Suppose we implement gather (see also Section Gather on page Gather ) as a spanning tree implemented on top of point-to-point routines. Since the receive buffer is only valid on the root process, one will need to allocate some temporary space for receiving data on intermediate nodes. However, the datatype extent cannot be used as an estimate of the amount of space that needs to be allocated, if the user has modified the extent using the MPI_UB and MPI_LB values. A function is provided which returns the true extent of the datatype.
| MPI_TYPE_GET_TRUE_EXTENT(datatype, true_lb, true_extent) | |
| IN datatype | datatype to get information on (handle) |
| OUT true_lb | true lower bound of datatype (integer) |
| OUT true_extent | true size of datatype (integer) |
int MPI_Type_get_true_extent(MPI_Datatype datatype, MPI_Aint *true_lb, MPI_Aint *true_extent)
MPI_TYPE_GET_TRUE_EXTENT(DATATYPE, TRUE_LB, TRUE_EXTENT, IERROR)
INTEGER DATATYPE, IERROR
INTEGER(KIND = MPI_ADDRESS_KIND) TRUE_LB, TRUE_EXTENT
{ void MPI::Datatype::Get_true_extent(MPI::Aint& true_lb, MPI::Aint& true_extent) const (binding deprecated, see Section Deprecated since MPI-2.2
) }
true_lb returns the offset of the lowest unit of store which
is addressed by the datatype, i.e., the lower bound of the
corresponding typemap, ignoring MPI_LB markers.
true_extent returns the
true size of
the datatype, i.e., the extent of the corresponding typemap, ignoring
MPI_LB and MPI_UB markers, and performing no
rounding for alignment. If the typemap associated with
datatype is
Typemap = { (type0, disp0), ... , (typen-1, dispn-1)}
Then
and
true_extent (Typemap) = true_ub(Typemap) - true_lb(typemap).
(Readers should compare this with the definitions in
Section Lower-Bound and Upper-Bound Markers
on page Lower-Bound and Upper-Bound Markers
and
Section Extent and Bounds of Datatypes
on page Extent and Bounds of Datatypes
,
which describe
the function
MPI_TYPE_GET_EXTENT.)
The true_extent is the minimum number of bytes of
memory necessary to hold a datatype, uncompressed.
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