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<title>Issue 4309: How are two seq_cst operations ordered in the single total order if these two operations are unsequenced?</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="4309"><a href="lwg-active.html#4309">4309</a>. How are two <code class='backtick'>seq_cst</code> operations ordered in the single total order if these two operations are unsequenced?</h3>
<p><b>Section:</b> 32.5.4 <a href="https://wg21.link/atomics.order">[atomics.order]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> jim x <b>Opened:</b> 2025-08-06 <b>Last modified:</b> 2025-08-09</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View other</b> <a href="lwg-index-open.html#atomics.order">active issues</a> in [atomics.order].</p>
<p><b>View all other</b> <a href="lwg-index.html#atomics.order">issues</a> in [atomics.order].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Consider this snippet code:
</p>
<blockquote><pre>
std::atomic&lt;int&gt; flag = {0};
std::atomic&lt;int&gt; turn = {0};

if(flag + turn){}
</pre></blockquote>
<p>
The loads of <code class='backtick'>flag</code> and <code class='backtick'>turn</code> as the operands of <code class='backtick'>+</code> are unsequenced according to [intro.execution] p10.
</p>
<blockquote style="border-left: 3px solid #ccc;padding-left: 15px;">
<p>
Except where noted, evaluations of operands of individual operators and of subexpressions of individual expressions are unsequenced.
</p>
</blockquote>
<p>
However, 32.5.4 <a href="https://wg21.link/atomics.order">[atomics.order]</a> p4 says:
</p>
<blockquote style="border-left: 3px solid #ccc;padding-left: 15px;">
<p>
There is a single total order <i>S</i> on all <code class='backtick'>memory_order::seq_cst</code> operations, including fences, 
that satisfies the following constraints.
</p></blockquote>
<p>
Then, it says that:
</p>
<blockquote style="border-left: 3px solid #ccc;padding-left: 15px;">
<p>
First, if <i>A</i> and <i>B</i> are <code class='backtick'>memory_order::seq_cst</code> operations and <i>A</i> strongly happens 
before <i>B</i>, then <i>A</i> precedes <i>B</i> in <i>S</i>.
</p></blockquote>
<p>
According to the first sentence, the load of <code class='backtick'>flag</code> and the load of <code class='backtick'>turn</code> do have an order in the single 
total order <i>S</i> since they are both <code class='backtick'>memory_order::seq_cst</code> operations. However, since they are unsequenced, 
the second sentence does not apply to them. So, what's the order of them in <i>S</i>? Is the order of them in 
<i>S</i> unspecified?
<p/>
<b>Suggested Resolution:</b>
<p/>
We may want to say the order of such operations is indeterminate in the single total order. That is,  
either <i>A</i> precedes <i>B</i> or <i>B</i> precedes <i>A</i>, but it is unspecified which.
</p>


<p id="res-4309"><b>Proposed resolution:</b></p>





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