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1 - Introduction to FLTK

The Fast Light Tool Kit ("FLTK", pronounced "fulltick") is a cross-platform C++ GUI toolkit for UNIX®/Linux® (X11), Microsoft® Windows®, and MacOS® X. FLTK provides modern GUI functionality without the bloat and supports 3D graphics via OpenGL® and its built-in GLUT emulation. 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.

History of FLTK

It has always been Bill's belief that the GUI API of all modern systems is much too high level. Toolkits (even FLTK) are not what should be provided and documented as part of an operating system. 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. NeXT (if you ignored NextStep) provided this, but they chose to hide it and tried to push their own baroque toolkit instead.

Many of the ideas in FLTK were developed on a NeXT (but not using NextStep) in 1987 in a C toolkit Bill called "views". Here he came up with passing events downward in the tree and having the handle routine return a value indicating whether it used the event, and the table-driven menus. In general he was trying to prove that complex UI ideas could be entirely implemented in a user space toolkit, with no knowledge or support by the system.

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 "views" atop that. NeWS did have an unnecessarily complex method of delivering events which hurt it. But the designers did admit that perhaps the user could write just as good of a button as they could, and officially exposed the lower level interface.

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.

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. He decided to use Forms, except he integrated his table-driven menus into it. Several very large programs were created using this version of Forms.

The need to switch to OpenGL and GLX, portability, and a desire to use C++ subclassing required a rewrite of 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 by simplifying the lower level interface and the event passing mechanism.

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.

Digital Domain has since withdrawn support for FLTK. While Bill is no longer able to actively develop it, he still contributes to FLTK in his free time and is a part of the FLTK development team.


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. 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. FLTK works fine as a shared library, though, and is now included with several Linux distributions.

Here are some of the core features unique to FLTK:

  • sizeof(Fl_Widget) == 64 to 92.
  • The "core" (the "hello" program compiled & linked with a static FLTK library using gcc on a 486 and then stripped) is 114K.
  • The FLUID program (which includes every widget) is 538k.
  • Written directly atop core libraries (Xlib, WIN32 or Carbon) for maximum speed, and carefully optimized for code size and performance.
  • Precise low-level compatability between the X11, WIN32 and MacOS versions - only about 10% of the code is different.
  • Interactive user interface builder program. Output is human-readable and editable C++ source code.
  • Support for overlay hardware, with emulation if none is available.
  • Very small & fast portable 2-D drawing library to hide Xlib, WIN32, or QuickDraw.
  • OpenGL/Mesa drawing area widget.
  • Support for OpenGL overlay hardware on both X11 and WIN32, with emulation if none is available.
  • Text widgets 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.


FLTK comes with complete free source code. FLTK is available under the terms of the GNU Library General Public License with exceptions that allow for static linking. Contrary to popular belief, it can be used in commercial software - even Bill Gates could use it!

What Does "FLTK" Mean?

FLTK was originally designed to be compatible with the Forms Library written for SGI machines. 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 was taken as the name of the library. It is almost impossible to search for "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", including a bogus excuse that it stands for "The Fast Light Toolkit".

Building and Installing FLTK Under UNIX and MacOS X

In most cases you can just type "make". This will run configure with the default of no options and then compile everything.

FLTK uses GNU autoconf to configure itself for your UNIX platform. The main things that the configure script will look for are the X11 and OpenGL (or Mesa) header and library files. If these cannot be found in the standard include/library locations you'll need to define the CFLAGS, CXXFLAGS, and LDFLAGS environment variables. For the Bourne and Korn shells you'd use:

    CFLAGS=-Iincludedir; export CFLAGS
    CXXFLAGS=-Iincludedir; export CXXFLAGS
    LDFLAGS=-Llibdir; export LDFLAGS

For C shell and tcsh, use:

    setenv CFLAGS "-Iincludedir"
    setenv CXXFLAGS "-Iincludedir"
    setenv LDFLAGS "-Llibdir"

By default configure will look for a C++ compiler named CC, c++, g++, or gcc in that order. To use another compiler you need to set the CXX environment variable:

    CXX=xlC; export CXX
    setenv CXX "xlC"

The CC environment variable can also be used to override the default C compiler (cc or gcc), which is used for a few FLTK source files.

You can run configure yourself to get the exact setup you need. Type "./configure <options>", where options are:

Enable the Cygwin libraries under WIN32
Enable debugging code & symbols
Disable OpenGL support
Enable generation of shared libraries
Enable multithreading support
Enable the X double-buffer extension
Enable the Xft library for anti-aliased fonts under X11
Set the location for executables [default = $prefix/bin]
Set the location for data files. [default = $prefix/share]
Set the location for libraries [default = $prefix/lib]
Set the location for include files. [default = $prefix/include]
Set the location for man pages. [default = $prefix/man]
Set the directory prefix for files [default = /usr/local]

When the configure script is done you can just run the "make" command. This will build the library, FLUID tool, and all of the test programs.

