Question

How do two values I pass into my JNI tossed down to C then C does its changes and updates the values. How do I get those two values(maxPower, index) and see then in Java? They always come back as zero.

JNIEXPORT jdouble JNICALL Java_com_TV (
     JNIEnv* env, jobject obj, jdouble maxPower, jint index)
{
    jdouble result = 0;

    result = Feature_TV(2, &maxPower, &index );

    return result;
}

// here is the c function it calls it actually does more than this but for
// demo it should return result 60 and maxPower and index should be -5.0 and -2..

double Feature_TV(double * maxPwr, int * maxPwrIdx ) {

    *maxPwr = -5.0;
    *maxPwrIdx = -2;

    /// do something 

     return 60;
}

Answer 1

There is no way to modify the variables that are passed to Java_com_TV because C is pass-by-value, just like Java. When you call Feature_TV and pass the addresses of maxPower and index, the addresses are of the local variables in Java_com_TV and no outside effect can be seen.

To do what you want to do you could either

1. make com.TV accept an object that has maxPower and index as fields, and modify the fields from JNI, or
2. make it return such an object, and create the object and set the fields in JNI.