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From: Bruno Hergott <bruno.hergott@9online.fr>
Newsgroups: comp.lang.ada
Subject: Protected Object: little question
Date: Fri, 30 Nov 2001 20:29:17 +0100
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Date: 2001-11-30T20:29:17+01:00
List-Id: <comp.lang.ada>

Hello,

Protected Object is really helpfull to provide read and/or write access to a ressource
in a multitask environnement.
I think i have understand the basis but something remains strange for me.

Before all, it could be better to write there the RM Ch. 9. 5.1 concerning protected
objects:
(1): A protected subprogram is a subprogram declared immediately within a
protected_definition.
Protected procedures provide exclusive read-write access to the data of a protected
object;
protected functions provide concurrent read-only access to the data.
(2): Within the body of a protected function (or a function declared immediately within a
protected_body), the current instance of the enclosing protected unit is defined to be a
constant
(that is, its subcomponents may be read but not updated). Within the body of a protected
procedure (or a procedure declared immediately within a protected_body), and within an
entry_body, the current instance is defined to be a variable (updating is permitted).
(4) A new protected action is not started on a protected object while another protected
action
on the same protected object is underway, unless both actions are the result of a call on
a
protected function. This rule is expressible in terms of the execution resource associated
with
the protected object :
 - (5) Starting a protected action on a protected object corresponds to acquiring the
execution
resource associated with the protected object, either for concurrent read-only access if
the
protected action is for a call on a protected function, or for exclusive read-write access

otherwise;
 - (6) Completing the protected action corresponds to releasing the associated execution
resource.
...

My question is:
If i want to implement an action wich does only read access to the protected data and wich
can
be called by 2 tasks at the same time, i must write a function.
But i may not need the return value of this function. Because what i want is not really a
function.
What i want is "the thing in a protected object to do only read access to the protected
data
and wich can be called by 2 tasks at the same time".
And so i must for example call this action like this:
My_Boolean_Return_Value_I_Dont_Want := My_Protected_Object.MyProtectedFunction(..);
It seems to strange to be true...

Or is there something i haven't understand ?