In addition to its role as an interface to services, the SCM has another totally unrelated responsibility: it notifies GUI applications in a system whenever the system creates or deletes a network drive-letter connection. The SCM waits for the Multiple Provider Router (MPR) to signal a named event, \BaseNamedObjects\ScNetDrvMsg, which MPR signals whenever an application assigns a drive letter to a remote network share or deletes a remote-share drive-letter assignment. (See Chapter 7, for more information on MPR.) When MPR signals the event, the SCM calls the GetDriveType Windows function to query the list of connected network drive letters. If the list changes across the event signal, the SCM sends a Windows broadcast message of type WM_DEVICECHANGE. The SCM uses either DBT_DEVICEREMOVECOMPLETE or DBT_DEVICEARRIVAL as the message’s subtype. This message is primarily intended for Windows Explorer so that it can update any open Computer windows to show the presence or absence of a network drive letter.
Service Startup
SvcCtrlMain invokes the SCM function ScAutoStartServices to start all services that have a Start value designating auto-start (except delayed auto-start services). ScAutoStartServices also starts auto-start device drivers. To avoid confusion, you should assume that the term services means services and drivers unless indicated otherwise. The algorithm in ScAutoStartServices for starting services in the correct order proceeds in phases, whereby a phase corresponds to a group and phases proceed in the sequence defined by the group ordering stored in the HKLM\SYSTEM\CurrentControlSet\Control\ServiceGroupOrder\List registry value. The List value, shown in Figure 4-13, includes the names of groups in the order that the SCM should start them. Thus, assigning a service to a group has no effect other than to fine-tune its startup with respect to other services belonging to different groups.
Figure 4-13. ServiceGroupOrder registry key
When a phase starts, ScAutoStartServices marks all the service entries belonging to the phase’s group for startup. Then ScAutoStartServices loops through the marked services seeing whether it can start each one. Part of this check includes seeing whether the service is marked as delayed auto-start, which causes the SCM to start it at a later stage. (Delayed auto-start services must also be ungrouped.) Another part of the check it makes consists of determining whether the service has a dependency on another group, as specified by the existence of the DependOnGroup value in the service’s registry key. If a dependency exists, the group on which the service is dependent must have already initialized, and at least one service of that group must have successfully started. If the service depends on a group that starts later than the service’s group in the group startup sequence, the SCM notes a “circular dependency” error for the service. If ScAutoStartServices is considering a Windows service or an auto-start device driver, it next checks to see whether the service depends on one or more other services, and if so, if those services have already started. Service dependencies are indicated with the DependOnService registry value in a service’s registry key. If a service depends on other services that belong to groups that come later in the ServiceGroupOrder\List, the SCM also generates a “circular dependency” error and doesn’t start the service. If the service depends on any services from the same group that haven’t yet started, the service is skipped.
When the dependencies of a service have been satisfied, ScAutoStartServices makes a final check to see whether the service is part of the current boot configuration before starting the service. When the system is booted in safe mode, the SCM ensures that the service is either identified by name or by group in the appropriate safe boot registry key. There are two safe boot keys, Minimal and Network, under HKLM\SYSTEM\CurrentControlSet\Control\SafeBoot, and the one that the SCM checks depends on what safe mode the user booted. If the user chose Safe Mode or Safe Mode With Command Prompt at the special boot menu (which you can access by pressing F8 early in the boot process), the SCM references the Minimal key; if the user chose Safe Mode With Networking, the SCM refers to Network. The existence of a string value named Option under the SafeBoot key indicates not only that the system booted in safe mode but also the type of safe mode the user selected. For more information about safe boots, see the section “Safe Mode” in Chapter 13 in Part 2.
