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A.12.1 The Package Streams.Stream_IO

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{heterogeneous input-output} [The subprograms in the child package Streams.Stream_IO provide control over stream files. Access to a stream file is either sequential, via a call on Read or Write to transfer an array of stream elements, or positional (if supported by the implementation for the given file), by specifying a relative index for an element. Since a stream file can be converted to a Stream_Access value, calling stream-oriented attribute subprograms of different element types with the same Stream_Access value provides heterogeneous input-output.] See 13.13 for a general discussion of streams. 

Static Semantics

1.1/1
  {8652/0055} {AI95-00026-01} The elements of a stream file are stream elements. If positioning is supported for the specified external file, a current index and current size are maintained for the file as described in A.8. If positioning is not supported, a current index is not maintained, and the current size is implementation defined.{Current index (of an open stream file)} {Current size (of a stream file)}
1.a.1/1
Implementation defined: Current size for a stream file for which positioning is not supported.
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The library package Streams.Stream_IO has the following declaration: 
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with Ada.IO_Exceptions;
package Ada.Streams.Stream_IO is
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    type Stream_Access is access all Root_Stream_Type'Class;
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    type File_Type is limited private;
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    type File_Mode is (In_File, Out_File, Append_File);
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    type    Count          is range 0 .. implementation-defined;
    subtype Positive_Count is Count range 1 .. Count'Last;
      -- Index into file, in stream elements.
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    procedure Create (File : in out File_Type;
                      Mode : in File_Mode := Out_File;
                      Name : in String    := "";
                      Form : in String    := "");
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    procedure Open (File : in out File_Type;
                    Mode : in File_Mode;
                    Name : in String;
                    Form : in String := "");
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    procedure Close  (File : in out File_Type);
    procedure Delete (File : in out File_Type);
    procedure Reset  (File : in out File_Type; Mode : in File_Mode);
    procedure Reset  (File : in out File_Type);
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    function Mode (File : in File_Type) return File_Mode;
    function Name (File : in File_Type) return String;
    function Form (File : in File_Type) return String;
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    function Is_Open     (File : in File_Type) return Boolean;
    function End_Of_File (File : in File_Type) return Boolean;
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    function Stream (File : in File_Type) return Stream_Access;
        -- Return stream access for use with T'Input and T'Output
14/1
This paragraph was deleted. 
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    -- Read array of stream elements from file
    procedure Read (File : in  File_Type;
                    Item : out Stream_Element_Array;
                    Last : out Stream_Element_Offset;
                    From : in  Positive_Count);
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    procedure Read (File : in  File_Type;
                    Item : out Stream_Element_Array;
                    Last : out Stream_Element_Offset);
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This paragraph was deleted. 
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    -- Write array of stream elements into file
    procedure Write (File : in File_Type;
                     Item : in Stream_Element_Array;
                     To   : in Positive_Count);
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    procedure Write (File : in File_Type;
                           Item : in Stream_Element_Array);
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This paragraph was deleted. 
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    -- Operations on position within file
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    procedure Set_Index(File : in File_Type; To : in Positive_Count);
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    function Index(File : in File_Type) return Positive_Count;
    function Size (File : in File_Type) return Count;
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    procedure Set_Mode(File : in out File_Type; Mode : in File_Mode);
25/1
{8652/0051} {AI95-00057-01}     procedure Flush(File : in out File_Type);
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    -- exceptions
    Status_Error : exception renames IO_Exceptions.Status_Error;
    Mode_Error   : exception renames IO_Exceptions.Mode_Error;
    Name_Error   : exception renames IO_Exceptions.Name_Error;
    Use_Error    : exception renames IO_Exceptions.Use_Error;
    Device_Error : exception renames IO_Exceptions.Device_Error;
    End_Error    : exception renames IO_Exceptions.End_Error;
    Data_Error   : exception renames IO_Exceptions.Data_Error;
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private
   ... -- not specified by the language
end Ada.Streams.Stream_IO;
27.1/2
   {AI95-00360-01} The type File_Type needs finalization (see 7.6).
28/2
 {AI95-00283-01} The subprograms given in subclause A.8.2 for the control of external files ( Create, Open, Close, Delete, Reset, Mode, Name, Form, and Is_Open) are available for stream files, and End_of_File have the same effect as the corresponding subprograms in Sequential_IO (see A.8.2).
28.1/2
   {AI95-00283-01} The End_Of_File function: 
28.2/2
28.3/2
28.4/2
28.5/2
   {8652/0055} {AI95-00026-01} {AI95-00085-01} The Set_Mode procedure sets changes the mode of the file. If the new mode is Append_File, the file is positioned to its end; otherwise, the position in the file is unchanged.
28.6/1
   {8652/0055} {AI95-00026-01} The Flush procedure synchronizes the external file with the internal file (by flushing any internal buffers) without closing the file or changing the position. Mode_Error is propagated if the mode of the file is In_File.
29/1
 {8652/0056} {AI95-00001-01} The Stream function returns a Stream_Access result from a File_Type object, thus allowing the stream-oriented attributes Read, Write, Input, and Output to be used on the same file for multiple types. Stream propagates Status_Error if File is not open.
30/2
 {AI95-00256-01} The procedures Read and Write are equivalent to the corresponding operations in the package Streams. Read propagates Mode_Error if the mode of File is not In_File. Write propagates Mode_Error if the mode of File is not Out_File or Append_File. The Read procedure with a Positive_Count parameter starts reading at the specified index. The Write procedure with a Positive_Count parameter starts writing at the specified index. For a file that supports positioning, Read without a Positive_Count parameter starts reading at the current index, and Write without a Positive_Count parameter starts writing at the current index.
30.1/1
   {8652/0055} {AI95-00026-01} The Size function returns the current size of the file.
31/1
 {8652/0055} {AI95-00026-01} The Index function returns the current file index, as a count (in stream elements) from the beginning of the file. The position of the first element in the file is 1
31.a/1
This paragraph was deleted.Ramification: The notion of Index for Stream_IO is analogous to that of Index in Direct_IO, except that the former is measured in Stream_Element units, whereas the latter is in terms of Element_Type values. 
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The Set_Index procedure sets the current index to the specified value.
32.1/1
   {8652/0055} {AI95-00026-01} If positioning is supported for the external file, the current index is maintained as follows:
32.2/1
32.3/1
32.4/1
32.5/1
32.6/1
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If positioning is not supported for the given file, then a call of Index or Set_Index propagates Use_Error. Similarly, a call of Read or Write with a Positive_Count parameter propagates Use_Error.
33.a/2
Implementation Note: {AI95-00085-01} It is permissible for an implementation to implement mode Append_File using the Unix append mode (the O_APPEND bit). Such an implementation does not support positioning when the mode is Append_File, and therefore the operations listed above must raise Use_Error. This is acceptable as there is no requirement that any particular file support positioning; therefore it is acceptable that a file support positioning when opened with mode Out_File, and the same file not support positioning when opened with mode Append_File. But it is not acceptable for a file to support positioning (by allowing the above operations), but to do something other than the defined semantics (that is, always write at the end, even when explicitly commanded to write somewhere else).
Paragraphs 34 through 36 were deleted. 
34/1
 {8652/0055} {AI95-00026-01} The Size function returns the current size of the file, in stream elements.
35/1
 {8652/0055} {AI95-00026-01} The Set_Mode procedure changes the mode of the file. If the new mode is Append_File, the file is positioned to its end; otherwise, the position in the file is unchanged.
36/1
 {8652/0055} {AI95-00026-01} The Flush procedure synchronizes the external file with the internal file (by flushing any internal buffers) without closing the file or changing the position. Mode_Error is propagated if the mode of the file is In_File. 