To install the library, become root and type "make install". This will copy the "fluid" executable to "bindir", the header files to "includedir", and the library files to "libdir".

Building FLTK Under Microsoft Windows

There are three ways to build FLTK under Microsoft Windows. The first is to use the Visual C++ 5.0 project files under the "visualc" directory. Just open (or double-click on) the "fltk.dsw" file to get the whole shebang.

The second method is to use the configure script included with the FLTK software; this has only been tested with the CygWin tools:

    sh configure --prefix=C:/FLTK

The final method is to use a GNU-based development tool with the files in the "makefiles" directory. To build using one of these tools simply copy the appropriate makeinclude and config files to the main directory and do a make:

    copy makefiles\Makefile.<env> Makefile

Using the Visual C++ DLL Library

The "fltkdll.dsp" project file builds a DLL-version of the FLTK library. Because of name mangling differences between PC compilers (even between different versions of Visual C++!) you can only use the DLL that is generated with the same version compiler that you built it with.

When compiling an application or DLL that uses the FLTK DLL, you will need to define the FL_DLL preprocessor symbol to get the correct linkage commands embedded within the FLTK header files.

Building FLTK Under OS/2

The current OS/2 build requires XFree86 for OS/2 to work. A native Presentation Manager version has not been implemented yet (volunteers are welcome!).

The current set of Makefiles/configuration failes assumes that EMX 0.9d and libExt (from is installed.

To build the XFree86 version of FLTK for OS/2, copy the appropriate makeinclude and config files to the main directory and do a make:

    copy makefiles\Makefile.os2x Makefile

Internet Resources

FLTK is available on the 'net in a bunch of locations:

WWW [for reporting bugs] [source code]
California, USA, (
California, USA (
Maryland, USA (
Espoo, Finland (
Germany (
Austria (
EMail [see instructions below] [for reporting bugs]
NNTP Newsgroups

To send a message to the FLTK mailing list ("") you must first join the list. Non-member submissions are blocked to avoid problems with unsolicited email.

To join the FLTK mailing list, send a message to "" with "subscribe fltk" in the message body. A digest of this list is available by subscribing to the "fltk-digest" mailing list.

Reporting Bugs

To report a bug in FLTK, send an email to "". Please include the FLTK version, operating system & version, and compiler that you are using when describing the bug or problem. We will be unable to provide any kind of help without that basic information.

Bugs can also be reported to the "fltk.bugs" newsgroup or on the SourceForge bug tracker pages.

For general support and questions, please use the FLTK mailing list at "" or one of the newsgroups.

User Comments [ Add Comment ]

From Anonymous, 15:09 Feb 03, 2003 (score=5)

If you are compiling for a cygwin application that uses OpenGL, be sure to enable cygwin in the fltk make file:

configure --enable-cygwin

Reply ]

From sparky, 09:17 Sep 01, 2005 (score=4)

To compile using Cygwin, I had to force use of local versions of the jpeg, zlib, and png libraries.

for example, when initially compiling FLTK 1.1.6 and FLUID type:
  sh configure --prefix=C:/fltk-1.1.6 --enable-local-jpeg    
    --enable-localzlib --enable-localpng

I previosly had to undefine HAVE_LIBZ and HAVELIBJPEG in "config.h" to compile correctly in MS VC++ 6.0 with FLTK 1.1.5.  This appears to have been fixed since FLTK 1.1.6.
Reply ]

From eng_ocoka, 14:55 Oct 12, 2005 (score=4)

I basically had the same problem in Cygwin.  The make would not find the jpeg header, but the options I used were

./configure --prefix=/usr/local/fltk --enable-localjpeg --enable-localzlib --enable-localpng

(notice no hyphen on the localjpeg)
Reply ]

From kanze, 10:12 Oct 03, 2008 (score=3)

When building for several systems from a common source tree, even make distclean isn't sufficient; it leaves some of the .o files.  I needed to do a find . -name '*.o' | xargs rm.  (This is for Unix based systems.  I haven't tried to build it yet for Windows.)
Reply ]

From Anonymous, 15:01 Oct 21, 2004 (score=2)

MSYS from the MinGW site also worked with: configure make make install
Reply ]

From Darius, 21:34 Nov 12, 2002 (score=2)

To compile against the DLL version of FLTK you also need to put fltkdll.lib in your Project->Settings->Link->Object/Library Modules list. (I found the file at fltk\test\fltkdll.lib)
Reply ]


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