A.9 The Generic Package Storage_IO
1
The generic package Storage_IO provides for reading
from and writing to an in-memory buffer. This generic package supports
the construction of user-defined input-output packages.
1.a
Reason: This package exists to allow
the portable construction of user-defined direct-access-oriented input-output
packages. The Write procedure writes a value of type Element_Type into
a Storage_Array of size Buffer_Size, flattening out any implicit levels
of indirection used in the representation of the type. The Read procedure
reads a value of type Element_Type from the buffer, reconstructing any
implicit levels of indirection used in the representation of the type.
It also properly initializes any type tags that appear within the value,
presuming that the buffer was written by a different program and that
tag values for the“same” type might vary from one executable
to another.
Static Semantics
2
The generic library
package Storage_IO has the following declaration:
3
with Ada.IO_Exceptions;
with System.Storage_Elements;
generic
type Element_Type
is private;
package Ada.Storage_IO
is
pragma Preelaborate(Storage_IO);
4
Buffer_Size :
constant System.Storage_Elements.Storage_Count :=
implementation-defined;
subtype Buffer_Type
is
System.Storage_Elements.Storage_Array(1..Buffer_Size);
5
-- Input and output operations
6
procedure Read (Buffer :
in Buffer_Type; Item :
out Element_Type);
7
procedure Write(Buffer :
out Buffer_Type; Item :
in Element_Type);
8
-- Exceptions
9
Data_Error :
exception renames IO_Exceptions.Data_Error;
end Ada.Storage_IO;
10
In each instance, the constant Buffer_Size has a
value that is the size (in storage elements) of the buffer required to
represent the content of an object of subtype Element_Type, including
any implicit levels of indirection used by the implementation. The Read
and Write procedures of Storage_IO correspond to the Read and Write procedures
of Direct_IO (see
A.8.4), but with the content
of the Item parameter being read from or written into the specified Buffer,
rather than an external file.
10.a
Reason: As
with Direct_IO, the Element_Type formal of Storage_IO does not have an
unknown_discriminant_part so that there is
a well-defined upper bound on the size of the buffer needed to hold the
content of an object of the formal subtype (i.e. Buffer_Size). If there
are no implicit levels of indirection, Buffer_Size will typically equal:
10.b
(Element_Type'Size + System.Storage_Unit - 1) / System.Storage_Unit
10.c
Implementation defined: The value of
Buffer_Size in Storage_IO.
11
22 A buffer used for Storage_IO holds only
one element at a time; an external file used for Direct_IO holds a sequence
of elements.
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