A.13 Exceptions in Input-Output
The package IO_Exceptions defines the exceptions
needed by the predefined input-output packages.
The library package
IO_Exceptions has the following declaration:
Status_Error : exception
Mode_Error : exception
Name_Error : exception
Use_Error : exception
Device_Error : exception
End_Error : exception
Data_Error : exception
Layout_Error : exception
If more than one error condition exists, the corresponding
exception that appears earliest in the following list is the one that
The exception Status_Error is propagated by an attempt
to operate upon a file that is not open, and by an attempt to open a
file that is already open.
The exception Mode_Error is propagated by an attempt
to read from, or test for the end of, a file whose current mode is Out_File
or Append_File, and also by an attempt to write to a file whose current
mode is In_File. In the case of Text_IO, the exception Mode_Error is
also propagated by specifying a file whose current mode is Out_File or
Append_File in a call of Set_Input, Skip_Line, End_Of_Line, Skip_Page,
or End_Of_Page; and by specifying a file whose current mode is In_File
in a call of Set_Output, Set_Line_Length, Set_Page_Length, Line_Length,
Page_Length, New_Line, or New_Page.
The exception Name_Error is propagated by a call
of Create or Open if the string given for the parameter Name does not
allow the identification of an external file. For example, this exception
is propagated if the string is improper, or, alternatively, if either
none or more than one external file corresponds to the string.
The exception Use_Error is propagated if an operation
is attempted that is not possible for reasons that depend on characteristics
of the external file. For example, this exception is propagated by the
procedure Create, among other circumstances, if the given mode is Out_File
but the form specifies an input only device, if the parameter Form specifies
invalid access rights, or if an external file with the given name already
exists and overwriting is not allowed.
The exception Device_Error is propagated if an input-output
operation cannot be completed because of a malfunction of the underlying
The exception End_Error is propagated by an attempt
to skip (read past) the end of a file.
The exception Data_Error can be propagated by the
procedure Read (or by the Read attribute) if the element read cannot
be interpreted as a value of the required subtype. This exception is
also propagated by a procedure Get (defined in the package Text_IO) if
the input character sequence fails to satisfy the required syntax, or
if the value input does not belong to the range of the required subtype.
The exception Layout_Error is propagated (in text
input-output) by Col, Line, or Page if the value returned exceeds Count'Last.
The exception Layout_Error is also propagated on output by an attempt
to set column or line numbers in excess of specified maximum line or
page lengths, respectively (excluding the unbounded cases). It is also
propagated by an attempt to Put too many characters to a string.
These exceptions are also propagated by various
other language-defined packages and operations, see the definition of
those entities for other reasons that these exceptions are propagated.
Reason: This clause
is based in Ada 95. Later versions of Ada (starting with Technical Corrigendum
1) have added a number of additional places and reasons that cause these
exceptions. In particular, TC1 says that stream attributes need to raise
End_Error in some circumstances; Amendment 1 adds Ada.Directories and
a number of new places and reasons that Name_Error and Use_Error are
raised. There are more. We don't want to try to update this text (or
even this note!) for every possible reason and place that might raise
one of these exceptions, so we add this blanket statement.
The implementation shall document the conditions
under which Name_Error, Use_Error and Device_Error are propagated.
The conditions under which Io_Exceptions.Name_Error,
Io_Exceptions.Use_Error, and Io_Exceptions.Device_Error are propagated.
If the associated check is too complex, an implementation
need not propagate Data_Error as part of a procedure Read (or the Read
attribute) if the value read cannot be interpreted as a value of the
Ramification: An example where the implementation
may choose not to perform the check is an enumeration type with a representation
clause with “holes” in the range of internal codes.
[If the element read by the procedure
Read (or by the Read attribute) cannot be interpreted as a value of the
required subtype, but this is not detected and Data_Error is not propagated,
then the resulting value can be abnormal, and subsequent references to
the value can lead to erroneous execution, as explained in 13.9.1
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