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.simo...@gmail.com> wrote: > On Mon, Aug 20, 2018 at 4:55 PM, Aleksey Shipilev <sh...@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 > > >
