It is often convenient to define explicitly the lower bound and upper bound of a type map, and override the definition given on page Lower-Bound and Upper-Bound Markers . This allows one to define a datatype that has ``holes'' at its beginning or its end, or a datatype with entries that extend above the upper bound or below the lower bound. Examples of such usage are provided in Section Examples . Also, the user may want to overide the alignment rules that are used to compute upper bounds and extents. E.g., a C compiler may allow the user to overide default alignment rules for some of the structures within a program. The user has to specify explicitly the bounds of the datatypes that match these structures.
To achieve this, we add two additional ``pseudo-datatypes,'' MPI_LB and MPI_UB, that can be used, respectively, to mark the lower bound or the upper bound of a datatype. These pseudo-datatypes occupy no space (extent(typeMPI_LB) = extent(typeMPI_UB) =0). They do not affect the size or count of a datatype, and do not affect the content of a message created with this datatype. However, they do affect the definition of the extent of a datatype and, therefore, affect the outcome of a replication of this datatype by a datatype constructor.
Example
Let D = (-3, 0, 6); T = (MPI_LB, MPI_INT, MPI_UB),
and B = (1, 1, 1). Then a call to
MPI_TYPE_STRUCT(3, B, D, T, type1)
creates a new datatype that has an
extent of 9 (from -3 to 5, 5 included), and contains an integer at
displacement 0. This is the datatype defined by the sequence
{(lb, -3), (int, 0), (ub, 6) } .
If this type is replicated twice by a call to
MPI_TYPE_CONTIGUOUS(2, type1, type2) then the newly created
type can
be described by the sequence
{(lb, -3), (int, 0), (int,9), (ub, 15) } .
(An entry of type
ub
can be deleted if there is another entry of type ub with a higher
displacement; an entry of type lb can be deleted if there is another
entry of type lb with a lower displacement.)
In general, if Typemap = { (type0 , disp0 ) , ... , (typen-1 , dispn-1) } , then the lower bound of Typemap is defined to be
Similarly, the upper bound of Typemap is defined to be
Then
extent(Typemap) = ub(Typemap) - lb(Typemap)
If typei requires alignment to a byte address that is a multiple of ki,
then
The formal definitions given for the various datatype constructors
apply now, with the amended definition of extent.
is the least non-negative increment needed to round
extent(Typemap) to the next multiple of 
.
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