... and a bit more docu and comments

Author: Thomas Weise

mpi/structScatter.c

@@ -2,54 +2,56 @@
 #include <stdlib.h>
 #include <mpi.h>
 
-typedef struct { int myIntA;
- int myIntB;
- short myShort;
- double myDouble;
- char myChar;
- float myFloat; } myStruct;
+typedef struct { int myIntA; // an integer (usually 4 bytes)
+ int myIntB; // another integer (usually 4 bytes)
+ short myShort; // a short integer (usually 2 bytes)
+ double myDouble; // a double-precision floating point number (usually 8 bytes)
+ char myChar; // a char = 1 single byte (usually 1 byte)
+ float myFloat; // a single-precision floating point number (usually 4 bytes)
+ } myStruct; // name of new data type = myStruct
+
 
 int main(int argc, char *argv[]) {
  int size, rank, cur, i, total, steps;
  myStruct data, *send;
  MPI_Datatype subtypes[6]; //the data types of the elements in myStruct
  int subblocks[6]; //how often they occur in a row
  MPI_Aint suboffsets[6]; //their address displacements
- MPI_Datatype myStructType;
+ MPI_Datatype myStructType; // the new data type
 
- //initializing communication
- MPI_Init(&argc, &argv);
- MPI_Comm_rank(MPI_COMM_WORLD, &rank);
+ MPI_Init(&argc, &argv); // initialize MPI
+ MPI_Comm_rank(MPI_COMM_WORLD, &rank); // get own rank/ID
 
  // create a new datatype description for the MPI job structure
- i = 0;
- subtypes[i] = MPI_INT; //myIntA + myIntB
+ i = 0; // index of first record is 0
+ subtypes[i] = MPI_INT; //myIntA and myIntB
  subblocks[i] = 2; //because there are 2 ints
- suboffsets[i++] = (((MPI_Aint)(&data.myIntA)) - ((MPI_Aint)(&data)));
+ suboffsets[i] = (((MPI_Aint)(&data.myIntA)) - ((MPI_Aint)(&data))); //offset from start
 
- subtypes[i]  = MPI_SHORT; //myShort
- subblocks[i] = 1;
- suboffsets[i++] = (((MPI_Aint)(&data.myShort)) - ((MPI_Aint)(&data)));
+ subtypes[++i] = MPI_SHORT; //second record (++i): myShort
+ subblocks[i] = 1; // there is one short
+ suboffsets[i] = (((MPI_Aint)(&data.myShort)) - ((MPI_Aint)(&data))); //offset from start
 
- subtypes[i]  = MPI_DOUBLE; //myDouble
- subblocks[i] = 1;
- suboffsets[i++] = (((MPI_Aint)(&data.myDouble)) - ((MPI_Aint)(&data)));
+ subtypes[++i] = MPI_DOUBLE; //third record (++i): myDouble
+ subblocks[i] = 1; // there is one double
+ suboffsets[i] = (((MPI_Aint)(&data.myDouble)) - ((MPI_Aint)(&data))); //offset from start
 
- subtypes[i]  = MPI_CHAR; //myChar
- subblocks[i] = 1;
- suboffsets[i++] = (((MPI_Aint)(&data.myChar)) - ((MPI_Aint)(&data)));
+ subtypes[++i] = MPI_CHAR; //fourth record (++i): myChar
+ subblocks[i] = 1; // one char
+ suboffsets[i] = (((MPI_Aint)(&data.myChar)) - ((MPI_Aint)(&data))); //offset from start
 
- subtypes[i]  = MPI_FLOAT; //myFloat
- subblocks[i] = 1;
- suboffsets[i++] = (((MPI_Aint)(&data.myFloat)) - ((MPI_Aint)(&data)));
+ subtypes[++i] = MPI_FLOAT; //fifth record (++i): myFloat
+ subblocks[i] = 1; // there is one float
+ suboffsets[i] = (((MPI_Aint)(&data.myFloat)) - ((MPI_Aint)(&data))); //offset from start
 
+ // register data type structure
  MPI_Type_create_struct(i, subblocks, suboffsets, subtypes, &myStructType);
- MPI_Type_commit(&myStructType);
+ MPI_Type_commit(&myStructType); // and commit it: it can now be used
 
- if(rank == 0) {
- MPI_Comm_size(MPI_COMM_WORLD, &size);
+ if(rank == 0) { // if we are root
+ MPI_Comm_size(MPI_COMM_WORLD, &size); // get number of processes
 
- send = (myStruct*)malloc(sizeof(myStruct) * size);
+ send = (myStruct*)malloc(sizeof(myStruct) * size); // allocate memory
  for(i = size; (--i) >= 0; ) {
  send[i].myIntA = rank;
  send[i].myIntB = i;

mpi/structTest.c

@@ -1,4 +1,4 @@
-#include <stdio.h>
+#include <stdio.h> // for printf
 
 typedef struct { int myIntA; // an integer (usually 4 bytes)
  int myIntB; // another integer (usually 4 bytes)