Where should I add my own code in the service template? - c++

Question

Microsoft provides a complete service sample to start with writing Windows services. However, I do not understand the following part from this file :
ReportSvcStatus( SERVICE_RUNNING, NO_ERROR, 0 );
// TO_DO: Perform work until service stops.
while(1)
{
// Check whether to stop the service.
WaitForSingleObject(ghSvcStopEvent, INFINITE);
ReportSvcStatus( SERVICE_STOPPED, NO_ERROR, 0 );
return;
}
I don't understand the point of the infinite loop containing a return statement. Doesn't it defeat the purpose ?
It seems to encourage to write the service content before the while loop but then, if we do not reach the following line :
WaitForSingleObject(ghSvcStopEvent, INFINITE);
... The service will not able to stop when the proper event is triggered, would it ?
Is this template flawed ? How can I make my service wait for an external trigger without making it impervious to stop calls ?

Answer 1

Yes, that example is not particularly well-written. The main loop of the service is better written (conceptually) as:
// Main processing loop
while (!quit)
do_work ();
ReportSvcStatus (SERVICE_STOPPED, NO_ERROR, 0);
return;
And in the service's control handler, you would have:
// CtrlHandler callback
DWORD WINAPI CtrlHandler (DWORD dwControl, DWORD dwEventType, LPVOID lpEventData, LPVOID lpContext)
{
...
if (dwControl == SERVICE_CONTROL_STOP)
quit = true;
return NO_ERROR;
}
Where quit is a global variable.
In practise (to avoid busy looping), the service probably normally sits around waiting on some sort of waitable object for something to do. Let's imagine, for the sake of argument, that's a HANDLE called, say, ghWakeupEvent, created via CreateEvent(), and again stored in a global variable.
Then, the code becomes something like:
// Main processing loop
while (1)
{
WaitForSingleObject (ghWakeupEvent, INFINITE);
if (quit)
{
ReportSvcStatus (SERVICE_STOPPED, NO_ERROR, 0);
return;
}
if (something_to_do)
do_work ();
}
// CtrlHandler callback
DWORD WINAPI CtrlHandler (DWORD dwControl, DWORD dwEventType, LPVOID lpEventData, LPVOID lpContext)
{
...
if (dwControl == SERVICE_CONTROL_STOP)
{
quit = true; // do this first!!
SetEvent (ghWakeupEvent);
}
return NO_ERROR;
}
Note: no need for (or point in) ghSvcStopEvent. The MSDN sample is a mixed-up mess.