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    CWG Issue 2970</TITLE>
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<P><EM>This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21
  Core Issues List revision 118b.
  See http://www.open-std.org/jtc1/sc22/wg21/ for the official
  list.</EM></P>
<P>2025-09-28</P>
<HR>
<A NAME="2970"></A><H4>2970.
  
Races with <TT>volatile sig_atomic_t</TT> bit-fields
</H4>
<B>Section: </B>6.10.2.2&#160; [<A href="https://wg21.link/intro.races">intro.races</A>]
 &#160;&#160;&#160;

 <B>Status: </B>CD7
 &#160;&#160;&#160;

 <B>Submitter: </B>Hubert Tong
 &#160;&#160;&#160;

 <B>Date: </B>2024-12-17<BR>


<P>[Accepted as a DR at the February, 2025 meeting.]</P>

<P>(From submission
<A HREF="https://github.com/cplusplus/CWG/issues/654">#654</A>.)
</P>

<P>Subclause 6.10.2.2 [<A href="https://wg21.link/intro.races#22">intro.races</A>] paragraph 22 specifies:</P>

<BLOCKQUOTE>

Two accesses to the same object of type <TT>volatile
std::sig_atomic_t</TT> do not result in a data race if both occur in
the same thread, even if one or more occurs in a signal handler. ...

</BLOCKQUOTE>

<P>This provision applies to bit-fields as well, because bit-fields
are objects (6.9.1 [<A href="https://wg21.link/basic.types.general#4">basic.types.general</A>] paragraph 4).  However, in
practice bit-fields are not updated atomically and are subject to
tearing.</P>

<P><B>Proposed resolution (2025-01-20, approved by CWG 2025-02-14):</B></P>

<P>Change in 6.10.2.2 [<A href="https://wg21.link/intro.races#22">intro.races</A>] paragraph 22 as follows:</P>

<BLOCKQUOTE>

Two accesses to the same <INS>non-bit-field</INS> object of
type <TT>volatile std::sig_atomic_t</TT> do not result in a data race
if both occur in the same thread, even if one or more occurs in a
signal handler. ...

</BLOCKQUOTE>

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