Memory Mapped file segment (file is empty)

[Maurizio Cimadamore]

Hi Johannes,
the specific condition you are talking about has been rectified in the 
upcoming Java 15.

The new code should be doing something like this:

```
if (bytesSize < 0) throw new IllegalArgumentException("Requested bytes 
size must be >= 0.");
if (bytesOffset < 0) throw new IllegalArgumentException("Requested bytes 
offset must be >= 0.");
```

So, hopefully, your code should work against the latest version of the API

Maurizio

On 25/06/2020 08:24, Johannes Lichtenberger wrote:
> Hi Paul,
>
> that's great. I guess usually there's a -dev and a -user Mailinglist,
> that's why I thought it's for internal development topics.
>
> I'm using AdpotOpenJDK, because I wanted to try Shenandoah, which is at
> least not in the Oracle binaries.
>
> IMPLEMENTOR="AdoptOpenJDK"
> IMPLEMENTOR_VERSION="AdoptOpenJDK"
> JAVA_VERSION="14.0.1"
> JAVA_VERSION_DATE="2020-04-14"
>
> BTW: My OS is Linux/Ubuntu.
>
> Kind regards
> Johannes
>
> Am Do., 25. Juni 2020 um 02:04 Uhr schrieb Paul Sandoz <
> paul.sandoz at oracle.com>:
>
>> Hi Johannes,
>>
>> This is the correct list. Feedback on the API and its usability is very
>> important, as well as whether the implementation works as expected.
>>
>> This was fixed in https://bugs.openjdk.java.net/browse/JDK-8246095
>>
>>
>> https://github.com/openjdk/panama-foreign/blob/foreign-memaccess/src/jdk.incubator.foreign/share/classes/jdk/internal/foreign/MappedMemorySegmentImpl.java#L102
>>
>> What JDK build are you using?
>>
>> Paul.
>>
>> On Jun 24, 2020, at 3:48 PM, Johannes Lichtenberger <
>> lichtenberger.johannes at gmail.com> wrote:
>>
>> Hi,
>>
>> I guess that's not the right mailinglist to ask for help with the API, but
>> maybe you can point me at least somewhere.
>>
>> Currently I face the problem, that I can't get a memory segment on an empty
>> file when I want to memory map a file. However, a writer would write to the
>> memory mapped file.
>>
>> I want to be able to swap a rather old RandomAccessFile based
>> implementation with a memory mapped file implementation. The two classes
>> (actually three, but only the FileReader and FileWriter matters) are rather
>> simple.
>>
>> The old package:
>>
>>
>> https://github.com/sirixdb/sirix/tree/master/bundles/sirix-core/src/main/java/org/sirix/io/file
>>
>>
>> and the new package
>>
>>
>> https://github.com/sirixdb/sirix/tree/master/bundles/sirix-core/src/main/java/org/sirix/io/memorymapped
>>
>> This line fails if the underlying file size is 0:
>>
>>
>> https://github.com/sirixdb/sirix/blob/7bdbd5d17a034f02902f8f7dd0ef7012d89c81fb/bundles/sirix-core/src/main/java/org/sirix/io/memorymapped/MemoryMappedFileWriter.java#L83
>>
>> Stacktrace  (Precondition fails, so I'm not using the API in the intended
>> way):
>>
>> java.lang.IllegalArgumentException: Requested bytes size must be > 0.
>>
>> at
>>
>> jdk.incubator.foreign/jdk.internal.foreign.Utils.makeMappedSegment(Utils.java:140)
>> at
>>
>> jdk.incubator.foreign/jdk.incubator.foreign.MemorySegment.mapFromPath(MemorySegment.java:398)
>> at
>>
>> org.sirix.io.memorymapped.MemoryMappedFileWriter.<init>(MemoryMappedFileWriter.java:83)
>>
>>
>> So, it might be easiest to write a more or less direct conversion to the
>> Foreign Memory API. Maybe someone can help or point me to a helpful
>> Mailinglist/Forum/whatever.
>>
>> Kind regards
>> Johannes
>>
>>
>>