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<H1> A.9 The Generic Package Storage_IO</H1>
<DIV Class="Paranum"><FONT SIZE=-2>1</FONT></DIV>
<DIV Class="Normal"> 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. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>1.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Reason: </B>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. </FONT></DIV>
<H4 ALIGN=CENTER>Static Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>2</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> The generic library
package Storage_IO has the following declaration: </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>3</FONT></DIV>
<DIV Class="Examples"><TT><B>with</B> Ada.IO_Exceptions;<BR>
<B>with</B> System.Storage_Elements;<BR>
<B>generic</B><BR>
<B>type</B> Element_Type <B>is</B> <B>private</B>;<BR>
<B>package</B> Ada.Storage_IO <B>is</B><A NAME="I5847"></A><A NAME="I5848"></A><A NAME="I5849"></A><BR>
<B>pragma</B> Preelaborate(Storage_IO);</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>4</FONT></DIV>
<DIV Class="Examples"><TT> <A NAME="I5850"></A>Buffer_Size : <B>constant</B> System.Storage_Elements.Storage_Count :=<BR>
<I>implementation-defined</I>;<BR>
<B>subtype</B> <A NAME="I5851"></A>Buffer_Type <B>is</B><BR>
System.Storage_Elements.Storage_Array(1..Buffer_Size);</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5</FONT></DIV>
<DIV Class="Examples"><TT> --<I> Input and output operations</I></TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>6</FONT></DIV>
<DIV Class="Examples"><TT> <B>procedure</B> <A NAME="I5852"></A><A NAME="I5853"></A>Read (Buffer : <B>in</B> Buffer_Type; Item : <B>out</B> Element_Type);</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>7</FONT></DIV>
<DIV Class="Examples"><TT> <B>procedure</B> <A NAME="I5854"></A><A NAME="I5855"></A>Write(Buffer : <B>out</B> Buffer_Type; Item : <B>in</B> Element_Type);</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>8</FONT></DIV>
<DIV Class="Examples"><TT> --<I> Exceptions</I></TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>9</FONT></DIV>
<DIV Class="Examples"><TT> <A NAME="I5856"></A>Data_Error : <B>exception</B> <B>renames</B> IO_Exceptions.Data_Error;<BR>
<B>end</B> Ada.Storage_IO;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>10</FONT></DIV>
<DIV Class="Normal"> 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 HREF="AA-A-8-4.html">A.8.4</A>), but with the content
of the Item parameter being read from or written into the specified Buffer,
rather than an external file.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>10.a</FONT></DIV>
<DIV Class="Annotations" Style="margin-bottom: 0.4em"><FONT SIZE=-1><B>Reason:
</B>As with Direct_IO, the Element_Type formal of Storage_IO does not
have an <FONT FACE="Arial, Helvetica">unknown_discriminant_part</FONT>
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: </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>10.b</FONT></DIV>
<DIV Class="SmallExamples"><TT>(Element_Type'Size + System.Storage_Unit - 1) / System.Storage_Unit</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>10.c</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Implementation defined: </B>The
value of Buffer_Size in Storage_IO.</FONT></DIV>
<DIV Class="NotesHeader"><FONT SIZE=-1>NOTES</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>11</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>21 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. </FONT></DIV>
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