I began this two-part series about Win32 services by presenting the structure of service applications, reviewing restrictions related to service user-account settings, and stepping through the first phases of the Service Control Manager's (SCM's) initialization. This time, I continue with a detailed description of how auto-start services initialize during the system boot. You'll also learn about the steps the SCM takes when a service fails during its startup, and about how the SCM shuts down services. Finally, I cover some new features of Windows 2000's services support, including service failure-recovery options.
SvcCtrlMain (the SCM's startup function) invokes the SCM function ScAutoStartServices to start all services that have an auto-start Start value. (ScAutoStartServices also starts auto-start device drivers, but for the purposes of this article, when I use the term services, I mean services and drivers, unless I indicate otherwise.) ScAutoStartServices' algorithm for starting services in the correct order proceeds in phases; each phase corresponds to a group, and phases proceed in the sequence that the group ordering stored in the HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\ Control\ServiceGroupOrder Registry value defines. As I explained in Part 1, a service belongs to a group if the service's Registry key has a Group value. The ServiceGroupOrder value, which Figure 1, page 58, shows, lists the names of groups in the order in which the SCM starts them. Thus, assigning a service to a group has no effect other than to fine-tune its startup with respect to other services that belong to different groups.
When a phase starts, ScAutoStartServices marks for startup all the service entries that belong to the phase's group. Then, ScAutoStartServices loops through the marked services to determine whether it can start each one. Part of the check ScAutoStartServices makes consists of determining whether a service has a dependency on another group; the DependOnGroup value in the service's Registry key specifies group dependencies. If a dependency exists, then the group on which the service is dependent must already have initialized, and at least one service of the group must have started successfully. If the service depends on a group that starts later in the group startup sequence than the service's group, the SCM notes a circular dependency error for the service and doesn't start the service. If the service depends on any services from its group that haven't yet started, then ScAutoStartServices skips over the service. If ScAutoStartServices is checking a Win32 service and not a device driver, ScAutoStartServices next tries to determine whether the service depends on one or more other services, and if so, whether those other services have already started. The DependOnService Registry value in a service's Registry key stores service dependencies.
Before ScAutoStartServices starts a service that has passed the function's dependencies check, ScAutoStartServices makes a final check to determine whether the service is part of the current boot configuration. When a user boots the system in safe mode, the SCM ensures that either a name or a group identifies the service in the appropriate safe-boot Registry key. Two safe-boot keys, Minimal and Network, exist under HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\SafeBoot; which safe-boot key the SCM checks depends on which safe mode the user booted. If the user chose standard safe mode or safe mode with command prompt at the special boot menu (to access the special boot menu, you press F8 when prompted in the boot process), the SCM references the Minimal key. If the user chose networking-enabled safe mode, the SCM references the Network key. The existence of the Option string value under the SafeBoot key signals that the system booted in safe mode and lists the safe-mode type the user selected. (For more information about safe-mode booting options, see “Inside Win2K Reliability Enhancements, Part 1,” August 1999.)
After the SCM decides to start a service, it calls ScStartService, which takes different steps to start services than to start device drivers. When ScStartService starts a Win32 service, the function first reads the ImagePath value from the service's Registry key to determine the name of the file that runs the service's process. Then, ScStartService examines the service's Type value; if this value is SERVICE_WIN32_SHARE_PROCESS, the SCM ensures that the process the service runs in, if already started, is logged in using the same account as specified for the service. A service's ObjectName Registry value stores the user account in which the service will run. A service with no ObjectName or with the LocalSystem ObjectName runs in the Local System account, an account with security privileges that I described in Part 1.
To verify that the service's process has not already started in a different account, the SCM checks to see whether the service's ImagePath value has an entry in the image database, an internal SCM database. If the image database doesn't have an entry for the service's ImagePath, the SCM creates the entry. Image database entries store a logon account name and an ImagePath value. When it creates a new image database entry, the SCM includes the logon account name for the service and the service's ImagePath value.
All services must have an ImagePath value; if not, the SCM doesn't start the service and generates an error stating that it couldn't find the service's path. If the SCM locates an existing image database entry with a matching ImagePath value, the SCM ensures that the user-account information for the service is the same as that stored in the database entry. Because a process can be logged on only as one account, the SCM reports an error if a service specifies an account name that is different from the account name of another service that has already started in the same process.
