source: CIVL/examples/mpi/c_ex02.c

/* Shows how to use probe & get_count to find the size of an */
/* incomming message                                         */
#include <stdio.h>
#include <stdlib.h>
#include <mpi.h>
#include <math.h>
int main(int argc,char **argv)
{
    int myid, numprocs;
    MPI_Status status;
    int mytag,ierr,icount,j,*i;
 
    MPI_Init(&argc,&argv);
    MPI_Comm_size(MPI_COMM_WORLD,&numprocs);
    MPI_Comm_rank(MPI_COMM_WORLD,&myid);
    printf(" Hello from c process: %d  Numprocs is %d\n",myid,numprocs);
    
    mytag=123;
    if(myid == 0) {
        j=200;
        icount=1;
        ierr=MPI_Send(&j,icount,MPI_INT,1,mytag,MPI_COMM_WORLD);
    }
    if(myid == 1){
        ierr=MPI_Probe(0,mytag,MPI_COMM_WORLD,&status);
        ierr=MPI_Get_count(&status,MPI_INT,&icount);
        i=(int*)malloc(icount*sizeof(int));
        printf("getting %d\n",icount);
            ierr = MPI_Recv(i,icount,MPI_INT,0,mytag,MPI_COMM_WORLD,&status);
        printf("i= ");
        for(j=0;j<icount;j++)
                printf("%d ",i[j]);
        printf("\n");
    }
    MPI_Finalize();
}