Learn Parallel Programming

This post talks about a MPI Broadcast function – MPI_Bcast. Broadcast operation is used to send data from one process to all other processes within a communicator. Every process receives the same data.

Syntax for MPI_Bcast

int MPI_Bcast(void *buffer, int count, MPI_Datatype datatype, int root, MPI_Comm comm)

Input Parameters

• count : number of elements/values to be Broadcasted (for example : 0)
• datatype : data type of elements/values to be sent (for example : 10)
• root : rank of root process (for example : 0)
• comm : communicator (for example : MPI_COMM_WORLD)

Input/Output Parameters

• buffer : Pointer to the data/array to be sent/received (for example : int *myarr)

Example code –

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

int main(int argc, char **argv)
{
	int myid, size;
	int data;
	
	//Initialize MPI environment 
	MPI_Init(&argc,&argv);
	
	//Get total number of processes
	MPI_Comm_size(MPI_COMM_WORLD, &size);
	
	//Get my unique identification among all processes
	MPI_Comm_rank(MPI_COMM_WORLD, &myid);
	
	//Initialize data to some value
	data = 20;
	
	//If root
	if(myid==0)
	{
		data = 10;			
	}
	//Bradcast the data
	MPI_Bcast(&data, 1, MPI_INT, 0, MPI_COMM_WORLD);
	
	//Every process prints the value of data
	printf("myid:%d \t data = %d\n",myid, data);
	
	//End MPI environment        
	MPI_Finalize();
}

To compile this code, use following command –

mpicc mpi_broadcast.c

To execute this code, run following command –

mpiexec -n 4 ./a.out

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

myid:0 	 data = 10
myid:1 	 data = 10
myid:2 	 data = 10
myid:3 	 data = 10

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_Bcast Syntax and other information