The SCM calls ScLogonAndStartImage to optionally log on a service and start the service's process. To log on services that don't run in the system account, the SCM calls the Local Security Authority Subsystem (LSASS?\winnt\system32\lsass.exe) function LsaLogonUser. LsaLogonUser requires a password, and the SCM signals to LSASS that the password for a service that doesn't run in the system account is stored as a services LSASS secret under the HKEY_LOCAL_MACHINE\SECURITY\Policy\Secrets Registry key. When the SCM calls LsaLogonUser, the SCM specifies a service logon as the logon type, so LSASS looks up the password as a name in the form _SC_<service name> under the Secrets subkey. The SCM directs LSASS to store a logon password as a secret when a Service Control Program (SCP) configures a service's logon information. When the logon is successful, LsaLogonUser returns a handle to an access token to the SCM. Win2K uses access tokens to represent a user's security context, and the SCM later associates the token it received from LsaLogonUser with the process that implements the service.
After a successful logon, the SCM calls the UserEnv DLL's (\winnt\system32\userenv.dll) LoadUserProfile function to load the account's profile information, if the information isn't already loaded. LoadUserProfile loads the Registry hive (i.e., Registry file) that the user's profile key under HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Winlogon\ProfileList points to, making the hive the HKEY_CURRENT_USER key for the service.
An interactive service must open the WinSta0 window station, but before ScLogonAndStartImage lets an interactive service access WinSta0, the function confirms whether the HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Windows\NoInteractiveServices value is set. Administrators set this value to prevent interactive services from displaying windows on the console. This option is desirable in unattended server environments, where no user is present to respond to pop-ups from interactive services.
Next, ScLogonAndStartImage launches the service's process if the process hasn't already started (e.g., for another service). The SCM uses the CreateProcessAsUser Win32 API to start the process in a suspended state. The SCM next creates a named pipe through which it communicates with the service process and assigns the pipe the name \Pipe\Net\NetControlPipex, where x is a number that increments each time the SCM creates a pipe. The SCM uses the ResumeThread API to resume the service process and waits for the service to connect to its SCM pipe. If the Registry value HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\ServicesPipeTimeout exists, it determines the length of time that the SCM waits for a service to call StartServiceCtrlDispatcher and connect before the SCM gives up, terminates the process, and concludes that the service failed to start. If the ServicesPipeTimeout value doesn't exist, the SCM uses a default timeout of 30 seconds. The SCM uses the same timeout value for all its service communications.
When a service connects to the SCM through the pipe that the SCM created, the SCM sends the service a start command. If the service fails to respond positively to the start command within the timeout period, the SCM moves on to start the next service. When a service doesn't respond to a start request, the SCM doesn't terminate the process (as the SCM does when a service doesn't call StartServiceCtrlDispatcher within the timeout period). Instead, the SCM records an error in the System event log that the service failed to start in a timely manner.
If the service the SCM starts with a call to ScStartService has a SERVICE_KERNEL_DRIVER or SERVICE_FILE_SYSTEM_DRIVER Value type, the service is actually a device driver. In that case, ScStartService calls ScLoadDeviceDriver to load the driver. ScLoadDeviceDriver enables the load driver security privilege for the SCM process and invokes the kernel service NtLoadDriver, passing the ImagePath value of the driver's Registry key to the function. Unlike services, drivers don't need to specify an ImagePath value. If a driver doesn't specify an ImagePath value, the SCM concatenates the driver's name with \winnt\system32\drivers to build an ImagePath.
ScAutoStartServices continues looping through the services in a group until all the group's services have either started or generated dependency errors. This looping process is how the SCM automatically orders services within a group according to their DependOnService dependencies. The SCM starts services that other services depend on in earlier loops, leaving the dependent services for subsequent loops. The SCM ignores Tag values for Win32 services. You might come across Tag values in Registry keys under HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services. The I/O Manager uses Tag values to order device-driver startup for boot and system-start drivers.
After the SCM completes the startup phases for all the groups that the ServiceGroupOrder value lists, the SCM performs a startup phase for services that belong to groups that the value doesn't list. Finally, the SCM completes a final startup phase for services that don't belong to any group.
