Patch (computing)

A patch is a specific type of data used to modify an existing software resource. This resource might be a computer program or a single file. The primary goal of patching is to fix bugs or address security vulnerabilities. However, developers also create patches to improve a program's functionality, usability, or performance. While a patch is often a small change, the term can sometimes describe larger updates. If a patch is very large in scope, it might be called a software update or a service pack.

To understand how patching works, we must look at how the data is applied. Most commonly, a tool compares two different versions of a resource. This tool generates data that can transform the old version into the new one. This process is often used for binary executables, which are the files that actually run on a computer. Patching a binary file can be technically difficult because it requires a deep understanding of how the executable works. However, it is useful when the original source code is unavailable. It also allows for a smaller distribution, which is more economical than sending entire files.

There are two main ways to apply these changes to a program's machine code. The first method is called an inline patch. This happens if the new code is small enough to fit in the exact space occupied by the old code. In this case, the patch utility simply overwrites the old bytes with the new ones. The second method is used if the new code is larger than the original space. The utility appends the new code to the end of the file. It then uses branch instructions, which are like jumps or calls, to direct the computer from the old code to the new code.

Programmers use different strategies to manage space during the patching process. Savvy programmers often plan for future changes by reserving unused memory when they first create the code. If a programmer is not involved in the original design, they must find ways to make room. They might shrink the existing code by using more efficient algorithms or more compact data. They might also remove less important features or move functions to mass storage. In some cases, a patch is applied to memory rather than permanent storage. These memory patches are lost once the resource is reloaded from the disk.

History shows how the concept of a "patch" has evolved with technology. In 1944, the Harvard Mark I used program tapes. To fix mistakes, people would physically cut out segments of the paper tape and patch in a replacement segment. This physical action is where the name comes from. Later, patches were distributed via magnetic tape, disks, or CD-ROMs sent through the mail. With the rise of the internet, users began downloading patches directly from websites. Modern operating systems, such as Mac OS 9 and Windows ME, introduced the ability to receive these updates automatically.

Different companies use specific names for large collections of patches. Microsoft uses the term "service pack" for its Windows NT systems and successors, such as Windows XP and Windows 7. Historically, IBM used terms like FixPak and Corrective Service Diskette. Large-scale projects also rely heavily on patching. For example, the Linux kernel was developed with hundreds of thousands of patches from various programmers. The Apache HTTP Server also grew from a collection of patches. Its name was even joked to imply it was a "patchy server" before the official meaning was established.

Managing these updates is a critical part of what experts call lifecycle management. This involves a strategy for deciding which patches to apply to which systems and when. System administrators often use automation to maintain consistency across many computers. This is especially important for applying security patches quickly. However, patching is not without risks. Sometimes, a patch can introduce new problems, a scenario known as software regression. Additionally, an update might intentionally disable certain features if a user no longer has the correct license.