mpi_get_elements.3

MPI stands for the Message Passing Interface. Written by the MPI Forum (a large committee comprising

.\"Copyright 2006, Sun Microsystems, Inc.
.\" Copyright (c) 1996 Thinking Machines Corporation
.TH MPI_Get_elements 3OpenMPI "September 2006" "Open MPI 1.2" " "
.SH NAME
\fBMPI_Get_elements \fP \- Returns the number of basic elements in a data type.

.SH SYNTAX
.ft R
.SH C Syntax
.nf
#include <mpi.h>
int MPI_Get_elements(MPI_Status *\fIstatus\fP, MPI_Datatype\fI datatype\fP,
	int\fI *count\fP)

.SH Fortran Syntax
.nf
INCLUDE 'mpif.h'
MPI_GET_ELEMENTS(\fISTATUS, DATATYPE, COUNT, IERROR\fP)
	INTEGER	\fISTATUS(MPI_STATUS_SIZE), DATATYPE, COUNT, IERROR\fP

.SH C++ Syntax
.nf
#include <mpi.h>
int Status::Get_elements(const Datatype& \fIdatatype\fP) const

.SH INPUT PARAMETERS
.ft R
.TP 1i
status
Return status of receive operation (status).
.TP 1i
datatype
Datatype used by receive operation (handle).

.SH OUTPUT PARAMETERS
.ft R
count     Number of received basic elements (integer).
.ft R
.TP 1i
IERROR
Fortran only: Error status (integer). 

.SH DESCRIPTION
.ft R
MPI_Get_elements behaves different from MPI_Get_count, which returns the number of "top-level entries" received, i.e., the number of "copies" of type datatype. MPI_Get_count may return any integer value k, where 0 =< k =< count. If MPI_Get_count returns k, then the number of basic elements received (and the value returned by  MPI_Get_elements) is n * k, where n is the number of basic elements in the type map of datatype. If the number of basic elements received is not a multiple of n, that is, if the receive operation has not received an integral number of datatype "copies," then MPI_Get_count returns the value MPI_UNDEFINED.
.sp
\fBExample:\fP Usage of MPI_Get_count and MPI_Get_element:  
.sp
.nf
  \&... 
  CALL MPI_TYPE_CONTIGUOUS(2, MPI_REAL, Type2, ierr) 
  CALL MPI_TYPE_COMMIT(Type2, ierr) 
  \&... 
  CALL MPI_COMM_RANK(comm, rank, ierr) 
  IF(rank.EQ.0) THEN 
        CALL MPI_SEND(a, 2, MPI_REAL, 1, 0, comm, ierr) 
        CALL MPI_SEND(a, 3, MPI_REAL, 1, 0, comm, ierr) 
  ELSE 
        CALL MPI_RECV(a, 2, Type2, 0, 0, comm, stat, ierr) 
        CALL MPI_GET_COUNT(stat, Type2, i, ierr)     ! returns i=1 
        CALL MPI_GET_ELEMENTS(stat, Type2, i, ierr)  ! returns i=2 
        CALL MPI_RECV(a, 2, Type2, 0, 0, comm, stat, ierr) 
        CALL MPI_GET_COUNT(stat, Type2, i, ierr) ! returns i=MPI_UNDEFINED
        CALL MPI_GET_ELEMENTS(stat, Type2, i, ierr)  ! returns i=3 
  END IF 
.fi
.sp
The function MPI_Get_elements can also be used after a probe to find the number of elements in the probed message. Note that the two functions MPI_Get_count and MPI_Get_elements return the same values when they are used with primitive data types. 
  
.SH ERRORS
Almost all MPI routines return an error value; C routines as the value of the function and Fortran routines in the last argument. C++ functions do not return errors. If the default error handler is set to MPI::ERRORS_THROW_EXCEPTIONS, then on error the C++ exception mechanism will be used to throw an MPI:Exception object.
.sp
Before the error value is returned, the current MPI error handler is
called. By default, this error handler aborts the MPI job, except for I/O function errors. The error handler may be changed with MPI_Comm_set_errhandler; the predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarantee that an MPI program can continue past an error.  

.SH SEE ALSO
.ft R
.sp
MPI_Get_count

' @(#)MPI_Get_elements.3 1.20 06/03/09