Erroneous Execution

36.1/1
   {8652/0056} {AI95-00001-01} {erroneous execution (cause) [partial]} If the File_Type object passed to the Stream function is later closed or finalized, and the stream-oriented attributes are subsequently called (explicitly or implicitly) on the Stream_Access value returned by Stream, execution is erroneous. This rule applies even if the File_Type object was opened again after it had been closed. 
36.a.1/1
Reason: These rules are analogous to the rule for the result of the Current_Input, Current_Output, and Current_Error functions. These rules make it possible to represent a value of (some descendant of) Root_Stream_Type which represents a file as an access value, with a null value corresponding to a closed file.

Inconsistencies With Ada 95

36.a/2
{AI95-00283-01} {inconsistencies with Ada 95} Amendment Correction: The description of the subprograms for managing files was corrected so that they do not require truncation of the external file — a stream file is not a sequential file. An Ada 95 program that expects truncation of the stream file may not work under Ada 2005. Note that the Ada 95 standard was ambiguous on this point (the normative wording seemed to require truncation, but didn't explain where; the AARM notes seemed to expect behavior like Direct_IO), and implementations varied widely. Therefore, as a practical matter, code that depends on stream truncation may not work even in Ada 95; deleting the file before opening it provides truncation that works in both Ada 95 and Ada 2005.

Incompatibilities With Ada 95

36.b/2
{AI95-00360-01} {incompatibilities with Ada 95} Amendment Correction: Stream_IO.File_Type is defined to need finalization. If the restriction No_Nested_Finalization (see D.7) applies to the partition, and File_Type does not have a controlled part, it will not be allowed in local objects in Ada 2005 whereas it would be allowed in original Ada 95. Such code is not portable, as another Ada compiler may have a controlled part in File_Type, and thus would be illegal. 

Wording Changes from Ada 95

36.c/2
{8652/0051} {AI95-00057-01} Corrigendum: Corrected the parameter mode of Flush; otherwise it could not be used on Standard_Output.
36.d/2
{8652/0055} {AI95-00026-01} {AI95-00256-01} Corrigendum: Added wording to describe the effects of the various operations on the current index. The Amendment adds an explanation of the use of current index for Read and Write.
36.e/2
{8652/0056} {AI95-00001-01} Corrigendum: Clarified that Stream can raise Status_Error, and clarified that using a Stream_Access whose file has been closed is erroneous.
36.f/2
{AI95-00085-01} Clarified that Set_Mode can be called with the current mode. 

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