Introduction to FLTK

This manual describes FLTK version 2.0, a cross-platform C++ Graphical User Interface (GUI) toolkit. Each of the "related pages" in this manual is designed as a tutorial for using FLTK, while the main sections provide a complete reference for all FLTK widgets and functions.

FLTK (pronounced "fulltick") is a LGPL'd C++ graphical user interface toolkit for X (UNIX®), OpenGL®, Microsoft® Windows®, OS/X, and several other platforms. It was originally developed by Mr. Bill Spitzak and is currently maintained by a small group of developers across the world with a central repository in the US.


FLTK comes with complete free source code. FLTK is ©1998-2011 by Bill Spitzak and others. Use and distribution of FLTK is governed by the FLTK Library License, which is the GNU Library General Public License with an exception added that allows you to distribute statically-linked programs using the library without providing source code to the program or the library). You can thus use FLTK in any software, whether or not you include the source code.

What Does "FLTK" Mean?

FLTK was originally designed to be compatible with the Forms Library written for SGI machines (a derivative of this library called "XForms" is still used quite often). In that library all the functions and structures started 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 it was impossible to search "FL" on the Internet, due to the fact that 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 came up with "FLTK", with the bogus excuse that it stands for the "Fast Light Tool Kit".


FLTK was designed to be statically linked. This was done by splitting it into many small objects and designing it so that functions that are not used do not have pointers to them in the parts that are used, and thus do not get linked in. It is also designed so that all data used by the GUI, such as images and widget layout, can be inlined into source code.

This allows you to make an easy-to-install program, or to modify FLTK to the exact requirements of your application, without worrying about bloat.

However, FLTK works fine as a shared library. It is often included in this form on Linux distributions.

Here are some of the core features unique to FLTK:

  • sizeof(fltk::Widget) == 60.
  • The "core" (the "hello" program compiled & linked with a static FLTK library using gcc on a 486 and then stripped) is 82K.
  • The FLUID program (which includes every widget) is 352k.
  • Written directly atop Xlib (or WIN32) for maximum speed, and carefully optimized for code size and performance.
  • Precise low-level compatibility between the X11 and WIN32 version (only about 10% of the code is different).
  • Interactive user interface builder program. Output is human-readable and editable C++ source code.
  • Support for X11 overlay hardware (emulation if none and under WIN32.)
  • Very small & fast portable 2-D drawing library with PostScript style functions, to hide Xlib and WIN32.
  • OpenGL/Mesa drawing area widget.
  • Support for OpenGL overlay hardware on both X11 and WIN32. Emulation if none.
  • Text input fields with Emacs key bindings, X cut & paste, and foreign letter compose!
  • Compatibility header file for the GLUT library.
  • Compatibility header file for the XForms library.
  • Much too much to list here...

History of FLTK

It has always been Bill's belief that the "operating system" does not have to provide any of what people call "GUI". Toolkits (even FLTK) are not what should be provided, the system only has to provide arbitrary shaped but featureless windows, a powerful set of graphics drawing calls, and a simple, unalterable method of delivering events to the owners of the windows. Much of the design of FLTK is to prove that complex UI ideas could be entirely implemented in a user space toolkit, with no knowledge or support by the system.

Many of the ideas in FLTK were developed on a NeXT (but not using NextStep) in 1987 in a C toolkit Bill called "viewkit". Here he came up with passing events downward in the tree and having the handle routine return a value indicating they used the event, which got rid of the need for "interests" that so complicated Motif and NeWS.

After going to film school for a few years, Bill worked at Sun Microsystems on the (doomed) NeWS project. Here he found an even better and cleaner windowing system, and he reimplemented "viewkit" atop that. NeWS did have an unnecessarily complex method of delivering events which hurt it.

With the death of NeWS Bill realized that he would have to live with X. The biggest problem with X is the "window manager", which means that the toolkit can no longer control the window borders or drag the window around. Indeed far more code is spent trying to talk to window managers than would be needed to draw the borders themselves. (fortunately the problems with X are also replicated on Windows, and thus solving them helped with the porting to Windows).

At Digital Domain Bill discovered another toolkit, "Forms". Forms was similar to his work, but provided many more widgets, since it was used in many real applications, rather then as theoretical work. Several large pieces of software were written using a version of Forms with the menus and file browser replaced with code from viewkit.

The need to switch to OpenGL, a desire to use C++, and the closed-source nature of XForms, all led to a requirement to rewrite Forms. This produced the first version of FLTK. The conversion to C++ required so many changes it made it impossible to recompile any Forms objects. Since it was incompatible anyway, Bill decided to incorporate his older ideas as much as possible.

Bill received permission to release it for free on the Internet, with the GNU general public license. Response from Internet users indicated that the Linux market dwarfed the SGI and high-speed GL market, so he rewrote it to use X for all drawing, greatly speeding it up on these machines. That is the version you have now.

FLTK 2.0 is a rewrite to make the interfaces to each widget more consistent, to use C++ more correctly, including the ability (but not the requirement) to support functor style callbacks, exceptions, and a namespace, and to support themeing of the GUI without having to set the color of every widget.

Internet Resources

FLTK is hosted on a web site, operated by Michael Sweet of the Easy Software Corporation (the developer of CUPS). An SVN repository of the source code, downloads of the fltk package, mailing list and newsserver, bug tracking, and programs using FLTK are all here.

FLTK also has a SourceForge page at, though currently all source code is actually on the site.

(Other stuff was here but I believe it was inaccurate. Need info on how to read the mailing lists using usenet, get svn access, post bugs, etc. This information is currently available at