cannot step into the JNI code itself. Having that capability may give me an idea of what is wrong.

Many thanks for any help.

Best regards, Jackie

Answer 1

When reading an instance (non-static) field you're supposed to pass the object, not the class.

So replace:

jdouble latitude = env->GetDoubleField(jcClass, iId);

with:

jdouble latitude = env->GetDoubleField(jc, iId);