<div dir="ltr"><div class="gmail_default" style="font-family:comic sans ms,sans-serif;color:#38761d">Right, so how do I formally request that the standard be updated to clarify this issue?<br></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Thu, Dec 8, 2022 at 4:56 PM gilles--- via mpi-comments <<a href="mailto:mpi-comments@lists.mpi-forum.org">mpi-comments@lists.mpi-forum.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><p>I have a different recollection.</p>
<p> </p>
<p>The link is broken, here is one that works for me</p>
<p><a href="https://lists.mpi-forum.org/pipermail/mpi-forum/2016-January/003152.html" target="_blank">https://lists.mpi-forum.org/pipermail/mpi-forum/2016-January/003152.html</a></p>
<p>TL;DR from Bill Gropp:</p>
<pre>"The standard says that the null handles are invalid for input unless explicitly permitted. There’s no exemption for a datatype argument where the associated count is zero."
</pre>
<p> </p>
<p>IIRC there was no objection to updating the standard in order to allow this, but unless this has been completed, I am afraid MPI_DATATYPE_NULL is not a valid datatype here</p>
<p> </p>
<p>Gilles</p>
<p>----- Original Message -----</p>
<blockquote style="margin:1em 0px;padding-left:1em;border-left:2px solid blue">
<div dir="ltr">You are correct. The MPI_DATATYPE_NULL is an acceptable datatype for cases where the target processor sends or receives no data. The standard uses MPI_DATATYPE_NULL in 2 instances (in example 5.23) to highlight no data movement, but fails to make a clear reference to the fact that this is an acceptable use.
<div> </div>
<div> George.</div>
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<div class="gmail_quote">
<div class="gmail_attr" dir="ltr">On Thu, Dec 8, 2022 at 5:31 PM Jim Edwards via mpi-comments <<a href="mailto:mpi-comments@lists.mpi-forum.org" target="_blank">mpi-comments@lists.mpi-forum.org</a>> wrote:</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
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<div class="gmail_default" style="color:rgb(56,118,29);font-family:"comic sans ms",sans-serif">There seems to be some ambiguity in the standard with respect to usage of the MPI_DATATYPE_NULL variable.</div>
<div class="gmail_default" style="color:rgb(56,118,29);font-family:"comic sans ms",sans-serif">It would seem to me that if an MPI routine such as MPI_Send or MPI_ALLTOALLW has a data count argument of 0</div>
<div class="gmail_default" style="color:rgb(56,118,29);font-family:"comic sans ms",sans-serif">then MPI_DATATYPE_NULL should be an acceptable value for the datatype. Here is a discussion from several years</div>
<div class="gmail_default" style="color:rgb(56,118,29);font-family:"comic sans ms",sans-serif">ago with openmpi developers. It references a discussion in the standards forum but I can no longer find it.</div>
<div class="gmail_default" style="color:rgb(56,118,29);font-family:"comic sans ms",sans-serif"> </div>
<div class="gmail_default" style="color:rgb(56,118,29);font-family:"comic sans ms",sans-serif"><span id="m_-7170094336555984817m_3259980529876526062gmail-init_selection"><a href="http://lists.mpi-forum.org/mpi-forum/2016/01/3152.php" target="_blank">http://lists.mpi-forum.org/mpi-forum/2016/01/3152.php</a></span></div>
<div> </div>
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<div class="gmail_default" style="color:rgb(56,118,29);font-family:"comic sans ms",sans-serif">Would it be possibly to clarify this case in the next iteration of the standard?</div>
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<div>Jim Edwards<br>
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<span style="font-size:x-small">CESM Software Engineer</span></div>
<span style="font-size:x-small">National Center for Atmospheric Research</span></div>
<span style="font-size:x-small">Boulder, CO</span></div>
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</blockquote></div><br clear="all"><br>-- <br><div dir="ltr" class="gmail_signature"><div dir="ltr"><div><div><div>Jim Edwards<br><br></div><font size="1">CESM Software Engineer<br></font></div><font size="1">National Center for Atmospheric Research<br></font></div><font size="1">Boulder, CO</font> <br></div></div>