If a file is shared between several tasks in an application, the file lock can be employed to guarantee that a series of file operations are executed atomically. fs_flockfile() (or its non-blocking equivalent fs_ftrylockfile()) acquires the lock for a task (if it does not already own it). Accesses from other tasks will be blocked until fs_funlockfile() is called.
Each file actually has a lock count associated with it. This allows nested calls by a task to acquire a file lock; each of those calls must be matched with a call to fs_funlockfile().