In Section 7.5, we introduced the concept of file locking and the two primary components: the RPC lock daemon and the status monitor. This chapter will delve more deeply into file locking and will examine the administrative aspects.
File locking is the act of ensuring that when you access a file, usually via a software application, no one can change the file until you are done examining it. If you want to modify the file, then file locking ensures that no one else can examine or modify the file until you are done modifying it.
The earliest versions of Unix had no way to lock files except to create lock files. The idea is that two or more processes would more or less simultaneously try to create a lock file in exclusive mode, via the O_EXCL flag of the open( ) system call. The operating system would return success to the process that won the race, and a "file exists" error to losing processes. One problem with this scheme is that it relies on the winning process to remove the lock file before it exits. If the process is running buggy software, this might not happen. Some applications mitigate this problem by recording the process ID of the winner into the contents of the lock file. A process that finds that it gets a "file exists" error can then read the lock file to see if the owning process is still running.
Still, lock files can be clumsy. In the 1980s, Unix versions were released with file locking support built into the operating ...