Client/Server Runtime Subsystem: Difference between revisions

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{{Short description|Windows NT operating system component}}

'''Client Server Runtime Subsystem''', or <code>csrss.exe</code>, is a component of the [[Windows NT]] family of [[operating system]]s that provides the [[User space|user mode]] side of the [[Windows API|Win32 subsystem]] and is included in [[Windows NT 3.1]] and later.<ref name="GDI" /> Because most of the Win32 subsystem operations have been moved to [[kernel mode]] [[Device driver|drivers]] in [[Windows NT 4]] and later, CSRSS is mainly responsible for [[Win32 console]] handling and GUI shutdown. It is critical to system operation; therefore, terminating this [[Process (computing)|process]] will result in system failure. Under normal circumstances, CSRSS cannot be terminated with the ''[[kill (command)|taskkill]]'' command or with [[Windows Task Manager]], although it is possible in [[Windows Vista]] if the Task Manager is run in Administrator mode. On [[Windows 7]] and later, Task Manager will inform the user that terminating the process may result in system failure, and prompt if they want to continue. In Windows NT 4.0 however, killing csrss without smss watching will not crash the system. However in Windows XP, killing csrss without smss watching will crash the system due to the critical bit being set in RAM for csrss.exe

The '''Client/Server Runtime Subsystem''', or <code>csrss.exe</code>, is a component of the [[Windows NT]] family of [[operating system]]s that provides the [[User space|user mode]] side of the [[Windows API|Win32 subsystem]]. In modern versions of Windows, it is primarily involved with process and thread management, [[Win32 console|console window]] handling, [[side-by-side assembly]] loading and the shutdown process. Historically, it had also been responsible for window management and graphics rendering, however, these operations have been moved to [[kernel mode]] starting with [[Windows NT 4.0]] to improve performance.<ref>{{cite web

== History ==

The [[Windows NT 3.x]] series of releases had placed the [[Graphics Device Interface]] component in CSRSS, but this was moved into kernel mode with Windows NT 4.0 to improve graphics performance.<ref name="GDI">{{cite web

|url=https://technet.microsoft.com/en-us/library/cc750820.aspx#XSLTsection124121120120

|title=The Windows NT 4.0 Kernel mode change

|accessdateaccess-date=2009-01-19

|work=MS Windows NT Kernel-mode User and GDI White Paper

|publisher=Microsoft

}}</ref>

}}</ref> The Windows startup process from Vista onward has changed significantly. Two instances of csrss.exe are running in Windows 7 and Vista.<ref>{{cite web

CSRSS instances are marked as critical processes, meaning that terminating one will [[blue screen of death|crash]] the system, if the critical status is removed and one is terminated, the system will freeze. Built-in process management tools in most Windows versions will also refuse to kill instances of CSRSS. Under normal operation, there is a CSRSS instance for each session (two in [[Windows Vista]] and newer, one in earlier versions,<ref>{{cite web

|url=https://technet.microsoft.com/en-us/magazine/2007.03.vistakernel.aspx

|title=Inside the Windows Vista Kernel – Startup Processes

|accessdateaccess-date=2010-10-01

|work=Inside the Windows Vista Kernel – Startup Processes

|publisher=Microsoft

}}</ref> both assuming there are no active [[Remote Desktop Protocol|RDP]] connections which spawn extra sessions).

}}</ref>

== Technical details ==

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|url=http://www.left-brain.com/tabId/65/itemId/1642/pageId/29/Undocumented-Windows-NT.aspx

|title=Detailed implementation of a system service in Windows NT

|accessdateaccess-date=2010-06-10

|work=Undocumented Windows NT

|archive-url=https://web.archive.org/web/20110717032622/http://www.left-brain.com/tabId/65/itemId/1642/pageId/29/Undocumented-Windows-NT.aspx

|archive-date=2011-07-17

|url-status=dead

}}</ref> Window manager and [[Graphics Device Interface|GDI]] services are handled by a kernel mode driver (win32k.sys) instead.<ref>{{cite book|last=Russinovich|first=Mark|authorlink=Mark Russinovich|title=Windows Internals, 5th Edition|year=2009|publisher=Microsoft Press|pages=54}}</ref>

CSRSS is called along with <code>winlogon.exe</code> from [[Session Manager Subsystem|smss.exe]] at Windows start-up. If either of the files is corrupted or otherwise inaccessible, SMSS will tell the kernel to shut down the start-up process with a [[Blue Screenscreen of Deathdeath]].<ref>{{Cite web|url=https://support.microsoft.com/en-us/help/156669/how-to-troubleshoot-a-stop-0xc000021a-error-in-windows-xp-or-windows-s|title=How to troubleshoot a "STOP 0xC000021A" error in Windows XP or Windows Server 2003|last=|first=|date=|website=support.microsoft.com|url-status=live|archive-url=|archive-date=|access-date=2020-03-15}}</ref> The error code for this fault is 0xc000021a (STATUS_SYSTEM_PROCESS_TERMINATED).

In Windows 7 and later, instead of drawing console windows itself, CSRSS spawns <code>conhost.exe</code> subprocesses to draw console windows for command line programs with the permissions of that user.

== Malware hoaxes ==

There are numerous [[virus hoax]]es that claim that csrss.exe is [[malware]] and should be removed to prevent damage to the system; these are false, as removing csrss.exe or killing the csrss.exe [[Process (computing)|process]] will result in a [[Bluesystem Screencrash ofin Death]]Windows applications.

In addition, [[technical support scam]]mers pretending to be Microsoft representatives are known to use csrss.exe as "proof" of a virus infection, and convince the user being scammed into purchasing their [[rogue security software]] to remove it.<ref>{{cite web|url=http://news.softpedia.com/news/symantec-disavows-business-partner-caught-running-a-tech-support-scam-499310.shtml|title=Symantec Disavows Business Partner Caught Running a Tech Support Scam|last=Cimpanu|first=Catalin|publisher=[[Softpedia]]|date=Jan 22, 2016|accessdateaccess-date=July 29, 2016}}</ref>

== See also ==

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* [https://technet.microsoft.com/en-us/library/bb457123.aspx Troubleshooting the Startup Process (Windows XP Professional Resource Kit)]

{{Microsoft Windows Componentscomponents}}

{{DEFAULTSORT:Client Server Runtime Subsystem}}