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Yes, you can send packed data and then receive it with a datatype. Example 4.21 (all references to MPI 3.1) does exactly that.
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<div class="">An implementation may include metadata in the packed buffer; that’s the reason for MPI_PACK_SIZE and the constraint that you can’t concatenate pack buffers. This does mean that an implementation that includes metadata must handle this case; it
can do so by noting that the data was sent with MPI_PACK. There isn’t a truncation problem because the metadata is not delivered; it would only be used to handle and necessary data transformations (e.g., big to little endian).</div>
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<div class="">Bill</div>
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William Gropp<br class="">
Director and Chief Scientist, NCSA<br class="">
Thomas M. Siebel Chair in Computer Science<br class="">
University of Illinois Urbana-Champaign</div>
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<div class="">On Jan 24, 2020, at 4:16 AM, Joachim Protze via mpiwg-p2p <<a href="mailto:mpiwg-p2p@lists.mpi-forum.org" class="">mpiwg-p2p@lists.mpi-forum.org</a>> wrote:</div>
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<div class="">Hi all,<br class="">
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the exception on MPI_PACKED in the type matching rules (3.3.1) suggests, that one side of the communication can pack/unpack the message, while the other side reads/writes natively.<br class="">
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I.e., is the following code valid MPI code?<br class="">
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<br class="">
#include <mpi.h><br class="">
#include <stdio.h><br class="">
#include <stdlib.h><br class="">
<br class="">
int main() {<br class="">
int A[100], B[10], psize;<br class="">
<br class="">
for (int i = 0; i < 100; i++)<br class="">
A[i] = i;<br class="">
<br class="">
MPI_Init(NULL, NULL);<br class="">
MPI_Pack_size(1, MPI_INT, MPI_COMM_SELF, &psize);<br class="">
void *parr = malloc(psize * 10);<br class="">
int pos = 0;<br class="">
for (int i = 0; i < 10; i++)<br class="">
MPI_Pack(&(A[i * 10]), 1, MPI_INT, parr, psize * 10, &pos,<br class="">
MPI_COMM_SELF);<br class="">
// Send the packed data and receive a contiguous buffer<br class="">
MPI_Sendrecv(parr, pos, MPI_PACKED, 0, 42, B, 10, MPI_INT, 0, <span class="Apple-tab-span" style="white-space:pre">
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42, MPI_COMM_SELF, MPI_STATUS_IGNORE);<br class="">
<br class="">
for (int i = 0; i < 10; i++)<br class="">
printf("%i\n", B[i]);<br class="">
MPI_Finalize();<br class="">
}<br class="">
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<br class="">
Can the packed buffer include meta data? This would be reflected by a larger value in /pos/, right? Can this lead to unexpected message truncation?<br class="">
<br class="">
Thanks<br class="">
Joachim<br class="">
<br class="">
-- <br class="">
Dipl.-Inf. Joachim Protze<br class="">
<br class="">
IT Center<br class="">
Group: High Performance Computing<br class="">
Division: Computational Science and Engineering<br class="">
RWTH Aachen University<br class="">
Seffenter Weg 23<br class="">
D 52074 Aachen (Germany)<br class="">
Tel: +49 241 80- 24765<br class="">
Fax: +49 241 80-624765<br class="">
<a href="mailto:protze@itc.rwth-aachen.de" class="">protze@itc.rwth-aachen.de</a><br class="">
www.itc.rwth-aachen.de<br class="">
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