.. _mpi_get_elements:

MPI_Get_elements
================

.. include_body

:ref:`MPI_Get_elements`, :ref:`MPI_Get_elements_x` - Returns the number of basic
elements in a data type.

Syntax
------

C Syntax
^^^^^^^^

.. code:: c

   #include <mpi.h>

   int MPI_Get_elements(const MPI_Status *status, MPI_Datatype datatype,
       int *count)

   int MPI_Get_elements_x(const MPI_Status *status, MPI_Datatype datatype,
       MPI_Count *count)

Fortran Syntax
^^^^^^^^^^^^^^

.. code:: fortran

   USE MPI
   ! or the older form: INCLUDE 'mpif.h'

   MPI_GET_ELEMENTS(STATUS, DATATYPE, COUNT, IERROR)
       INTEGER STATUS(MPI_STATUS_SIZE), DATATYPE, COUNT, IERROR

   MPI_GET_ELEMENTS_X(STATUS, DATATYPE, COUNT, IERROR)
       INTEGER STATUS(MPI_STATUS_SIZE), DATATYPE
           INTEGER(KIND=MPI_COUNT_KIND) COUNT
           INTEGER IERROR

Fortran 2008 Syntax
^^^^^^^^^^^^^^^^^^^

.. code:: fortran

   USE mpi_f08

   MPI_Get_elements(status, datatype, count, ierror)
       TYPE(MPI_Status), INTENT(IN) :: status
       TYPE(MPI_Datatype), INTENT(IN) :: datatype
       INTEGER, INTENT(OUT) :: count
       INTEGER, OPTIONAL, INTENT(OUT) :: ierror

   MPI_Get_elements_x(status, datatype, count, ierror)
       TYPE(MPI_Status), INTENT(IN) :: status
       TYPE(MPI_Datatype), INTENT(IN) :: datatype
       INTEGER(KIND = MPI_COUNT_KIND), INTENT(OUT) :: count
       INTEGER, OPTIONAL, INTENT(OUT) :: ierror

Input Parameters
----------------

-  status : Return status of receive operation (status).
-  datatype : Datatype used by receive operation (handle).

Output Parameters
-----------------

-  IERROR : Fortran only: Error status (integer).

Description
-----------

:ref:`MPI_Get_elements` and :ref:`MPI_Get_elements_x` behave different from
:ref:`MPI_Get_count`, which returns the number of "top-level entries" received,
i.e., the number of "copies" of type datatype. :ref:`MPI_Get_count` may return
any integer value k, where 0 =< k =< count. If :ref:`MPI_Get_count` returns k,
then the number of basic elements received (and the value returned by
:ref:`MPI_Get_elements` and MPI_Get_elements_x) 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
:ref:`MPI_Get_count` returns the value MPI_UNDEFINED. For both functions, if
the count parameter cannot express the value to be returned (e.g., if
the parameter is too small to hold the output value), it is set to
MPI_UNDEFINED.

Example: Usage of :ref:`MPI_Get_count` and MPI_Get_element:

fortran //... MPI_TYPE_CONTIGUOUS(2, MPI_REAL, Type2, ierr)
MPI_TYPE_COMMIT(Type2, ierr) // ... 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 :ref:`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
:ref:`MPI_Get_count` and :ref:`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.

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
:ref:`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.

Fortran 77 Notes
----------------

The MPI standard prescribes portable Fortran syntax for the COUNT
argument of :ref:`MPI_Get_elements_x` only for Fortran 90. FORTRAN 77 users may
use the non-portable syntax

Fortran INTEGERMPI_COUNT_KIND COUNT

where MPI_COUNT_KIND is a constant defined in mpif.h and gives the
length of the declared integer in bytes.


.. seealso:: :ref:`MPI_Get_count`
