Alreadyrunning

Alreadyrunning is a software concept describing a situation in which a program, service, or component detects that an instance is already running before starting a new one. The primary goal is to prevent multiple concurrent instances, which can cause data corruption, resource conflicts, or user confusion.

The most common mechanisms involve some form of single-instance enforcement, such as a lock. This can be a lock file or PID file on disk, or an operating system primitive like a mutex, named pipe, or port binding. On startup, the program attempts to acquire the lock. If the lock is already held by another process, the new instance typically exits or delegates control to the existing instance, for example by sending a message to bring the running application to the foreground or by passing command-line arguments.

Interprocess communication is often used when a second launch should interact with the first. The existing instance may receive data or commands from the new invocation and respond accordingly, rather than starting a separate copy.

Stale locks pose a common challenge. If a running process crashes or terminates unexpectedly, a lock or PID file may remain. Robust implementations check whether the PID in the lock file corresponds to an active process and may remove or reclaim the lock after a timeout or manual cleanup.

Platform considerations vary. Windows frequently uses a named mutex to enforce a single instance, while Linux and macOS commonly rely on lock files, flock, or PID files, each with its own edge cases.

Alternatives include allowing multiple instances or using a launcher or central manager process. See also single-instance applications, interprocess communication, lock files, PID files, and mutex.