Qt framework

Mostly used as GUI toolkit but that is far from the only functionality it offers. It has networking, file management, memory management, image manipulation, unicode string handling, and much more that help porting and extending software.

The basic Qt toolkit is for C++, but there are bindings to other languages (core Qt is still for C++, regardless of that).

Some subsets of QT are done in pure C

Note that KDE, Opera (the Internet browser), Google Earth, Skype, Qtopia, and OPIE depend on Qt. So tagging games with the Qt tag that depend on these programs also, is redundant.

Qt was originally created by Trolltech, who changed their name to Qt Software and were subsequently bought by Nokia. For a long while, Qt was not distributed under a totally free license (under Trolltech). Currently, it is dual licensed as GPL2 or 'GPL3 with the Nokia Exception'.

At all times, Qt has been available under a commercial license that allows developing proprietary derivatives with no restrictions on licensing. In addition, Qt has been gradually made available under several increasingly free licenses.

The FreeQt license applies up to v1.45 and applies to the source code (not the binaries) and did not allow redistribution. Notably, Qt is a core component of KDE Software Compilation. Many Linux distributions made use of KDE and therefore were inherently proprietary. It wasn't until 1998 when KDE gained popularity that this became a priority issue in the Linux and Open Source communities. It had always been a priority for Free Software community. Just to note a practical problem where Open Source has a consistent disadvantage over Free Software. v2.0 was released under the Q Public License which was also not compliant with Free Software but the community successfully negotiated a deal whereby (non-commercial) Qt would never be under a more restrictive license than QPL even independent of Trolltech's involvement. The KDE Free Qt foundation took ownership of Qt and implemented BSD licensing as a fallback in the event that more than 12 months passed after a proprietary Qt release with no corresponding QPL release. This was never used and in 2000 *nix Qt was made GPL v2 in response to QPL Qt being ported to Windows (a potential liability to the copyright holders of Qt). Trolltech was making it's own proprietary Qt/Windows and felt this version was not covered under the agreement. 2002 saw the release of the 'unauthorised' (but legally protected) KDE for Windows. June 2005, Trolltech released Qt/Windows as GPL. This also resolved many cross-platform compatibility issues users had been having. Currently the project is GPL v3, or optionally LGPL v3, or users can optionally use a GPL-incompatible Free Software license for derivative works. The proprietary version was sold to Nokia and they sold it to Digia who uses Qt Commercial License.