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From: "Alex R. Mosteo" <devnull@mailinator.com>
Newsgroups: comp.lang.ada
Subject: Re: question about tasks, multithreading and multi-cpu machines
Date: Thu, 16 Mar 2006 11:04:34 +0100
Message-ID: <44193832.1070702@mailinator.com>
References: <dv6qsd$lg0$00$1@news.t-online.com> <4416F491.6080309@obry.net>
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Xref: g2news1.google.com comp.lang.ada:3377
Date: 2006-03-16T11:04:34+01:00
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Norbert Caspari wrote:
> Pascal Obry wrote:
> 
> 
>>Use Tasks.
> 
> 
> Thank you very much for answering my questions. But how can I use
> multitasking with Ada? There is no such thing like "fork" available in
> this programming language.

Multitasking is built-in in the language. Basically, each object of type 
Task is a new thread of execution. Check some tutorials on Ada tasking 
for details. Sorry, I don't have references handy. Tasking is an 
important feature of Ada and so can't be properly explained in a single 
post.

Maybe:

http://en.wikibooks.org/wiki/Ada_Programming/Tasking

So, in Ada instead of "forking" the current process into two, you simply 
create a new task. You'll see that Ada tasking features are much more 
confortable than low-level APIs for multithreading.

Two examples:

procedure Main is

    task Second;

    task body Second is
    begin
       loop
          null;
       end loop;
    end Second;

begin

    --  whatever

end;

Here you have your main task, whose code is in the body of the main 
procedure, and a second "thread" which is the code inside the body of 
Second. The Second task starts running during the elaboration (or 
"loading", if you're not familiar with the elaboration concept) of the 
main program.

Second example:

procedure Main is

    task type Child; -- Note the "type" keyword here, missing in 1st ex.

    task body Child is
    begin
       --  whatever
    end;

    type Access_Child is access all Child;

    C : Access_Child;

begin

    --  some code

    C := new Child;

    --  more code

end;

Here, you have your main task/thread embodied, as before, in the Main 
procedure. You create a new task/thread with the line

C := new Child;

and the code inside Child starts running at that point. The difference 
with the 1st example is that we are declaring a "task type", so no task 
is created until some object of that type is created. In the first 
example we directly declared a task object (not type), so the task 
started to exist right there.

Each task has its own stack, but all tasks share the global variables. 
You can then communicate amongst them with rendez-vous or protected objects.