I got the above mentioned error on running the following program in C. It uses the MPI library.

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

int main (int argc, char *argv)
{
int numranks, rank, dest, tag, source, rc, count;
char inmsg, outmsg='x';
MPI_Status Stat;

MPI_Init(&argc,&argv);
MPI_Comm_size(MPI_COMM_WORLD, &numranks);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
printf("Task %d starting...n",rank);

if (rank == 0) {
if (numranks > 2)
printf("Numranks=%d. Only 2 needed. Ignoring extra...n",numranks);
dest = rank + 1;
source = dest;
tag = rank;
rc = MPI_Send(&outmsg, 1, MPI_CHAR, dest, tag, MPI_COMM_WORLD);
printf("Sent to task %d...n",dest);
rc = MPI_Recv(&inmsg, 1, MPI_CHAR, source, tag, MPI_COMM_WORLD, &Stat);
printf("Received from task %d...n",source);
}
else if (rank == 1) {
dest = rank - 1;
source = dest;
tag = rank;
rc = MPI_Recv(&inmsg, 1, MPI_CHAR, source, tag, MPI_COMM_WORLD, &Stat);
printf("Received from task %d...n",source);
rc = MPI_Send(&outmsg, 1, MPI_CHAR, dest, tag, MPI_COMM_WORLD);
printf("Sent to task %d...n",dest);
}

if (rank < 2) {
rc = MPI_Get_count(&Stat, MPI_CHAR, &count);
printf("Task %d: Received %d char(s) from rank %d with tag %d n",rank, count, Stat.MPI_SOURCE, Stat.MPI_TAG);
}

MPI_Finalize();
}


Used 4 processes by calling

mpirun -n 8 ./a.out


Runtime error message

Task 0 starting...
Numranks=4. Only 2 needed. Ignoring extra...
Task 2 starting...
Task 3 starting...
Sent to task 1...
Task 1 starting...


After this it just hangs. This leads me to believe that there is an issue with MPI_Recv because it says sent to task 1... but does not receive it.

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