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Portability requires standards. Open-source reference implementations are the most effective method known for both promulgating a standard and for pressuring proprietary vendors into conforming. If you are a developer, open-source implementations of a published standard can both tremendously reduce your coding workload and allow your product to benefit (in ways both expected and unexpected) from the labor of others.
Let's suppose, for example, you are designing image-capture software for a digital camera. Why write your own format for saving image bits or buy proprietary code when (as we noted in Chapter═5) there is a well-tested, full-featured library for writing PNGs in open source?
The (re)invention of open source has had a significant impact on the standards process as well. Though it is not formally a requirement, the IETF has since around 1997 grown increasingly resistant to standard-tracking RFCs that do not have at least one open-source reference implementation. In the future, it seems likely that conformance to any given standard will increasingly be measured by conformance to (or outright use of!) open-source implementations that have been blessed by the standard's authors.
The flip side of this is that often the best way to make something a standard is to distribute a high-quality open-source implementation of it.
In the end, the most effective step you can take to ensure the portability of your code is to not rely on proprietary technology. You never know when the closed-source library or tool or code generator or network protocol you are depending on will be end-of-lifed, or when the interface will be changed in some backwards-incompatible way that breaks your project. With open-source code, you have a path forward even if the leading-edge version changes in a way that breaks your project; because you have access to source code, you can forward-port it to new platforms if you need to.
Until the late 1990s this advice would have been impractical. The few alternatives to relying on proprietary operating systems and development tools were noble experiments, academic proofs-of-concept, or toys. But the Internet changed everything; in mid-2003 Linux and the other open-source Unixes exist and have proven their mettle as platforms for delivering production-quality software. Developers have a better option now than being dependent on short-term business decisions designed to protect someone else's monopoly. Practice defensive design — build on open source and don't get stranded!