File: MPI_Allreduce.3

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.TH MPI_Allreduce 3 "1/6/1997" " " "MPI"
.SH NAME
MPI_Allreduce \-  Combines values from all processes and distribute the result back to all processes 
.SH SYNOPSIS
.nf
#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;
.fi
.SH INPUT PARAMETERS
.PD 0
.TP
.B sendbuf 
- starting address of send buffer (choice) 
.PD 1
.PD 0
.TP
.B count 
- number of elements in send buffer (integer) 
.PD 1
.PD 0
.TP
.B datatype 
- data type of elements of send buffer (handle) 
.PD 1
.PD 0
.TP
.B op 
- operation (handle) 
.PD 1
.PD 0
.TP
.B comm 
- communicator (handle) 
.PD 1

.SH OUTPUT PARAMETER
.PD 0
.TP
.B recvbuf 
- starting address of receive buffer (choice) 
.PD 1

.SH NOTES FOR FORTRAN
All MPI routines in Fortran (except for 
.I MPI_WTIME
and 
.I MPI_WTICK
) have
an additional argument 
.I ierr
at the end of the argument list.  
.I 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
.I call
statement.

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

.SH NOTES ON COLLECTIVE OPERATIONS

The reduction functions (
.I MPI_Op
) do not return an error value.  As a result,
if the functions detect an error, all they can do is either call 
.I MPI_Abort
or silently skip the problem.  Thus, if you change the error handler from
.I MPI_ERRORS_ARE_FATAL
to something else, for example, 
.I 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.

.SH ERRORS

All MPI routines (except 
.I MPI_Wtime
and 
.I 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 
.I MPI_Errhandler_set
; the predefined error handler
.I MPI_ERRORS_RETURN
may be used to cause error values to be returned.
Note that MPI does 
.B not
guarentee that an MPI program can continue past
an error.

.PD 0
.TP
.B MPI_ERR_BUFFER 
- Invalid buffer pointer.  Usually a null buffer where
one is not valid.
.PD 1
.PD 0
.TP
.B MPI_ERR_COUNT 
- Invalid count argument.  Count arguments must be 
non-negative; a count of zero is often valid.
.PD 1
.PD 0
.TP
.B MPI_ERR_TYPE 
- Invalid datatype argument.  May be an uncommitted 
.PD 1
MPI_Datatype (see 
.I MPI_Type_commit
).
.PD 0
.TP
.B MPI_ERR_OP 
- Invalid operation.  MPI operations (objects of type 
.I MPI_Op
)
must either be one of the predefined operations (e.g., 
.I MPI_SUM
) or
created with 
.I MPI_Op_create
.
.PD 1
.PD 0
.TP
.B MPI_ERR_COMM 
- Invalid communicator.  A common error is to use a null
communicator in a call (not even allowed in 
.I MPI_Comm_rank
).
.PD 1
.SH LOCATION
allreduce.c