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Point-to-point communications, the MPI_Status structure

You might remember that, in the previous exercise, we overlooked the MPI_Status value when receiving. The MPI_Status is a struct that you can access if necessary to have more information on the message you just received. For instance, when receiving a message with MPI_ANY_SOURCE or MPI_ANY_TAG you disregard the origin and tag of the sender. However, you might still want to get this information to post-process the data you just received. The MPI_Status struct is defined as followed in OpenMPI :

struct MPI_Struct {
  int MPI_TAG;
  int MPI_ERROR;
  int _cancelled;
  size_t _ucount;

MPI_SOURCE indicates where the message comes from, MPI_TAG the tag of the message, and MPI_ERROR if an error has occured during the reception of the message. The two last attributes are not supposed to be used, or, as the OpenMPI developers put it in the source code :

/* The following two fields are internal to the Open MPI
       implementation and should not be accessed by MPI applications.
       They are subject to change at any time.  These are not the
       droids you're looking for. */

Here is an example of reception of 5 doubles from any process, with any tag, but using the MPI_Status structure to retrieve the information :

double values[5];
MPI_Status status;
MPI_Recv(&values, 5, MPI_DOUBLE, MPI_ANY_SOURCE, MPI_ANY_TAG, &status);
std::cout << "Received from process " << status.MPI_SOURCE
          << "; with tag " << status.MPI_TAG << std::endl;

We will see later in this chapter how to use the MPI_STATUS information to get access to more information. For instance, the status can be used to probe how many elements are/were being sent in a communication before even receiving it.

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