If a driver or service reports an error in response to the SCM's startup command, the ErrorControl value of the service's Registry key determines how the SCM reacts. If the ErrorControl value is SERVICE_ERROR_IGNORE (0) or isn't specified, the SCM simply ignores the error and continues processing service startups. If the ErrorControl value is SERVICE_ERROR_NORMAL (1), then the SCM writes an event to the System event log stating that the service failed to start and specifies the reason. The SCM includes in the event log the textual representation of the Win32 error code that the service returns to the SCM as the reason for the startup failure. Figure 2 shows the Event Viewer utility displaying an example event-log entry that reports a service startup error.
If a service with an ErrorControl value of SERVICE_ERROR_SEVERE (2) or SERVICE_ERROR_CRITICAL (3) reports a startup error, the SCM logs a record to the event log, then calls the internal function ScRevertToLastKnownGood. ScRevertToLastKnownGood switches the system's Registry configuration to the Last Known Good version, which is the boot configuration with which the system last booted successfully. Then, the SCM restarts the system using the NtShutdownSystem system service, which implements in the kernel. If the Registry configuration was already set to Last Known Good, then the system simply reboots.
In addition to making the SCM responsible for starting services, the system charges the SCM with determining when the system's Registry configuration, HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet, should be saved as the LastKnownGood control set. CurrentControlSet contains the Services key as a subkey, so the CurrentControlSet key includes the Registry representation of the SCM database. CurrentControlSet also contains the Control key, which stores many kernel- and user-mode subsystem configuration settings. By default, a successful boot consists of auto-start services successfully starting and a successful user logon. A boot fails if a device driver crashes the system during the boot, or if an auto-start service with an ErrorControl value of SERVICE_ERROR_SEVERE or SERVICE_ERROR_ CRITICAL reports a startup error.
The SCM obviously knows when it has successfully started auto-start services, but Winlogon (\winnt\system32\winlogon.exe) must notify the SCM when a user has successfully logged on. Winlogon invokes the ADVAPI32 (\winnt\system32\advapi32.dll) function NotifyBootConfigStatus when a user logs on, and NotifyBootConfigStatus sends a message to the SCM. Following the successful startup of auto-start services and receipt of the NotifyBootConfigStatus message (whichever comes last), SCM calls the system function NtInitializeRegistry to save the current Registry startup configuration.
Third-party software developers can supersede Winlogon's definition of a successful logon with their own definition. For example, a system running Microsoft SQL Server might not consider a boot successful until after SQL Server can accept and process transactions. A developer imposes its successful-boot definition by writing a boot verification program and installs the program by pointing to the program's on-disk location with the HKEY_LOCAL_MACHINESYSTEM\CurrentControlSet\Control\Boot VerificationProgram Registry value. In addition, a proprietary boot verification program's installation must set HKEY_ LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Win logon\ReportBootOk to 0 to disable Win logon's call to NotifyBootConfigStatus. When a proprietary boot verification program exists, the SCM launches the program after starting auto-start services, then waits for the program's call to NotifyBootConfigStatus before saving the LastKnownGood control set.
Win2K maintains several copies of CurrentControlSet, and the CurrentControl Set key is actually a symbolic Registry link that points to one of the copies. Control sets have names in the form HKEY_LOCAL_MACHINE\SYSTEM\ControlSet nnn, where nnn is a number such as 001 or 002. The HKEY_LOCAL_MACHINESYSTEM\Select key contains values that identify the role of each control set. For example, if CurrentControlSet points to ControlSet001, then Current under Select has a value of 1. Last Known Good contains the number of the Last KnownGood control set, which is the control set the system last used to boot successfully. You might have the Failed control set on your system. Failed points to the last control set for which the boot was unsuccessful and indicates that the system aborted in favor of attempting to boot with the LastKnownGood control set. Figure 3 displays a system's control sets and Select values.
NtInitializeRegistry synchronizes the contents of the LastKnownGood control set with CurrentControlSet's tree. After a system's first successful boot, LastKnownGood doesn't exist, so the system creates a LastKnownGood control set. If the LastKnownGood tree already exists, the system simply updates the tree by synchronizing it with CurrentControlSet.
The Win2K SCM's reliance on NtInitializeRegistry to update Last Known Good differs from the NT 4.0 SCM's behavior. In NT 4.0, the SCM is fully responsible for managing control sets. Before the NT 4.0 SCM starts auto-start services, it copies the CurrentControlSet to a new key, Clone. After the boot succeeds, the SCM makes a copy of the Clone key and labels the key LastKnownGood. Win2K thereby optimizes performance because whereas NT 4.0 copies CurrentControlSet twice, Win2K usually performs only an update, not a copy operation.
