[fpc-pascal] Acitve Object Messaging Library

Werner Van Belle werner at yellowcouch.org
Mon Apr 5 08:19:51 CEST 2010


After the couple of questions I had about reference counting and
cmpxchg, I can finally share what we have been working on, this would be
the eternal gratitude part. A freepascal implementation of active objects.


From the abstract: Active objects seperates various tasks through means
of a message queue whereby the queue will activate the underlying task
whenever a new message comes in. Using this abstraction it becomes
possible to have time speration between threads (they will not influence
each others execution/not wait for each other), space separation
(different tasks will not write into each others data space), scheduling
independence (we can run multiple tasks with one, two or more
processes), syntactical separation (each task is declared in its own
source file) and finally lockfree programming, which thus also leads to
a noticable lack of deadlock and race-conditions

The library consists of a precompiler and a runtime. Originally, both
were written in C++. Recently I modified the compiler to generate pascal
code. In general, to write an active object description one writes a
littl  file:

active DemoSender
  integer tosend = 20;
  DemoReceiver recv;
  message startSending(DemoReceiver vrecv, integer nr);

active DemoReceiver
  message printNumber(integer nr);

This can then be compiled to a forward include, an interface include and
an implementation include.-

   aoc demo2.ao demo2.pas --pascal

This generates

- demo2_ifc.inc: contains the most important part of the interface
section. It can be included in other interface sections.
- demo2_imp.inc: contains all the function, constructor and producedure
implementations. Can be included in -guess what- an  implementation section.
- demo2_fwd.inc: if you need them, you get a collection of forward
declarations. Can also be included in an interface section.

Of course, the messages startSending and printNumber must be
implemented. This is done in the file demo2.pas

program demo2;
uses cthreads, sysutils, active_objects;

{$include demo2_fwd.inc}
{$include demo2_ifc.inc}
{$include demo2_imp.inc}

Function ActiveDemoSender.startSending(vrecv: DemoReceiver; nr:
Integer): ElementResult;
   i : integer;
   tosend:= nr; 
   for i := 0 to tosend-1 do vrecv.printNumber(i);
   Result:= Done;  

Function ActiveDemoReceiver.printNumber(nr: Integer): elementResult;
   if nr = 20 then deactivate;
   Result:= Done;

An then of course, in the end, we want to start these two task after
allocating them-

   sender : DemoSender;
   recv      : DemoReceiver;     
   sender:= DemoSender.Create;
   recv:= DemoReceiver.Create; 
   WriteLn('Loaded Objects. Press enter...');

So, obviously a very quick example. If you are interested, it is
probably best to read the article mentioned above and to download both
the C++ version (it contains the compiler) and the fpc version (which
contains the runtime) from


With kind regards,


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