Is returning a value != '0' or '1' as jboolean from a JNI function legal?

[Krystal Mok]

Hi guys,

Haha this is fun. I actually hit this issue the hard way and had to tweak a
bit of my code to accommodate that: I had to return a jint from a function
that I wanted to return a jbool at first:
http://hg.openjdk.java.net/hsx/hsx25/hotspot/diff/8f37087fc13f/src/share/vm/c1/c1_Runtime1.cpp

- Kris

On Mon, Aug 20, 2018 at 8:38 AM Volker Simonis <volker.simonis at gmail.com>
wrote:

> On Mon, Aug 20, 2018 at 4:55 PM, Aleksey Shipilev <shade at redhat.com>
> wrote:
> > On 08/20/2018 12:22 PM, Volker Simonis wrote:
> >> So to summarize, my current view on this topic is:
> >>  - JNI functions returning a jboolean are only allowed to return
> >> JNI_TRUE/JNI_FALSE (or 1/0) according to the current JNI spcification.
> >
> > Now *I* am having trouble seeing where exactly the JNI spec says the
> domain of jboolean is
> > (JNI_FALSE, JNI_TRUE). In "Primitive Types" [1] it says "The following
> definition is provided for
> > convenience: JNI_FALSE, JNI_TRUE", but that does not restrict the
> domain, because those are
> > "convenience" defines. And "Description" in the table says jboolean is
> "unsigned 8 bits", which
> > seems to invite interpretation that all 8 bits are usable.
> >
> > John says [2]:
> >
> > "The JNI documents specify that, at least for returning values from
> native methods, a Java boolean
> > (T_BOOLEAN) value is converted to the value-set 0..1 by first truncating
> to a byte (0..255 or maybe
> > -128..127) and then testing against zero."
> >
> > ...which is what I am looking for, but I cannot find the "JNI document"
> that actually says that. I
> > can see the idea of that in JVMS [3], but that seems to only apply to
> on-heap booleans, does that
> > also extend to jboolean's? Maybe John can point out the JNI document
> where it is said explicitly?
> >
>
> Yes, you're right - there's no exact documentation for neither of the
> two possible interpretations. A colleague just pointed me to the
> definition of invokestatic in the JVMS [4] which has the following
> sentence:
>
> "If the native method returns a value, the return value of the
> platform-dependent code is converted in an implementation-dependent
> way to the return type of the native method and pushed onto the
> operand stack."
>
> But then again, it has this unfortunate "implementation-dependent"
> which can be interpreted either way :(
>
> [4]
> https://docs.oracle.com/javase/specs/jvms/se10/html/jvms-6.html#jvms-6.5.invokestatic
>
> >
> >>  - to code in Java_java_io_Console_echo() should be fixed (as
> >> confirmed by Sherman later in this thread)
> >
> > Yes, that's a bug waiting to happen anyway.
> >
> >
> >>  - normalization of native, off-heap 8-bit values to Java booleans as
> >> currently implemented in the HotSpot (and fixed by JDK-8161720) is (1)
> >> only for convenience to simply access to off-heap data in Unsafe, (2)
> >> to implement better Java/Native integration in projects like Panama
> >> and (3) to fix legacy JNI code which was developed under the
> >> assumption that the advice in the "JNI Programmer's Guide &
> >> Specification" book is specification relevant.
> >
> > Yes, the intent seems to be what you describe. But see above about the
> spec.
> >
> >
> > -Aleksey
> >
> > [1]
> https://docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/types.html#primitive_types
> > [2]
> http://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2016-August/024263.html
> > [3]
> https://docs.oracle.com/javase/specs/jvms/se10/html/jvms-2.html#jvms-2.3.4
> >
>