72. Lower-Bound and Upper-Bound Markers

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 ) , ... , (type_n-1 , disp_n-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 type_i requires alignment to a byte address that is a multiple of k_i, then is the least nonnegative increment needed to round extent(Typemap) to the next multiple of .

The formal definitions given for the various datatype constructors apply now, with the amended definition of extent.