Patch (computing)

In computing, a patch is a software update meant to fix problems with a computer program. This can range from fixing bugs to replacing graphics to improving the usability or performance of a previous version. The term probably originates from the Unix patch command written by Larry Wall. Though meant to fix problems, patches can sometimes introduce new problems.

Types

Programmers publish and apply patches in various forms. Because proprietary software writers keep their source code, their patches usually circulate in binary form. This type of patch modifies the program executable -- the program the user actually runs -- either modifying the binary file to include the fixes or just completely replacing it.

Patches can also be applied to a program's source code. In these cases, the patches are mere textual difference between two source code files. These types of patches are common with open source projects. In these cases, the users are expected to compile the new or changed files themselves.

Since patching has a connotation of small fixes, patches that are large in a size or significant in terms of change are not referred as patches, but rather service packs or software updates. The Microsoft Windows operating system uses this terminology.

Sometimes patches are released to eliminate certain functionality or to prevent users from performing a certain activity. Some companies employ a tactic of issuing patches that are automatically applied, mostly to obfuscate the protocol for blocking third-party products. For example, in 2003 and 2004 AOL issued updates for its instant messenger whose only functionality was to block clients like Trillian, Gaim and KaZaA.

History

Applying a patch used to be a tedious, error-frought process done by following an often ill-documented set of procedures. Missing or misapplying a step would usually result in having to re-install both the application and patch. Today, patch installtion is usually done automatically by special programs.

Historically, patches were distributed on paper tape or punched cards, and the recipient was expected to cut out the indicated part of the original tape (or deck), and patch in (hence the name) the replacement segment. Then they were distributed on magnetic tape. Then, after removeable disk drives had been invented, patches were obtained from the software developer via a disk or, later, CD-ROM via mail. Today, with almost ubiquitous Internet access, most patches are available only through download via the developer's web site.

Often today, computer programs can co-ordinate patches to update a target program. Automation simplifies the end-users' task -- they need only to execute an update program, whereupon that program makes sure updating the target is done completely and correctly. Service packs for Microsoft Windows and for commercial software products adopt such automated strategies.

Though not common, some programs can update themselves via the Internet with very little or no intervention on the part of users. The maintenance of server software and of operating systems often takes place in this manner. In situations where system administrators control a number of computers, this sort of automation helps to maintain consistency. The application of security patches commonly occurs in this manner.

Application

The size of patches vary from a few kilobytes to hundreds of megabytes — mostly more significant changes imply a larger size. In particular, patches can become quite large when the changes add or replace non-program data, such as graphics and sounds files. Such situations occur common in the patching of computer games.

Compared with the initial installation of software, patches usually do not take long to apply. Patched acquired via the Internet may take longer (depending on Internet connection speed).

In the case of operating systems and computer server software, patches have the particularly important role of fixing security holes. To facilitate updates, operating systems often provide automatic or semi-automatic update facilities.

Complete automatic update has not succeeded in gaining mainstream popularity, partly because of the aforementioned glitches, but also because users fear that software companies may gain unlimited control over their computers. Package management systems can offer various degrees of patch automation.

Cautious users, particularly system administrators, tend to put off applying patches until they can verify the stability of the fixes. When the patch is large and can cause significant changes, patches are often only distributed to qualified developers as a beta test.

Applying patches to firmware poses special challenges, because programs embedded on hardware devices are typically so small that totally new program code is provided, instead of simply differences from the previous version. Often the patch consists of bare binary data and a special program that replaces the previous version with the new version is provided. Any unexpected error or interruption, like a power outage, might spell disaster. A motherboard BIOS update exemplifies a common firmware patch.

Computer games

Unlike applications such as word processors, patches play a unique role in computer games. Computer games often — almost ubiquitously today — require patches to fix compatibility problems after their initial release.

Uncommonly, suppliers release patches not to fix technical problems, but to change game rules or algorithms. It is particularly necessary with network multiplayer games as some find a tactic that the game designers and testers never exploited, giving some users unfair advantages. Often in network games (played over the Internet), any program that is not updated to the latest version is rejected to avoid conflicts. This also gives users an incentive to update their versions.

Tools

Several software manufacturers develop tools to aid in the patch application process, such as Pocket Soft's RTPatch, which only delivers changes to the binary executable. WinZip has a self-extraction utility that will launch a program that can apply a patch.

Patches in software development

Patches are sometimes needed to fix problems with libraries or portions of source code for programs that are used frequently or are in maintenance. This commonly occurs on very large-scale software projects, but rarely in small-scale development.

In open source projects, it is common that the authors receive patches or many people publish patches that fix particular problems or add certain functionality, like support for local languages outside the project's locale. An example is from the early development of the Linux operating system, which is noted for publishing its complete source code. Linus Torvalds, the original author, received hundreds of thousands of patches from many programmers to apply against his original version.

The Apache HTTP Server originally evolved as a number of patches that a webmaster created to add new features to NCSA HTTPd, hence the name that implies that it is a collection of patches: a patchy server.

See also

• Patch (Unix)
• Port