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A.8 Sequential and Direct Files
Static Semantics
1
{sequential file}
{direct 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 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.
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