Last Known Good is helpful in situations in which a change to CurrentControlSet, such as the modification of a system performance-tuning value under HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control or the addition of a service or device driver, causes the subsequent boot to fail. Users can press F8 early in the boot process to bring up a menu that lets them direct the boot to use the LastKnownGood control set, which rolls the system's Registry configuration back to the way it was the last time the system booted successfully.
In NT 4.0, a service that fails after it starts successfully does so silently. An administrator has no way of knowing, without checking manually or using third-party service-monitoring utilities, that the service's process has exited. Win2K introduces service failure-action capability, a feature that the SCM implements. A Win2K service can have optional FailureAction and FailureCommand values in its Registry key; the SCM records these values during the service's startup. The SCM registers with the system so that the system signals the SCM when a service process exits. When a service process terminates unexpectedly, the SCM determines which services ran in the process and takes the recovery steps that the services' failure-related Registry values specify.
Actions that a service can configure for the SCM include restarting the service, running a program, or rebooting the computer. In addition, a service can specify what failure actions take place the first, second, and subsequent times the service process fails and can indicate a period during which the SCM must wait before restarting the service if the service asks to be restarted. For example, the IIS Administrator service's failure action results in the SCM running the IISReset application, which performs cleanup work and restarts the service. You can easily manage a service's recovery actions from the Recovery tab of the service's Properties dialog box in the Services Microsoft Management Console (MMC) snap-in, as Figure 4, page 62, shows.
When Winlogon calls the Win32 ExitWindowsEx API, ExitWindowsEx sends a message to CSRSS, the Win32 subsystem process, that invokes CSRSS's shutdown routine. CSRSS loops through all active processes and notifies them that the system is shutting down. For every system process except the SCM, CSRSS waits for the number of seconds that HKEY_USERS\.DEFAULT\Control Panel\Desktop\WaitToKillAppTimeout specifies (the default is 20 seconds) for the process to exit before moving to the next process. When CSRSS encounters the SCM process, CSRSS notifies the SCM that the system is shutting down and employs a timeout specific to the SCM. The process ID that the SCM used when it registered with CSRSS during system initialization lets CSRSS recognize the SCM. The SCM's timeout differs from that of other processes because the SCM communicates with services that need to perform cleanup when they shut down; thus, an administrator might need to tune only the SCM's timeout. The SCM's timeout value resides in the HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\WaitToKillServicesTimeout Registry value and defaults to 30 seconds.
The SCM's shutdown handler sends shutdown notifications to all services that request shutdown notification when they initialize with the SCM. The SCM function ScShutdownAllServices loops through the SCM services database searching for services that request shutdown notification and sends each such service a shutdown command. For each service to which ScShutdownAllServices sends a shutdown command, the SCM records the value of the service's wait hint, a value that a service also specifies when it registers with the SCM. The SCM keeps track of the largest wait hint it records. After ScShutdownAllServices sends shutdown messages, the SCM waits until one of the services notified of shutdown exits or until the largest wait hint's timeout passes.
If the wait hint expires before a service exits, the SCM determines whether one or more of the services upon which it was waiting has sent a message notifying the SCM that the service is progressing in its shutdown process. If at least one service made progress toward shutdown, the SCM waits again for the duration of the largest wait hint's timeout. The SCM continues this wait loop until either all the services have exited or none of the services upon which it is waiting has sent notification of shutdown progress within the wait hint's timeout.
After the SCM has directed services to shut down and is waiting for the services to exit, CSRSS waits for the SCM to exit. If CSRSS's wait ends before the SCM exits (i.e., the WaitToKillServicesTimeout expires), CSRSS simply continues the shutdown process. Thus, CSRSS leaves running those services (as well as the SCM) that fail to shut down in a timely manner as the system shuts down. Unfortunately, administrators have no way of knowing whether they should raise the WaitToKillServicesTimeout value on systems on which services are not getting a chance to completely shut down before the system shuts down.
Running every service in its own process, rather than having services share a process when sharing is possible, wastes system resources. However, when services share a process, any service in the shared process that has a bug that causes the process to exit also causes all the services in the process to terminate. Win2K includes many built-in services, so Microsoft chose a mix of approaches to maximize system stability and minimize resource usage.
