Re: Byte streams

["Jeffrey R. Carter" <spam.jrcarter.not@acm.nospam.org>]

shaunpatterson@gmail.com wrote:
> 
> I've written a socket functions to send and receive data in C and then
> created a spec in Ada so I could use those same functions:
> 
> -- C functions --
> 
> unsigned char *readBytes (const unsigned int numBytes);
> void sendBytes (unsigned char *data, const unsigned int numBytes);
> 
> 
> --- Ada side
> type Byte is mod 256;
> for Byte'Size use 8;
> 
> type ByteStream is array (Integer range <>) of Byte;
> 
> 
> function readBytes (numBytes : Integer) return System.Address;
> pragma import (C, readBytes, "readBytes");

Integer? What would Readbytes (-7) mean?

> procedure sendBytes (data : ByteStream; numBytes : Integer);
> pragma import (C, sendBytes, "sendBytes");

For a direct translation, you should use the types in Interfaces.C and 
Interfaces.C.Strings (ARM B.3[.1]):

function Read_Bytes (Num_Bytes : in Interfaces.C.Unsigned)
return Interfaces.C.Strings.Chars_Ptr;

procedure Send_Bytes (Data      : in Interfaces.C.Char_Array;
                       Num_Bytes : in Interfaces.C.Unsigned);

More in the spirit rather than the letter of what the C is trying to 
say, you probably want to use System.Storage_Elements.Storage_Array:

procedure Send_Bytes
    (Data      : in System.Storage_Elements.Storage_Array;
     Num_Bytes : in Interfaces.C.Unsigned);

You send data by creating a subtype of 
System.Storage_Elements.Storage_Array with a length equal to the value 
you'll pass to Num_Bytes, and instantiating Ada.Unchecked_Conversion 
with this subtype as the target.

>                 type IntegerPtr is access all Integer;
>                 function to_IntegerPtr is new Unchecked_Conversion
>                       (source => System.Address, target =>
> IntegerPtr);
> 
> --- this works...and I can do all the conversions I need using an
> unchecked_conversion

This is highly compiler dependent. There is no guarantee that a 
System.Address and a C pointer are the same size, much less the same 
representation. There is no guarantee that a System.Address and an 
access value are the same size, much less the same representation. There 
are compilers where these things aren't and this won't work.

For situations where you must convert between System.Address and access 
values, there's System.Address_To_Access_Conversions, but this isn't one 
of those cases.

Reading is a little more complicated:

with System;

type Value is ...

type Value_Ptr is access all Value;
pragma Convention (C, Value_Ptr);

The pragma Convention is important; it tells the compiler to use the 
same representation as C. That may be different from Ada's default 
representation.

function Get (Num_Bytes : in Interfaces.C.Unsigned :=
                  Value'Size / System.Storage_Unit)
return Value_Ptr;

This is a way to do it at the low level (streams are another, and 
perhaps better, way). But Ada is usually about hiding the low level. You 
shouldn't want your application declaring access types and imported 
functions and doing unchecked conversions all over the place.

generic -- Socket_IF
    type Value (<>) is private;
package Socket_IF is
    procedure Send (Data : in Value);

    function Read return Value;
end Socket_IF;

with Ada.Unchecked_Conversion;
with Interfaces.C;
with System.Storage_Elements;

package Body Socket_IF is
    subtype Unsigned is Interfaces.C.Unsigned;
    use type Unsigned;

    Num_Bytes : constant Unsigned := Value'Size / System.Storage_Unit;

    procedure Send (Data : in Value) is
       subtype List is
          System.Storage_Elements.Storage_Array (1 .. Num_Bytes);

       function To_List is new Ada.Unchecked_Conversion
          (Source => Value, Target => List);

       procedure C_Send (Data : in List; Size : in Unsigned);
       pragma Import (C, C_Send, ...);
    begin -- Send
       C_Send (Data => To_List (Data), Size => Num_Bytes);
    end Send;

    function Read return Value is
       type Value_Ptr is access all Value;
       pragma Convention (C, Value_Ptr);

       function C_Read (Size : in Unsigned) return Value_Ptr;
       pragma Import (C, C_Read, ...);
    begin -- Read
       return C_Read (Num_Bytes).all;
    end Read;
end Socket_IF;

I haven't tested this, but it might even work.

Now your application only has to instantiate Socket_IF for each type of 
interest, and call the resulting Send and Read operations. Much 
replication of code is eliminated, and you only have to get this right once.

Something similar for the streams approach is left as an exercise for 
the reader.

-- 
Jeff Carter
"We call your door-opening request a silly thing."
Monty Python & the Holy Grail
17