Re: FIFO

[Niklas Holsti <niklas.holsti@tidorum.invalid>]

On 17-09-17 10:39 , Jacob Sparre Andersen wrote:
> Frank Buss wrote:
>
>> And everyone feel free to criticize my code, as I'm very new to
>> Ada.

    [snip]

>> generic
>>    Size : Positive;
>>    type Item is private;
>> package Ringbuffers is
>
> Another option is:
>
>    generic
>       type Index   is mod <>;
>       type Element is private;
>    package Ring_Buffer is
>       type Instance is tagged private;
>       subtype Class is Instance'Class;
>
>       procedure Write (Buffer   : in out Instance;
>                        New_Item : in     Element);
>       function Read (Buffer : in out Instance) return Element;
>       function Is_Empty (Buffer : Instance) return Boolean;
>    private
>       type Element_Array is array (Index) of Element
>         with Volatile; --  I'm not sure this does what you want.
>
>       type Instance is tagged
>          record
>             Head, Tail : Index := Index'First;
>             Elements   : Element_Array;
>          end record
>            with Volatile; --  I'm not sure this does what you want either.
>    end Ring_Buffer;
>
> Notice that "Volatile" is a bit tricky.  I'm not sure if that is what
> you need.  Nor if it makes sense to declare both types volatile.

I would use pragma Atomic on Head and Tail, pragma Volatile_Components 
on Element_Array, and no pragma on the (whole) Instance record type.

In May of this year there was a long and profound thread on 
comp.lang.ada about the Atomic/Volatile issue, with the subject 
"Portable memory barrier?". Worth reading if you intend to use those 
pragmas.

On the functional level, it seems to me that the ring-buffer package 
examples given so far are missing a check for overflow (full queue) in 
the "write" (enqueue) operation. I would augment the Write procedure to 
make this check and return a success/failure result to the caller (the 
interrupt handler) and/or increment a "lost elements" counter in the 
queue Instance, for use by the queue-reading task,

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
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