Correct use of addresses

Correct use of addresses


Up: Derived datatypes Next: Examples Previous: Use of general datatypes in communication

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_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.
A correct program uses only valid addresses to identify the locations of entries in communication buffers. Furthermore, if u and v are two valid addresses, then the (integer) difference u - v can be computed only if both u and v are in the same sequential storage. No other arithmetic operations can be meaningfully executed on addresses.

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.)
Note that in Fortran, Fortran INTEGERs may be too small to contain an address (e.g., 32 bit INTEGERs on a machine with 64bit pointers). Because of this, in Fortran, implementations may restrict the use of absolute addresses to only part of the process memory, and restrict the use of relative displacements to subranges of the process memory where they are constrained by the size of Fortran INTEGERs.


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