Added Some Comments

Author: Thomas Weise

mpi/bareBones.c

@@ -1,7 +1,7 @@
-#include <mpi.h>
+#include <mpi.h> // import MPI header
 
 int main(int argc, char **argv) {
-  MPI_Init(&argc, &argv);
-  MPI_Finalize();
+  MPI_Init(&argc, &argv); // initialize MPI
+  MPI_Finalize();         // shut down MPI
   return 0;
-  }
+}

mpi/basicInfo.c

@@ -1,20 +1,23 @@
-#include <mpi.h>
-#include <stdio.h>
+#include <mpi.h>   // import MPI header
+#include <stdio.h> // import for printf
 
 int main(int argc, char **argv) {
   int size, rank;
 
-  MPI_Init(&argc, &argv);
+  MPI_Init(&argc, &argv); // initialize MPI
 
-  MPI_Comm_size(MPI_COMM_WORLD, &size);
-  MPI_Comm_rank(MPI_COMM_WORLD, &rank);
+  MPI_Comm_size(MPI_COMM_WORLD, &size); // get number of program instances
+  MPI_Comm_rank(MPI_COMM_WORLD, &rank); // get own ID/address
 
-  if(rank == 0) {
+  // often, an MPI application has a master and some slaves
+  // master distributes tasks and combine partial results to final results
+  // slaves receive partial task, compute partial result, and send to master
+  if(rank == 0) { // the instance with rank=0 is often chosen as master
     printf("Hi from Master\n");
-  } else {
+  } else {        // the others are often slaves 
     printf("Just Slave %d out of %d\n", rank, size);
   }
 
-  MPI_Finalize();
+  MPI_Finalize(); // finalize = shut down MPI
   return 0;
 }

mpi/simplePointToPoint1.c

@@ -1,28 +1,28 @@
-#include <mpi.h>
-#include <stdio.h>
+#include <mpi.h>   // import MPI header
+#include <stdio.h> // needed for printf
 
 int main(int argc, char **argv) {
   int size, rank, s_msg, r_msg, next, prev;
   MPI_Status status;
 
-  MPI_Init(&argc, &argv);
-  MPI_Comm_size(MPI_COMM_WORLD, &size);
-  MPI_Comm_rank(MPI_COMM_WORLD, &rank);
+  MPI_Init(&argc, &argv); // initialize MPI
+  MPI_Comm_size(MPI_COMM_WORLD, &size); // get number of program instances
+  MPI_Comm_rank(MPI_COMM_WORLD, &rank); // get own ID/address
 
-  next  = ((rank + 1) % size);
-  prev  = ((rank + size - 1) % size);
-  s_msg = ((size * rank) + next);
+  next  = ((rank + 1) % size);  // next higher id, wrap from size-1 to 0
+  prev  = ((rank + size - 1) % size); // next lower id, wrap from 0 to size-1
+  s_msg = ((size * rank) + next); // the example message, just some number
 
-  if((rank % 2) == 0) {
+  if((rank % 2) == 0) { // even rank: message to next, receive from prev
     MPI_Send(&s_msg, 1, MPI_INT, next, 42, MPI_COMM_WORLD);
     MPI_Recv(&r_msg, 1, MPI_INT, prev, 42, MPI_COMM_WORLD, &status);
-  } else {
+  } else { // otherwise: receive from rev, send to next
     MPI_Recv(&r_msg, 1, MPI_INT, prev, 42, MPI_COMM_WORLD, &status);
     MPI_Send(&s_msg, 1, MPI_INT, next, 42, MPI_COMM_WORLD);
   }
 
   printf("id: %d, next: %d, prev: %d, send: %d, recv: %d\n", rank, next, prev, s_msg, r_msg);
 
-  MPI_Finalize();
+  MPI_Finalize(); // shut down MPI
   return 0;
 }

mpi/simplePointToPoint2.c

@@ -1,6 +1,6 @@
-#include <mpi.h>
-#include <stdio.h>
-#include <string.h>
+#include <mpi.h>    // import MPI header
+#include <stdio.h>  // needed for printf
+#include <string.h> // needed for strlen
 
 int main(int argc, char *argv[]) {
   char message[20];