File locking is a mechanism that restricts access to a computer file by allowing only one user or process access at any specific time.Systems implement locking to prevent the classic interceding update scenario, which is a typical example of race condition, by enforcing the serialization of update processes to any given file.The following example illustrates the interceding update problem: Most operating systems support the concept of record locking, which means that individual records within any given file may be locked, thereby increasing the number of concurrent update processes.
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Although this does prevent any other process from accessing the file, it can be more efficient than individually locking a large number of regions in the file by removing the overhead of acquiring and releasing each lock.
Poor use of file locks, like any computer lock, can result in poor performance or in deadlocks.
File locking may also refer to additional security applied by a computer user either by using Windows security, NTFS permissions or by installing a third party file locking software.
IBM pioneered file locking in 1963 for use in mainframe computers using OS/360, where it was termed "exclusive control".
Microsoft Windows uses three distinct mechanisms to manage access to shared files: Windows inherits the semantics of share-access controls from the MS-DOS system, where sharing was introduced in MS–DOS 3.3.
Thus, an application must explicitly allow sharing; otherwise an application has exclusive read, write, and delete access to the file (other types of access, such as those to retrieve the attributes of a file are allowed.) For a file with shared access, applications may then use byte-range locking to control access to specific regions of the file.
Such byte-range locks specify a region of the file (offset and length) and the type of lock (shared or exclusive).
Note that the region of the file being locked is not required to have data within the file, and applications sometimes exploit this ability to implement their functionality.
For applications that use the file read/write APIs in Windows, byte-range locks are enforced (also referred to as mandatory locks) by the file systems that execute within Windows.