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In this article we explore how to turn a single threaded Perl/Tk GUI application into a multi-threaded one; examining the steps, the obstacles and the benefits. Along the way we cover concepts such as callbacks, events, windows messages, thread affinity, thread safety and more.
For code demonstration purposes we will use Perl, the Tk toollkit, and an open source application of mine, Unrar Extract and Recover, which is the real world example that this article is based on.
This application started as single thread and evolved into a threaded one.Using Perl also provides an opportunity to look at the workings from a low level view since frameworks like .NET hide a lot of the underlying complexity.
The UE&R application
First a few words on the Unrar Extract and Recover application.
I was always finding the operation of choosing a compressed file or files by left clicking on them, then right clicking to extract the contents, choosing an extraction directory, and then having to provide a password (if it was password protected), at least cumbersome. Though there are other ways for doing the same thing, none of them was intuitive and I thought that there could be an easier way where the UI won't get in your way and won't waste so much time.
Nowadays, with time being a commodity and compression becoming an integral part of everybody's computing life, the need for automating repetitive tasks is more pressing than ever.
Thus UE&R was born to help with the repetitive operation of extracting .rar files while saving valuable time. And since I am a fan of 'the keep it simple' principle I gave the application 'fire and forget' functionality. Place your files together, choose an input and output directory and just click 'Go'. Of course this basic functionality is coupled with other convenient options such as to 'Delete extracted files' or 'Map directory to archive name'.
So what does Unrar Extract and Recover do?
1. Do you compress your files to save storage space? Then, when you want to extract your files are you frustrated by having to go through them one by one?
Use fire-and-forget functionality. Just point to the directory holding the archives and the software will automatically extract each one (optionally into its own subdirectory) with no user intervention. It saves you the time and the dull task of going through the files manually allowing you to use your precious time productively.
2. Do you compress and protect your valuable data/files by using a password?
UE&R frees you from having to remember individual file passwords by keeping a single password depository so letting you choose as many passwords as needed. All you have to do is keep your passwords into one file (password_file.txt), essentially called a pass-wordlist, and when about to extract just feed the program the wordlist; it will check each password one by one against all files until it finds the correct one; then it continues with the extraction as expected.
3. Another use of this software is that of password recovery. Have you forgotten the password of a password protected archive?
UE&R can retrieve the password by using the dictionary approach. Give it a wordlist with your common passwords and it will attempt all words against the archive. Once it retrieves the password it continues with extracting the archive. UE&R cannot guess the password required since it has no brute force functionality. Personally I think that the time the brute force technique takes to break a password renders the effort unworthy. I prefer using a dictionary, despite its limitations of course.
4. A pleasant side effect of its design is that it can be used as an automated file integrity validation tool. When batch processing your files it checks for broken multipart files, files with corrupted headers, CRC errors, etc and logs all errors into a text file; hence it enables you to check that your files are valid without requiring your attention.
The interface indicates most of the facilities.
Extracting a RAR
(Click images to expand)
You can download Unrar Extract and Recover here.