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Printing from Service's thread

Posted on 2004-11-03
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Last Modified: 2010-04-05
Hi Experts,

I have a Service Application written in Delphi 6. I need to print a page to the default printer, but this has to be done in a thread.
Procedure doprint;
Begin
 Printer.BeginDoc;
 Printer.Canvas.TextOut(x,y, 'My text to print');
 Printer.EndDoc;
End;
This seems to be ok, but sometimes, printing stops. Exactly, the document is put into the print spooler's queue, but never printed out. If I delete the documents from the print queue it works again.

I think the problem is due the fact, the Canvas object is not thread-safe, so I've put it into synchronize like this:
Synchronize(doprint);
The problem still exists. Delphi's help tells that, the method called with synchronize will be executed in the main VCL thread.
But I have not VCL thread, I have a service which is also a thread...
Moreover, I have no error message or exception at all...


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Question by:spbltd
2 Comments
 
LVL 17

Accepted Solution

by:
Wim ten Brink earned 250 total points
ID: 12495401
This could be a security related issue. Personally, I've never tried to print from a service anyway, because then I would need to provide a user account for this service and make sure that account has access to at least a default printer. I'm not even sure if a service actually has any real access rights to the printer in the first place.

About synchronizations, this is required if you're working with a GUI-based application that depends on the Forms unit. When you call the Synchronize method of a thread, it will actually send a Windows message to the main thread. But a service doesn't have a message handler thus this would just fail. You would have to use a critical section to make sure printer access is synchronized.
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Author Comment

by:spbltd
ID: 12509440
Finally we have found the problem, it was a hardware failure.

But your comments about the synchronization is still useful, so I accept your answer
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