userspace

Userspace, also written as user space or userland, refers to the portion of an operating system that runs outside the kernel and operates with limited privileges. It contrasts with kernel space, where the trusted, privileged parts of the OS execute. The separation is intended to protect the system from faulty or malicious code by preventing direct access to hardware and core OS data structures.

In practice, most user-space components include applications, libraries, and runtime environments. Programs run in their own isolated address spaces, provided by the system’s virtual memory, which helps protect one process from another. When a user-space program needs to perform privileged actions, it makes a controlled request to the kernel through system calls or other inter-process interfaces. The kernel then performs the requested action on behalf of the program and returns the results.

Architectures vary in how they distribute functionality. In monolithic kernels, the majority of OS services and device drivers reside in kernel space for performance. In microkernel designs, many services run in user space and communicate with the kernel via well-defined IPC mechanisms, reducing kernel complexity and increasing modularity at the potential cost of some efficiency.

Examples of user-space components include standard libraries (such as the C library), shells, graphical applications, and utilities in Unix-like systems, or the Win32 user-mode components in Windows. Overall, userspace is a fundamental concept in OS design, enabling process isolation, system stability, and security by restricting direct interaction with hardware and core kernel data.