Understanding the Linux Kernel, 3rd Edition

File Locking

When a file can be accessed by more than one process, a synchronization problem occurs. What happens if two processes try to write in the same file location? Or again, what happens if a process reads from a file location while another process is writing into it?

In traditional Unix systems, concurrent accesses to the same file location produce unpredictable results. However, Unix systems provide a mechanism that allows the processes to lock a file region so that concurrent accesses may be easily avoided.

The POSIX standard requires a file-locking mechanism based on the fcntl( ) system call. It is possible to lock an arbitrary region of a file (even a single byte) or to lock the whole file (including data appended in the future). Because a process can choose to lock only a part of a file, it can also hold multiple locks on different parts of the file.

This kind of lock does not keep out another process that is ignorant of locking. Like a semaphore used to protect a critical region in code, the lock is considered “advisory” because it doesn’t work unless other processes cooperate in checking the existence of a lock before accessing the file. Therefore, POSIX’s locks are known as advisory locks .

Traditional BSD variants implement advisory locking through the flock( ) system call. This call does not allow a process to lock a file region, only the whole file. Traditional System V variants provide the lockf( ) library function, which is simply an interface to fcntl( ).