An issue recently surfaced with a customer regarding the use of an ElevateDB external module in a Delphi application that was using multiple threads to execute some SQL scripts. The external module was called within the scripts to do some text processing, and there were only two threads being launched to execute the scripts in the background. Much to the customer's dismay, the external module kept blowing up in various places, with no real pattern or indication of the cause.

For those that aren't aware of what an ElevateDB external module is, this section of the manual will serve as a good primer:

External Modules

The customer was kind enough to send me a cut-down version of the application, and after playing around with the application and the external module in the debugger for a few hours, the lightbulb finally came on. We had experienced the same issue with our DBISAM ODBC Driver a few years ago, and the cause was very simple: DLLs that are used in multi-threaded Delphi applications that use TThread objects, but don't themselves use TThread objects, will not have the global IsMultiThread variable (System unit) set properly. The IsMultiThread variable is only set to True by Delphi automatically when the TThread class is instantiated, or when the BeginThread procedure (System unit) is called manually. The BeginThread procedure is what sets the IsMultiThread variable, but TThread calls the BeginThread procedure when a TThread instance is created.

So, why is this bad ? Without the IsMultiThread variable set to True, the Delphi memory manager does not know that it needs to use mutual exclusion techniques such as critical sections or interlocked calls to ensure that the multiple threads don't step on each other when allocating or deallocating memory.

So, the solution was to simply add this code in the initialization section of the external module's main unit:

initialization
   IsMultiThread := True;
end

Having said all that, this variable should be set already in the template projects that we provide for Delphi in the Object Repository, so we will be including this as a fix in the next build (2.03 Build 9). The flag does result in a little extra overhead in the memory manager, but saves a lot of grief trying to track down random AVs and memory overwrites.

---

Tags: ElevateDB, External Modules, DLLs, Threads

Permanent Link

Comments (2)	You must be logged in to comment
kavyadelhi said...	Reply
Newcomers to the city face difficulties when they attempt to discover the local social scene. A Call Girl Service in Delhi provides you with a dependable partner who will introduce you to a verified Call Girl who will fulfill your requirements while delivering a respectful experience which will create lasting satisfaction between both of you. Visit here : https://delhi.kavyaarora.in/
Shehnaaz khan said...	Reply
Feel the best kind of pleasure with premium Delhi Escorts, they totally get what you really want deep down. Our Escorts in Delhi are gorgeous, a bit classy, and ready to take care of your fantasies, however you imagine them. Booking Delhi Escorts Service is easy, just check the profiles and pick the one you like most, your dream companion. If you want a romantic vibe, or something more intense, Escorts Service in Delhi promises complete satisfaction. So don’t wait anymore, book your Delhi Escort tonight for moments you won’t forget. https://golf-links.kavyamehra.com https://govindpuri.kavyamehra.com https://greater-kailash.kavyamehra.com https://green-park.kavyamehra.com https://gulmohar-park.kavyamehra.com