userlands

In computing, userland (or user space) denotes the part of an operating system where user applications run, as opposed to the kernel, which manages hardware and core services. The plural form "userlands" is occasionally used to describe multiple distinct user-space environments on a system or network, such as separate containers, sandboxed runtimes, or virtual machines, each with its own libraries and binaries.

Within a typical OS, the kernel exposes interfaces (system calls, libraries) that allow userland processes to perform operations like I/O, networking, and process management. Userland programs execute in protected memory and rely on the kernel for resource isolation; a fault in userland generally cannot corrupt the kernel.

Modern software deployment often relies on multiple userlands to achieve isolation and portability. Containers create lightweight, isolated userlands that share a common kernel but have separate filesystem trees, libraries, and configurations. Tools like chroot, Linux namespaces, and container runtimes implement these boundaries. Virtual machines provide entirely separate userlands with their own guest OS, running atop a hypervisor while maintaining hardware-level isolation.

The concept traces to Unix design, which separated user-space utilities from kernel code. In practice, "userland" is used to discuss portability of software (for example, a GNU userland running on different Unix-like systems) and debugging where issues reside in user-space rather than in the kernel.