Låsefiler

Låsefiler, or lock files, are a mechanism used by software to coordinate access to shared resources by multiple processes. A lock file is typically a small file placed in the resource’s directory or in a dedicated lock directory. The presence of the file signals that another process currently uses the resource, helping to prevent data corruption from concurrent writes.

There are two main ways lock files are implemented. One common method is to create a file atomically with exclusive access, often containing the owner’s process ID. If the file already exists, other processes know that the resource is locked. The lock is released when the owning process completes its work and removes the file. A second, broader approach is to use file descriptor locks (advisory locks) via mechanisms such as fcntl or flock, which lock an open file and require all participants to cooperate by using the same locking mechanism.

Lock files are widely used in various areas, including package management, databases, and system services. For example, package managers may create a lock file to prevent simultaneous installations, while a database may use a lock or a PID file to indicate an active instance.

Common issues include stale locks if a process crashes or terminates abnormally, race conditions during lock creation, and portability concerns across different file systems (for example, NFS can complicate certain lock strategies). Best practices involve using atomic lock creation where possible, preferring OS-supported locking primitives like flock, and implementing cleanup logic and checks for stale locks (such as verifying whether the recorded PID is still running). Proper permissions and clear documentation about lock ownership help reduce misuse and deadlocks. See also: lock management, pid files, and advisory vs mandatory locks.