<![CDATA[/*
* Laboratory for Scientific Computing
* http://www.lam-mpi.org/tutorials/
* University of Notre Dame
*
* MPI Tutorial
* The cannonical ring program
*
* Mail questions regarding tutorial material to mpi@lam-mpi.org
*/
#include <stdio.h>
#include "mpi.h"
int main(int argc, char *argv[])
{
MPI_Status status;
int num, rank, size, tag, next, from;
/* Start up MPI */
MPI_Init(&argc, &argv);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
MPI_Comm_size(MPI_COMM_WORLD, &size);
/* Arbitrarily choose 201 to be our tag. Calculate the */
/* rank of the next process in the ring. Use the modulus */
/* operator so that the last process "wraps around" to rank */
/* zero. */
tag = 201;
next = (rank + 1) % size;
from = (rank + size - 1) % size;
/* If we are the "console" process, get a integer from the */
/* user to specify how many times we want to go around the */
/* ring */
if (rank == 0) {
printf("Enter the number of times around the ring: ");
scanf("%d", &num);
printf("Process %d sending %d to %d\n", rank, num, next);
MPI_Send(&num, 1, MPI_INT, next, tag, MPI_COMM_WORLD);
}
/* Pass the message around the ring. The exit mechanism works */
/* as follows: the message (a positive integer) is passed */
/* around the ring. Each time is passes rank 0, it is decremented. */
/* When each processes receives the 0 message, it passes it on */
/* to the next process and then quits. By passing the 0 first, */
/* every process gets the 0 message and can quit normally. */
while (1) {
MPI_Recv(&num, 1, MPI_INT, from, tag, MPI_COMM_WORLD, &status);
printf("Process %d received %d\n", rank, num);
if (rank == 0) {
num--;
printf("Process 0 decremented num\n");
}
printf("Process %d sending %d to %d\n", rank, num, next);
MPI_Send(&num, 1, MPI_INT, next, tag, MPI_COMM_WORLD);
if (num == 0) {
printf("Process %d exiting\n", rank);
break;
}
}
/* The last process does one extra send to process 0, which needs */
/* to be received before the program can exit */
if (rank == 0)
MPI_Recv(&num, 1, MPI_INT, from, tag, MPI_COMM_WORLD, &status);
/* Quit */
MPI_Finalize();
return 0;
}
]]>