Question

So this is my question , threads are so confusing for me , let’s say I have 5 threads , and 50 or 100 or more sites. So as far as I’ve learned about threads , I can make constructor create (link:string) and start new threads with different links , but than I wold need to make as much threads as the number of links I need to parse.So how can I make variable link shared between threads , so when thread one downloads link listbox1.items[0] it tells others that number 0 is downloaded and next thread should ask what link should I download and get answer listbox1.items[1] and so on until they download all links when they should terminate.
Can anyone provide me with simple example of how can this be done. Threads are killing me 🙁

Answer 1

Sounds like you simply need to set up a critical section.

This needs to be set up around the code segment which reads the next URL. To do this you would typically place a semaphore at the start of the code so that only one thread can enter it at any time. The semaphore is reset at the end of the code. As each new thread sees the URL list has expired, then it terminates.

Typically semaphores are boolean, but they can be integers for example if you want to allow a specific number of threads to enter the region at any time.

In your case you can simply set up a global boolean variable (visible to all threads), say “fSemaphore”.

At the start of the region, the thread checks the flag. If it is false it sets it to true and enters the region (to get the next URL).
If it is true, then it loops – e.g. repeat sleep(0) until (not fSemaphore).
When it exits the region it set fSemaphore := False;

Obviously you need to make sure you guard against a possible infinite loop scenario…