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From: Ludovic Brenta <ludovic@ludovic-brenta.org>
Newsgroups: comp.lang.ada
Subject: Re: Writing Data to a file
Date: Thu, 15 Dec 2011 19:32:37 +0100
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Date: 2011-12-15T19:32:37+01:00
List-Id: <comp.lang.ada>

awdorrin <awdorrin@gmail.com> writes:
> I have code that imports the unix 'write' function in order to write a
> stream of byte data.
>
> I had thought that replacing this call with the POSIX.IO.Write would
> make more sense, so that I was using more of a standard method.
>
> Unfortunately, I cannot access the write function within POSIX.IO, so
> I'm stuck with using the Write procedures.
>
> I have the following code, but I'm not sure that I'm doing this the
> right way...
>
> declare
>   Buf_Last : Stream_Element_Count;
>   Buf_Addr : System.Address := AREA_PTR + Storage_Offset(READ_OFFSET);
>   Buf : Stream_Element_Array( 1 .. Stream_Element_Count( WRITE_OFFSET
> - READ_OFFSET) );
>   for Buf'Address use Buf_Addr;
> begin
>   POSIX.IO.Write( File=>FileHandle, Buffer=>Buf, Last=>Buf_Last);
> end;
>
> The original code, using the imported function was:
>
> C_WRITE( File => FileHandle, Buffer=> (Q_AREA_PTR +
> Storage_Offset(READ_OFFSET), Num_Bytes=> (WRITE_OFFSET -
> READ_OFFSET));
>
> having to use the declare section to overlay the Stream_Element_Array
> onto the memory segment of the buffer seems much more complicated; so
> I feel like there is likely an easier way to do this, using standard
> Ada, and not having to import the C write function.
>
> Anyone have any suggestions?

with Ada.Streams.Stream_IO;

declare
   File : Ada.Streams.Stream_IO.File_Type;
   Stream : Ada.Streams.Stream_IO.Stream_Access;
begin
   Ada.Streams.Stream_IO.Open (File => File,
                               Mode => Ada.Streams.Stream_IO.Out_File,
                               Name => "my_output.bin");
   Stream := Ada.Streams.Stream_IO.Stream (File);
   Ada.Streams.Write (Stream => Stream,
                      Item => Buf (1 .. Write_Offset - Read_Offset));
end;

See ARM A.12.1 and 13.13.

The problem you may (or may not) face is converting a POSIX file
descriptor (File_Handle) to a Ada.Streams.Stream_IO.File_Type.  If you
know the name of the file, then the problem disappears, as above.

Note that you have not specified what it is that Buf is supposed to
contain; you seem to assume that an explicit conversion to
Stream_Element_Array is necessary.  It is not; given a stream you might
as well use the stream-oriented attributes (ARM 13.13.2):

package Data is
   type T is private;
private
   -- omitted
end Data;

with Ada.Streams.Stream_IO;
with Data;

declare
   -- as above
   Item : Data.T;
begin
   Data.T'Output (Stream, Item);  -- emits bounds and discriminants
   Data.T'Write (Stream, Item); -- does not emit bounds or discriminants
end;

-- 
Ludovic Brenta.