CODEWITHSUNDEEP
androidarraysandroid-ndkjava-native-interfaceinteger
torger
torger

Reputation: 2328

Why does copying native arrays back to Java via the JNI using the Android NDK not fill correctly?

Here are the global initializers:

int width = 100;
int height = 100;
int cells = width * height;
int* pixels = (int*) malloc(sizeof(int) * cells);

int i;

for (i = 0; i < cells; i++) {

  pixels[i] = 255;

}

Here is the code to copy the array back up to Java via JNI call:

void Java_com_example_app_setPixels(
        JNIEnv *env, jobject obj, jintArray arr) {

    // initializations, declarations, etc
    jint *c_array;
    jint i = 0;

    // get a pointer to the array
    c_array = (*env)->GetIntArrayElements(env, arr, NULL);

    // do some exception checking
    if (c_array == NULL) {
        return; /* exception occurred */
    }

    for (i = 0; i < cells; i++) {

        c_array[i] = (jint) pixels[i];

    }

    // release the memory so java can have it again
    (*env)->ReleaseIntArrayElements(env, arr, c_array, 0);

}

This leads to a mostly filled Java array - however it stops about 80% of the way through.

However if I change: 

        c_array[i] = (jint) pixels[i];

to

        c_array[i] = 255;

The modified Java array is filled.

Upvotes: 1

Views: 859

Answers (1)

torger
torger

Reputation: 2328

As always it was my own fault. The array being allocated (pixels) and the array it was copied into were not of the same size.

Once fixed the array sizes everything worked correctly.

Upvotes: 1

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