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code/distributed/OpenMP_and_MPI/main.cpp

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+/**
+ * Author      : Dr Carey Pridgeon
+ * Copyright   : Dr Carey Pridgeon 2021
+ * Licence     : Licensed under the Apache License, Version 2.0 (the "License");
+ *             : you may not use this file except in compliance with the License.
+ *             : You may obtain a copy of the License at
+ *             : http://www.apache.org/licenses/LICENSE-2.0
+ *             :
+ *             : Unless required by applicable law or agreed to in writing,
+ *             : software distributed under the License is distributed on
+ *             : an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND,
+ *             : either express or implied. See the License for the specific
+ *             : language governing permissions and limitations under the License.
+ */
+
+#include <iostream>
+#include <string.h>
+#include "mpi.h"
+#include "omp.h"
+
+
+int main(int argc, char **argv)
+{
+
+    MPI_Init(NULL, NULL);
+
+    // Find out rank, size
+    int world_rank, world_size, namelen;
+    MPI_Comm_rank(MPI_COMM_WORLD, &world_rank);
+    MPI_Comm_size(MPI_COMM_WORLD, &world_size);
+
+    char node_name[MPI_MAX_PROCESSOR_NAME];
+
+    MPI_Get_processor_name(node_name, &namelen);
+    memset(node_name + namelen, 0, MPI_MAX_PROCESSOR_NAME - namelen);
+    const float floaty = 42;
+    int source = 0;
+    int destination = 5;
+
+    if (world_rank == source){
+      std::cout << "> hello from "<< node_name<< std::endl; //standard debugging code, remove later
+      MPI_Send(&floaty,1, MPI_FLOAT, destination, 0, MPI_COMM_WORLD); 
+    }
+
+    if (world_rank == destination){
+    float local_floaty = 0.0; // just so we can be certain it's local to this node, and it's initialised with z, so we'll know if nothing's happening.
+    float second_var;
+    float third_var;
+
+     // this of course means you can't use z.
+     std::cout << "> hello from "<< node_name<< std::endl; //standard debugging code, remove later
+     MPI_Recv(&local_floaty, 1, MPI_FLOAT, source, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
+     std::cout << "> Float: " << local_floaty << " Received by "<< node_name<< std::endl;
+
+     /// OpenMP Code Begins that does almost nothing really
+     int i;
+     second_var = local_floaty;
+     #pragma omp parallel for
+     for (i=0;i<10;i++) {
+       local_floaty = local_floaty*10;
+     }
+     /// OpenMP Code Ends
+
+     // This produces the output (in various notations)    
+     // 4.2 × 109 (scientific notation)
+
+     // 4.2e9 (scientific e notation)
+
+     // 4.2 × 109 (engineering notation)
+
+     // 4200000000 (real number)
+
+
+     std::cout << "> Transform finished, sending the result back to origin node: " << source <<  std::endl;
+
+     // now it's been changed we need to send it back to the source node, or rather, put it back into the communicator addressed to the head node ready to be collected
+       MPI_Send(&local_floaty,1, MPI_FLOAT, source, 0, MPI_COMM_WORLD);
+    }
+
+    // and now we repet the receive code, but this time for the source node
+
+    if (world_rank == source){
+
+    float local_floaty = 0.0; // same variable name, because this is a different scope, and will also be on a different machine
+     std::cout << "> hello from "<< node_name<< std::endl; //standard debugging code, remove later
+     MPI_Recv(&local_floaty, 1, MPI_FLOAT, destination, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
+     std::cout << "> Float: " << local_floaty << " Received back from the the node tasked with transform "<< std::endl;
+    }
+
+
+    MPI_Finalize();
+
+    return 0;
+}