[DUG] Program crashes - anyone knows why?

[Gajo Csaba]

Hello,

I wrote a multithreaded application for a client and it crashes on the  
client's computer, while it works ok on mine. Can someone give me a clue  
as to what might be the problem?

The program works like this: I have two TThreadLists: one contains a list  
of keywords (~20000) while the other a list of proxy servers (~500). When  
the user clicks on the button, 21 threads are created. 1 thread directly  
accesses Google, enters the keyword and processes the result. The other 20  
threads do the same, except they use a proxy server.

OK, so each thread grabs a keyword from the list and then removes it from  
the list, so no one else could use it again. It tries to access the  
internet. If the connection fails then it means the proxy doesn't work, so  
it gets another proxy address from the list. No two threads ever share the  
same proxy. If there are no more free proxies left, the thread dies (and  
returns the unprocessed keyword to the list).

They do this until all the keywords are processed. When it seems like  
there are no more keywords left, the 1st thread (the one not using a  
proxy) changes its priority to tpLowest and loops forever, waiting for  
another keyword to show up.

Like I said, this works perfectly on my computer. On the client's computer  
(Windows XP I think) an error dialog pops up saying:

<program name> has encountered a problem and needs to close. We are sorry  
for the inconvenience.
If you were in the middle of something, the information you were working  
on might be lost.

If the client clicks on the Close button, the entire application will shut  
down. However, if he ignores the message, the application will keep on  
working and it will eventually finish it's job, and then the client can  
export the result to a file.

Because the application works fine, I'm guessing this is just one thread  
that has crashed. The problem is the error dialog shouldn't appear. I  
didn't make it appear with MessageDlg, so it must be Windows' internal  
something. The error occurs somewhere at the start, around the 1000th  
keyword.

I tried lowering the number of threads to 5, but this didn't help either.

Any idea what may be causing this problem? The only two shared global  
variables are the lists, though they are in the main unit, not in the same  
unit where the threads are. But these are TThreadLists so they should be  
thread-safe. There are no other global variables.

The thread's Execute method itself does not contain a try-except block. I  
guess I could try this and then when an exception occurs I do not display  
an error message - but I'm not sure if this would solve the problem.

To download a file from the Internet I use a modified version of the  
GetInetFile method found at  
http://delphi.about.com/od/internetintranet/l/aa013001a.htm However this  
method is also sorrounded with a try-except block which should prevent  
this kind of error.

I don't know if there's anything else. Anyone has an idea what went wrong?  
Why it works perfectly on my computer and doesn't on the clients?

I'm using Delphi 5, no custom components. I'm also using Windows XP with  
service pack 2.

If you've read the whole mail then thank you :)
If you could also suggest what the error might be then I would be most  
grateful. I've looked through the entire code several times and can't see  
anything that's wrong in it. Everything seems to be synchronized, when I  
access the items of the ThreadList I use LockList-UnlockList... The only  
thing that isn't synchronized is the GetInetFile method, but I'm guessing  
that shouldn't be a problem. Or should it?

Thanks,
Csaba