Section 1.3. Open Source and the GPL

1.3. Open Source and the GPL

One of the fundamental factors driving the adoption of Linux is the fact that it is open source. The Linux kernel is licensed under the terms of the GNU GPL^[1] (General Public License), which leads to the popular myth that Linux is free.^[2] In fact, the second paragraph of the GNU GPL declares: "When we speak of free software, we are referring to freedom, not price." The GPL license is remarkably short and easy to read. Among the most important key characteristics:

^[1] See Appendix A, "GNU Public License," for the complete text of the license.

^[2] Most professional development managers agree: You can download Linux without charge, but there is a cost (often a substantial one) for development and deployment of any OS on an embedded platform. See Section 1.3.1, "Free Versus Freedom," for a discussion of cost elements.

• The license is self-perpetuating.

• The license grants the user freedom to run the program.

• The license grants the user the right to study and modify the source code.

• The license grants the user permission to distribute the original code or his modifications.

• The license grants these same rights to anyone to whom you distribute GPL software.

When a software work is released under the terms of the GPL, it must forever carry that license.^[3] Even if the code is highly modified, which is allowed and even encouraged by the license, the GPL mandates that it must be released under the same license. The intent of this feature is to guarantee access to everyone, even of modified versions of the software (or derived works, as they are commonly called).

^[3] If all the copyright holders agreed, the software could, in theory, be released under a new license, a very unlikely scenario indeed!

No matter how the software was obtained, the GPL grants the licensee unlimited distribution rights, without the obligation to pay royalties or per-unit fees. This does not mean that a vendor can't charge for the GPL softwarethis is a very reasonable common business practice. It means that once in possession of GPL software, it is permissible to modify and redistribute it, whether it is a derived (modified) work or not. However, as defined by the GPL license, the author(s) of the modified work are obligated to release the work under the terms of the GPL if they decide to do so. Any distribution of a derived work, such as shipment to a customer, triggers this obligation.

For a fascinating and insightful look at the history and culture of the open source movement, read Eric S. Raymond's book referenced at the end of this chapter.

1.3.1. Free Versus Freedom

Two popular phrases are often repeated in the discussion about the free nature of open source: "free as in freedom" and "free as in beer." (The author is particularly fond of the latter.) The GPL license exists to guarantee "free as in freedom" of a particular body of software. It guarantees your freedom to use it, study it, and change it. It also guarantees these freedoms for anyone to whom you distribute your modified code. This concept has become fairly widely understood.

One of the misconceptions frequently heard is that Linux is "free as in beer." Sure, you can obtain Linux free of cost. You can download a Linux kernel in a few minutes. However, as any professional development manager understands, certain costs are associated with any software to be incorporated into a design. These include the costs of acquisition, integration, modification, maintenance, and support. Add to that the cost of obtaining and maintaining a properly configured toolchain, libraries, application programs, and specialized cross-development tools compatible with your chosen architecture, and you can quickly see that it is a nontrivial exercise to develop the needed software components to deploy your embedded Linux-based system.