A.8 Sequential and Direct Files
Static Semantics
{
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 is described in
A.12.1.
{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.
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.
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.)
{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.
{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
{
AI95-00283-01}
Italicized “stream file” to clarify that this is another
kind of file.
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