# MPI_Reduce_local

Applies a reduction operator to local arguments.

## Synopsis

    int MPI_Reduce_local(const void *inbuf, void *inoutbuf, int count, MPI_Datatype datatype, MPI_Op op)

## Input Parameters

inbuf
  address of the input buffer (choice)
count
  number of elements in each buffer (integer)
datatype
  data type of elements in the buffers (handle)
op
  reduction operation (handle)

## Output Parameters

inoutbuf
  address of input-output buffer (choice)

## 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.

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

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

MPI_ERR_BUFFER
  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).