A short example MPI code in C

Here is a "Hello World" program in MPI written in C. In this example, we send a "hello" message to each processor, manipulate it trivially, return the results to the main process, and print the messages.

 /*
  "Hello World" MPI Test Program
 */
 #include <mpi.h>
 #include <stdio.h>
 #include <string.h>
 
 #define BUFSIZE 128
 #define TAG 0
 
 int main(int argc, char *argv[])
 {
   char idstr[32];
   char buff[BUFSIZE];
   int numprocs;
   int myid;
   int i;
   MPI_Status stat;
   /* MPI programs start with MPI_Init; all 'N' processes exist thereafter */
   MPI_Init(&argc,&argv);
   /* find out how big the SPMD world is */
   MPI_Comm_size(MPI_COMM_WORLD,&numprocs);
   /* and this processes' rank is */
   MPI_Comm_rank(MPI_COMM_WORLD,&myid);
 
   /* At this point, all programs are running equivalently, the rank
      distinguishes the roles of the programs in the SPMD model, with
      rank 0 often used specially... */
   if(myid == 0)
   {
     printf("%d: We have %d processors\n", myid, numprocs);
     for(i=1;i<numprocs;i++)
     {
       sprintf(buff, "Hello %d! ", i);
       MPI_Send(buff, BUFSIZE, MPI_CHAR, i, TAG, MPI_COMM_WORLD);
     }
     for(i=1;i<numprocs;i++)
     {
       MPI_Recv(buff, BUFSIZE, MPI_CHAR, i, TAG, MPI_COMM_WORLD, &stat);
       printf("%d: %s\n", myid, buff);
     }
   }
   else
   {
     /* receive from rank 0: */
     MPI_Recv(buff, BUFSIZE, MPI_CHAR, 0, TAG, MPI_COMM_WORLD, &stat);
     sprintf(idstr, "Processor %d ", myid);
     strncat(buff, idstr, BUFSIZE-1);
     strncat(buff, "reporting for duty", BUFSIZE-1);
     /* send to rank 0: */
     MPI_Send(buff, BUFSIZE, MPI_CHAR, 0, TAG, MPI_COMM_WORLD);
   }
 
   /* MPI programs end with MPI Finalize; this is a weak synchronization point */
   MPI_Finalize();
   return 0;
 } 

Compile and run:

#mpicc test_mpi.c

#mpirun -np 2 a.out

When run with two processors this gives the following output.

0: We have 2 processors 

0: Hello 1! Processor 1 reporting for duty 

 If you run it with 4 processors:
#mpirun -np 4 a.out

0: We have 4 processors 
0: Hello 1! Processor 1 reporting for duty 
0: Hello 2! Processor 2 reporting for duty
0: Hello 3! Processor 3 reporting for duty