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

Volker Simonis volker.simonis at gmail.com
Mon Aug 20 10:22:16 UTC 2018

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On Fri, Aug 17, 2018 at 7:25 PM, Aleksey Shipilev <shade at redhat.com> wrote:

On 08/17/2018 05:12 PM, Volker Simonis wrote:

The offending code in Consolemd.c looks as follows:

#define ECHO 8 JNIEXPORT jboolean JNICALL JavajavaioConsoleecho(...) { jboolean old; ... old = (tio.clflag & ECHO); ... return old; } The intention of this code is to return "true" if the ECHO flag was set but it really returns the value of ECHO (which is defined as '8' in a system header). The question now is, if a Java SE compatible VM guarantees that any arbitrary, non-zero valued jboolean will be interpreted as "JNITRUE" and only a zero valued jboolean will be interpreted as "JNIFALSE"? Or, the other way round, is the normalization performed by the HotSpot result handlers necessary (i.e. enforced by the specification) or just a convenience to fix broken code like the above from Console.echo()? I think this is intentional aftermath of boolean value normalization: https://bugs.openjdk.java.net/browse/JDK-8161720

OMG - I'm getting old :)

Thanks for the link to JDK-8161720! Funny enough I was one of the reviewers of JDK-8161720 and almost exactly two years ago I already asked the exactly same question [1]:

"And I have a question about JNI: is it only a convention that JNI functions should return JNI_TRUE/JNI_FALSE or is this required by the specification?"

Zoltan answered to that question by citing an excerpt from the JNI Programmer's Guide & Specification [2]:

But the "JNI Programmer's Guide & Specification" is from 1999 and it only contains the cited sentence in its "Traps and Pitfalls" section and not in the "Specification" part. The latest JNI Specification [3] doesn't seem to contain such a rule.

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.
to code in Java_java_io_Console_echo() should be fixed (as confirmed by Sherman later in this thread)
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.

Do you agree?

Thank you and best regards, Volker

[1] http://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2016-August/024192.html [2] https://web.archive.org/web/20120626012047/http://java.sun.com/docs/books/jni/html/pitfalls.html#30066 [3] https://docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/jniTOC.html

So, JavajavaioConsoleecho looks broken and needs to be fixed, by e.g.:

- old = (tio.clflag & ECHO); + old = (tio.clflag & ECHO) != 0; Thanks, -Aleksey

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