Motivations for using and developing open source software are mixed, ranging from philosophical and ethical reasons to pure practical issues. In this subsection, some of the most widely proposed practical advantages will be introduced. For a discussion on some of the ethical issues (which is not covered in this document), refer to [25].

Usually, the first perceived advantage of open source models is the fact that open source software is made available gratis or at a low cost. But this characteristic is not exclusive to open source software, and several proprietary software products are made available in similar ways (a prominent case could be Microsoft’s Internet Explorer). What really distinguishes open source software from software available without fee is the combination of effects due to the characteristics discussed in section 3.1. All of them combined produce a synergistic impact which is the cause of the real advantages of the open source model. Let us provide some more detail on how do these characteristics turn into advantages:

• The availability of the source code and the right to modify it is very important. It enables the unlimited tuning and improvement of a software product. It also makes it possible to port the code to new hardware, to adapt it to changing conditions, and to reach a detailed understanding of how the system works. This is why many experts are reaching the conclusion that to really extend the lifetime of an application, it must be available in source form. In fact, no binary-only application more than 10 years old now survives in unmodified form, while several open source software systems from the 1980s are still in widespread use (although in many cases conveniently adapted to new environments). Source code availability also makes it much easier to isolate bugs, and (for a programmer) to fix them.
• The right to redistribute modifications and improvements to the code, and to reuse other open source code, permits all the advantages due to the modifiability of the software to be shared by large communities. This is usually the point that differentiates open source software licences from “nearly free” ones. In substance, the fact that redistribution rights cannot be revoked, and that they are universal, is what attracts a substantial crowd of developers to work around open source software projects.
• The right to use the software in any way. This, combined with redistribution rights, ensures (if the software is useful enough), a large population of users, which helps in turn to build up a market for support and customization of the software, which can only attract more and more developers to work in the project. This in turn helps to improve the quality of the product, and to improve its functionality. Which, once more, will cause more and more users to give the product a try, and probably to use it regularly.

The issue about non-exclusive rights on the software, which has just being mentioned, deserves some more attention. When no one holds exclusive rights on a given code (sometimes mentioned as “life or death rights”), several traditional problems of the proprietary software model can be overcome:

• There is no one with the power to restrict in a unilateral way how the software is used, even in a retroactive way. Such a power manifests, for instance, when a proprietary software vendor decides not to upgrade some software product for some old platform. In this case, customers can only stick to the old version of the software, or switch to another product. If open source software is used, customers can also fund some development for the desired platform, or look for other vendors to provide the upgrades (of the very same product).
• There is no single entity on which the future of the software depends. This is a very common concern with proprietary software. Let us say that a company uses a software product, and relies on the software manufacturer for upgrades and continued development. If the software manufacturer closes doors, or decides to discontinue development of the product, no one has the right to take the program and continue development on it, effectively killing its usability in the market. This has happened many times, and this problem is amplified by the recent mergers in the software market, that usually lead to “cannibalization” of some software product to allow just one or two to get to the market. Open source software effectively protects against this, because if the group or company that originated the code decides to stop development, it is always possible to fund another software group to continue the maintenance and improvement, without legal nor practical limitations.
• No “black boxes” are possible. This point is so important that open source is now considered by many experts as one of the necessary conditions for dependable applications. There are several reasons for this importance. One of them is related to the dependability of the services provided by a given software. By having the source code available, it is possible to perform a thorough inspection and verify the correctness of the algorithm and the implementation scheme used. This is also possible in part even with closed source or nearly free licences. The difference lies in the fact that users are allowed to modify everything they find appropriate to suit their needs. A glaring example is the Linux kernel and its “international patches”, a set of enhancements with legal problems in some countries. These patches include support for encrypted communications, and can be legally used in large parts of the world. The patches have being developed by concerned parties in countries where such a development is allowed, and therefore users in those countries can use those enhancements. With binary only products no inspection is possible, with closed source or nearly free licences inspection is possible, but modifications are forbidden, so the inherent advantage of having source code available is rendered ineffective.
• There is always the possibility of “forking”, or creating an alternative code base if the current one is in some way perceived as wrongly managed. This is sometimes considered a disadvantage, having to manage not only one code base, but two. A “fork” is a subdivision of the code base in two different parts, each managed by a different group. Forks happens for technical or licence reasons, for example because a particular release is made under a non-free licence, the previous one is used as a base for subsequent free releases. Technical motivations are common, because there are sometimes many different way to perform a task, and it is not possible to decide which is better. So if the two camps cannot reach a consensus and the user base is large enough the code splits in two, and both continue development. If the reasons for the split are overcome, usually the two camps agree on a reunification. A recent example is the decision to reunify the compilers gcc and egcs, and to make one of them (egcs) the new base compiler. In other cases a fork is used as a way to coordinate work. An example is the Linux kernel, where two distinct code bases are used, one “stable” and one “experimental”. This way it is possible to introduce new and potentially dangerous technologies without disrupting the stable ones. So, people interested in leading edge technologies can try them, and people that uses the Linux kernel in production environments can count on stable and tested features.But the main point about forking is that it introduces several levels of competition within the model. For instance, before forking, several programmers can work harder to keep everybody happy integrating as many well-engineered features as possible, to prevent a fork by people whose needs are not addressed. After a fork, both branches tend to compete for the user base with very similar products: only good quality and quick improvement can maintain them in the market.
• No per-copy fees can be asked for modified versions, and anyone can use the current code base to start new projects. Working knowledge can be gathered at a minimal cost. This is what made Internet software systems such an important factor in the new economy: students, and people trying new technologies were able to integrate and adopt them immediately, without the hurdles of commercial or non-disclosure licence agreements. In addition, the right to freely modify them allowed for the incredible expansion in the number of communication protocols and systems, each perfectly tailored to the needs of their users. This is also a reason for the overwhelming success of the Linux kernel, widely employed by students thanks to its near-zero cost, and subsequently used by the same students in the startups originated by them, when they turn into entrepreneurs after leaving University.
• There are fewer conflicting priorities due to marketing pressures. This is a simple consequence of the fact that there is no single commercial entity pushing for precise delivery dates or features that must be supported. Usually open source software is delivered “when it is ready”, and when the development team feels that its quality is good enough. This means that software usually does not need as many “service packs”, updates and such, reducing the maintenance cost. Of course this could be turned into disadvantage if a product is indefinitely delayed, or if some feature is missing one release after another. But in this case, the competition between projects may help. If a project starts failing to meet the expectations of its users, it often happens that a new project is forked, using the same code base, to fill this gap. This happens especially if a market exists for some new features, or for better quality versions of the application.