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

[Roger Riggs]

Hi Volker,

This may be more of a hotspot question than core-libs.

This seems like a practical feature of the implementation and a convenience
to match the conventions of C/C++ with JNI.

It might be worth adding a test to verify the assumptions between Java 
and Native.

$.02, Roger


On 8/17/18 11:12 AM, Volker Simonis wrote:
> Hi,
>
> we've recently discovered a problem with Console.readPassword() which
> failed to restore the correct echo mode on Linux/s390x [1]. We could
> finally track this down to a "bug" [2] in the s390x template
> interpreter which failed to call the "result handler" after a native
> call. For jboolean return types, the "result handler" merges all
> values > 0 into 1 (i.e. JNI_TRUE). Not calling the result handler
> after returning from native calls resulted in cases where non-zero
> jboolean values returned from native functions where interpreted as
> zero (i.e. JNI_FALSE).
>
> The offending code in Console_md.c looks as follows:
>
> #define ECHO    8
>
> JNIEXPORT jboolean JNICALL
> Java_java_io_Console_echo(...) {
>
>      jboolean old;
>      ...
>      old = (tio.c_lflag & 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 "JNI_TRUE"
> and only a zero valued jboolean will be interpreted as "JNI_FALSE"?
>
> 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()?
>
> All I could find from the Specification so far is the following, which
> seems to suggest that only JNI_TRUE/JNI_FASLE are valid values for a
> jboolean, but I'm not completely sure...
>
> From: JNI Types and Data Structures [3]
>
> Java "boolean" == JNI "jboolean" == unsigned 8 bits
>
> The following definition is provided for convenience.
>
> #define JNI_FALSE  0
> #define JNI_TRUE   1
>
> From: JVMLS, § 2.3.4. The boolean Type [4]
>
> Although the Java Virtual Machine defines a boolean type, it only
> provides very limited support for it. There are no Java Virtual
> Machine instructions solely dedicated to operations on boolean values.
> Instead, expressions in the Java programming language that operate on
> boolean values are compiled to use values of the Java Virtual Machine
> int data type.
>
> The Java Virtual Machine encodes boolean array components using 1 to
> represent true and 0 to represent false. Where Java programming
> language boolean values are mapped by compilers to values of Java
> Virtual Machine type int, the compilers must use the same encoding.
>
> Thank you and best regards,
> Volker
>
> [1] http://mail.openjdk.java.net/pipermail/core-libs-dev/2018-July/thread.html#54479
> [2] https://bugs.openjdk.java.net/browse/JDK-8209637
> [3] https://docs.oracle.com/javase/7/docs/technotes/guides/jni/spec/types.html
> [4] https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-2.html#jvms-2.3.4