Fast Light Toolkit (FLTK, pronounced fulltick)[3] is a cross-platform widget (graphical control element) library for graphical user interfaces (GUIs), developed by Bill Spitzak and others. Made to accommodate 3D graphics programming, it has an interface to OpenGL, but it is also suitable for general GUI programming.

Initial release1998 (1998)
Stable release1.3.5 (March 3, 2019 (2019-03-03)[1]) [±]
Preview release1.4.x-20190329-7a3af40e (March 29, 2019 (2019-03-29)) [±]
Written inC++
Operating systemUnix-like with X11, macOS, Microsoft Windows, AmigaOS 4
TypeUser interface library
LicenseGNU Lesser General Public License version 2[2] (with an exception to allow static linking)

Using its own widget, drawing and event systems abstracted from the underlying system-dependent code, it allows for writing programs which look the same on all supported operating systems.

FLTK is free and open-source software, licensed under GNU Lesser General Public License (LGPL) with an added clause permitting static linking from applications with incompatible licenses.

In contrast to user interface libraries like GTK+, Qt, and wxWidgets, FLTK uses a more lightweight design and restricts itself to GUI functionality. Because of this, the library is very small (the FLTK "Hello World" program is around 100 KiB), and is usually statically linked. It also avoids complex macros, separate code preprocessors, and use of some advanced C++ features: templates, exceptions, and run-time type information (RTTI) or, for FLTK 1.x, namespaces. Combined with the modest size of the package, this makes it relatively easy to learn for new users.[4]

These advantages come with corresponding disadvantages. FLTK offers fewer widgets than most GUI toolkits and, because of its use of non-native widgets, does not have native look-and-feel on any platform.

Meaning of the name

FLTK was originally designed to be compatible with the Forms Library written for Silicon Graphics (SGI) machines (a derivative of this library called XForms is still used quite often). In that library, all functions and structures start with fl_. This naming was extended to all new methods and widgets in the C++ library, and this prefix FL was taken as the name of the library. After FL was released as open source, it was discovered that searching "FL" on the Internet was a problem, because it is also the abbreviation for Florida. After much debating and searching for a new name for the toolkit, which was already in use by several people, Bill Spitzak came up with Fast Light Tool Kit (FLTK).[5]


FLTK is an object-oriented widget toolkit written in the programming language C++. While GTK+ is mainly for the X Window System, FLTK works on other platforms, including Microsoft Windows (interfaced with the Windows API), and OS X (interfaced with Quartz). HTML5 and Wayland back-ends are in development. FLTK2 has gained experimental support for optionally using the cairo graphics library.

Language bindings

A library written in one programming language may be used in another language if language bindings are written. FLTK has a range of bindings for various languages.[6]

FLTK was mainly designed for, and is written in, the programming language C++. However, bindings exist for other languages, for example Lua,[7] Perl,[8] Python,[9] Ruby[10] and Tcl.[11]

For FLTK 1.x, this example creates a window with an Okay button:

#include <FL/Fl.H>
#include <FL/Fl_Window.H>
#include <FL/Fl_Button.H>

int main(int argc, char *argv[]) {
   Fl_Window* w = new Fl_Window(330, 190);
   new Fl_Button(110, 130, 100, 35, "Okay");
   w->show(argc, argv);
   return Fl::run();

GUI designers

FLTK includes Fast Light User Interface Designer (FLUID), a graphical GUI designer that generates C++ source and header files.


Many programs and projects use FLTK, including:


This version history is an example of the sometimes tumultuous nature of open-source development.[34]


This is a prior stable version, now unmaintained.


This is a prior stable version, now unmaintained.

2.0 branch

This was a development branch, long thought to be the next step in FLTK's evolution, with many new features and a cleaner programming style. It never achieved stability, and development has largely ceased. The branch is inactive now.


This was an attempt to take some of the best features of 2.0 and merge them back into the more popular 1.1 branch. It is no longer developed.


Current stable release.[35] Provides UTF-8 support.


Current development branch. Adds more features to 1.3.[36]

3.0 branch

This branch is mostly a conceptual model for future work. Now inactive.

See also


  1. https://www.fltk.org/articles.php?L1635
  2. "FLTK License Agreement".
  3. "FLTK home".
  4. "Archived copy". Archived from the original on 2015-05-18. Retrieved 2015-05-13.CS1 maint: archived copy as title (link)
  5. http://www.fltk.org/articles.php?L363+I0+TFAQ+P1+Q
  6. "GTK+ Language Bindings".
  7. "murgaLua homepage".
  8. http://www.fltk.org/wiki.php?LC+P139+TC+Q
  9. "pyFLTK homepage".
  10. "Ruby/FLTK homepage".
  11. "Tcl/Fltk homepage".
  12. "Nanolinux Official Website".
  13. https://eerielinux.wordpress.com/2014/11/30/tiny-to-the-extreme-nanolinux/
  14. https://eerielinux.wordpress.com/2014/12/30/an-interview-with-the-nanolinux-developer/
  15. "Archived copy". Archived from the original on 1999-04-20. Retrieved 2019-08-15.CS1 maint: archived copy as title (link)
  16. http://sites.utoronto.ca/webdocs/HTMLdocs/PCTOOLS/pc_editors.html
  17. http://www.georgpotthast.de/
  18. https://eerielinux.wordpress.com/2012/10/12/an-extraordinary-tk-example/
  19. https://eerielinux.wordpress.com/2014/12/30/an-interview-with-the-nanolinux-developer/
  20. "Archived copy". Archived from the original on 1999-04-20. Retrieved 2019-08-15.CS1 maint: archived copy as title (link)
  21. http://www.georgpotthast.de/
  22. YPH
  23. "Giada, hardcore loopmachine". Archived from the original on 2011-09-25. Retrieved 2011-09-29.
  24. Prodatum
  25. DiSTI GL Studio
  26. ForcePAD
  27. RoboCIM - Robotics software
  28. FlBurn
  29. Open Movie Editor
  30. OpenVSP
  31. PosteRazor
  32. Fifth browser
  33. NetRider
  34. "What are the Versions of FLTK?". Retrieved 2010-07-23.
  35. http://www.fltk.org/articles.php?L825 taken 2017 march 5.
  36. http://www.fltk.org/articles.php?L825 Taken 2017 March 5.
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