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Android native C++ function call cause application crash

I have Android app, where I need to use C++ code. But I realised, I have problem to call C++ function. I have read something about it and tried to write something, I thought it could work. Here is my code:
private native int test(int a);
private void process() {
int ret=test(5);
Toast.makeText(this, String.valueOf(ret), Toast.LENGTH_LONG).show();
}
And my C++ code is:
#include <jni.h>
extern "C"
JNIEXPORT jint JNICALL
Java_com_example_woodem_woodem_1opencvgrains_Main_test(JNIEnv *env, jint a)
{
return a*a;
}
Of course, my real function is much more complicated and I need to pass about 6 arguments, but I hope, this can illustrate.
NOTE: Even this code doesn't work to me.
My application crash immediately after calling process(). Could you please advice me, what am I doing wrong? Compiler tells me nothing and I have no idea, where the problem is.

You are forgetting about the second argument. For example method void test() will have JNI signature JNIEXPORT void JNICALL Java..._test(JNIEnv *env, jobject thiz). In your case, edit signature to (JNIEnv *env. jobject thiz, jint a).
Edit
The previous version showed a signature for a static function. I've updated it to match instance function.
For static signature is (JNIEnv *env, jclass clazz).

You've declared your native function as private native int test(); passing an integer argument to it.
It should be private native int test(int num);

SIGILL in GetMethodID while "Evaluate Expression" is OK

I want to call back Java code from native code.
The Java code:
public final class Underlying {
public static native int setOnEventListener(OnEventListener listener);
public interface OnEventListener {
int EVENT_TEST = 1;
int onEvent(int code, String msg);
}
}
The C++ code (I omit some checks to make it clear):
extern "C" JNIEXPORT jint JNICALL
Java_packageName_Underlying_setOnEventListener(JNIEnv* env, jclass type, jobject listener) {
jclass clz = env->GetObjectClass(listener);
// assign to static jobject
eventListener = env->NewGlobalRef(listener);
// assign to static jmethodID
onEventMethodID = env->GetMethodID(clz, "onEvent", "(ILjava/lang/String;)I");
}
SIGILL (signal SIGILL: illegal instruction operand) occurs in GetMethodID. But I evaluate the same sentence by Evaluate Expression in Android Studio and everything is OK.
There must be some differences between reality and Evaluate Expression. One difference I could think of is that the calling thread may be different. setOnEventListener was originally called in UI thread, so I created a new thread to do this, but nothing changed.

I found two things lead to this error.
The return type is jint, then a jint must be returned.
In Android Studio, Make Project sometimes doesn't rebuild the C++ code to APK, Rebuild Project is needed in this situation.

Android/Cocos2d-x unsatisfiedlinkerror using JNI using Java to call C++

EDIT: I've solved this problem (and found the next issue in the long line of JNI tribulations!) by changing the following:
Delete "static" declaration of the native method in both Java and C++, add a Java method to get an instance of my SimpleGame class, and calling
public static void callCppApiResponse (String result, String token, long context) {
**getInstance()**.cppAndroidApiResponse(token, 200, result, result.length(), context);
}
Now the information successfully makes it from Java to C++. Hopefully this may help someone else with a similar issue.
ORIGINAL POST:
I'm using the Cocos2d-x platform and writing C++ code for a cross-platform iOS/Android app, and there are certain methods that I need to be handled by the respective native iOs/Android systems, so I need to be able to call Java from C++ then call C++ from Java.
I am able to load the shared library for the app and trigger a Java response from C++, but when I try to call C++ from Java I get the following logcat output and the app crashes:
dalvik vm No implementation found for native Lorg/cocos2dx/simplegame/SimpleGame; cppSideAndroidApiResponse:(Ljava/lang/String;ILJava/lang/String;IJ)V
AndroidRuntime java.lang.UnsatisfiedLinkError: Native method not found: org.cocos2dx.simplegame.SimpleGame.cppAndroidApiResponse:(Ljava/lang/String;ILJava/lang/String;IJ)V
Here is the relevant Java code:
package org.cocos2dx.simplegame;
private static native void cppAndroidApiResponse(String token, int response, String data, int dataLen, long context);
static {
System.loadLibrary("sb");
}
public static void callCppApiResponse (String result, String token, long context) {
//This is where the error is triggered
cppAndroidApiResponse(token, 200, result, result.length(), context);
}
and in the C++ file cppSide.h:
# if (CC_TARGET_PLATFORM == CC_PLATFORM_ANDROID)
#include "platform/android/jni/JniHelper.h"
#include <jni.h>
#endif
#ifdef __cplusplus
extern "C" {
#endif
# if (CC_TARGET_PLATFORM == CC_PLATFORM_ANDROID)
JNIEXPORT static void JNICALL
Java_org_cocos2dx_simplegame_SimpleGame_cppAndroidApiResponse(JNIEnv *, jobject, jstring, jint, jstring, jint, jlong);
#endif
#ifdef __cplusplus
}
#endif
Then finally the cppSide.cpp file
#include "cppSide.h"
extern "C" {
JNIEXPORT static void JNICALL
Java_org_cocos2dx_simplegame_SimpleGame_cppAndroidApiResponse(JNIEnv *env, jobject obj, jstring token, jint response, jstring data, jint length, jlong context)
{ ...do stuff...}
}
I know that my shared library is loading because I get my C++ based loading screen, and I know that JNI is working at least partially because I am able to call Java from that same cppSide.cpp file in another method. I have been researching the JNI and Cocos2d-X documentation, but to no avail.
Thanks in advance!

