Aim: Design a distributed application using Message Passing Interface (MPI) for remote computation where client submits a string to the server and server returns the reverse of it to the client.

Steps:

sudo apt-get update
sudo apt-get install libopenmpi-dev
sudo apt-get install openmpi-bin
mkdir exp3
cd exp3

gedit server.c

#include <stdlib.h> 
#include <stdio.h> 
#include "mpi.h" 
#include<string.h> 
int main(int argc, char **argv) 
{ 
 MPI_Comm client; 
 MPI_Status status; //return status for send or receive
 char port_name[MPI_MAX_PORT_NAME],str[50],ch,temp; 
 int size, again, i,j; 
 MPI_Init(&argc, &argv); //Let the system do what it needs to start up MPI
 MPI_Comm_size(MPI_COMM_WORLD, &size); //find size of msg or no of processes
 if (size != 1) 
 { 
  fprintf(stderr, "Server too big"); 
  exit(EXIT_FAILURE); 
 } 
 MPI_Open_port(MPI_INFO_NULL, port_name); 
 printf("Server available at port: %s\n", port_name); 
 i=0; 
 while (1) 
 { 
  MPI_Comm_accept(port_name, MPI_INFO_NULL, 0, MPI_COMM_WORLD, &client); 
  again = 1; 
  while (again) 
  { 
   MPI_Recv(&ch, 1, MPI_CHAR, MPI_ANY_SOURCE, MPI_ANY_TAG, client, &status);
   switch (status.MPI_TAG) 
   { 
    case 0: 
     MPI_Comm_free(&client); 
     MPI_Close_port(port_name); 
     MPI_Finalize(); 
     return 0; 
    case 1: 
     printf("\nReceived String: %s\n",str); 
     // reverse the string 
     i = 0; 
     j = strlen(str) - 1; 
     while (i < j) 
     { 
      temp = str[i];        str[i] = str[j]; 
      str[j] = temp; 
      i++; 
      j--; 
     } 
     printf("\nReversed string is : %s\n",str); 
     // send the reversed string to client (character by character) 
     for (i = 0; i < strlen(str); i++)
      { 
      ch=str[i]; 
      MPI_Send(&ch, 1, MPI_CHAR, 0, 2, client); 
     } 
     //send tag=1 to indicate end of string 
     MPI_Send(&ch, 1, MPI_CHAR, 0, 1, client); 
     MPI_Comm_disconnect(&client); 
     again = 0; 
     strcpy(str,""); 
     i=0; 
     break; 
    case 2: 
     printf("Received character: %c\n", ch); 
     str[i]=ch; 
     i++;// add null character at the end of string 
     str[i]='\0'; 
     break; 
    default: 
     /* Unexpected message type */ 
     MPI_Abort(MPI_COMM_WORLD, 1); 
   } 
  } 
 } 
}
# save and exit the file
gedit client.c

#include <stdlib.h> 
#include <stdio.h> 
#include <string.h> 
#include "mpi.h" 
int main( int argc, char **argv ) 
{ 
 MPI_Comm server; 
 MPI_Status status; //return status for send or receive
 char port_name[MPI_MAX_PORT_NAME],str[50],ch; 
 int i, tag,again; 
 if (argc < 2) 
 { 
  fprintf(stderr, "server port name required.\n"); 
  exit(EXIT_FAILURE); 
 } 
 MPI_Init(&argc, &argv); //Let the system do what it needs to start up MPI
 strcpy(port_name, argv[1]); 
 MPI_Comm_connect(port_name, MPI_INFO_NULL, 0, MPI_COMM_WORLD, &server); 

 // accept input string 
 printf("\nEnter the string :\n"); 
 scanf("%s",str); 

 //send string to server (character by character) 
 for (i = 0; i < strlen(str); i++) 
 { 
  if(str[i]!='\0') 
  ch=str[i]; 
  tag=2; //tag- used to classify messages

  MPI_Send(&ch, 1, MPI_CHAR, 0, tag, server); 
 }

 // done sending string to the server 
 MPI_Send(&i, 0, MPI_INT, 0, 1, server); 

 // Receive the reversed string from server and display it 
 i=0; 
 again=1; 
 while (again) 
 { 
  MPI_Recv(&ch, 1, MPI_CHAR, MPI_ANY_SOURCE, MPI_ANY_TAG, server, &status); 
  switch (status.MPI_TAG) 
  { 
   case 2: 
    str[i]=ch; 
    i++; 
    break; 
   case 1: again=0; 
    break; 
  } 
 } 
  printf("\nReversed string is : %s\n\n",str); 
  MPI_Comm_disconnect(&server); 
  MPI_Finalize(); //Once MPI_FINALIZE has been called, no other MPI routines may be called. 

  return 0; 
}
# save and exit the file

# compile
mpicc server.c -o server
mpicc client.c -o client

# run server
mpirun -np 1 ./server

# it will display output similar to below (not necessarily the same)
Server available at port: 4290510848.0;tcp://192.168.1.101:35820;tcp://192.168.122.1:35820+4290510849.0;tcp://192.168.1.101:40208;tcp://192.168.122.1:40208:300

# copy the port-string from the terminal output (e.g. the highlighted portion above)

# we are going to supply this port-string as a first command line argument to the client

# open another terminal
mpirun -np 1 ./client '4290510848.0;tcp://192.168.1.101:35820;tcp://192.168.122.1:35820+4290510849.0;tcp://192.168. 1.101:40208;tcp://192.168.122.1:40208:300' 

# Don't forget to insert single quotes at the start & end of the port-string.