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MPI_Type_create_f90_real

MPI_Type_create_f90_real - Returns a bounded MPI real datatype

SYNTAX

C Syntax

#include <mpi.h>

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

Fortran Syntax

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

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 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 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 MPI_Pack_external, MPI_Unpack_external and many MPI_File functions - is subtle. The external32 representation of returned datatypes is as follows.

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 MPI_Sizeof and MPI_Type_match_size.

ERRORS

* MPI_Pack_external * MPI_Sizeof * MPI_Type_match_size * MPI_Unpack_external * SELECTED_REAL_KIND