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<p>They're not. All Java arrays in HotSpot have similar layouts:
object header, some padding (this varies between array types), and
then the actual array data (no indirection). In other words, the
same restrictions would apply to any Java array type.</p>
<p>Jorn</p>
<div class="moz-cite-prefix">On 18-11-2025 20:43, Jonathan Rosenne
wrote:<br>
</div>
<blockquote type="cite" cite="mid:PAVPR10MB6789A36DED8A8FEFC2FB61B084D6A@PAVPR10MB6789.EURPRD10.PROD.OUTLOOK.COM">
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<p class="MsoNormal"><span style="font-size:14.0pt;font-family:"Arial",sans-serif;color:#1F497D">Why
is an array of bytes any different from any other array?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:14.0pt;font-family:"Arial",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<div>
<p class="MsoNormal"><span style="font-size:14.0pt;font-family:"Arial",sans-serif;color:#1F497D">Best
Regards,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:14.0pt;font-family:"Arial",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:14.0pt;font-family:"Arial",sans-serif;color:#1F497D">Jonathan
Rosenne</span><span style="font-size:14.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p></o:p></span></p>
</div>
<p class="MsoNormal"><span style="font-size:14.0pt;font-family:"Arial",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<div>
<div style="border:none;border-top:solid #E1E1E1 1.0pt;padding:3.0pt 0in 0in 0in">
<p class="MsoNormal"><b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">
Jorn Vernee <a class="moz-txt-link-rfc2396E" href="mailto:jorn.vernee@oracle.com"><jorn.vernee@oracle.com></a>
<br>
<b>Sent:</b> Tuesday, November 18, 2025 8:34 PM<br>
<b>To:</b> Jonathan Rosenne <a class="moz-txt-link-rfc2396E" href="mailto:jr@qsm.co.il"><jr@qsm.co.il></a>;
<a class="moz-txt-link-abbreviated" href="mailto:panama-dev@openjdk.org">panama-dev@openjdk.org</a><br>
<b>Subject:</b> Re: [External] : Re: MemorySegment APIs
for reading and writing strings with known lengths<o:p></o:p></span></p>
</div>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
<p>You mean 'arrays of bytes' as in byte[]? The FFM API supports
off-heap memory as well, which can't just be treated as a
byte[]. For one, it's missing a Java object header. Even if we
added a way for a byte[] to just be a pointer to some other
memory, which would let us wrap a byte[] object around a
native pointer, the entire JVM would need to be updated to
handle that alternative format (the current byte[] layout
contains no such indirection). In other words: the two
in-memory representations are incompatible.<o:p></o:p></p>
<p>Jorn<o:p></o:p></p>
<div>
<p class="MsoNormal">On 18-11-2025 19:20, Jonathan Rosenne
wrote:<o:p></o:p></p>
</div>
<blockquote style="margin-top:5.0pt;margin-bottom:5.0pt">
<p class="MsoNormal"><span style="font-size:14.0pt;font-family:"Arial",sans-serif;color:#1F497D">Excuse
my ignorance, but why can't fixed length C strings be
treated as arrays of bytes? Java has all the necessary
options to convert between byte arrays and strings.</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:14.0pt;font-family:"Arial",sans-serif;color:#1F497D"> </span><o:p></o:p></p>
<div>
<p class="MsoNormal"><span style="font-size:14.0pt;font-family:"Arial",sans-serif;color:#1F497D">Best
Regards,</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:14.0pt;font-family:"Arial",sans-serif;color:#1F497D"> </span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:14.0pt;font-family:"Arial",sans-serif;color:#1F497D">Jonathan
Rosenne</span><o:p></o:p></p>
</div>
<p class="MsoNormal"><span style="font-size:14.0pt;font-family:"Arial",sans-serif;color:#1F497D"> </span><o:p></o:p></p>
<div>
<div style="border:none;border-top:solid #E1E1E1 1.0pt;padding:3.0pt 0in 0in 0in">
<p class="MsoNormal"><b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">
panama-dev
<a href="mailto:panama-dev-retn@openjdk.org" moz-do-not-send="true"><panama-dev-retn@openjdk.org></a>
<b>
On Behalf Of </b>Jorn Vernee<br>
<b>Sent:</b> Tuesday, November 18, 2025 6:52 PM<br>
<b>To:</b> Liam Miller-Cushon <a href="mailto:cushon@google.com" moz-do-not-send="true"><cushon@google.com></a>;
Maurizio Cimadamore
<a href="mailto:maurizio.cimadamore@oracle.com" moz-do-not-send="true"><maurizio.cimadamore@oracle.com></a><br>
<b>Cc:</b> <a href="mailto:panama-dev@openjdk.org" moz-do-not-send="true" class="moz-txt-link-freetext">panama-dev@openjdk.org</a><br>
<b>Subject:</b> Re: [External] : Re: MemorySegment
APIs for reading and writing strings with known
lengths</span><o:p></o:p></p>
</div>
</div>
<p class="MsoNormal"> <o:p></o:p></p>
<p>Coming back to this, I think we've settled on the following
three methods:<br>
<br>
In MemorySegment:<o:p></o:p></p>
<p><span style="font-family:"Courier New"">
