The Great and Terrible implementation of MPI-2

function index


Computes the exclusive scan (partial reductions) of data on a collection of processes
int MPI_Exscan(
  void *sendbuf,
  void *recvbuf,
  int count,
  MPI_Datatype datatype,
  MPI_Op op,
  MPI_Comm comm


[in] starting address of send buffer (choice)
[out] starting address of receive buffer (choice)
[in] number of elements in input buffer (integer)
[in] data type of elements of input buffer (handle)
[in] operation (handle)
[in] communicator (handle)


MPI_Exscan is like MPI_Scan, except that the contribution from the calling process is not included in the result at the calling process (it is contributed to the subsequent processes, of course).

MPI_EXSCAN is used to perform a prefix reduction on data distributed across the group. The value in recvbuf on the process with rank 0 is undefined, and recvbuf is not signficant on process 0. The value in recvbuf on the process with rank 1 is defined as the value in sendbuf on the process with rank 0. For processes with rank i > 1, the operation returns, in the receive buffer of the process with rank i, the reduction of the values in the send buffers of processes with ranks 0,...,i-1 (inclusive). The type of operations supported, their semantics, and the constraints on send and receive buffers, are as for MPI_REDUCE.

No "in place" option is supported.

Advice to users.

As for MPI_SCAN, MPI does not specify which processes may call the operation, only that the result be correctly computed. In particular, note that the process with rank 1 need not call the MPI_Op, since all it needs to do is to receive the value from the process with rank 0. However, all processes, even the processes with ranks zero and one, must provide the same op.


The exclusive scan is more general than the inclusive scan provided in MPI-1 as MPI_SCAN. Any inclusive scan operation can be achieved by using the exclusive scan and then locally combining the local contribution. Note that for non-invertable operations such as MPI_MAX, the exclusive scan cannot be computed with the inclusive scan.

The reason that MPI-1 chose the inclusive scan is that the definition of behavior on processes zero and one was thought to offer too many complexities in definition, particularly for user-defined operations.

Thread and Interrupt Safety

This routine is thread-safe. This means that this routine may be safely used by multiple threads without the need for any user-provided thread locks. However, the routine is not interrupt safe. Typically, this is due to the use of memory allocation routines such as malloc or other non-MPICH runtime routines that are themselves not interrupt-safe.

Notes for Fortran

All MPI routines in Fortran (except for MPI_WTIME and MPI_WTICK) have an additional argument ierr at the end of the argument list. ierr is an integer and has the same meaning as the return value of the routine in C. In Fortran, MPI routines are subroutines, and are invoked with the call statement.

All MPI objects (e.g., MPI_Datatype, MPI_Comm) are of type INTEGER in Fortran.

Notes on collective operations

The reduction functions (MPI_Op) do not return an error value. As a result, if the functions detect an error, all they can do is either call MPI_Abort or silently skip the problem. Thus, if you change the error handler from MPI_ERRORS_ARE_FATAL to something else, for example, MPI_ERRORS_RETURN, then no error may be indicated.

The reason for this is the performance problems in ensuring that all collective routines return the same error value.


All MPI routines (except MPI_Wtime and MPI_Wtick) return an error value; C routines as the value of the function and Fortran routines in the last argument. Before the value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job. The error handler may be changed with MPI_Comm_set_errhandler (for communicators), MPI_File_set_errhandler (for files), and MPI_Win_set_errhandler (for RMA windows). The MPI-1 routine MPI_Errhandler_set may be used but its use is deprecated. The predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarentee that an MPI program can continue past an error; however, MPI implementations will attempt to continue whenever possible.

No error; MPI routine completed successfully.
Invalid communicator. A common error is to use a null communicator in a call (not even allowed in MPI_Comm_rank).
Invalid count argument. Count arguments must be non-negative; a count of zero is often valid.
Invalid datatype argument. May be an uncommitted MPI_Datatype (see MPI_Type_commit).
Invalid buffer pointer. Usually a null buffer where one is not valid.
This error class is associcated with an error code that indicates that two buffer arguments are aliased; that is, the describe overlapping storage (often the exact same storage). This is prohibited in MPI (because it is prohibited by the Fortran standard, and rather than have a separate case for C and Fortran, the MPI Forum adopted the more restrictive requirements of Fortran).

Example Code

The following sample code illustrates MPI_Exscan.

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

int main( int argc, char *argv[] )
    int errs = 0;
    int rank, size;
int minsize = 2, count;
int *sendbuf, *recvbuf, i;
    MPI_Comm comm;

    MPI_Init( &argc, &argv );
    comm = MPI_COMM_WORLD;
    MPI_Comm_rank( comm, &rank );
    MPI_Comm_size( comm, &size );
    for (count = 1; count < 65000; count = count * 2)
        sendbuf = (
int *)malloc( count * sizeof(int) );
        recvbuf = (
int *)malloc( count * sizeof(int) );
for (i=0; i<count; i++) {
            sendbuf[i] = rank + i * size;
            recvbuf[i] = -1;
        MPI_Exscan( sendbuf, recvbuf, count, MPI_INT, MPI_SUM, comm );
/* Check the results. rank 0 has no data */
if (rank > 0) {
            int result;
            for (i=0; i<count; i++) {
                result = rank * i * size + ((rank) * (rank-1))/2;
if (recvbuf[i] != result) {
                    if (errs < 10) {
                        fprintf( stderr, "Error in recvbuf[%d] = %d on %d, expected %d\n", i, recvbuf[i], rank, result );fflush(stderr);
        free( sendbuf );
        free( recvbuf );
return 0;