Patch (computing)

A patch is data for modifying an existing software resource such as a program or a file, often to fix bugs and security vulnerabilities.^[1][2] Patch is also the process of applying the data to the existing resource. Patching a system involves applying a patch. A patch may be created to improve functionality, usability, or performance. A patch may be created manually, but commonly it is created via a tool that compares two versions of the resource and generates data that can be used to transform one to the other.

Typically, a patch needs to be applied to the specific version of the resource it is intended to modify, although there are exceptions. Some patching tools can detect the version of the existing resource and apply the appropriate patch, even if it supports multiple versions. As more patches are released, their cumulative size can grow significantly, sometimes exceeding the size of the resource itself. To manage this, the number of supported versions may be limited, or a complete copy of the resource might be provided instead.

Patching allows for modifying a binary executable. Although this can be technically challenging (requires a thorough understanding of the workings of the executable), it may be feasible when the source code is unavailable to build a full executable, and it allows for a smaller distribution which can be more economical than distributing full files.

Although often intended to fix problems, a patch can introduce new problems. In some cases, an update intentionally disables functionality, for instance, by removing aspects for which the consumer is no longer licensed. Patch management is a part of lifecycle management, and involves a strategy and planning of what patches should be applied to which systems and at what times. Typically, a patch is applied in a permanent way (i.e. to storage), but in some cases, a patch is applied to memory (i.e. via a tool such as a debugger) in which case the change is lost when the resource is reloaded from storage.

Software update is sometimes conflated with patch even though they are not synonyms. An update can be implemented using patch files and the patching process. Also, some may contend that patching is not limited to modifying file content; that adding, removing and replacing whole files is patching. Typically, patch connotates a relatively small change, so a patch that is large in size or scope may be called the more general software update or another more specific name such as service pack. Windows NT and its successors (including Windows 2000, Windows XP, Windows Vista and Windows 7) use service pack.^[3] Historically, IBM used the terms FixPak and Corrective Service Diskette for such updates.^[4]