.TH MPI_Reduce 3 "11/12/2019" " " "MPI"

.SH NAME

MPI_Reduce \-  Reduces values on all processes to a single value 

.SH SYNOPSIS

.nf

int MPI_Reduce(const void *sendbuf, void *recvbuf, int count, MPI_Datatype datatype,

MPI_Op op, int root, MPI_Comm comm)

.fi

.SH INPUT PARAMETERS

.PD 0

.TP

.B sendbuf 

- 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 

- reduce operation (handle)

.PD 1

.PD 0

.TP

.B root 

- rank of root process (integer)

.PD 1

.PD 0

.TP

.B comm 

- communicator (handle)

.PD 1

.SH OUTPUT PARAMETERS

.PD 0

.TP

.B recvbuf 

- address of receive buffer (choice,

significant only at 

.I root

)

.PD 1

.SH 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 

.I malloc

or other non-MPICH runtime routines that are themselves not interrupt-safe.

.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_Comm_set_errhandler

(for communicators),

.I MPI_File_set_errhandler

(for files), and 

.I MPI_Win_set_errhandler

(for

RMA windows).  The MPI-1 routine 

.I MPI_Errhandler_set

may be used but

its use is deprecated.  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; however, MPI implementations will attempt to continue whenever

possible.

.PD 0

.TP

.B MPI_SUCCESS 

- No error; MPI routine completed successfully.

.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

.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.  Additionally, this error can

occur if an uncommitted MPI_Datatype (see 

.I MPI_Type_commit

) is used

in a communication call.

.PD 1

.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_BUFFER 

- This error class is associcated with an error code that

indicates that two buffer arguments are 

.B 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).

.PD 1