A simple example of MPI communication is the sending of a single value from one process to another. In order to do this, one needs to use the following listed code and run the compiled binary to start at least two processes. It does not matter whether these processes run locally or on two compute nodes.

The following code was gratefully borrowed from http://mpitutorial.com/tutorials/mpi-hello-world/:

#include <mpi.h> 
#include <stdio.h> 
#include <stdlib.h> 
 
int main(int argc, char** argv) { 
   // Initialize the MPI environment. 
   MPI_Init(NULL, NULL); 
    
   // Find out rank, size. 
   int world_rank; 
   MPI_Comm_rank(MPI_COMM_WORLD, &world_rank); 
   int world_size; 
   MPI_Comm_size(MPI_COMM_WORLD, &world_size); 
 
   // We are assuming at least 2 processes for this task. 
   if (world_size < 2) { 
               fprintf(stderr, "World size must be greater than 1 for %s.n", argv[0]); 
               MPI_Abort(MPI_COMM_WORLD, 1); 
   } 
 
   int number; 
   if (world_rank == 0) { 
         // If we are rank 0, set the number to -1 and send it to process 1. 
               number = -1; 
               MPI_Send(&number, 1, MPI_INT, 1, 0, MPI_COMM_WORLD); 
   }  
   else if (world_rank == 1) { 
               MPI_Recv(&number, 1, MPI_INT, 0, 0,  
                           MPI_COMM_WORLD,  
                           MPI_STATUS_IGNORE); 
               printf("Process 1 received number %d from process 0.n", number); 
   } 
    
   MPI_Finalize(); 
} 

There isn't a lot to this code. We work through the usual MPI initialization, followed by a check to ensure that our world size is at least two processes large.

The process with rank 0 will then send an MPI message of data type MPI_INT and value -1. The process with rank 1 will wait to receive this message. The receiving process specifies for MPI_Status MPI_STATUS_IGNORE to indicate that the process will not be checking the status of the message. This is a useful optimization technique.

Finally, the expected output is the following:

    $ mpirun -n 2 ./send_recv_demo
    Process 1 received number -1 from process 0

Here we start the compiled demo code with a total of two processes. The output shows that the second process received the MPI message from the first process, with the correct value.