oneinstance

oneinstance is a mechanism designed to guarantee that only a single running instance of an application exists on a system. It is used by desktop programs, services, and CLI tools to prevent conflicts over resources such as files, ports, or user interface focus.

Implementation typically relies on a cross-process lock. Common techniques include a lock file or PID file, operating system synchronization primitives such as a Windows named mutex or POSIX file locks, and an inter-process communication channel (for example a local socket or UNIX domain socket). The first instance creates the lock and registers itself; subsequent launches detect the lock, connect to the existing process, and often hand off work or raise the existing window.

Behavior after detection varies by design. Some oneinstance implementations terminate the new process after notifying the user or delegating work to the first instance. Others forward command-line arguments to the running instance so it can perform tasks on behalf of the new invocation.

Design considerations include cross-platform compatibility, handling of stale locks after crashes, race conditions during startup, and user experience in multi-user environments. Security and proper release of locks on normal and abnormal termination are important.

See also: single-instance application, singleton pattern, inter-process communication, lock file, mutex.