Definitions

The following terms are used to describe partitioning data among processes. Figure 4-1 illustrates some of these concepts.

• An elementary data type (or etype) is the unit of data access and positioning. It can be any MPI basic or derived data type. Data access is performed in elementary-data-type units, and offsets (see below) are expressed as a count of elementary data types.

• The file type (or filetype) is used to partition a file among processes; that is, a file type defines a template for accessing the file. It is either a single elementary data type or a derived MPI data type constructed from elementary data types. A file type may contain "holes," or extents of bytes that will not be accessed by this process.

• A file displacement (or disp) is an absolute byte position relative to the beginning of a file. The displacement defines the location where a view begins (see below).

• A view defines the current set of data visible and accessible by a process from an open file in terms of a displacement, an elementary data type, and a file type. The pattern described by a file type is repeated, beginning at the displacement, to define the view.

• An offset is a position relative to the current view, expressed as a count of elementary data types. Holes in the view's file type are ignored when calculating this position.

Figure 4-1 Displacement, the Elementary Data Type, the File Type, and the View

For a more detailed description of MPI I/O, see Chapter 9, "I/O," of the MPI-2 standard.

Note for Fortran Users

When writing a Fortran program, you must declare the variable ADDRESS as

INTEGER*MPI_ADDRESS_KIND
ADDRESS

MPI_ADDRESS_KIND is a constant defined in mpi.h. This constant defines the length of the declared integer.