Solved

Multithreaded

Posted on 2003-11-09
2
524 Views
Last Modified: 2010-04-05
Hi,

How can I handle variable accessing (read and write) simultaneously in a multithreaded application ?

Thanks,
Andrew
0
Comment
Question by:andrewyu
[X]
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions
2 Comments
 
LVL 7

Accepted Solution

by:
jconde earned 80 total points
ID: 9712790
You can do this with a Mutex ... In delphi terms, a TCriticalSection will take care of this.

You should NEVER try to access variables simultaneously (specially for write) ... its certain that your app will crash sooner rather than later.  By using a TCriticalSection you  protect each variable so that only one threat can access it at a time.  As soon as you finish updating the variable (writing to it) you should call TCriticalSection.leave.  At that point, another thread will enter the critical section and so on.   ... for example

type

  someclass = class
  protected:
    CriticalSection: TCriticalSection;
  public
   constructor create;
  destructor destroy;
    procedure SomeClass.bcISAPIFilterException(Sender: TObject; E: Exception);
  end;

constructor create;
begin
  CriticalSection:= TCriticalSection.Create;
end;

destructor destroy;
begin
  CriticalSection.Free;
end;

 procedure SomeClass.bcISAPIFilterException(Sender: TObject; E: Exception);
 begin
  CriticalSection.Enter; //protect the log file from multiple threads
  bcLog.LogInfo('An exception occured: ' + E.Message);
  CriticalSection.Leave;
end;

Read the delphi documentation on TCriticalSection as it will explain to you exactly what its used for and why is it needed.

You can also search google for "pthread Mutex" and that will explain in more detail what mutexes (TCriticalSections) are used for.
0
 
LVL 8

Expert Comment

by:gmayo
ID: 9713146
Simple variables are not a problem. Records, strings, objects, lists, arrays etc are a problem. Several objects have a thread-safe version, such as a Lock/Unlock method. TThreadList is a thread-safe version of TList which does not explicitly need to be locked/unlocked.

Failing that, if the data is more complex, use a CriticalSection as jconde suggests.

Geoff M.
0

Featured Post

Technology Partners: We Want Your Opinion!

We value your feedback.

Take our survey and automatically be enter to win anyone of the following:
Yeti Cooler, Amazon eGift Card, and Movie eGift Card!

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

A lot of questions regard threads in Delphi.   One of the more specific questions is how to show progress of the thread.   Updating a progressbar from inside a thread is a mistake. A solution to this would be to send a synchronized message to the…
Introduction The parallel port is a very commonly known port, it was widely used to connect a printer to the PC, if you look at the back of your computer, for those who don't have newer computers, there will be a port with 25 pins and a small print…
Do you want to know how to make a graph with Microsoft Access? First, create a query with the data for the chart. Then make a blank form and add a chart control. This video also shows how to change what data is displayed on the graph as well as form…
This tutorial will teach you the special effect of super speed similar to the fictional character Wally West aka "The Flash" After Shake : http://www.videocopilot.net/presets/after_shake/ All lightning effects with instructions : http://www.mediaf…

696 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question