JNI: converting unsigned int to jint

Question

How do I convert an unsigned int to jint? Do I have to convert it at all, or can I just return it without any special treatment? This is basically my code right now, but I can't test it, as I haven't setup JNI locally.

JNIEXPORT jint JNICALL
Java_test_test(JNIEnv* env, jobject obj, jlong ptr)
{
    MyObject* m = (MyObject*) ptr; 
    unsigned int i = m->get(); 
    return i; 
}

Tonguelashing

Answer 1

In the general case, jint is equivalent to int, and so can hold about half the values of unsigned int. Conversion will work silently, but if a jint value is negative or if an unsigned int value is larger than the maximum value a jint can hold, the result will not be what you are expecting.

Autolysis

I am storing only small numbers, that will never exceed the limits of a jint, so it should be fine. – Tonguelashing

@Jonathan Once the value reaches half the capacity of an unsigned int, it will give a negative int due to how the types are implemented; small values are fine – Leaper

Yes, I know how int works. I'm asking Pedro if his data includes negative numbers. – Autolysis

Answer 2

jint is a typedef for int32_t so all the usual casting rules apply.

Bdellium

Answer 3

Depending on your compiler settings, there may or may not be a warning about mixing signed/unsigned integers. There will be no error. All the caveats from the answers above apply - unsigned int values of 0x80000000 (2,147,483,648) and above will end up as negative integers on the Java side.

If it's imperative that those large numbers are preserved in Java, use a jlong as a return datatype instead, and convert like this:

return (jlong)(unsigned long long)i;

The point is to first expand to 64 bits, then to cast away unsigned-ness. Doing it the other way around would produce a 64-bit negative number.

Pest