[mpiwg-p2p] Question on MPI_PACKED

Gropp, William D wgropp at illinois.edu
Sun Jan 26 19:04:58 CST 2020

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.

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).


William Gropp
Director and Chief Scientist, NCSA
Thomas M. Siebel Chair in Computer Science
University of Illinois Urbana-Champaign

On Jan 24, 2020, at 4:16 AM, Joachim Protze via mpiwg-p2p <mpiwg-p2p at lists.mpi-forum.org<mailto:mpiwg-p2p at lists.mpi-forum.org>> wrote:

Hi all,

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.

I.e., is the following code valid MPI code?

#include <mpi.h>
#include <stdio.h>
#include <stdlib.h>

int main() {
 int A[100], B[10], psize;

 for (int i = 0; i < 100; i++)
   A[i] = i;

 MPI_Pack_size(1, MPI_INT, MPI_COMM_SELF, &psize);
 void *parr = malloc(psize * 10);
 int pos = 0;
 for (int i = 0; i < 10; i++)
   MPI_Pack(&(A[i * 10]), 1, MPI_INT, parr, psize * 10, &pos,
 // Send the packed data and receive a contiguous buffer
 MPI_Sendrecv(parr, pos, MPI_PACKED, 0, 42, B, 10, MPI_INT, 0,
              42, MPI_COMM_SELF, MPI_STATUS_IGNORE);

 for (int i = 0; i < 10; i++)
   printf("%i\n", B[i]);

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?


Dipl.-Inf. Joachim Protze

IT Center
Group: High Performance Computing
Division: Computational Science and Engineering
RWTH Aachen University
Seffenter Weg 23
D 52074  Aachen (Germany)
Tel: +49 241 80- 24765
Fax: +49 241 80-624765
protze at itc.rwth-aachen.de<mailto:protze at itc.rwth-aachen.de>

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