In both Win2K and NT 4.0, the SCM process hosts many services, including the EventLog service, the file-server service (LanmanServer), and the LAN Manager name-resolution service. Table 1 lists the services that the SCM hosts in Win2K (not every service is active on every system). You can use the Tlist program from the Win2K support tools CD-ROM with the /s option to obtain a list of which services run in processes.
TABLE 1: Windows 2000 Services That Run in the SCM
|Alerter||Notifies selected users and computers of administrative alerts.|
|AppMgmt||Provides software installation services such as Assign, Publish, and Remove.|
|Browser||Maintains an up-to-date list of computers on your network and supplies the list to programs that request it.|
|Dhcp||Manages network configuration by registering and updating IP addresses and DNS names.|
|DmServer||Enables the Logical Disk Manager (LDM) Watchdog Service.|
|DnsCache||Resolves and caches DNS names.|
|EventLog||Logs event messages that programs and Windows issue. EventLog reports contain information that can be useful in diagnosing problems. Event Viewer displays reports.|
|LanmanServer||Provides Remote Procedure Call (RPC) support and file, print, and named pipe sharing.|
|LanmanWorkstation||Provides network connections and communications.|
|LmHosts||Enables support for NetBIOS over TCP/IP (NetBT) service and NetBIOS name resolution.|
|Messenger||Sends and receives messages that administrators or the Alerter service transmit.|
|PlugPlay||Manages device installation and configuration and notifies programs of device changes.|
|ProtectedStorage||Provides protected storage for sensitive data (e.g., private keys) to prevent access by unauthorized services, processes, or users.|
|SecLogon||Enables starting processes under alternative credentials.|
|TrkSvr||Stores information to track files moved between volumes for each volume in the domain.|
|TrkWks||Sends notifications of files moving between NTFS volumes in a network domain.|
|W32Time||Sets the computer clock.|
|Wmi Provides||systems management information to and from drivers.|
In NT 4.0, the SCM is the only process that the system uses to host multiple built-in services. Several other services, such as the Remote Procedure Call Subsystem service (RpcSs), the Telephony API service (TapiSrv), and Remote Access Manager (RasMan), use separate processes.
Because of the increased number of built-in services in Win2K, Microsoft decided to include more services in the Win2K SCM and to add new processes to act as service hosts. In NT 4.0, the LSASS process included only the Netlogon service. In Win2K, several security-related services (e.g., the Security Accounts Manager Subsystem?SamSs?the Netlogon service, and the Win2K Policy Agent service) share the LSASS process.
Win2K also introduces Service Host (winnt\system32\svchost.exe), an application that exists solely to execute in processes that host services. A Win2K system can start multiple instances of SvcHost running in different processes. Services that run in SvcHost processes include TapiSrv, RpcSs, and RasMan. Win2K implements services that run in SvcHost as DLLs and includes an ImagePath definition of the form \%systemroot%\system32\svchost.exe -k netsvcs in the services' Registry key. The services' Registry key must also have a Registry value of ServiceDll under a Parameters subkey that points to the services' DLL file.
All services that share a common SvcHost process specify the same parameter (-k netsvcs in the previous example) so that they each have the same entries in the SCM's image database. When the SCM first encounters a service during service startup with a SvcHost ImagePath and a particular parameter, the SCM creates a new image database entry and launches a SvcHost process with the parameter. The new SvcHost process looks under HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\SvcHost for a value that has the same name as the service's parameter. SvcHost interprets the contents of the value as a list of service names; when it registers with the SCM, SvcHost notifies the SCM that the SvcHost process is hosting those services. Figure 5 presents an example of a SvcHost Registry key that shows that a SvcHost process started with the -k netsvcs parameter is prepared to host many different network-related services.
When the SCM encounters a SvcHost service during service startup with an ImagePath that matches an entry that already appears in the SCM's image database, the SCM doesn't launch a second process. Rather, the SCM sends a start command for the service to the SvcHost process that is already started for that ImagePath value. The existing SvcHost process reads the ServiceDll parameter in the service's Registry key and loads the DLL into the service's process to start the service.
