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This post talks about MPI_Comm_size function in MPI. MPI_Comm_size is a function in the Message Passing Interface (MPI) used to determine the total number of processes in a communicator.

Syntax for MPI_Comm_size

int MPI_Comm_size( MPI_Comm comm, int *size )

Input Parameters

• comm : communicator (for example : MPI_COMM_WORLD)

Output Parameter

• size : number of processes in the group of comm (integer)

Example code –

#include"stdio.h"
#include"mpi.h"

int main(int argc, char **argv)
{
	int myid, size;
	
	//Initialize MPI environment 
	MPI_Init(&argc,&argv);
	
	//Get total number of processes
	MPI_Comm_size(MPI_COMM_WORLD, &size);
	
	//Get my unique ID among all processes
	MPI_Comm_rank(MPI_COMM_WORLD, &myid);
	
	printf("Hello World from %d out of %d.\n",myid, size);
	
	//End MPI environment        
	MPI_Finalize();
}

To compile this code, use following command –

mpicc hello_world.c

To execute this code, run following command –

mpiexec -n 4 ./a.out

Output of this code will be something similar to the following –


Hello World from 0 out of 4.
Hello World from 1 out of 4.
Hello World from 2 out of 4.
Hello World from 3 out of 4.

To know more about MPI, visit our dedicated page for MPI here.

If you are new to Parallel Programming / HPC, visit our dedicated page for beginners.

References :

• MPI Standard Website
• MPI_Comm_size and other information