Question about global SegmentScope

[刘希晨]

Thank you for the clarification. I am stuck with my mind because I thought
that if the SegmentScope() is global, then even if the MemorySegment Object
has been GCed, the underlying memory won't be released, not until the JVM
exit. (Is that correct? I think that's the difference between the global
scope and the auto scope).  Since there's no destructor in Java, there's no
way to get notified that the MemorySegment  is not available anymore, thus
some cleanup could be done. Perhaps this operation should not be suggested,
since it make things much more complicated.

Maurizio Cimadamore <maurizio.cimadamore at oracle.com> 于2023年4月17日周一 16:53写道：

> What Jorn says is correct. There is no way for the FFM API to reason about
> the lifetime of pointers returned by native functions. Sometimes these
> pointers will need to be freed, manually, by the caller, other times they
> will be freed by the library, other times (as in this case) they will point
> to some constant data that will remain valid as long as the library is
> loaded.
>
> It is up to the client to “override” the lifetime of the returned segment
> as needed.
>
> The Java 21 API makes this a bit clearer, as segments with unknown
> lifetime are modelled as segment that have an *unbounded* scope (e.g. a
> scope that is always alive), but that is *different* from the scope of
> the global arena. This is demonstrated below:
>
> jshell> MemorySegment.ofAddress(42).scope().equals(Arena.global().scope())
> $2 ==> false
>
> Also, you might find the new javadoc for the Linker useful, as it contains
> a section on native functions returning pointers:
>
>
> https://cr.openjdk.org/~pminborg/panama/21/v2/javadoc/api/java.base/java/lang/foreign/Linker.html#by-ref
>
> Which shows how, where needed, new temporal bounds can be given to
> segments returned by a downcall method handle.
>
> Hope this helps.
>
> Maurizio
>
> On 17/04/2023 08:25, Jorn Vernee wrote:
>
> I think they are asking not about the library symbols, but about the
> pointer returned by OPENSSL_version?
>
> Whether such pointers should be 'freed' depends on the native library.
> Some native libraries might allocate memory and return a pointer to the
> user. It is up to the native library to specify how the memory should be
> freed in that case. Usually if the user is expected to free the memory a
> special function to do that is provided by the library.
>
> In this case, OPENSSL_version seems to be returning a pointer to a
> constant string (string literal), which doesn't have to be freed by the
> user.
>
> Jorn
> On 16/04/2023 14:32, Glavo wrote:
>
> The zero-length MemorySegment associated with the global scope is a
> wrapper of the raw pointer.
> It indicates that the lifetime is unknown, rather than indicating that the
> lifetime is truly infinite.
>
> The true lifetime of these symbols is associated with SymbolLookup. They
> are also invalid when the scope of the SymbolLookup is invalid.
>
> Glavo
>
> On Sun, Apr 16, 2023 at 7:52 PM 刘希晨 <benrush0705 at gmail.com> wrote:
>
>> Thank you for the clarification. I didn' mean the  SymbolLookup itself,
>> but the MemorySegment that MethodHandle returns, the  MethodHandle is
>> created by  , I already tested no matter what kind
>> of SegmentScope SymbolLookup used, the native MemorySegment that
>> MethodHandle returns would always be Global scope.
>>
>> A example would be like below :
>>
>> ```
>> SymbolLookup crypto = SymbolLookup.libraryLookup(Path.of("C:\\Program
>> Files\\OpenSSL-Win64\\libcrypto-3-x64.dll"), SegmentScope.auto());
>> MemorySegment methodPointer = crypto.find("OPENSSL_version")
>> .orElseThrow(() -> new RuntimeException("Unable to load target native
>> method")); MethodHandle openSSLVersion = Linker.nativeLinker().downcallHandle(methodPointer,
>> FunctionDescriptor.of(ValueLayout.ADDRESS, ValueLayout.JAVA_INT));
>> MemorySegment version = (MemorySegment) openSSLVersion.invokeExact(0);
>> System.out.println(version);
>> ```
>> the version Segment is global somehow and I don't know how to release it
>>
>> Glavo <zjx001202 at gmail.com> 于2023年4月16日周日 19:38写道：
>>
>>> SymbolLookup.libraryLookup takes a SegmentScope as a parameter, which
>>> represents the lifecycle of the returned SymbolLookup.
>>>
>>> It looks like you are using an old JDK? Panama has critical changes
>>> every Java update, please use the latest JDK.
>>>
>>> Glavo
>>>
>>> On Sun, Apr 16, 2023 at 6:36 PM 刘希晨 <benrush0705 at gmail.com> wrote:
>>>
>>>> Hi I have a question about global segment scope.
>>>>
>>>> If I use SymbolLookup.libraryLookup to load a C library and call its
>>>> function which returns a pointer , in java it's represented by a memory
>>>> segment, the memorysegment's scope would be global scope. Does this mean
>>>> that the memory segment object wouldn't be released by anything until the
>>>> VM exits?
>>>>
>>> ​
>
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