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<p style="margin: 0px 0px 1.2em !important;">Some additional
notes, as we dug into this more.</p>
<p style="margin: 0px 0px 1.2em !important;">The main issue you
describe is this:</p>
<blockquote style="margin: 1.2em 0px;border-left: 4px solid rgb(221, 221, 221); padding: 0px 1em; color: rgb(119, 119, 119); quotes: none;">
<blockquote style="margin: 1.2em 0px;border-left: 4px solid rgb(221, 221, 221); padding: 0px 1em; color: rgb(119, 119, 119); quotes: none;">
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<p style="margin: 0px 0px 1.2em !important;">What makes this
worse is that if you are just given a<br>
SymbolLookup, or even a MemorySegment obtained from it,
you don’t<br>
have access to the original Arena used to load the
library, so<br>
you cannot even manually fix this unsafety this by using<br>
<code style="font-size: 0.85em; font-family: Consolas, Inconsolata, Courier, monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;">MemorySegment#reinterpet</code>
to change the Arena.</p>
<p style="margin: 0px 0px 1.2em !important;">A similar
problem to the one you cite was mentioned in this document
we shared a while ago:</p>
</blockquote>
</blockquote>
</blockquote>
<p style="margin: 0px 0px 1.2em !important;"><a href="https://cr.openjdk.org/~mcimadamore/panama/scoped_arenas.html" class="moz-txt-link-freetext">https://cr.openjdk.org/~mcimadamore/panama/scoped_arenas.html</a></p>
<p style="margin: 0px 0px 1.2em !important;">See section on
“co-allocation”.</p>
<p style="margin: 0px 0px 1.2em !important;">At the time we
concluded (based on feedback) that the usefulness of the
proposed factory to “convert” a scope back into an Arena was
limited. As such we decided to leave that out. That is, even if
we could see that there were cases where a client might expect
to e.g. be able to allocate a new segment with the same lifetime
of an existing segment, the cases where this came up seemed
rather obscure, and even then, it wasn’t clear as to whether it
was something that was 100% ok for the client to be wanting to
do.</p>
<p style="margin: 0px 0px 1.2em !important;">You seem to have use
case where you need to perform a reinterpret - but have no
Arena. Can you please share more about your use case? E.g. are
you really in a situation where you don’t have an Arena, or are
you concerned about a case where you <em>might</em> not have
one?</p>
<p style="margin: 0px 0px 1.2em !important;">For instance, in the
case you mention there is a native library. This is loaded using
a SymbolLookup - probably SymbolLookup::libraryLookup. This
lookup takes an Arena. If you are using jextract, jextract will
emit a constant called <code style="font-size: 0.85em; font-family: Consolas, Inconsolata, Courier, monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;">LIBRARY_ARENA</code>
which will give you the arena used by jextract to generate the
bindings (although, I see that constant is currently
package-private, so probably not accessible). So, at least in
the common case of jextract-generated code, something along the
lines of what you want to do seems already possible.</p>
<p style="margin: 0px 0px 1.2em !important;">If you did not use
jextract, but created your own bindings manually, the same
approach could in principle be replicated: there’s an arena for
the entire library - and you can access it in order to allocate,
reinterpret, etc. Is there a specific reason as to why you think
this approach is not viable in your case? If so, we’re genuinely
interested in knowing more about your code.</p>
<p style="margin: 0px 0px 1.2em !important;">Cheers<br>
Maurizio</p>
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