What is "jobject thiz" in JNI and what is it used for?

I am having a hard time finding an answer to this. But, what is "jboject thiz" used for in JNI function calls? For example:
jobjectArray Java_com_gnychis_awmon_Test( JNIEnv* env, jobject thiz ) {
I use env to allocate objects often, but I've never used thiz and I'm not sure what it is for. Just for knowledge purposes.

The following is a JNI wrapper function which has two parameters, and returns a primitive array of objects:
jobjectArray Java_com_gnychis_awmon_Test( JNIEnv* env, jobject thiz );
From the function name you have given I don't think it is complete, that is, you haven't respected the obligatory function name convention which is:
Start the function with Java_
Append the package name separated by _ (undescores) i.e. com_company_awesomeapp. So far the function name is composed of: Java_com_company_awesomeapp
Append the Java class name where the native method has been defined,
followed by the actual function name. So at this point we should have the following function name: Java_com_company_awesomeapp_MainActivity_Test
The first parameter is a pointer to a structure storing all JNI function pointers, i.e. all the predefined functions you have available after you #include <jni.h>.
The second parameter is a reference to the Java object inside which this native method has been declared in. You can use it to call the other methods of the Java object from the current JNI function, i.e. Call Java instance methods from JNI code written in C or C++.
If for example you have the following Java class inside the MainActivity.java file:
public class MainActivity extends Activity
{
static
{
try
{
System.loadLibrary("mynativelib");
}
catch (UnsatisfiedLinkError ule)
{
Log.e(TAG, "WARNING: Could not load native library: " + ule.getMessage());
}
}
public static native Object[] Test();
}
Then, the jobject thiz parameter of the JNI function would be a reference to an object of type MainActivity.

I found this link that should help clarify the question.
https://library.vuforia.com/articles/Solution/How-To-Communicate-Between-Java-and-C-using-the-JNI
Here is an example in it that uses the "jobject".
JNIEXPORT void JNICALL
Java_com_qualcomm_QCARSamples_ImageTargets_ImageTargets_initApplicationNative(
JNIEnv* env, jobject obj, jint width, jint height)
{
...
jclass activityClass = env->GetObjectClass(obj);
jmethodID getTextureCountMethodID = env->GetMethodID(activityClass,
"getTextureCount", "()I");
if (getTextureCountMethodID == 0)
{
LOG("Function getTextureCount() not found.");
return;
}
textureCount = env->CallIntMethod(obj, getTextureCountMethodID);
...
}

jobject thiz means the this in java class.
Sometimes if you create a static native method like this.
void Java_MyClass_method1 (JNIEnv *, jclass);
jclass means the class itself.

