User space and kernel space

Overview

The term userland (or user space) refers to all code which runs outside the operating system's kernel. Userland usually refers to the various programs and libraries that the operating system uses to interact with the kernel: software that performs input/output, manipulates file system objects, application software etc.

Each user space process normally runs in its own virtual memory space, and, unless explicitly allowed, cannot access the memory of other processes. This is the basis for memory protection in today's mainstream operating systems, and a building block for privilege separation. A separate user mode can also be used to build efficient virtual machines – see Popek and Goldberg virtualization requirements. Depending on the privileges, processes can request the kernel to map part of another process's memory space to its own, as is the case for debuggers. Programs can also request shared memory regions with other processes, although other techniques are also available to allow inter-process communication.

Various layers within Linux, also showing separation between the userland and kernel space

User mode	User applications	bash, LibreOffice, GIMP, Blender, 0 A.D., Mozilla Firefox, ...
	System components	init 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 library	fopen, execv, malloc, memcpy, localtime, pthread_create... (up to 2000 subroutines) glibc aims to be fast, musl aims to be lightweight, uClibc targets embedded systems, bionic was written for Android, etc. All aim to be POSIX/SUS-compatible.
Kernel mode	Linux kernel	stat, splice, dup, read, open, ioctl, write, mmap, close, exit, etc. (about 380 system calls) The Linux kernel System Call Interface (SCI), aims to be POSIX/SUS-compatible
		Process scheduling subsystem	IPC subsystem	Memory management subsystem	Virtual files subsystem	Networking 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.)

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See also

• BIOS
• CPU modes
• Memory protection

External links

• Linux Kernel Space Definition
• Low Level Guide To Getting into User Space