MPI/C – A Simple Send & Receive

/* send_receive_simple.c
 * PARALLEL [MPI] C PROGRAM TO DEMONSTRATE MPI_Send & MPI_Recv FUNCTIONS.
 * MASTER [proc_id = 0] SENDS SOME DATA TO A WORKER [proc_id = 1].  
 * WORKER DISPLAYS THE DATA RECEIVED. 
 *
 * TESTED SUCCESSFULLY WITH MPICH2 (1.3.1) COMPILED AGAINST GCC (4.1.2) 
 * IN A LINUX BOX WITH QUAD CORE INTEL XEON PROCESSOR (1.86 GHz) & 4GB OF RAM.
 *
 * FIRST WRITTEN: GOWTHAM; Sat, 27 Nov 2010 16:01:01 -0500
 * LAST MODIFIED: GOWTHAM; Sat, 27 Nov 2010 16:49:11 -0500
 *
 * URL:
 * http://sgowtham.net/blog/2010/11/28/mpi-c-a-simple-send-receive/
 *
 * COMPILATION:
 * mpicc -g -Wall -lm send_receive_simple.c -o send_receive_simple.x
 *
 * EXECUTION:
 * mpirun -machinefile MACHINEFILE -np NPROC ./send_receive_simple.x
 *
 * NPROC       : NUMBER OF PROCESSORS ALLOCATED TO RUNNING THIS PROGRAM;
 *               MUST BE EQUAL TO 2
 * MACHINEFILE : FILE LISTING THE HOSTNAMES OF PROCESSORS ALLOCATED TO
 *               RUNNING THIS PROGRAM
 *
*/
 
/* STANDARD HEADERS AND DEFINITIONS 
 * REFERENCE: http://en.wikipedia.org/wiki/C_standard_library
*/
#include <stdio.h>  /* Core input/output operations                         */
#include <stdlib.h> /* Conversions, random numbers, memory allocation, etc. */
#include <math.h>   /* Common mathematical functions                        */
#include <time.h>   /* Converting between various date/time formats         */
#include <mpi.h>    /* MPI functionality                                    */
 
#define MASTER  0   /* Process ID for MASTER                                */
#define WORKER  1   /* Process ID for WORKER                                */
 
/* MAIN PROGRAM BEGINS */
int main(int argc, char **argv) {
 
  /* VARIABLE DECLARATION */
  int    proc_id,       /* Process identifier                    */
         n_procs;       /* Number of processors                  */
 
  double x;             /* Information sent from MASTER
                           Information received by WORKER        */ 
 
  MPI_Status status;    /* MPI structure containing return codes
                           for message passing operations        */
 
  /* INITIALIZE MPI */
  MPI_Init(&argc, &argv);
 
  /* GET THE PROCESS ID AND NUMBER OF PROCESSORS */
  MPI_Comm_rank(MPI_COMM_WORLD, &proc_id);
  MPI_Comm_size(MPI_COMM_WORLD, &n_procs);
 
  /* IF MASTER, THEN DO THE FOLLOWING:
   * INITIALIZE x
   * SEND IT TO THE WORKER 
  */
  if (proc_id == MASTER) {
 
    /* INITIALIZE x */
    x = 100.001;
 
    /* SEND x TO WORKER */
    printf("\n  Sending x to WORKER [proc_id = 1] with TAG = 0\n");
 
    /* MPI_Send syntax:
     * MPI_Send(buf, count, datatype, dest, tag, comm)
     * [IN buf]      initial address of send buffer (choice)
     * [IN count]    number of elements in send buffer (nonnegative integer)
     * [IN datatype] datatype of each send buffer element (handle)
     * [IN dest]     rank of destination (integer)
     * [IN tag]      message tag (integer)
     *               must be unique for a pair of Send/Recv statements
     * [IN comm]     communicator (handle) 
    */
    MPI_Send(&x, 1, MPI_DOUBLE, 1, 0, MPI_COMM_WORLD);
 
  } /* MASTER LOOP ENDS */
 
 
  /* IF WORKER, THEN DO THE FOLLOWING:
   * RECEIVE x FROM MASTER
   * DISPLAY IT 
  */
  if (proc_id == WORKER) {
 
    /* RECEIVE x FROM MASTER */
    printf("\n  Receiving x from MASTER [proc_id = 0] with TAG = 0\n");
 
    /* MPI_Recv syntax:
     * MPI_Recv(buf, count, datatype, source, tag, comm, status)
     * [OUT buf]     initial address of receive buffer (choice)
     * [IN count]    number of elements in receive buffer (integer)
     * [IN datatype] datatype of each receive buffer element (handle)
     * [IN source]   rank of source (integer)
     * [IN tag]      message tag (integer)
     *               must be same as the tag in Send statement
     * [IN comm]     communicator (handle)
     * [OUT status]  status object (Status) 
    */
    MPI_Recv(&x, 1, MPI_DOUBLE, 0, 0, MPI_COMM_WORLD, &status);
 
    /* DISPLAY x */
    printf("\n    x = %lf\n", x);
 
  } /* WORKER LOOP ENDS */
 
  /* FINALIZE MPI */
  MPI_Finalize();
 
  /* INDICATE THE TERMINATION OF THE PROGRAM */
  return 0;
 
} /* MAIN PROGRAM ENDS */