hw.c

#include <mpi.h>
#include <stdio.h>
#include <stdlib.h>

#ifdef DEBUG
	#define dbg printf ("R%d:I%d: ", rank, i); 
#else 
	#define dbg if(0)
#endif /* DEBUG */

// compile with 
// mpicc hw.c -o hw.out
//
// run with 
// mpirun -np 4 hw.out


int tagit(int a, int b) {
	return (a<<8)+b;
}

int main(int argc, char * argv[])
{
	int numtasks, rank, tmp;
	int i = 0;
	MPI_Status stat;
	MPI_Request handle;
	
	// Initialize the parallel stuff	
	tmp = MPI_Init(&argc, &argv);
	if (tmp != MPI_SUCCESS) {
		printf("MPI Initialization faild!\n");
		MPI_Abort(MPI_COMM_WORLD, tmp);
	}

	// Get the info about the parallel stuff
	MPI_Comm_size(MPI_COMM_WORLD, &numtasks);
	MPI_Comm_rank(MPI_COMM_WORLD, &rank);
	dbg printf("[INFO] Hello from the %d / %d\n", rank, numtasks);

	// Check the call accuracy and show the quick help
	if (argc < 2 && rank == 0) {
		printf("Usage: %s <n>\n"
			"where:\n"
			"<n>	number of iterations\n",
		argv[0]);
		MPI_Barrier(MPI_COMM_WORLD);
		MPI_Abort(MPI_COMM_WORLD, tmp);
	}


	// initialize parameters from the command line
	const int msg_size = 2 + atoi(argv[1]);

	// Prepare buffers
	char * buff_in = (char*) malloc(msg_size);
	char * buff_out = (char*) malloc(msg_size);
	char * swap;

	// Prepare useful values
	int prev = (rank == 0 ? numtasks - 1 : rank - 1);
	int next = (rank + 1) % numtasks;
	int tag_recv = tagit(prev, rank);
	int tag_send = tagit(rank, next);

	// LET THE GAMES BEGIN
	for (i = 0; i < msg_size; ++i) {
		buff_in[i] = rank;
	dbg printf("Data writed\n");
		swap = buff_out; buff_out = buff_in; buff_in = swap;
	dbg printf("Buffers prepared\n");
		MPI_Isend(buff_out, msg_size, MPI_CHAR, next, tag_send, MPI_COMM_WORLD, &handle);	
	dbg printf("Outcoming buffer sended to %d with tag %02d\n", next, tag_send);
		MPI_Irecv(buff_in, msg_size, MPI_CHAR, prev, tag_recv, MPI_COMM_WORLD, &handle);
	dbg printf("Incoming buffer recived from %d with tag %02d\n", prev, tag_recv);
		MPI_Wait(&handle, &stat);
	dbg printf("Transaction completed\n");
	}

	// dumping the message
	printf("%d : %d", rank, buff_in[0]);
	for (i = 1; i < msg_size; ++i) 
		printf(" >> %d", buff_in[i]);
	printf("\n");

	MPI_Finalize();
	return 0;
}