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    CWG Issue 785</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="785"></A><H4>785.
  
&#8220;Execution sequence&#8221; is inappropriate phraseology
</H4>
<B>Section: </B>6.10.1&#160; [<A href="https://wg21.link/intro.execution">intro.execution</A>]
 &#160;&#160;&#160;

 <B>Status: </B>CD2
 &#160;&#160;&#160;

 <B>Submitter: </B>US/UK
 &#160;&#160;&#160;

 <B>Date: </B>3 March, 2009<BR><BR>


<A href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2010/n3086.html#US16">N2800 comment
  US&#160;16<BR></A>
<A href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2010/n3086.html#UK8">N2800 comment
  UK&#160;8<BR></A>
<A href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2010/n3086.html#UK7">N2800 comment
  UK&#160;7<BR></A>

<P>[Voted into WP at October, 2009 meeting.]</P>

<P>In the presence of threads, it is no longer appropriate to characterize
the abstract machine as having an &#8220;execution sequence.&#8221;</P>

<P><B>Proposed resolution (September, 2009):</B></P>

<OL>
<LI><P>Change 6.10.1 [<A href="https://wg21.link/intro.execution#3">intro.execution</A>] paragraph 3 as follows:</P></LI>

<BLOCKQUOTE>

...An instance of the abstract machine can thus have more than one
possible execution <DEL>sequence</DEL> for a given program and a given input.

</BLOCKQUOTE>

<LI><P>Change 6.10.1 [<A href="https://wg21.link/intro.execution#5">intro.execution</A>] paragraph 5 as follows:</P></LI>

<BLOCKQUOTE>

A conforming implementation executing a well-formed program shall
produce the same observable behavior as one of the possible
<DEL>execution sequences</DEL> <INS>executions</INS> of the
corresponding instance of the abstract machine with the same program
and the same input. However, if any such execution <DEL>sequence</DEL>
contains an undefined operation, this International Standard places no
requirement on the implementation executing that program with that
input (not even with regard to operations preceding the first
undefined operation).

</BLOCKQUOTE>

<LI><P>Delete 6.10.1 [<A href="https://wg21.link/intro.execution#6">intro.execution</A>] paragraph 6, including the
footnote:</P></LI>

<BLOCKQUOTE>

<DEL>The observable behavior of the abstract machine is its sequence
of reads and writes to <TT>volatile</TT> data and calls to library I/O
functions. [<I>Footnote:</I> An implementation can offer additional
library I/O functions as an extension.  Implementations that do so
should treat calls to those functions as &#8220;observable
behavior&#8221; as well. &#8212;<I>end footnote</I>]</DEL>

</BLOCKQUOTE>

<LI><P>Change 6.10.1 [<A href="https://wg21.link/intro.execution#9">intro.execution</A>] paragraph 9 as follows:</P></LI>

<BLOCKQUOTE>

<P>The least requirements on a conforming implementation are:</P>

<UL>
<LI><P>Access to volatile objects are evaluated strictly according
to the rules of the abstract machine.</P></LI>

<LI><P>At program termination, all data written into files shall be
identical to one of the possible results that execution of the program
according to the abstract semantics would have produced.</P></LI>

<LI><P>The input and output dynamics of interactive devices shall take
place in such a fashion that <DEL>prompting messages actually appear
prior to a program waiting</DEL> <INS>prompting output is actually
delivered before a program waits</INS> for input. What constitutes an
interactive device is implementation-defined.</P></LI>

</UL>

<P>
<INS>These collectively are referred to as the observable behavior
of the program.</INS> [<I>Note:</I> more stringent correspondences
between abstract and actual semantics may be defined by each
implementation. &#8212;<I>end note</I>]</P>

</BLOCKQUOTE>

</OL>

<P>(Note; this resolution also resolves <A HREF="612.html">issue 612</A>.)</P>

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