The displacements in a general datatype are relative to some initial buffer address. Absolute addresses can be substituted for these displacements: we treat them as displacements relative to ``address zero,'' the start of the address space. This initial address zero is indicated by the constant MPI_BOTTOM. Thus, a datatype can specify the absolute address of the entries in the communication buffer, in which case the buf argument is passed the value MPI_BOTTOM.
The address of a location in memory can be found by invoking the
function
MPI_ADDRESS.
MPI_ADDRESS(location, address)
[ IN location] location in caller memory (choice)
[ OUT address] address of location (integer)
int MPI_Address(void* location, MPI_Aint *address)
MPI_ADDRESS(LOCATION, ADDRESS, IERROR)
<type> LOCATION(*)
INTEGER ADDRESS, IERROR
Returns the (byte) address of location.
Example
Using MPI_ADDRESS for an array.
REAL A(100,100) INTEGER I1, I2, DIFF CALL MPI_ADDRESS(A(1,1), I1, IERROR) CALL MPI_ADDRESS(A(10,10), I2, IERROR) DIFF = I2 - I1 ! The value of DIFF is 909*sizeofreal; the values of I1 and I2 are ! implementation dependent.
[] Advice to users.
C users may be tempted to avoid the usage of MPI_ADDRESS and rely on
the availability of the address operator &. Note, however, that
& cast-expression is a pointer, not an
address. ANSI C does not require that the value of a pointer
(or the pointer cast to int) be
the absolute address of the object pointed at --- although this is
commonly the case.
Furthermore, referencing may not have a unique
definition on machines with a segmented address space.
The use of MPI_ADDRESS to ``reference'' C
variables guarantees portability to such machines as well.
( End of advice to users.)
The following auxiliary functions provide useful information on
derived datatypes.
MPI_TYPE_EXTENT(datatype, extent)
[ IN datatype] datatype (handle)
[ OUT extent] datatype extent (integer)
int MPI_Type_extent(MPI_Datatype datatype, MPI_Aint *extent)
MPI_TYPE_EXTENT(DATATYPE, EXTENT, IERROR)
INTEGER DATATYPE, EXTENT, IERROR
Returns the extent of a datatype, where extent is as defined.
MPI_TYPE_SIZE(datatype, size)
[ IN datatype] datatype (handle)
[ OUT size] datatype size (integer)
int MPI_Type_size(MPI_Datatype datatype, int *size)
MPI_TYPE_SIZE(DATATYPE, SIZE, IERROR)
INTEGER DATATYPE, SIZE, IERROR
MPI_TYPE_SIZE returns the total size, in bytes, of the entries in the type signature associated with datatype; i.e., the total size of the data in a message that would be created with this datatype. Entries that occur multiple times in the datatype are counted with their multiplicity.