Combines values from all processes and distribute the result back to all processes


#include "mpi.h"
int MPI_Allreduce ( sendbuf, recvbuf, count, datatype, op, comm )
void             *sendbuf;
void             *recvbuf;
int               count;
MPI_Datatype      datatype;
MPI_Op            op;
MPI_Comm          comm;

Input Parameters

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

Output Parameter

starting address of receive buffer (choice)

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_Errhandler_set; 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.

Invalid buffer pointer. Usually a null buffer where one is not valid.
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 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.
Invalid communicator. A common error is to use a null communicator in a call (not even allowed in MPI_Comm_rank).