Re: gnat: time-slicing

[Robert A Duff <bobduff@shell01.TheWorld.com>]

Jan Prazak <janp9@gmx.net> writes:

> Hello,
> 
> I have seen this example program in a tutorial:
> 
> ------------
> with Ada.Text_IO; use Ada.Text_IO;
> 
> procedure Task_Demo is
>   task A;
>   task body A is
>   begin
>     Put_Line("b");
>     Put_Line("b");
>   end A;
> begin
>   Put_Line("a");
>   Put_Line("a");
> end Task_Demo;
> ---------------

The above example is erroneous, which means that the output is totally
unpredictable.  The reason it's erroneous is that two tasks are messing
with the same shared variable (the standard output file) at the same
time.  There's been a lot of confusing discussion about what a file is,
and whether this is a "device" or a "file" and so forth.  You're correct
in thinking that writing to standard output writes to the screen (by
default).  But that's all irrelevant.  The point is, you have two tasks
messing with the same variable (in this case, what Ada calls an
"internal file"), and that's wrong.

Note that "erroneous" is an Ada technical term.  It doesn't mean exactly
the same thing as it does in plain English (unfortunately).  Look it up
in the RM, if you like.

> The output is:
> a
> a
> b
> b
> 
> But I want the output to be:
> 
> ab
> 
> ab

It doesn't matter what you *want* -- this program could do *anything*,
since it's erroneous.  As Robert Dewar pointed out, it could erase your
entire hard disk.  Or, as the C folks like to put it, it might make
demons fly from your nose.

There's no standard Ada way to make this program do anything in
particular, because it is erroneous (i.e. unpredictable).

> (the tutorial says that this is the output with enabled time-slicing)

I haven't seen this tutorial.  Perhaps its point is that you can get
different outputs from erroneous programs, depending on compiler
switches.  Perhaps the author means that it is *possible* to get the
above output when time slicing is turned on (and perhaps some experiment
showed that to be true).  True, but there is certainly no way to
guarantee that output (in standard Ada).

I suspect the tutorial is confusing.  You shouldn't be trying to get
things to behave in a certain way by fiddling with compiler switches.
You should be writing code that obeys the rules (unlike the above
example), and therefore gives the results you want.

> I have tried to use -gnatT100, and -gnatT1, and pragma
> Time_Slice(0.000001) etc., but none of this works.
> I have gnat for Linux.
> 
> Jan

- Bob