Ring Example

ring.c

+#include <mpi.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+int main(int argc, char** argv) {
+  // Initialize the MPI environment
+  MPI_Init(NULL, NULL);
+  // Find out rank, size
+  int world_rank;
+  MPI_Comm_rank(MPI_COMM_WORLD, &world_rank);
+  int world_size;
+  MPI_Comm_size(MPI_COMM_WORLD, &world_size);
+
+  int token;
+  // Receive from the lower process and send to the higher process. Take care
+  // of the special case when you are the first process to prevent deadlock.
+  if (world_rank != 0) {
+    MPI_Recv(&token, 1, MPI_INT, world_rank - 1, 0, MPI_COMM_WORLD,
+             MPI_STATUS_IGNORE);
+    printf("Process %d received token %d from process %d\n", world_rank, token,
+           world_rank - 1);
+  } else {
+    // Set the token's value if you are process 0
+    token = -1;
+  }
+  MPI_Send(&token, 1, MPI_INT, (world_rank + 1) % world_size, 0,
+           MPI_COMM_WORLD);
+  // Now process 0 can receive from the last process. This makes sure that at
+  // least one MPI_Send is initialized before all MPI_Recvs (again, to prevent
+  // deadlock)
+  if (world_rank == 0) {
+    MPI_Recv(&token, 1, MPI_INT, world_size - 1, 0, MPI_COMM_WORLD,
+             MPI_STATUS_IGNORE);
+    printf("Process %d received token %d from process %d\n", world_rank, token,
+           world_size - 1);
+  }
+  MPI_Finalize();
+}
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