Patch (computing)

A program tape for the 1944 Harvard Mark I, one of the first digital computers. Note physical patches used to correct punched holes by covering them.

Historically, software suppliers distributed patches on paper tape or on punched cards, expecting the recipient to cut out the indicated part of the original tape (or deck), and patch in (hence the name) the replacement segment. Later patch distributions used magnetic tape. Then, after the invention of removable disk drives, patches came from the software developer via a disk or, later, CD-ROM via mail. With widely available Internet access, downloading patches from the developer's web site or through automated software updates became often available to the end-users. Starting with Apple's Mac OS 9 and Microsoft's Windows ME, PC operating systems gained the ability to get automatic software updates via the Internet.

Computer programs can often coordinate patches to update a target program. Automation simplifies the end-user's task – they need only to execute an update program, whereupon that program makes sure that updating the target takes place completely and correctly. Service packs for Microsoft Windows NT and its successors and for many commercial software products adopt such automated strategies.

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.

The size of patches may vary from a few bytes to hundreds of megabytes; thus, more significant changes imply a larger size, though this also depends on whether the patch includes entire files or only the changed portion(s) of files. In particular, patches can become quite large when the changes add or replace non-program data, such as graphics and sounds files. Such situations commonly occur in the patching of computer games. Compared with the initial installation of software, patches usually do not take long to apply.

In the case of operating systems and computer server software, patches have the particularly important role of fixing security holes. Some critical patches involve issues with drivers.[5] Patches may require prior application of other patches, or may require prior or concurrent updates of several independent software components. To facilitate updates, operating systems often provide automatic or semi-automatic updating facilities. Completely automatic updates have not succeeded in gaining widespread popularity in corporate computing environments, partly because of the aforementioned glitches, but also because administrators fear that software companies may gain unlimited control over their computers. Package management systems can offer various degrees of patch automation.

Usage of completely automatic updates has become far more widespread in the consumer market, due largely to the fact that Microsoft Windows added support for them, and Service Pack 2 of Windows XP (available in 2004) enabled them by default. Cautious users, particularly system administrators, tend to put off applying patches until they can verify the stability of the fixes. Microsoft (W)SUS supports this. In the cases of large patches or of significant changes, distributors often limit availability of patches to qualified developers as a beta test.

Applying patches to firmware poses special challenges, as it often involves the provisioning of totally new firmware images, rather than applying only the differences from the previous version. The patch usually consists of a firmware image in form of binary data, together with a supplier-provided special program that replaces the previous version with the new version; a motherboard BIOS update is an example of a common firmware patch. Any unexpected error or interruption during the update, such as a power outage, may render the motherboard unusable. It is possible for motherboard manufacturers to put safeguards in place to prevent serious damage; for example, the update procedure could make and keep a backup of the firmware to use in case it determines that the primary copy is corrupt (usually through the use of a checksum, such as a CRC).

Video games receive patches to fix compatibility problems after their initial release just like any other software, but they can also be applied to change game rules or algorithms. These patches may be prompted by the discovery of exploits in the multiplayer game experience that can be used to gain unfair advantages over other players. Extra features and gameplay tweaks can often be added. These kinds of patches are common in first-person shooters with multiplayer capability, and in MMORPGs, which are typically very complex with large amounts of content, almost always rely heavily on patches following the initial release, where patches sometimes add new content and abilities available to players. Because the balance and fairness for all players of an MMORPG can be severely corrupted within a short amount of time by an exploit, servers of an MMORPG are sometimes taken down with short notice in order to apply a critical patch with a fix.

Companies sometimes release games knowing that they have bugs. Computer Gaming World's Scorpia in 1994 denounced "companies—too numerous to mention—who release shoddy product knowing they can get by with patches and upgrades, and who make 'pay-testers of their customers".[6]

Patches sometimes become mandatory to fix problems with libraries or with portions of source code for programs in frequent use or in maintenance. This commonly occurs on very large-scale software projects, but rarely in small-scale development.

In open-source projects, the authors commonly 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. In an example from the early development of the Linux kernel (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 Brian Behlendorf collated to improve NCSA HTTPd, hence a name that implies that it is a collection of patches ("a patchy server"). The FAQ on the project's official site states that the name 'Apache' was chosen from respect for the Native American Indian tribe of Apache. However, the 'a patchy server' explanation was initially given on the project's website.[7]

Hot patching, also known as live patching or dynamic software updating, is the application of patches without shutting down and restarting the system or the program concerned. This addresses problems related to unavailability of service provided by the system or the program.[13] Method can be used to update Linux kernel without stopping the system.[14][15] A patch that can be applied in this way is called a hot patch or a live patch. This is becoming a common practice in the mobile app space.[16] Companies like Rollout.io use method swizzling to deliver hot patches to the iOS ecosystem.[17] Another method for hot-patching iOS apps is JSPatch.[18]

Cloud providers often use hot patching to avoid downtime for customers when updating underlying infrastructure.[19]

In computing, slipstreaming is the act of integrating patches (including service packs) into the installation files of their original app, so that the result allows a direct installation of the updated app.[20][21]

The nature of slipstreaming means that it involves an initial outlay of time and work, but can save a lot of time (and, by extension, money) in the long term. This is especially significant for administrators that are tasked with managing a large number of computers, where typical practice for installing an operating system on each computer would be to use the original media and then update each computer after the installation was complete. This would take a lot more time than starting with a more up-to-date (slipstreamed) source, and needing to download and install the few updates not included in the slipstreamed source.

However, not all patches can be applied in this fashion and one disadvantage is that if it is discovered that a certain patch is responsible for later problems, said patch cannot be removed without using an original, non-slipstreamed installation source.

Typical modern Software Update prompt on macOS

Software update systems allow for updates to be managed by users and software developers. In the 2017 Petya cyberpandemic, the financial software "MeDoc"'s update system is said to have been compromised to spread malware via its updates.[22][23] On the Tor Blog, cybersecurity expert Mike Perry states that deterministic, distributed builds are likely the only way to defend against malware that attacks the software development and build processes to infect millions of machines in a single, officially signed, instantaneous update.[24] Update managers also allow for security updates to be applied quickly and widely. Update managers of Linux such as Synaptic allow users to update all software installed on their machine. Applications like Synaptic use cryptographic checksums to verify source/local files before they are applied to ensure fidelity against malware.[25][26]

• Software release life cycle
• Software maintenance
• Backporting
• Dribbleware
• Patch (Unix)
• Porting
• Vulnerability database
• Delta encoding
• SMP/E
• Automatic bug fixing
• Shavlik Technologies
• White hat (computer security)

• The Jargon File version 4.4.7 entry for patch
• A detailed masters dissertation dealing with security patches
• Official Linux kernel patch format
• 0-Day Patch - Exposing Vendors (In)security Performance. Archived 2014-01-31 at the Wayback Machine. A metric comparing patch performance of Microsoft and Apple.