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<title>Issue 4193: &sect;[time.zone.db] the specification uses the undefined term "thread-safe"</title>
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<p><em>This page is a snapshot from the LWG issues list, see the <a href="lwg-active.html">Library Active Issues List</a> for more information and the meaning of <a href="lwg-active.html#New">New</a> status.</em></p>
<h3 id="4193"><a href="lwg-active.html#4193">4193</a>. &sect;[time.zone.db] the specification uses the undefined term "thread-safe"</h3>
<p><b>Section:</b> 30.11.2 <a href="https://wg21.link/time.zone.db">[time.zone.db]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> S. B. Tam <b>Opened:</b> 2025-01-16 <b>Last modified:</b> 2025-02-07</p>
<p><b>Priority: </b>3
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
<a href="https://wg21.link/N5001" title=" Working Draft, Programming Languages — C++">N5001</a> 30.11.2.2 <a href="https://wg21.link/time.zone.db.list">[time.zone.db.list]</a> p3:
</p>
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<p>
<i>Synchronization</i>: This operation is thread-safe with respect to <code class='backtick'>reload_tzdb()</code>.
</p>
</blockquote>
<p>
<a href="https://wg21.link/N5001" title=" Working Draft, Programming Languages — C++">N5001</a> 30.11.2.3 <a href="https://wg21.link/time.zone.db.access">[time.zone.db.access]</a> p2:
</p>
<blockquote style="border-left: 3px solid #ccc;padding-left: 15px;">
<p>
<i>Synchronization</i>: It is safe to call this function from multiple threads at one time.
</p>
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<p>
<a href="https://wg21.link/N5001" title=" Working Draft, Programming Languages — C++">N5001</a> 30.11.2.4 <a href="https://wg21.link/time.zone.db.remote">[time.zone.db.remote]</a> p3:
</p>
<blockquote style="border-left: 3px solid #ccc;padding-left: 15px;">
<p>
<i>Synchronization</i>: This function is thread-safe with respect to <code class='backtick'>get_tzdb_list().front()</code> and 
<code class='backtick'>get_tzdb_list().erase_after()</code>.
</p>
</blockquote>
<p>
The standard does not define what "thread-safe" means, which makes the meaning of these sentences unclear. 
Does it mean that "concurrent calls do not introduce data races", or does it additionally require a single 
total order on these operations? In either case, it should be specified clearly.
</p>


<p><i>[2025-02-07; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll.
</p>
<p>
"We're missing a guarantee that <code class='backtick'>reload_tzdb()</code> strongly happens before
a call to <code class='backtick'>get_tzdb_list().front()</code> that retrieves a newly added element.
Otherwise accessing any <code class='backtick'>time_zone</code> obtained from <code class='backtick'>get_tzdb_list().front()</code>
could race with the call to <code class='backtick'>reload_tzdb()</code> that writes the front of the list."
</p>



<p id="res-4193"><b>Proposed resolution:</b></p>





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