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<H1> A.7 External Files and File Objects</H1>
<H4 ALIGN=CENTER>Static Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>1</FONT></DIV>
<DIV Class="Normal"> <A NAME="I5736"></A><A NAME="I5737"></A><A NAME="I5738"></A>Values
input from the external environment of the program, or output to the
external environment, are considered to occupy <I>external files</I>.
An external file can be anything external to the program that can produce
a value to be read or receive a value to be written. An external file
is identified by a string (the <I>name</I>). A second string (the <I>form</I>)
gives further system-dependent characteristics that may be associated
with the file, such as the physical organization or access rights. The
conventions governing the interpretation of such strings shall be documented.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>2</FONT></DIV>
<DIV Class="Normal"> <A NAME="I5739"></A>Input and output operations
are expressed as operations on objects of some <I>file type</I>, rather
than directly in terms of the external files. In the remainder of this
section, the term <I>file</I> is always used to refer to a file object;
the term <I>external file</I> is used otherwise.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>3</FONT></DIV>
<DIV Class="Normal"> Input-output for sequential files of values of
a single element type is defined by means of the generic package Sequential_IO.
In order to define sequential input-output for a given element type,
an instantiation of this generic unit, with the given type as actual
parameter, has to be declared. The resulting package contains the declaration
of a file type (called File_Type) for files of such elements, as well
as the operations applicable to these files, such as the Open, Read,
and Write procedures.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>4</FONT></DIV>
<DIV Class="Normal"> Input-output for direct access files is likewise
defined by a generic package called Direct_IO. Input-output in human-readable
form is defined by the (nongeneric) packages Text_IO for Character and
String data, and Wide_Text_IO for Wide_Character and Wide_String data.
Input-output for files containing streams of elements representing values
of possibly different types is defined by means of the (nongeneric) package
Streams.Stream_IO.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5</FONT></DIV>
<DIV Class="Normal"> Before input or output operations can be performed
on a file, the file first has to be associated with an external file.
While such an association is in effect, the file is said to be <I>open</I>,
and otherwise the file is said to be <I>closed</I>.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>6</FONT></DIV>
<DIV Class="Normal"> The language does not define what happens to external
files after the completion of the main program and all the library tasks
(in particular, if corresponding files have not been closed). <A NAME="I5740"></A><A NAME="I5741"></A><A NAME="I5742"></A>The
effect of input-output for access types is unspecified.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>7</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> <A NAME="I5743"></A>An
open file has a <I>current mode</I>, which is a value of one of the following
enumeration types: </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>8</FONT></DIV>
<DIV Class="Examples"><TT><B>type</B> File_Mode <B>is</B> (In_File, Inout_File, Out_File); --<I> for Direct_IO</I></TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>9</FONT></DIV>
<DIV Class="CodeIndented">These values correspond respectively to the
cases where only reading, both reading and writing, or only writing are
to be performed. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>10</FONT></DIV>
<DIV Class="Examples"><TT><B>type</B> File_Mode <B>is</B> (In_File, Out_File, Append_File);<BR>
--<I> for Sequential_IO, Text_IO, Wide_Text_IO, and Stream_IO</I></TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>11</FONT></DIV>
<DIV Class="CodeIndented">These values correspond respectively to the
cases where only reading, only writing, or only appending are to be performed.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>12</FONT></DIV>
<DIV Class="CodeIndented" Style="margin-bottom: 0.9em">The mode of a
file can be changed. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>13</FONT></DIV>
<DIV Class="Normal"> Several file management operations are common
to Sequential_IO, Direct_IO, Text_IO, and Wide_Text_IO. These operations
are described in subclause <A HREF="AA-A-8-2.html">A.8.2</A> for sequential
and direct files. Any additional effects concerning text input-output
are described in subclause <A HREF="AA-A-10-2.html">A.10.2</A>.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>14</FONT></DIV>
<DIV Class="Normal"> The exceptions that can be propagated by the
execution of an input-output subprogram are defined in the package IO_Exceptions;
the situations in which they can be propagated are described following
the description of the subprogram (and in clause <A HREF="AA-A-13.html">A.13</A>).
<A NAME="I5744"></A><A NAME="I5745"></A>The exceptions Storage_Error
and Program_Error may be propagated. (Program_Error can only be propagated
due to errors made by the caller of the subprogram.) Finally, exceptions
can be propagated in certain implementation-defined situations. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>14.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Implementation defined: </B>Any
implementation-defined characteristics of the input-output packages.</FONT></DIV>
<DIV Class="NotesHeader"><FONT SIZE=-1>NOTES</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>15</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>18 Each instantiation of
the generic packages Sequential_IO and Direct_IO declares a different
type File_Type. In the case of Text_IO, Wide_Text_IO, and Streams.Stream_IO,
the corresponding type File_Type is unique.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>16</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>19 A bidirectional device
can often be modeled as two sequential files associated with the device,
one of mode In_File, and one of mode Out_File. An implementation may
restrict the number of files that may be associated with a given external
file. </FONT></DIV>
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