Address Function

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Address Function


As shown in Example gif, page gif, one sometimes needs to be able to find the displacement, in bytes, of a structure component relative to the structure start. In C, one can use the sizeof operator to find the size of C objects; and one will be tempted to use the & operator to compute addresses and then displacements. However, the C standard does not require that (int)& be the byte address of variable v: the mapping of pointers to integers is implementation dependent. Some systems may have ``word'' pointers and ``byte'' pointers; other systems may have a segmented, noncontiguous address space. Therefore, a portable mechanism has to be provided by MPI to compute the ``address'' of a variable. Such a mechanism is certainly needed in Fortran, which has no dereferencing operator. addressderived datatype, address

MPI_ADDRESS(location, address)

  IN    location           variable representing a memory location
  OUT   address            address of location
MPI_Address(void* location, MPI_Aint *address)


MPI_ADDRESS is used to find the address of a location in memory. It returns the byte address of location.

Jack Dongarra
Fri Sep 1 06:16:55 EDT 1995