String getString(long offset, Charset charset, long
length); // as in Liam's PR<br>
void copy(String src, Charset dstEncoding, int
srcIndex, MemorySegment dst, int numChars);</span><o:p></o:p></p>
<p>And in SegmentAllocator:<o:p></o:p></p>
<p><span style="font-family:"Courier New"">
MemorySegment allocateFrom(String src, Charset
dstEncoding, int srcIndex, int numChars);</span><o:p></o:p></p>
<p>For encoding directly into a memory segment without the
need to go to an intermediate buffer, it looks like we can
use the internal StringCharBuffer class, in combination with
the `CharsetEncoder::encode` method. But of course we can
skip encoding altogether when the internal string encoding
matches the target, and just do a bulk copy.<o:p></o:p></p>
<p>For allocateFrom, since we don't yet have a way to
determine the encoded length of a String, I think we'd still
have to go to an intermediate byte[], and then allocate the
result segment based on its length. We can still avoid the
intermediate byte[] in most cases where the encoding of the
String's internal buffer is compatible with the target
encoding, and again just do a bulk copy from the string's
internal buffer.<o:p></o:p></p>
<p>Note on the length parameter for getString: we thought that
it might be possible to open this up to any charset, not
just the standard ones we support now, in which case having
the length be specified as a byte length would be more
flexible, since not every charset might have a notion of
'code unit' (and associated unit size). For charsets with a
code unit size, converting to a byte length would be trivial
any ways (Sorry for the back-and-forth on that). Right now
we can't handle a length > Integer.MAX_VALUE because of
limitations of ByteBuffer used in the decoding
(CharsetDecoder::decode takes ByteBuffer as input), but we
wanted to keep this option open for the future, so that's
why the length is a `long` above.<o:p></o:p></p>
<p>Liam, would you be interested in working on these as part
of your PR [1]?<o:p></o:p></p>
<p>Jorn<o:p></o:p></p>
<p>[1]: <a href="https://github.com/openjdk/jdk/pull/28043" moz-do-not-send="true" class="moz-txt-link-freetext">https://github.com/openjdk/jdk/pull/28043</a><br>
[2]: <o:p></o:p></p>
<div>
<p class="MsoNormal">On 12-11-2025 15:54, Liam Miller-Cushon
wrote:<o:p></o:p></p>
</div>
<blockquote style="margin-top:5.0pt;margin-bottom:5.0pt">
<div>
<p class="MsoNormal">Thanks. I am convinced :)<o:p></o:p></p>
</div>
<p class="MsoNormal"> <o:p></o:p></p>
<div>
<div>
<p class="MsoNormal">On Wed, Nov 12, 2025 at 3:30<span style="font-family:"Arial",sans-serif"> </span>PM
Maurizio Cimadamore <<a href="mailto:maurizio.cimadamore@oracle.com" moz-do-not-send="true" class="moz-txt-link-freetext">maurizio.cimadamore@oracle.com</a>>
wrote:<o:p></o:p></p>
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<div>
<p> <o:p></o:p></p>
<div>
<p class="MsoNormal">On 12/11/2025 11:40, Liam
Miller-Cushon wrote:<o:p></o:p></p>
</div>
<blockquote style="margin-top:5.0pt;margin-bottom:5.0pt">
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<div>
<p>For the non-\0 terminated strings, you
have the String-based MemorySegment::copy
I described - e.g.<o:p></o:p></p>
<pre>void copy(String srcString, Charset srcCharset, int srcIndex, MemorySegment dstSegment, long dstOffset, int length);<o:p></o:p></pre>
<p>With this, we also have two cases:<o:p></o:p></p>
<p>* if the charset is compatible with the
string buffer, we just bulk-copy the
string buffer (or a portion of it) into
the dest segment<br>
* otherwise we can encode the srcString
directly into the dest segment<o:p></o:p></p>
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</blockquote>
<div>
<p class="MsoNormal">Thanks! I think I'm
caught up now. My misunderstanding was
whether MS::ofString was being suggested
instead of and not in addition to the bulk
copy.<o:p></o:p></p>
</div>
</div>
</div>
</blockquote>
<p>Ah, gotcha.<o:p></o:p></p>
<p>I think MS::ofString is a possible add-on. To be
fair, since writing the document I think we've grown
a little colder on it, as such a view would make for
a pretty big footgun, as it would allow a native
function (invoked via critical downcall handle) to
directly modify the string buffer (at least in some
cases). There's also some question about how
`MemorySegment::equals` should work in this case, as
`equals` for heap segments takes into account the
identity of the underlying heap object.<o:p></o:p></p>
<p>So, if we could get there with the new
`getString`/`copy` + maybe some way to determine the
length of an encoded string, I think it would be
preferrable/less risky. We could always add
`ofString` later, if we find a way to address and/or
mitigate the issues above.<o:p></o:p></p>
<p>Maurizio<o:p></o:p></p>
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