The GPL is basically a reciprocal agreement. If you improve or add to a GPL program, or if you build an application that includes software licensed under the GPL, then you must make the source code for your application available, too. Share and share alike. That's not to say you can't sell GPL software; you can. Put simply, the Free Software Foundation promotes the concept of software that is free as in "free speech," not necessarily free as in "free beer." Free means open and unrestricted by pre-existing proprietary claims; it doesn't mean without cost.
March 8, 2004
Opinion: When those who use free software are really freeloading
The spirit of open-source might once have been summed up as "share and share alike." This philosophy has its roots in the GNU General Public License (GPL), which is the license for the Linux kernel (the operating system engine) and most of the core operating system utilities that come with Linux.
- Free Software