Using jni in Android: UNsatisfiedLinkError

I'm new to jni, and I was going over a tutorial to implement a simple native method, but I'm getting an unsatisfiedlinkerror. As far as I know, I followed the steps in the tutorial exactly. Please help me.
Here is the java wrapper code:
package com.cookbook.jni;
public class SquaredWrapper {
// Declare native method (and make it public to expose it directly)
public static native int squared(int base);
// Provide additional functionality, that "extends" the native method
public static int to4(int base)
{
int sq = squared(base);
return squared(sq);
}
// Load library
static {
System.loadLibrary("squared");
}
}
Here's what my Android.mk file looks like:
LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE := squared
LOCAL_SRC_FILES := squared.c
include $(BUILD_SHARED_LIBRARY)
Here's what my .c file looks like:
#include "squared.h"
#include <jni.h>
JNIEXPORT jint JNICALL Java_org_edwards_1research_demo_jni_SquaredWrapper_squared
(JNIEnv * je, jclass jc, jint base)
{
return (base*base);
}
And here is what my .h file looks like:
enter code here/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class com_cookbook_jni_SquaredWrapper */
#ifndef _Included_com_cookbook_jni_SquaredWrapper
#define _Included_com_cookbook_jni_SquaredWrapper
#ifdef __cplusplus
extern "C" {
#endif
/*
* Class: com_cookbook_jni_SquaredWrapper
* Method: squared
* Signature: (I)I
*/
JNIEXPORT jint JNICALL Java_com_cookbook_jni_SquaredWrapper_squared
(JNIEnv *, jclass, jint);
#ifdef __cplusplus
}
#endif
#endif

Your JNI signature doesn't match. In your .c file, change:
JNIEXPORT jint JNICALL Java_org_edwards_1research_demo_jni_SquaredWrapper_squared
to
JNIEXPORT jint JNICALL Java_com_cookbook_jni_SquaredWrapper_squared
Generally there are two ways to "glue" native C through JNI to a Java function. The first is what you're attempting to do here, that is use a predetermined signature that JNI will recognize and associate with your appropriate Java code. The second is to pass function pointers, signatures, and Java class names into JNI when you include the library.
Here's the second method that would bind the native function to the appropriate Java code (this would be your .c file):
#include "squared.h"
#include <jni.h>
static const char* SquaredWrapper = "com/cookbook/jni/SquaredWrapper";
jint squared(JNIEnv * env, jobject this, jint base) {
return (base*base);
}
// Methods to register for SquaredWrapper
static JNINativeMethod SquareWrapperMethods[] = {
{"squared", "(I)I", squared}
};
jint JNI_OnLoad(JavaVM* vm, void* reserved) {
JNIEnv* env;
if ( (*vm)->GetEnv(vm, (void **) &env, JNI_VERSION_1_6) != JNI_OK)
return JNI_ERR;
jclass class = (*env)->FindClass(env, SquaredWrapper);
(*env)->RegisterNatives(env, class, SquaredWrapperMethods, sizeof(SquaredWrapperMethods)/sizeof(SquaredWrapperMethods[0]));
return JNI_VERSION_1_6;
}
void JNI_OnUnload(JavaVM* vm, void* reserved) {
JNIEnv* env;
if ((*vm)->GetEnv(vm, (void**) &env, JNI_VERSION_1_6) != JNI_OK)
return;
jclass class = (*env)->FindClass(env, SquaredWrapper);
(*env)->UnregisterNatives(env, class);
return;
}
It's a good deal longer but it gives you a lot more flexibility when binding native code. The definition for squared and the includes are as you would expect. on the 4th line, the static const char* SquaredWrapper is an escaped string with the fully qualified package name of the class you want to bind squared to. Near the bottom are the JNI_OnLoad and JNI_OnUnLoad functions that take care of binding and unbinding the functions on library load and unload. The last piece is the JNINativeMethod array. This array contains as each entry an array of size 3 whose components are the Java name of the method as a const char*, the JNI signature of the Java method, and the native C function pointer to bind to that method. The JNI function signature tells the environment the argument list format and return value of the Java function. The format is "(Arg1Arg2Arg3...)Ret", so a function that takes an int and double and returns a float would have a signature of "(ID)F", and a function that takes no arguments and returns void would be "()V". I use this handy cheat sheet to remember most of the shorthand:
http://dev.kanngard.net/Permalinks/ID_20050509144235.html
Good luck :)
Edit: Oh, BTW, you'll likely want to add the signatures for JNI_OnLoad and JNI_UnOnLoad to your header, and change the name of your native function prototype to reflect the new .c file.

This is kind of an obscure case, but if you get an access violation in your native code, Android will cover up the thrown exception and throw the error you got. In my case, the native code threw an access violation but Java kept running. It then tried to call a JNI method on the crashed NDK.
To find the access violation, I ended up moving the offending JNI method to another IDE to debug.
I hope this saves someone the amount of time it took me to figure this out.