Windows® Internals, Sixth Edition, Part 2

This time, Csrss sees that the request is from Winlogon and loops through all the processes in the logon session of the interactive user (again, not the user who requested a shutdown) in reverse order of their shutdown level. A process can specify a shutdown level, which indicates to the system when it wants to exit with respect to other processes, by calling SetProcessShutdownParameters. Valid shutdown levels are in the range 0 through 1023, and the default level is 640. Explorer, for example, sets its shutdown level to 2 and Task Manager specifies 1. For each process that owns a top-level window, Csrss sends the WM_QUERYENDSESSION message to each thread in the process that has a Windows message loop. If the thread returns TRUE, the system shutdown can proceed. Csrss then sends the WM_ENDSESSION Windows message to the thread to request it to exit. Csrss waits the number of seconds defined in HKCU\Control Panel\Desktop\HungAppTimeout for the thread to exit. (The default is 5,000 milliseconds.)

If the thread doesn’t exit before the timeout, Csrss fades out the screen and displays the hung-program screen shown in Figure 13-11. (You can disable this screen by creating the registry value HKCU\Control Panel\Desktop\AutoEndTasks and setting it to 1.) This screen indicates which programs are currently running and, if available, their current state. Windows indicates which program isn’t shutting down in a timely manner and gives the user a choice of either killing the process or aborting the shutdown. (There is no timeout on this screen, which means that a shutdown request could wait forever at this point.) Additionally, third-party applications can add their own specific information regarding state—for example, a virtualization product could display the number of actively running virtual machines.

Figure 13-11. Hung program screen

EXPERIMENT: Witnessing the HungAppTimeout

You can see the use of the HungAppTimeout registry value by running Notepad, entering text into its editor, and then logging off. After the amount of time specified by the HungAppTimeout registry value has expired, Csrss.exe presents a prompt that asks you whether or not you want to end the Notepad process, which has not exited because it’s waiting for you to tell it whether or not to save the entered text to a file. If you click the Cancel button, Csrss.exe aborts the shutdown.

As a second experiment, if you try shutting down again (with Notepad’s query dialog box still open), Notepad will display its own message box to inform you that shutdown cannot cleanly proceed. However, this dialog box is merely an informational message to help users—Csrss.exe will still consider that Notepad is “hung” and display the user interface to terminate unresponsive processes.

If the thread does exit before the timeout, Csrss continues sending the WM_QUERYENDSESSION/WM_ENDSESSION message pairs to the other threads in the process that own windows. Once all the threads that own windows in the process have exited, Csrss terminates the process and goes on to the next process in the interactive session.

If Csrss finds a console application, it invokes the console control handler by sending the CTRL_LOGOFF_EVENT event. (Only service processes receive the CTRL_SHUTDOWN_EVENT event on shutdown.) If the handler returns FALSE, Csrss kills the process. If the handler returns TRUE or doesn’t respond by the number of seconds defined by HKCU\Control Panel\Desktop\WaitToKillAppTimeout (the default is 20,000 milliseconds), Csrss displays the hung-program screen shown in Figure 13-11.

Next, Winlogon calls ExitWindowsEx to have Csrss terminate any COM processes that are part of the interactive user’s session.

At this point, all the processes in the interactive user’s session have been terminated. Wininit next calls ExitWindowsEx, which this time executes within the system process context. This causes Wininit to send a message to the Csrss part of session 0, where the services live. Csrss then looks at all the processes belonging to the system context and performs and sends the WM_QUERYENDSESSION/WM_ENDSESSION messages to GUI threads (as before). Instead of sending CTRL_LOGOFF_EVENT, however, it sends CTRL_ SHUTDOWN_EVENT to console applications that have registered control handlers. Note that the SCM is a console program that does register a control handler. When it receives the shutdown request, it in turn sends the service shutdown control message to all services that registered for shutdown notification. For more details on service shutdown (such as the shutdown timeout Csrss uses for the SCM), see the “Services” section in Chapter 4 in Part 1.