The GPL is essentially a hack on copyright law that rewrote the rights and responsibilities of the end user, giving the user the right to use, copy, share, repackage or sell the software with the reservation that any changes to the code be made available under the same conditions.
The GPL’s recursive adaptation of copyright law, or copyleft, is the reason why opponents refer to the GPL as viral – the GPL is made to apply to additions and changes to the code, and all rights are reversed. The benefits of copyleft are that it encourages the rollback of code into a project, discourages forking, and gives assurance that code won’t be hijacked for proprietary means and ends.
The name copyleft was inspired by a sticker that Richard Stallman, the man who wrote the GPL, saw on the on the back of an envelope mailed from Don Hopkins to Richard in 1984. The envelope contained a 68000 manual that Don borrowed from Richard, that he was returning. the sticker inspired Richard to use the word ‘Copyleft’ for licensing free software.
The effect of copyleft is to preserve the continuity and integrity of a project, as was demonstrated after the take up of the Linux kernel. Without the GPL, the corporate user (be it SGI, IBM, HP or any other) could find advantage in hijacking and forking the kernel, as happened with Unix, thus discouraging wider contributions from other corporate users. Forking would have been inevitable, because advantages could be found in pushing proprietary enhancements that were unavailable to competitors. In the long run, the proprietary divisions of Unix were damaging to the individual Unix companies. Each proprietary Unix had its own distinctive advantages over the others, but each was weaker because of the collective disassociation from the whole.
The advantage for the Unix companies of adopting Linux was that there was a collective reduction in the cost of development – the code that one company gave came back in the contributions of others. Companies felt enabled to share their code contributions, and the Linux kernel became a collaborative project across several industries, which almost certainly would not have been the case without the protection of the GPL.
The collective advantage to everybody is obvious; costs are shared and development goes at a much quicker pace. It is highly unlikely that the Linux kernel would have retained its current level of integrity if it had been released under a more liberal licence. The GPL secures the source for all participants, and everybody is allowed to be a participant. The standardisation of industry on the Linux kernel made it possible to write applications for a wider market, and helped to drive open standards, which are to the advantage of all concerned.
The myth used to be that the GPL is hostile to business because business can’t appropriate the code. Also, that the BSD licence is business-friendly because the corporate user can do what they like with the code, close it, spin it, market and enhance it, without any obligation to return any enhancements to the community).
In the real world, the opposite has proved to be true. The GPL was not only the best protector of the principles of free software, but was also the most business-friendly of the licences available, for one reason: companies such as IBM, HP, and SGI could openly contribute to the kernel, releasing large chunks of code under the GPL, in the knowledge that the developments of their competitors would also be fed back to the community, and their contributions could not be laid open to exclusive development by third parties. The ‘viral’ element of the licence, which so many people objected to, made the licence more business friendly, and worked to the advantage of all contributors and the project as a whole.
The GPL was not created with the intention of encouraging the participation of business in the development of free software, but like the widely perceived notion that the open-source model of distributed software development produces better software, its beneficial relationship with business was an accidental side effect of the licence.