Why can't I open get 8 processes at a time? - parallel-processing

Question

I'm a beginner to MPI. When I coded my first program, I came into a tough problem to me.
MPI_Init(&argc, &argv) ;
MPI_Comm_rank( MPI_COMM_WORLD, &rank) ;
MPI_Comm_size( MPI_COMM_WORLD, &size) ;
printf("Process: %d\n", rank);
printf("Procs_num: %d\n", size);
and:
mpicc main.c -o main
mpirun -np 8 main
But all I get is 8 duplicates of:
Process: 0
Procs_num: 1
What I expect to get is:
Process: 0~7
Procs_num: 8
I guess it is because there is no 8 processes in MPI_COMM_WORLD, but I cannot figure out the reason and have no idea about how to solve it.
And I hope you will catch what I want to express. Many thx!!

Answer 1

Apparently you forgot to call MPI_Finalize
Here is the correct code:
#include <stdio.h>
#include <stdlib.h>
#include <mpi.h>
int main (int argc, char *argv[]) {
int rank, size;
MPI_Init(&argc, &argv) ;
MPI_Comm_rank( MPI_COMM_WORLD, &rank) ;
MPI_Comm_size( MPI_COMM_WORLD, &size) ;
printf("Process: %d\n", rank);
printf("Procs_num: %d\n", size);
MPI_Finalize();
return(0);
}
Then you get your 8 fifferents ranks:
~/mpi_tests/examples > mpirun -np 8 ~/mpi_tests/examples/t
Process: 4
Procs_num: 8
Process: 5
Procs_num: 8
Process: 0
Procs_num: 8
Process: 1
Procs_num: 8
Process: 7
Procs_num: 8
Process: 2
Procs_num: 8
Process: 3
Procs_num: 8
Process: 6
Procs_num: 8