SCPs are standard Win32 applications that use SCM APIs that ADVAPI32 exports. APIs that the SCM implements include CreateService, OpenService, StartService, ControlService, QueryServiceStatus, and DeleteService. To use a SCM API, an SCP must first call the OpenSCManager API to open a communications channel to the SCM. At the time of the open call, the SCP must specify the types of actions it wants to perform. For example, if an SCP wants to enumerate and display the services in the SCM's database, the SCP requests enumerate-service access in its call to OpenSCManager. As the SCM initializes, it creates an internal object that represents the SCM database. The SCM uses the Win2K security APIs to protect the internal object, a security descriptor that specifies which accounts can open the object with which access permissions. For example, the security descriptor specifies that the Everyone group (of which every account is a member) has permission to open the SCM internal object with enumerate-service access. However, only administrators have permission to open the object with the access required to create or delete a service.
The SCM implements security for services, as it does for the SCM database. When an SCP uses the CreateService API to create a service, the SCP specifies a security descriptor that the SCM internally associates with the service's entry in the service database. The SCM stores the security descriptor in the service's Registry key as the Security value and reads the value when it scans the Registry's Services key during initialization. This setup ensures that the security settings persist across reboots. Just as an SCP must specify in its call to OpenSCManager what types of access the SCP wants to the SCM database, an SCP must tell the SCM in a call to OpenService what access the SCP wants to a service. Accesses that an SCP can request include the ability to query a service's status and to configure, stop, and start a service.
The SCP that you're likely most familiar with is the Control Panel Services applet in NT 4.0 and the Services MMC snap-in in Win2K. The NT 4.0 Control Panel Services applet implements its SCP in the \winnt\system32\srvmgr.cpl library, and in Win2K, the SCP resides in \winnt\system32\filemgr.dll. The Microsoft Windows NT Server 4.0 Resource Kit and the Microsoft Windows 2000 Resource Kit include sc.exe, a command-line SCP.
SCPs sometimes layer service policy over services that the SCM implements. A good example is the timeout that the Services MMC snap-in implements when you start a service manually. The snap-in displays a progress bar that represents the service's startup progress. Whereas the SCM waits indefinitely for a service to respond to a start command, the Services snap-in waits only 2 minutes before the progress bar reaches 100 percent and the snap-in announces that the service didn't start in a timely manner. Services indirectly interact with SCPs by adjusting their configuration status to reflect their progress as they respond to SCM commands such as the start command. SCPs query services' status with the QueryServiceStatus API. Thus, SCPs can tell when a service actively updates its status or appears to be hung, and the SCM can take action to notify users about the service's activity.
The Win2K and NT 4.0 resource kits include the SrvAny utility, which lets you run any application as a service. (For information about how to use SrvAny, see Mark Minasi, This Old Resource Kit, February 2000 and March 2000.) SrvAny is similar to SvcHost?both are generic service-host applications. As in SvcHost, a SrvAny process reads the path of the service file that it loads from the Parameters subkey of the service's Registry key. When SrvAny starts, it notifies the SCM that it is hosting a particular service. Then, when SrvAny receives a start command, it launches the service executable file as a child process. Because the child process receives a copy of the SrvAny process' access token and a reference to the same window station, the executable file runs in the same security account and with the same interactivity setting that you specified when you configured the SrvAny process. Unlike SvcHost, however, SrvAny services don't have the share-process Type value. Therefore, each application you install as a service in SrvAny runs in a separate process with a different instance of the SrvAny host program.
In addition to its role as a services interface, the SCM has another responsibility: It notifies GUI applications in a system whenever the system creates or deletes a network drive-letter connection. The SCM waits for the LAN Manager Workstation service to signal the ScNetDrvMsg named event. (The Workstation service signals ScNetDrvMsg whenever an application assigns a drive letter to a remote network share or deletes a remote-share drive letter assignment.) When the Workstation service signals the event, the SCM uses the GetDriveType Win32 API to query the list of connected network drive letters. If the list changes across the event signal, the SCM sends a type WM_DEVICECHANGE Windows broadcast message. The SCM uses either DBT_DEVICEREMOVECOMPLETE or DBT_DEVICEARRIVAL as the message's subtype. This message is intended primarily for Windows Explorer, so that it can update any open My Computer windows to show the presence or absence of a network drive letter.
Although the SCM remained mostly unchanged from NT 4.0 to Win2K, it picked up the powerful new capability of detecting service process failures and taking recovery steps to restart services or run arbitrary programs. In addition, Win2K's use of the SvcHost application reduces the overhead otherwise incurred with the increase in the number of built-in Win2K services. This information about service startup and shutdown can help you better understand what is going on behind the scenes so that you can troubleshoot service-related problems you might encounter.