[python-win32] Using signal handlers in a Windows Service

Justin Johnson justinjohnson at gmail.com
Mon Dec 5 18:19:38 CET 2005

I'm trying to setup an application that runs as a Windows service to rotate
logs upon receiving a certain signal.  Whenever I try to do this I get the
following error.

Exception in TestService.SvcStart: signal only works in main thread

Attached is a very trimmed down example.  To run the example and see the
error message, do the following.

   1. In one DOS window run
   PYTHONHOME\Lib\site-packages\win32\lib\win32traceutil.py to begin collecting
   trace output.
   2. In a separate DOS window run "python tsvc.py --username domain\user
   --password yourpassword --startup auto install" using your own user and
   3. Using Start > Control Panel > Administrative Tools > Services,
   modify the service properties, modifying the user and password again (even
   though you already specified them) and click OK.  Accept any prompts about
   granting the appropriate rights to run as a service.
   4. Run "net start tsvc"
   5. Observe the error in the win32traceutil window.

I realize this is a known problem but am unsure how to get around it to make
sure my log rotation works when receiving the signal.  Does anyone have any
ideas on how to get around this limitation?

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import win32serviceutil
import win32service
import sys
import os

SVC_NAME = "tsvc"
SVC_DISPLAY_NAME = "Test Service"

class TestService(win32serviceutil.ServiceFramework):

    _svc_name_ = SVC_NAME
    _svc_display_name_ = SVC_DISPLAY_NAME

    def SvcDoRun(self):
        ## Import inside the method, so we redirect all output
        ## to the trace output collector.  If you need to debug this service,
        ## run win32traceutil.py as a script.  It will receive all output from
        ## this service.
        import win32traceutil
            import signal
            SIGUSR1 = 10 # Windows doesn't care what the number is
            def rotateLog(signal, frame):
            signal.signal(SIGUSR1, rotateLog)
        except Exception, e:
            print "Exception in TestService.SvcStart:", str(e)

    def SvcStop(self):
        ## See above for why we import this.
        import win32traceutil
        except Exception, e:
            print "Exception in TestService.SvcStop:", str(e)

if __name__ == '__main__':

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