[SOLVED] Callback function's byte parameter has value out of bounds in project Panama function interface

Callback function's byte parameter has value out of bounds in project Panama function interface

I'm using the new Foreign function & memory API which is part of project Panama and available in preview in JAVA 19.

This is a simple Java callback function

public static byte addFive(byte t) {
    Byte test = (Byte) t;
    return (byte) (Long.valueOf(test) + 5);
}

I have a native library function that is called with parameters addFive and (byte) -41.

uint8_t callback_test(uint8_t (*fn)(uint8_t), uint8_t arg) {
    uint8_t res = fn(arg + 1) * 2;
    return res;
}

I expect t to have value -40, and this does show in the Intellij debugger. But when you print it, it prints 216 instead. In addition, casting to Byte gives an ArrayIndexOutOfBoundsException.

I do realize that the native function is an unsigned int, so the returned value of 216 makes sense. However, a java byte should not be allowed that value, so, I expect java to automatically convert it to -41.

Is this behavior expected?

What is the correct way to get the unsigned int from the native code passed to the java function as a byte in the range [-128, 127]?

Solution

There was a bug in JDK19 such that there was no value normalization in upcalls.

This is now fixed.