how to fix this MPI code program - debugging

Question

This program demonstrates an unsafe program, because sometimes it will execute fine, and other times it will fail. The reason why the program fails or hangs is due to buffer exhaustion on the receiving task side, as a consequence of the way an MPI library has implemented an eager protocol for messages of a certain size. One possible solution is to include an MPI_Barrier call in the both the send and receive loops.
how its program code is correct???
#include "mpi.h"
#include <stdio.h>
#include <stdlib.h>
#define MSGSIZE 2000
int main (int argc, char *argv[])
{
int numtasks, rank, i, tag=111, dest=1, source=0, count=0;
char data[MSGSIZE];
double start, end, result;
MPI_Status status;
MPI_Init(&argc,&argv);
MPI_Comm_size(MPI_COMM_WORLD, &numtasks);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
if (rank == 0) {
printf ("mpi_bug5 has started...\n");
if (numtasks > 2)
printf("INFO: Number of tasks= %d. Only using 2 tasks.\n", numtasks);
}
/******************************* Send task **********************************/
if (rank == 0) {
/* Initialize send data */
for(i=0; i<MSGSIZE; i++)
data[i] = 'x';
start = MPI_Wtime();
while (1) {
MPI_Send(data, MSGSIZE, MPI_BYTE, dest, tag, MPI_COMM_WORLD);
count++;
if (count % 10 == 0) {
end = MPI_Wtime();
printf("Count= %d Time= %f sec.\n", count, end-start);
start = MPI_Wtime();
}
}
}
/****************************** Receive task ********************************/
if (rank == 1) {
while (1) {
MPI_Recv(data, MSGSIZE, MPI_BYTE, source, tag, MPI_COMM_WORLD, &status);
/* Do some work - at least more than the send task */
result = 0.0;
for (i=0; i < 1000000; i++)
result = result + (double)random();
}
}
MPI_Finalize();
}

Answer 1

Ways to improve this code so that the receiver doesn't end up with an unlimited number of unexpected messages include:
Synchronization - you mentioned MPI_Barrier, but even using MPI_Ssend instead of MPI_Send would work.
Explicit buffering - the use of MPI_Bsend or Brecv to ensure adequate buffering exists.
Posted receives - the receiving process posts IRecvs before starting work to ensure that the messages are received into the buffers meant to hold the data, rather than system buffers.
In this pedagogical case, since the number of messages is unlimited, only the first (synchronization) would reliably work.