Question

I’m studying a bit of MPI, and decided to do a test by making a program that calls objects, eg main.c -> main program, function.c -> any function

function.c that will only use the MPI. compiling I as follows:

gcc-c main.c

to create main.o, mpicc-c to create function.c function.o, of course I create the file function.h too.

I compile with mpicc-o program main.o function.o

Here is main.c

#include <stdio.h>
#include "function.h"

void main(int argc, char *argv[])
{
  printf("Hello\n");
  function();
  printf("Bye\n");
}

just function has the MPI code, but when I’m running the program mpiexe -np 2 I get

Hello
Hello
----- function job here -----
Bye
Bye

But I wanted it to be

Hello
------ function job -----
Bye

What can I do?

Answer 1

Your whole program is run on both of the two processors you set with the -np 2. A common way to prevent duplicates of printouts, final results, etc., is have one thread do those things just by checking the thread id first. Like:

int id;
MPI_Comm_rank(MPI_COMM_WORLD, &id);
if (id == 0) {
  printf("only process %d does this one\n", id);
}

printf("hello from process %d\n", id);  // all processes do this one

When starting out in MPI I found it helpful to print out those id numbers along with whatever partial results or data each thread was dealing with. Helped me make more sense out of what was happening.