fcntl: Locks and Other File Operations

• The fcntl system call is the access point for several advanced operations on file descriptors.
• The first argument to fcntl is an open file descriptor, and the second is a value that indicates which operation is to be performed.
• The fcntl system call allows a program to place a read lock or a write lock on a file, somewhat analogous to the mutex locks.
• A read lock is placed on a readable file descriptor, and a write lock is placed on a writable file descriptor.
• More than one process may hold a read lock on the same file at the same time, but only one process may hold a write lock, and the same file may not be both locked for read and locked for write.
• Note that placing a lock does not actually prevent other processes from opening the file, reading from it, or writing to it, unless they acquire locks with fcntl as well. The http://siber.cankaya.edu.tr/SystemsProgramming/cfiles/lock-file.c program in Fig. 2 opens a file for writing whose name is provided on the command line, and then places a write lock on it.

  Figure 2: Create a Write Lock with fcntl.

  $ gcc -o lock-file lock-file.c
  $ touch /tmp/test-file
  $ ./lock-file /tmp/test-file
  

  Now, in another window, try running it again on the same file.

  $ ./lock-file /tmp/test-file
  

  opening /tmp/test-file
• Linux provides another implementation of file locking with the flock call. The fcntl version has a major advantage: It works with files on NFS file systems.