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<title>Issue 1524: Allocation functions are missing happens-before requirements and guarantees</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#C++11">C++11</a> status.</em></p>
<h3 id="1524"><a href="lwg-defects.html#1524">1524</a>. Allocation functions are missing <i>happens-before</i> requirements and guarantees</h3>
<p><b>Section:</b> 17.6.3.5 <a href="https://wg21.link/new.delete.dataraces">[new.delete.dataraces]</a> <b>Status:</b> <a href="lwg-active.html#C++11">C++11</a>
 <b>Submitter:</b> Hans Boehm <b>Opened:</b> 2011-02-26 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#new.delete.dataraces">issues</a> in [new.delete.dataraces].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++11">C++11</a> status.</p>
<p><b>Discussion:</b></p>
<p><b>Addresses US-34</b></p>

<p>Technical details:
<p/>
When the same unit of storage is allocated and deallocated repeatedly, operations on it can't be allowed to
race between the allocator and the user program. But I don't see any mention of <i>happens-before</i> in the
descriptions of allocation and deallocation functions.
<p/>
Proposed resolution (not wording yet):
</p>
<ul>
<li><p>The call to an allocation function returning a pointer <code>P</code> must happen-before the matching
deallocation call with <code>P</code> as a parameter. Otherwise the behavior is undefined. I don't know whether
receiving <code>P</code> with <code>memory_order_consume</code> fits this requirement. <code>memory_order_relaxed</code> does not.</p>
</li>
<li><p>If some memory is passed to a deallocation function, the implementation must ensure that the
deallocation call happens-before any allocation call that returns the same memory address.</p>
</li>
</ul>

<p><i>[2011-02-26: Hans comments and drafts wording]</i></p>


<p>The second requirement already exists, almost verbatim, as 17.6.3.5 <a href="https://wg21.link/new.delete.dataraces">[new.delete.dataraces]</a> p. 1. 
I think this is where the statement belongs.  However, this paragraph requires work to correctly address 
the first part of the issue.
</p>

<p><i>[Adopted at Madrid, 2011-03]</i></p>




<p id="res-1524"><b>Proposed resolution:</b></p>
<p>Change 17.6.3.5 <a href="https://wg21.link/new.delete.dataraces">[new.delete.dataraces]</a> p. 1 as follows:</p>

<blockquote><p>
1 <del>The library versions of <code>operator new</code> and <code>operator delete</code>, user replacement versions of global 
<code>operator new</code> and <code>operator delete</code>, and the C standard library functions <code>calloc</code>, <code>malloc</code>, 
<code>realloc</code>, and <code>free</code> shall not introduce data races (6.10.2 <a href="https://wg21.link/intro.multithread">[intro.multithread]</a>) as a result 
of concurrent calls from different threads.</del><ins> For purposes of determining the existence of data races,
the library versions of <code>operator new</code>, user replacement versions of global <code>operator new</code>, and the C 
standard library functions <code>calloc</code> and <code>malloc</code> shall behave as though they accessed and modified only 
the storage referenced by the return value. The library versions of <code>operator delete</code>, user replacement
versions of <code>operator delete</code>, and the C standard library function <code>free</code> shall behave as though they 
accessed and modified only the storage referenced by their first argument. The C standard library <code>realloc</code> 
function shall behave as though it accessed and modified only the storage referenced by its first argument and by 
its return value.</ins> Calls to these functions that allocate or deallocate a particular unit of storage 
shall occur in a single total order, and each such deallocation call shall happen before the next allocation 
(if any) in this order.
</p></blockquote>





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