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<H1> A.8.3 Sequential Input-Output Operations</H1>
<H4 ALIGN=CENTER>Static Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>1</FONT></DIV>
<DIV Class="Normal"> The operations available for sequential input
and output are described in this subclause. The exception Status_Error
is propagated if any of these operations is attempted for a file that
is not open. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>2</FONT></DIV>
<DIV Class="Examples"><TT><B>procedure</B> Read(File : <B>in</B> File_Type; Item : <B>out</B> Element_Type);</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>3</FONT></DIV>
<DIV Class="CodeIndented">Operates on a file of mode In_File. Reads an
element from the given file, and returns the value of this element in
the Item parameter. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>3.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Discussion: </B>We considered
basing Sequential_IO.Read on Element_Type'Read from an implicit stream
associated with the sequential file. However, Element_Type'Read is a
type-related attribute, whereas Sequential_IO should take advantage of
the particular constraints of the actual subtype corresponding to Element_Type
to minimize the size of the external file. Furthermore, forcing the implementation
of Sequential_IO to be based on Element_Type'Read would create an upward
incompatibility since existing data files written by an Ada 83 program
using Sequential_IO might not be readable by the identical program built
with an Ada 95 implementation of Sequential_IO.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>3.b</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>An Ada 95 implementation might
still use an implementation-defined attribute analogous to 'Read to implement
the procedure Read, but that attribute will likely have to be subtype-specific
rather than type-related, and it need not be user-specifiable. Such an
attribute will presumably be needed to implement the generic package
Storage_IO (see <A HREF="AA-A-9.html">A.9</A>). </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>4</FONT></DIV>
<DIV Class="CodeIndented" Style="margin-bottom: 0.9em">The exception
Mode_Error is propagated if the mode is not In_File. The exception End_Error
is propagated if no more elements can be read from the given file. The
exception Data_Error can be propagated if the element read cannot be
interpreted as a value of the subtype Element_Type (see <A HREF="AA-A-13.html">A.13</A>,
``<A HREF="AA-A-13.html">Exceptions in Input-Output</A>''). </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>4.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Discussion: </B>Data_Error
need not be propagated if the check is too complex. See <A HREF="AA-A-13.html">A.13</A>,
``<A HREF="AA-A-13.html">Exceptions in Input-Output</A>''. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5</FONT></DIV>
<DIV Class="Examples"><TT><B>procedure</B> Write(File : <B>in</B> File_Type; Item : <B>in</B> Element_Type);</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>6</FONT></DIV>
<DIV Class="CodeIndented">Operates on a file of mode Out_File or Append_File.
Writes the value of Item to the given file.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>7</FONT></DIV>
<DIV Class="CodeIndented" Style="margin-bottom: 0.9em">The exception
Mode_Error is propagated if the mode is not Out_File or Append_File.
The exception Use_Error is propagated if the capacity of the external
file is exceeded.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>8</FONT></DIV>
<DIV Class="Examples"><TT><B>function</B> End_Of_File(File : <B>in</B> File_Type) <B>return</B> Boolean;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>9</FONT></DIV>
<DIV Class="CodeIndented">Operates on a file of mode In_File. Returns
True if no more elements can be read from the given file; otherwise returns
False.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>10</FONT></DIV>
<DIV Class="CodeIndented">The exception Mode_Error is propagated if the
mode is not In_File. </DIV>
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