NDK JNI functions call

Question

I just started using NDK, since my Android SDK code did not give satisfactory results, though I have never tried to code in C/C++. Until now, I generated the headers of my two native functions, fft_transform() and convolve().

I want to have use of the code written here. The problem is that fft_transform calls other C functions, and being called itself in another C function.

JNIEXPORT void JNICALL Java_com_example_ffttest_FFTActivity_transform
(JNIEnv *env, jobject obj, jdoubleArray real, jdoubleArray imag, jint n)
{
if (n == 0)
    return 1;
else if ((n & (n - 1)) == 0)  // Is power of 2
    return transform_radix2(real, imag, n);
else  // More complicated algorithm for arbitrary sizes
    return transform_bluestein(real, imag, n);
}

Should I declare the non-JNI functions in the header as they are, or change them to JNI functions? How to add JNI *env and jobj obj variables in the non-JNI functions?

For example:

int inverse_transform(double real[], double imag[], size_t n) {
    return Java_com_example_ffttest_FFTActivity_transform(env, obj, imag, real, n);
}

Please consider that I never used C/C++ neither NDK before starting to vote down.

Answer 1

1. The Java method FFTActivity.transform(..) seems to have no return type. Therefore you can not return something in it's JNI implementation like you do: if (n == 0) return 1; Change the Java method and re-create/update your JNI headers accordingly.

2. Use Java_com_example_ffttest_FFTActivity_transform to convert from Java JNI types into C/C++ types used in the C/C++ implementation you want to use. Check JNI documentation for the available functions for doing so (see e.g. GetDoubleArrayElements method).

3. You should not mix up regular C/C++ functions with JNI C/C++ function. In general this possible but a bit complicated. Therefore you should not call the JNI function anywhere in your c/C++ code. This method is only called from the Java side of your application.