# MPI_Exscan

Computes the exclusive scan (partial reductions) of data on a collection of processes

## Synopsis

    int MPI_Exscan(const void *sendbuf, void *recvbuf, int count,
                   MPI_Datatype datatype, MPI_Op op, MPI_Comm comm)

    int MPI_Exscan_c(const void *sendbuf, void *recvbuf, MPI_Count count,
                     MPI_Datatype datatype, MPI_Op op, MPI_Comm comm)

## Input Parameters

sendbuf
  starting address of send buffer (choice)
count
  number of elements in input buffer (non-negative integer)
datatype
  data type of elements of input buffer (handle)
op
  operation (handle)
comm
  intra-communicator (handle)

## Output Parameters

recvbuf
  starting address of receive buffer (choice)

## Notes

`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).

## 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.

## 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.

## Errors

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 guarantee that an MPI program can continue past an error; however, MPI implementations will attempt to continue whenever possible.

MPI_SUCCESS
  No error; MPI routine completed successfully.

MPI_ERR_BUFFER
  Invalid buffer pointer. Usually a null buffer where one is not valid.

MPI_ERR_COMM
  Invalid communicator. A common error is to use a null communicator in a call (not even allowed in `MPI_Comm_rank`).

MPI_ERR_COUNT
  Invalid count argument. Count arguments must be non-negative; a count of zero is often valid.

MPI_ERR_OP
  Invalid operation. MPI operations (objects of type `MPI_Op`) must either be one of the predefined operations (e.g., `MPI_SUM`) or created with `MPI_Op_create`.

MPI_ERR_TYPE
  Invalid datatype argument. Additionally, this error can occur if an uncommitted MPI_Datatype (see `MPI_Type_commit`) is used in a communication call.

MPI_ERR_OTHER
  Other error; use `MPI_Error_string` to get more information about this error code.