sendRecvC.c

/** -*- fill-column:110 -*-
   @file      sendRecvC.c
   @author    Mitch Richling <https://www.mitchr.me/>
   @Copyright Copyright 1999 by Mitch Richling.  All rights reserved.
   @brief     Demo MPI functions MPI_Send and MPI_Recv@EOL
   @Keywords  LAM MPI
   @Std       C99 MPI1

              This program demonstrates the following functions:

                    MPI_Send
                    MPI_Recv

              Note that this program makes the same mistake that many real world MPI programs do.  It makes
              the assumption that no process will die and that all will end up sending back the part of the
              computation they were responsible for. In this program, if that doesn't happen, then the rank 0
              process will wait forever for a communication that may never come.  See sendRecvErrC.c for a way
              out of this situation.

              This code requires that the rank zero process have stdout capabilities -- which most MPI
              implementations provide.
*/

#include <unistd.h>                                                      /* UNIX std stf            POSIX    */
#include <stdio.h>                                                       /* I/O lib                 C89      */
#include <mpi.h>                                                         /* MPI Std                 MPI      */

int main(int argc, char *argv[]) {
  int rank, size, n, rslt, i, flags;
  MPI_Status status;
  int needData, msgProbeCount=0;

  MPI_Init(&argc, &argv);

  MPI_Comm_rank(MPI_COMM_WORLD, &rank);
  MPI_Comm_size(MPI_COMM_WORLD, &size);

  if(rank == 0) {
    printf("Mr. Zero has come on line....\n");
    printf("Mr. Zero waiting for %d slaves to check in.\n", size);

    /* NOTE: If a slave process were to die before sending it's results back, this loop would hang forever.
       See sendRecvErrC.c for an example of how to recover in that situation.*/
    for(i=1;i<size;i++) {
      
      /* Receive a message. */
      rslt = MPI_Recv(&n, 1, MPI_INT, // Location of data, number of elements, type
                      MPI_ANY_SOURCE, // Who to receive data from
                      MPI_ANY_TAG,    // Tags to receive
                      MPI_COMM_WORLD, // Group from which to receive
                      &status);       // Status of call
      printf("Process %d has checked in (%d,%d,%d,%d)\n", 
             n, status.MPI_SOURCE, status.MPI_TAG, status.MPI_ERROR, rslt);
    } /* end for */
  } else {
    /* Sleep a bit to help demonstrate the probe and wait code in the rank 0 process. */
    sleep(rank/2+1);

    /* Send a message. MPI_Ssend(...) is the same thing, but it blocks until the receiver starts to read
       data. */
    MPI_Send(&rank, 1, MPI_INT, // The data, number of elements, and type
             0,                 // Who to send data too
             999,               // Tag to use (random number in this case)
             MPI_COMM_WORLD);   // The group we are sending to.
  } /* end if/else */

  MPI_Finalize();
  return 0;
} /* end func main */