.. _mpi_type_create_f90_real:


MPI_Type_create_f90_real
========================

.. include_body

::

   MPI_Type_create_f90_real - Returns a bounded MPI real datatype


SYNTAX
------


C Syntax
^^^^^^^^

.. code-block:: c

   #include <mpi.h>

   int MPI_Type_create_f90_real(int p, int r, MPI_Datatype *newtype)


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

.. code-block:: fortran

   USE MPI
   ! or the older form: INCLUDE 'mpif.h'
   MPI_TYPE_CREATE_F90_REAL (P, R, NEWTYPE, IERROR)
   	INTEGER	P, R, NEWTYPE, IERROR


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

.. code-block:: fortran

   USE mpi_f08
   MPI_Type_create_f90_real(p, r, newtype, ierror)
   	INTEGER, INTENT(IN) :: p, r
   	TYPE(MPI_Datatype), INTENT(OUT) :: newtype
   	INTEGER, OPTIONAL, INTENT(OUT) :: ierror


INPUT PARAMETERS
----------------
* ``p``: Precision, in decimal digits (integer).
* ``r``: Decimal exponent range (integer).

OUTPUT PARAMETERS
-----------------
* ``newtype``: New data type (handle).
* ``ierror``: Fortran only: Error status (integer).

DESCRIPTION
-----------

This function provides a way to declare KIND-parameterized REAL MPI
datatypes. The arguments are interpreted in a similar fashion to the F90
function SELECTED_REAL_KIND. The parameters *p* and *r* must be scalar
integers. The argument *p* represents the required level of numerical
precision, in decimal digits. The *r* parameter indicates the range of
exponents desired: the returned datatype will have at least one exponent
between +\ *r* and -*r* (inclusive).

Either *p* or *r*, but not both, may be omitted from calls to
SELECTED_REAL_KIND. Similarly, either argument to
:ref:`MPI_Type_create_f90_real` may be set to MPI_UNDEFINED.


NOTES
-----

It is erroneous to supply values for *p* and *r* not supported by the
compiler.

The Fortran function SELECTED_REAL_KIND maps a large number of (*p,r*)
pairs to a much smaller number of KIND parameters supported by the
compiler. KIND parameters are not specified by the language and are not
portable. From the point of view of the language, variables of the same
base type and KIND parameter are equivalent, even if their KIND
parameters were generated by different (*p,r*) arguments to
SELECTED_REAL_KIND. However, to help facilitate interoperability in a
heterogeneous environment, equivalency is more strictly defined for
datatypes returned by :ref:`MPI_Type_create_f90_real`. Two MPI datatypes, each
generated by this function, will match if and only if they have
identical values for both *p* and *r*.

The interaction between the datatypes returned by this function and the
external32 data representation - used by :ref:`MPI_Pack_external`,
:ref:`MPI_Unpack_external` and many MPI_File functions - is subtle. The
external32 representation of returned datatypes is as follows.

.. code-block:: c

   	if (p > 33) and/or (r > 4931):
   		external32 size = n/a (undefined)
   	else if (p > 15) and/or (r > 307):
   		external32 size = 16
   	else if (p > 6) and/or (r > 37):
   		external32 size = 8
   	else:
   		external32 size = 4

If the external32 representation of a datatype is undefined, so are the
results of using that datatype in operations that require the external32
format. Care should be taken not to use incompatible datatypes
indirectly, e.g., as part of another datatype or through a duplicated
datatype, in these functions.

If a variable is declared specifying a nondefault KIND value that was
not obtained with SELECTED_REAL_KIND (i.e., *p* and/or *r* are unknown),
the only way to obtain a matching MPI datatype is to use the functions
:ref:`MPI_Sizeof` and :ref:`MPI_Type_match_size`.


ERRORS
------

.. include:: ./ERRORS.rst

.. seealso::
   * :ref:`MPI_Pack_external`
   * :ref:`MPI_Sizeof`
   * :ref:`MPI_Type_match_size`
   * :ref:`MPI_Unpack_external`
   * SELECTED_REAL_KIND
