80. Correct Use of Addresses

Successively declared variables in C or Fortran are not necessarily stored at contiguous locations. Thus, care must be exercised that displacements do not cross from one variable to another. Also, in machines with a segmented address space, addresses are not unique and address arithmetic has some peculiar properties. Thus, the use of addresses, that is, displacements relative to the start address MPI_BOTTOM, has to be restricted.

Variables belong to the same sequential storage if they belong to the same array, to the same COMMON block in Fortran, or to the same structure in C. Valid addresses are defined recursively as follows:

1. The function MPI_GET_ADDRESS returns a valid address, when passed as argument a variable of the calling program.
2. The buf argument of a communication function evaluates to a valid address, when passed as argument a variable of the calling program.
3. If v is a valid address, and i is an integer, then v+i is a valid address, provided v and v+i are in the same sequential storage.
4. If v is a valid address then MPI_BOTTOM + v is a valid address.

The rules above impose no constraints on the use of derived datatypes, as long as they are used to define a communication buffer that is wholly contained within the same sequential storage. However, the construction of a communication buffer that contains variables that are not within the same sequential storage must obey certain restrictions. Basically, a communication buffer with variables that are not within the same sequential storage can be used only by specifying in the communication call buf = MPI_BOTTOM, count = 1, and using a datatype argument where all displacements are valid (absolute) addresses.

Advice to users.

It is not expected that MPI implementations will be able to detect erroneous, ``out of bound'' displacements --- unless those overflow the user address space --- since the MPI call may not know the extent of the arrays and records in the host program. ( End of advice to users.)

Advice to implementors.

There is no need to distinguish (absolute) addresses and (relative) displacements on a machine with contiguous address space: MPI_BOTTOM is zero, and both addresses and displacements are integers. On machines where the distinction is required, addresses are recognized as expressions that involve MPI_BOTTOM. ( End of advice to implementors.)