source: CIVL/examples/mpi/collective/scan.c

/**
 * This program illustrates MPI_Scan.
 * online source: http://mpi.deino.net/mpi_functions/MPI_Scan.html
 */
#include "mpi.h"
#include <stdio.h>
 
void addem ( int *, int *, int *, MPI_Datatype * );
void assoc ( int *, int *, int *, MPI_Datatype * );
 
void addem( int *invec, int *inoutvec, int *len, MPI_Datatype *dtype)
{
    int i;
    for ( i=0; i<*len; i++ ) 
    inoutvec[i] += invec[i];
}
#define BAD_ANSWER 100000
/*
    The operation is inoutvec[i] = invec[i] op inoutvec[i] 
    (see 4.9.4). The order is important.
    Note that the computation is in process rank (in the communicator) order, independant of the root.
*/
void assoc( int *invec, int *inoutvec, int *len, MPI_Datatype *dtype)
{
    int i;
    for ( i=0; i<*len; i++ ) {
        if (inoutvec[i] <= invec[i] ) {
            int rank;
            MPI_Comm_rank( MPI_COMM_WORLD, &rank );
            fprintf( stderr, "[%d] inout[0] = %d, in[0] = %d\n", rank, inoutvec[0], invec[0] );fflush(stderr);
            inoutvec[i] = BAD_ANSWER;
        }
        else 
            inoutvec[i] = invec[i];
    }
}
 
int main( int argc, char **argv )
{
    int rank, size, i;
    int data;
    int errors=0;
    int result = -100;
    int correct_result;
    MPI_Op op_assoc, op_addem;
    MPI_Comm comm=MPI_COMM_WORLD;
 
    MPI_Init( &argc, &argv );
    MPI_Op_create( (MPI_User_function *)assoc, 0, &op_assoc );
    MPI_Op_create( (MPI_User_function *)addem, 1, &op_addem );
 
    /* Run this for a variety of communicator sizes */
 
    MPI_Comm_rank( comm, &rank );
    MPI_Comm_size( comm, &size );
 
    data = rank;

    correct_result = 0;
    for (i=0;i<=rank;i++)
        correct_result += i;

    MPI_Scan ( &data, &result, 1, MPI_INT, MPI_SUM, comm );
    if (result != correct_result) {
        fprintf( stderr, "[%d] Error suming ints with scan\n", rank );fflush(stderr);
        errors++;
    }
    MPI_Scan ( &data, &result, 1, MPI_INT, MPI_SUM, comm );
    if (result != correct_result) {
        fprintf( stderr, "[%d] Error summing ints with scan (2)\n", rank );fflush(stderr);
        errors++;
    }

    data = rank;
    result = -100;
    MPI_Scan ( &data, &result, 1, MPI_INT, op_addem, comm );
    if (result != correct_result) {
        fprintf( stderr, "[%d] Error summing ints with scan (userop)\n", rank );fflush(stderr);
        errors++;
    }

    MPI_Scan ( &data, &result, 1, MPI_INT, op_addem, comm );
    if (result != correct_result) {
        fprintf( stderr, "[%d] Error summing ints with scan (userop2)\n", rank );fflush(stderr);
        errors++;
    }
    result = -100;
    data = rank;
    MPI_Scan ( &data, &result, 1, MPI_INT, op_assoc, comm );
    if (result == BAD_ANSWER) {
        fprintf( stderr, "[%d] Error scanning with non-commutative op\n", rank );fflush(stderr);
        errors++;
    }
 
    MPI_Op_free( &op_assoc );
    MPI_Op_free( &op_addem );

    MPI_Finalize();
    return errors;
}