<!DOCTYPE html><html><head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
</head>
<body>
<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.</p>
<p>Jorn</p>
<div class="moz-cite-prefix">On 18-11-2025 19:20, Jonathan Rosenne
wrote:<br>
</div>
<blockquote type="cite" cite="mid:PAVPR10MB6789E76BB0813A5F7CDC9EAC84D6A@PAVPR10MB6789.EURPRD10.PROD.OUTLOOK.COM">
<meta name="Generator" content="Microsoft Word 15 (filtered medium)">
<style>@font-face
{font-family:"Cambria Math";
panose-1:2 4 5 3 5 4 6 3 2 4;}@font-face
{font-family:Calibri;
panose-1:2 15 5 2 2 2 4 3 2 4;}@font-face
{font-family:Aptos;}@font-face
{font-family:Consolas;
panose-1:2 11 6 9 2 2 4 3 2 4;}p.MsoNormal, li.MsoNormal, div.MsoNormal
{margin:0in;
font-size:12.0pt;
font-family:"Aptos",sans-serif;}a:link, span.MsoHyperlink
{mso-style-priority:99;
color:blue;
text-decoration:underline;}pre
{mso-style-priority:99;
mso-style-link:"HTML Preformatted Char";
margin:0in;
font-size:10.0pt;
font-family:"Courier New";}span.HTMLPreformattedChar
{mso-style-name:"HTML Preformatted Char";
mso-style-priority:99;
mso-style-link:"HTML Preformatted";
font-family:Consolas;}span.EmailStyle21
{mso-style-type:personal-reply;
font-family:"Arial",sans-serif;
color:#1F497D;
font-weight:normal;
font-style:normal;}.MsoChpDefault
{mso-style-type:export-only;
font-size:10.0pt;
mso-ligatures:none;}div.WordSection1
{page:WordSection1;}</style>
<div class="WordSection1">
<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.<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">
panama-dev <a class="moz-txt-link-rfc2396E" href="mailto:panama-dev-retn@openjdk.org"><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 class="moz-txt-link-rfc2396E" href="mailto:cushon@google.com"><cushon@google.com></a>;
Maurizio Cimadamore
<a class="moz-txt-link-rfc2396E" href="mailto:maurizio.cimadamore@oracle.com"><maurizio.cimadamore@oracle.com></a><br>
<b>Cc:</b> <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>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>
</div>
<blockquote style="border:none;border-right:solid #CCCCCC 1.0pt;padding:0in 0in 0in 6.0pt;margin-left:4.8pt;margin-right:0in">
<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">
<div>
<div>
<blockquote style="border:none;border-right:solid #CCCCCC 1.0pt;padding:0in 0in 0in 6.0pt;margin-left:4.8pt;margin-right:0in">
<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>
</div>
</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>
</div>
</blockquote>
</div>
</blockquote>
</div>
</blockquote>
</body>
</html>