File locking is a mechanism that enforces access to a computer file by only one user at any specific time. The purpose of locking is to prevent the classic "interceding update" scenario.

The interceding update problem may be illustrated as in the following example: Process 'A' reads a customer record from a file containing account information, including the customer's account balance. Process 'B' now reads the same record from the same file. Process 'A' changes the account balance and writes the new value back to the file. However, process 'B' - which still has the original values of that customer record, makes a different change to its copy of the data (for example, changes the customer's phone number) and re-writes this data to the file. Process B's copy of the data, still containing the original account balance, overwrites the changes made by process A, reverting the account balance to its previous value and causing the update of the customer balance by process 'A' to be lost.

File locking prevents this problem by enforcing the serialization of update processes to any given file. Most operating systems support the concept of record locking which means that individual records within any given file may be locked, so increasing the number of concurrent update processes.