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Date: Wed, 16 Jul 2014 22:18:18 -0700 (PDT)
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Subject: Timeouts in Ada
From: NiGHTS <nights@unku.us>
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Xref: news.eternal-september.org comp.lang.ada:20986
Date: 2014-07-16T22:18:18-07:00
List-Id: <comp.lang.ada>

First of all I just want to say how much I appreciate the help you have giv=
en me so far with the questions I have had.

For this next question I am interested in creating a "wrapper" around certa=
in tasks that should have a timeout period. I feel this feature is part of =
what makes Ada so good at keeping a software running stable. There are plen=
ty of network and I/O tasks that I would love to have a "fallback" plan in =
the event that they take too long to execute.=20

For study on this subject I am using this as a reference:

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

I am looking at Ex. 10.=20

  task Password_Server is
     entry Check (User, Pass : in String; Valid : out Boolean);
     entry Set (User, Pass : in  String);
  end Password_Server;
  ...
  User_Name, Password : String (1 .. 8);
  ...
  Put ("Please give your new password:");
  Get_Line (Password);
  select
     Password_Server.Set (User_Name, Password);
     Put_Line ("Done");
  or
     delay 10.0;
     Put_Line ("The system is busy now, please try again later.");
  end select;


Now as an Ada beginner with a strong background in C/Asm I interpret this e=
xample in the following way...

1. "select" creates a thread which runs the body of "select" or the body of=
 "or". Or maybe it creates two threads for each and blocks its own thread u=
ntil one finishes.

2. In either case the variables in scope where "select" was run will be acc=
essible to both the thread created to deal with the body of "select" and th=
e body of "or"

3. The "Password_Server" task is run in its very own thread so it has its o=
wn internal scope of variables. So this would mean that the "select" body i=
s dealing with two nested threads.

4. If the "select" body completes before the "or" body is completed, the "o=
r" thread is terminated. If the "or" body completes before the "select" bod=
y completes, the "select" body and the Password_Server (if applicable) is t=
erminated.

5.a. If both threads happen to reach the "Put_Line" command of each body, b=
oth threads will terminate and there would be a conflict of information -- =
It will confuse the user with printing both "Done" AND "The system is busy"=
. If this was a more mission-critical operation, both the main operation an=
d its fallback plan may execute at the same time. In other words I don't se=
e how this is atomic, and if its not atomic how could this possibly be usef=
ul to anyone?

5.b. Terminating threads while its doing things (while writing to shared me=
mory? performing network operations mid-way?) sounds disastrous, but it see=
ms like the only way this could possibly function in Ada.

Now keep in mind these are all assumptions I made in order to make sense of=
 what I am reading. So far in my self-study of Ada 80% of everything had to=
 be assumed with the help of trial and error because no literature that I h=
ave read explained WHY things happen, only superficial usage syntax. Its ra=
ther annoying but that is why I am so thrilled to be able to ask this quest=
ion here.

Thank you again for all your help!