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JNA and Unsigned Integers
2016-01-09

I’ve written an Argon2 binding for the JVM (in case you’re interested, you can find it here). This binding uses JNA to call the Argon2 C library. The C library uses the uint32_t datatype, that’s an unsigned 32-bit integer. The JNA documentation states:

Unsigned values may be passed by assigning the corresponding two’s-complement representation to the signed type of the same size.

Haha, you expect me to bitshift my integers around to get rid of the two’s-complement representation?

I’ve found a simpler way: JNA provides an abstract class called IntegerType. If you want an unsigned 32-bit integer, just create this class:

public class Uint32_t extends IntegerType {
    public Uint32_t() {
        this(0);
    }

    public Uint32_t(int value) {
        super(4, value, true);
    }
}

The first constructor is needed because JNA (otherwise it fails at runtime), the second constructor accepts the value of the unsiged integer. The 4 is 32 bit in bytes, the true argument tells the superclass to handle the value as an unsigned integer.

You can use this trick to pass 8-bit, 16-bit, 32-bit and 64-bit values without hassle.


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