The following test fails:

in libTestBool.c:

    int get_bool() {
        return 0x80000000;
    }

In TestBool.java:

    Addressable ma = findNativeOrThrow("get_bool");
    FunctionDescriptor fd = FunctionDescriptor.of(C_BOOL);

    MethodHandle GET_BOOL = LINKER.downcallHandle(ma, fd);
    boolean result = (boolean) GET_BOOL.invokeExact();
    assertTrue(result);

It seems that, even though the value of the int is non-zero, it is not converted to a boolean correctly (according to C rules).