MPI_Get_elements(3)

NAME
MPI_Get_elements - Returns the number of basic elements in a data type.

SYNTAX

C Syntax
#include <mpi.h>
int MPI_Get_elements(MPI_Status *status, MPI_Datatype datatype,
	int *count)

Fortran Syntax
INCLUDE 'mpif.h'
MPI_GET_ELEMENTS(STATUS, DATATYPE, COUNT, IERROR)
	INTEGER	STATUS(MPI_STATUS_SIZE), DATATYPE, COUNT, IERROR

C++ Syntax
#include <mpi.h>
int Status::Get_elements(const Datatype& datatype) const

INPUT PARAMETERS

status
    Return status of receive operation (status).

datatype
    Datatype used by receive operation (handle).

OUTPUT PARAMETERS
count Number of received basic elements (integer).

IERROR
    Fortran only: Error status (integer).

DESCRIPTION
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.

Example: Usage of MPI_Get_count and MPI_Get_element:

  ...
  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

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.

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.

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.

SEE ALSO

MPI_Get_count