How to Send/Receive in MPI using all processors

Question

This program written using C Lagrange and MPI. I am new to MPI and want to use all processors to do some calculations, including process 0. To learn this concept, I have written

Answer 1

I don't really get what would be wrong with using 2 if statements, surrounding the working domain. But anyway, here is an example of what could be done.

I modified your code to use collective communications as they make much more sense than the series of send/receive you used. Since the initial communications are with a uniform value, I use a MPI_Bcast() which does the same in one single call.
Conversely, since the result values are all different, a call to MPI_Gather() is perfectly appropriate.
I also introduce a call to sleep() just to simulate that the processes are working for a while, prior to sending back their results.

The code now looks like this:

#include 
#include    // for malloc and free
#include     // for printf
#include    // for sleep

int main( int argc, char *argv[] ) {

    MPI_Init( &argc, &argv );
    int world_rank;
    MPI_Comm_rank( MPI_COMM_WORLD, &world_rank );
    int world_size;
    MPI_Comm_size( MPI_COMM_WORLD, &world_size );

    // sending the same number to all processes via broadcast from process 0
    int number = world_rank == 0 ? -2 : 0;
    MPI_Bcast( &number, 1, MPI_INT, 0, MPI_COMM_WORLD );
    printf( "Process %d received %d from process 0\n", world_rank, number );

    // Do something usefull here
    sleep( 1 );
    int my_result = world_rank + 1;

    // Now collecting individual results on process 0
    int *results = world_rank == 0 ? malloc( world_size * sizeof( int ) ) : NULL;
    MPI_Gather( &my_result, 1, MPI_INT, results, 1, MPI_INT, 0, MPI_COMM_WORLD );

    // Process 0 prints what it collected
    if ( world_rank == 0 ) {
        for ( int i = 0; i < world_size; i++ ) {
            printf( "Process 0 received result %d from process %d\n", results[i], i );
        }
        free( results );
    }

    MPI_Finalize();

    return 0;
}

After compiling it as follows:

$ mpicc -std=c99 simple_mpi.c -o simple_mpi

It runs and gives this:

$ mpiexec -n 4 ./simple_mpi
Process 0 received -2 from process 0
Process 1 received -2 from process 0
Process 3 received -2 from process 0
Process 2 received -2 from process 0
Process 0 received result 1 from process 0
Process 0 received result 2 from process 1
Process 0 received result 3 from process 2
Process 0 received result 4 from process 3