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A.8 Sequential and Direct Files

Static Semantics

1/2
{AI95-00283-01} {sequential file} {direct file} {stream file} Two kinds of access to external files are defined in this subclause: sequential access and direct access. The corresponding file types and the associated operations are provided by the generic packages Sequential_IO and Direct_IO. A file object to be used for sequential access is called a sequential file, and one to be used for direct access is called a direct file. Access to stream files stream files is described in A.12.1.
2
{sequential access} For sequential access, the file is viewed as a sequence of values that are transferred in the order of their appearance (as produced by the program or by the external environment). When the file is opened with mode In_File or Out_File, transfer starts respectively from or to the beginning of the file. When the file is opened with mode Append_File, transfer to the file starts after the last element of the file. 
2.a
Discussion: Adding stream I/O necessitates a review of the terminology. In Ada 83, `sequential' implies both the access method (purely sequential — that is, no indexing or positional access) and homogeneity. Direct access includes purely sequential access and indexed access, as well as homogeneity. In Ada 95, streams allow purely sequential access but also positional access to an individual element, and are heterogeneous. We considered generalizing the notion of `sequential file' to include both Sequential_IO and Stream_IO files, but since streams allow positional access it seems misleading to call them sequential files. Or, looked at differently, if the criterion for calling something a sequential file is whether it permits (versus requires) purely sequential access, then one could just as soon regard a Direct_IO file as a sequential file.
2.b
It seems better to regard `sequential file' as meaning `only permitting purely sequential access'; hence we have decided to supplement `sequential access' and `direct access' with a third category, informally called `access to streams'. (We decided against the term `stream access' because of possible confusion with the Stream_Access type declared in one of the stream packages.)
3
{direct access} {index (of an element of an open direct file)} {current size (of an external file)} For direct access, the file is viewed as a set of elements occupying consecutive positions in linear order; a value can be transferred to or from an element of the file at any selected position. The position of an element is specified by its index, which is a number, greater than zero, of the implementation-defined integer type Count. The first element, if any, has index one; the index of the last element, if any, is called the current size; the current size is zero if there are no elements. The current size is a property of the external file.
4
{current index (of an open direct file)} An open direct file has a current index, which is the index that will be used by the next read or write operation. When a direct file is opened, the current index is set to one. The current index of a direct file is a property of a file object, not of an external file. 

Wording Changes from Ada 95

4.a/2
{AI95-00283-01} Italicized “stream file” to clarify that this is another kind of file. 

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