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 >
