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<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>
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<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>
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<p class="MsoNormal"><span style="font-size:14.0pt;font-family:"Arial",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
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<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 <panama-dev-retn@openjdk.org>
<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 <cushon@google.com>; Maurizio Cimadamore <maurizio.cimadamore@oracle.com><br>
<b>Cc:</b> panama-dev@openjdk.org<br>
<b>Subject:</b> Re: [External] : Re: MemorySegment APIs for reading and writing strings with known lengths<o:p></o:p></span></p>
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<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">https://github.com/openjdk/jdk/pull/28043</a><br>
[2]: <o:p></o:p></p>
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<p class="MsoNormal">On 12-11-2025 15:54, Liam Miller-Cushon wrote:<o:p></o:p></p>
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<p class="MsoNormal">Thanks. I am convinced :)<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<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">maurizio.cimadamore@oracle.com</a>> wrote:<o:p></o:p></p>
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<p><o:p> </o:p></p>
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<p class="MsoNormal">On 12/11/2025 11:40, Liam Miller-Cushon wrote:<o:p></o:p></p>
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<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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<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>
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<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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