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User space and kernel space

From Wikipedia, the free encyclopedia
Way of using computer memory
"Kernel space" and "User space" redirect here. For the mathematical definition, seeNull space. For the user space on Wikipedia, seeWikipedia:User pages.
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A modern computeroperating system usually usesvirtual memory to provide separate address spaces or separate regions of a single address space, calleduser space and kernel space.[1][a] Primarily, this separation serves to providememory protection and hardware protection from malicious or errant software behaviour.

Kernel space is strictly reserved for running a privilegedoperating system kernel, kernel extensions, and mostdevice drivers. In contrast, user space is the memory area whereapplication software and some drivers execute, typically one address space per process.

Overview

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The termuser space (oruserland) refers to all code that runs outside the operating system's kernel.[2] User space usually refers to the various programs andlibraries that the operating system uses to interact with the kernel: software that performsinput/output, manipulatesfile system objects,application software, etc.

Each user spaceprocess normally runs in its ownvirtual memory space, and, unless explicitly allowed, cannot access the memory of other processes. This is the basis formemory protection in today's mainstream operating systems, and a building block forprivilege separation. A separate user mode can also be used to build efficient virtual machines – seePopek and Goldberg virtualization requirements. With enough privileges, processes can request the kernel to map part of another process's memory space to its own, as is the case fordebuggers. Programs can also requestshared memory regions with other processes, although other techniques are also available to allowinter-process communication.

Various layers within Linux, also showing separation between theuserland andkernel space
User modeUser applicationsbash,LibreOffice,GIMP,Blender,0 A.D.,Mozilla Firefox, ...
System componentsinit daemon:
OpenRC,runit,systemd...
System daemons:
polkitd,smbd,sshd,udevd...
Windowing system:
X11,Wayland,SurfaceFlinger (Android)
Graphics:
Mesa,AMD Catalyst, ...
Other libraries:
GTK,Qt,EFL,SDL,SFML,FLTK,GNUstep, ...
C standard libraryfopen,execv,malloc,memcpy,localtime,pthread_create... (up to 2000subroutines)
glibc aims to be fast,musl aims to be lightweight,uClibc targets embedded systems,bionic was written forAndroid, etc. All aim to bePOSIX/SUS-compatible.
Kernel modeLinux kernelstat,splice,dup,read,open,ioctl,write,mmap,close,exit, etc. (about 380 system calls)
The Linux kernelSystem Call Interface (SCI), aims to bePOSIX/SUS-compatible[3]
Process scheduling subsystemIPC subsystemMemory management subsystemVirtual files subsystemNetworking subsystem
Other components:ALSA,DRI,evdev,klibc,LVM,device mapper,Linux Network Scheduler,Netfilter
Linux Security Modules:SELinux,TOMOYO,AppArmor,Smack
Hardware (CPU,main memory,data storage devices, etc.)

Implementation

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The most common way of implementing auser mode separate fromkernel mode involves operating systemprotection rings.Protection rings, in turn, are implemented usingCPU modes.Typically, kernel space programs run inkernel mode, also calledsupervisor mode;normal applications in user space run in user mode.

Some operating systems aresingle address space operating systems—they have a single address space for all user-mode code. (The kernel-mode code may be in the same address space, or it may be in a second address space).Other operating systems have a per-process address space, with a separate address space for each user-mode process.

Another approach taken in experimental operating systems is to have a singleaddress space for all software, and rely on a programming language's semantics to make sure that arbitrary memory cannot be accessed – applications cannot acquire anyreferences to the objects that they are not allowed to access.[4][5] This approach has been implemented inJXOS, Unununium as well as Microsoft'sSingularity research project.

See also

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Notes

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  1. ^Older operating systems, such asDOS andWindows 3.1x, do not use this architecture.

References

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  1. ^"Address space". Address space options for 32bit systems.
  2. ^"userland, n."TheJargon File.Eric S. Raymond. Retrieved2016-08-14.
  3. ^"Admin Guide README".Kernel.org git repositories.
  4. ^"Unununium System Introduction". Archived fromthe original on 2001-12-15. Retrieved2016-08-14.
  5. ^"uuu/docs/system_introduction/uuu_intro.tex".UUU System Introduction Guide. 2001-06-01. Retrieved2016-08-14.

External links

[edit]
General
Variants
Kernel
Architectures
Components
Process management
Concepts
Scheduling
algorithms
Memory management,
resource protection
Storage access,
file systems
Supporting concepts
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