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Service Control Manager (SCM) is a special system process under the
Windows NT Windows NT is a proprietary graphical operating system produced by Microsoft, the first version of which was released on July 27, 1993. It is a processor-independent, multiprocessing and multi-user operating system. The first version of Wi ...
family of
operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ef ...
s, which starts, stops and interacts with Windows service processes. It is located in the %SystemRoot%\System32\services.exe executable. Service processes interact with SCM through a well-defined API, and the same API is used internally by the interactive Windows service management tools such as the
MMC MMC may stand for: Business *Mail.com Media Corp., a former name of the US digital media company PMC * Marsh & McLennan Companies, an American-based global professional services firm *Master Municipal Clerk, a certification in the US for a munic ...
snap-in Services.msc and the command-line Service Control utility sc.exe. Terminating this file is used as a method of causing the Blue Screen of Death.


Implementation

The SCM executable, Services.exe, runs as a Windows console program and is launched by the
Wininit Service Control Manager (SCM) is a special system process under the Windows NT family of operating systems, which starts, stops and interacts with Windows service processes. It is located in the %SystemRoot%\System32\services.exe executable. Service ...
process early during the system startup. Its main function, SvcCtrlMain(), launches all the services configured for automatic startup. First an internal database of installed services is initialized by reading the following two registry keys: * HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\ServiceGroupOrder\List, containing the names and order of service groups. Each service's registry key contains an optional Group value which governs the order of initialization of a respective service or a
device driver In computing, a device driver is a computer program that operates or controls a particular type of device that is attached to a computer or automaton. A driver provides a software interface to hardware devices, enabling operating systems and o ...
, with respect to other service groups. * HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services, which contains the actual database of services and device drivers and is read into SCM's internal database. SCM reads every service's Group value as well as load-order dependencies from their DependOnGroup and DependOnService registry keys. In the next step, SCM's main function SvcCtrlMain() calls the function ScGetBootAndSystemDriverState() function which checks whether the device drivers that should be started during the boot or system startup were successfully loaded, and those that have failed to do so are stored in a list called ScFailedDrivers. Then a
named pipe In computing, a named pipe (also known as a FIFO for its behavior) is an extension to the traditional pipe concept on Unix and Unix-like systems, and is one of the methods of inter-process communication (IPC). The concept is also found in OS/ ...
\Pipe\Ntsvcs is created as a remote procedure call interface between the SCM and the SCPs (Service Control Processes) that interact with specific services. Next, it calls the ScAutoStartServices() function which loops through all the services marked as auto-start, paying attention to the calculated load-order dependencies. In case of a circular dependency an error is noted and the service depending on a service that belongs to a group coming later in the load order is skipped. For delayed auto-start services, grouping has no effect, and those are loaded at a later stage of system startup. For each service it wants to start, the SCM calls the ScStartService() function which checks the name of the file that runs the service's process, ensuring that the account specified for the service is same as the account that the service process runs in. Every service that does not run in the System account is logged in by calling the LSASS function LogonUserEx()
/code>, for which LSASS process looks up "secret" passwords stored in the HKLM\SECURITY\Policy\Secrets\ registry key, which were stored by the SCP using the LsaStorePrivateData()
/code> API, when the service was originally configured. Next, the ScLogonAndStartImage() function is called for every service whose service process has not been already launched. Service processes are created in a suspended state via the CreateProcessAsUser()
/code> API. Before the service process' execution is resumed, a named pipe \Pipe\Net\NtControlPipeX (where X is a number incremented for each service iteration) is created which serves as a communication channel between the SCM and the service process. Service process connects to the pipe by calling the StartServiceCtrlDispatcher()
/code> function, after which the SCM sends the service a "start" command.


Delayed auto-start services

Delayed auto-start services have been added in
Windows Vista Windows Vista is a major release of the Windows NT operating system developed by Microsoft. It was the direct successor to Windows XP, which was released five years before, at the time being the longest time span between successive releases of ...
, in order to solve the problem of a prolonged system startup, as well as to speed-up the start of critical services that cannot be delayed. Originally the auto-start method of service initialization was designed for essential system services upon which other applications and services depend. The SCM initializes the delayed services only after handling all the non-delayed auto-start services, by invoking the ScInitDelayStart() function. This function queues a delayed (120 seconds by default) work item associated with a corresponding worker thread. Other than being initialized after a delay, there are no other differences between delayed and non-delayed services.


Device drivers

Services whose Type registry value is SERVICE_KERNEL_DRIVER or SERVICE_FILE_SYSTEM_DRIVER are handled specially: these represent device drivers for which ScStartService() calls the ScLoadDeviceDriver() function which loads the appropriate driver (usually a file with an extension .sys) which must be located in the %SystemRoot%\System32\Drivers\ directory. For that purpose, the NtLoadDriver
/code>
system call In computing, a system call (commonly abbreviated to syscall) is the programmatic way in which a computer program requests a service from the operating system on which it is executed. This may include hardware-related services (for example, acc ...
is invoked, and the SeLoadDriverPrivilege is added to the SCM's process.


Network drive letters

SCM provides an additional functionality completely unrelated to Windows services: it notifies GUI applications such as the
Windows Explorer File Explorer, previously known as Windows Explorer, is a file manager application that is included with releases of the Microsoft Windows operating system from Windows 95 onwards. It provides a graphical user interface for accessing the file ...
when a network drive-letter connection has been created or deleted, by broadcasting Windows
messages A message is a discrete unit of communication intended by the source for consumption by some recipient or group of recipients. A message may be delivered by various means, including courier, telegraphy, carrier pigeon and electronic bus. A ...
WM_DEVICECHANGE.


See also

* Windows service * List of Windows Services * Operating system service management *
svchost.exe Svchost.exe (Service Host, or SvcHost) is a system process that can host from one or more Windows services in the Windows NT family of operating systems. Svchost is essential in the implementation of ''shared service processes'', where a number o ...


Notes


References

* {{Windows Components Windows services Windows components