# MPI_Get_accumulate

Perform an atomic, one-sided read-and-accumulate operation.

## Synopsis

    int MPI_Get_accumulate(const void *origin_addr, int origin_count,
                           MPI_Datatype origin_datatype, void *result_addr, int result_count,
                           MPI_Datatype result_datatype, int target_rank, MPI_Aint target_disp,
                           int target_count, MPI_Datatype target_datatype, MPI_Op op, MPI_Win win)

Accumulate origin_count elements of type origin_datatype from the origin buffer (origin_addr) to the buffer at offset target_disp, in the target window specified by target_rank and win, using the operation op and return in the result buffer result_addr the content of the target buffer before the accumulation.

## Input Parameters

origin_addr
  initial address of buffer (choice)
origin_count
  number of entries in buffer (nonnegative integer)
origin_datatype
  datatype of each buffer entry (handle)
result_addr
  initial address of result buffer (choice)
result_count
  number of entries in result buffer (non-negative integer)
result_datatype
  datatype of each entry in result buffer (handle)
target_rank
  rank of target (nonnegative integer)
target_disp
  displacement from start of window to beginning of target buffer (nonnegative integer)
target_count
  number of entries in target buffer (nonnegative integer)
target_datatype
  datatype of each entry in target buffer (handle)
op
  predefined reduce operation (handle)
win
  window object (handle)

## Notes

This operations is atomic with respect to other "accumulate" operations.

The get and accumulate steps are executed atomically for each basic element in the datatype (see MPI 3.0 Section 11.7 for details). The predefined operation `MPI_REPLACE` provides fetch-and-set behavior.

The origin and result buffers (origin_addr and result_addr) must be disjoint. Each datatype argument must be a predefined datatype or a derived datatype where all basic components are of the same predefined datatype. All datatype arguments must be constructed from the same predefined datatype. The operation op applies to elements of that predefined type. target_datatype must not specify overlapping entries, and the target buffer must fit in the target window or in attached memory in a dynamic window.

Any of the predefined operations for `MPI_Reduce`, as well as `MPI_NO_OP` or `MPI_REPLACE` can be specified as op. User-defined functions cannot be used. A new predefined operation, `MPI_NO_OP`, is defined. It corresponds to the associative function f (a, b) = a; i.e., the current value in the target memory is returned in the result buffer at the origin and no operation is performed on the target buffer. `MPI_NO_OP` can be used only in `MPI_Get_accumulate`, `MPI_Rget_accumulate`, and `MPI_Fetch_and_op`. `MPI_NO_OP` cannot be used in `MPI_Accumulate`, `MPI_Raccumulate`, or collective reduction operations, such as `MPI_Reduce` and others.

## 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 guarentee 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_ARG
  Invalid argument. Some argument is invalid and is not identified by a specific error class (e.g., `MPI_ERR_RANK`).

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

MPI_ERR_RANK
  Invalid source or destination rank. Ranks must be between zero and the size of the communicator minus one; ranks in a receive (`MPI_Recv`, `MPI_Irecv`, `MPI_Sendrecv`, etc.) may also be `MPI_ANY_SOURCE`.

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_WIN
  Invalid MPI window object

## See Also

MPI_Rget_accumulate MPI_Fetch_and_op