Add demo for nonblocking send and recv

mpi/demos/send_and_recv_nonblocking.c

@@ -0,0 +1,34 @@
+#include <stdio.h>
+#include <mpi.h>
+
+int main(int argc, char *argv[])
+{
+  int size, rank;
+  MPI_Init(&argc, &argv);
+  MPI_Comm_size(MPI_COMM_WORLD, &size);
+  MPI_Comm_rank(MPI_COMM_WORLD, &rank);
+
+  printf("Hello from rank %d of %d\n", rank, size);
+
+  if (rank == 0) {
+    // Send with rank 0
+    double data = 42.0;
+    MPI_Send(&data, 1, MPI_DOUBLE, 1, 0, MPI_COMM_WORLD);
+    printf("Rank %d sent %f\n", rank, data);
+
+  } else if (rank == 1) {
+    // Receive with rank 1
+    double data = 0.0;
+    MPI_Request request;
+    MPI_Irecv(&data, 1, MPI_DOUBLE, 0, 0, MPI_COMM_WORLD, &request);
+
+    printf("Rank %d data before wait %f\n", rank, data);
+
+    MPI_Status status;
+    MPI_Wait(&request, &status);
+    printf("Rank %d received %f\n", rank, data);
+
+  }
+
+  MPI_Finalize();
+}