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FLTK 1.5.0
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The Fast Light Tool Kit ("FLTK") is a cross-platform C++ GUI toolkit for UNIX®/Linux® (X11 and Wayland), Microsoft® Windows®, and Apple® macOS®. FLTK provides modern GUI functionality without 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 on GitHub.
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 than 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.
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 later ported to Windows and macOS. FLTK 1.4 added a "driver based" system of virtual device drivers that enabled porting to Wayland as well. Drawing features include Windows GDI+, Cairo (Wayland and X11), and improved text layout with Pango.
There have been experiments using this driver system to build FLTK based on SDL2, Android, and other graphics systems based solely on simple pixel drawing, but this experimental code is not included in FLTK 1.4. There are thoughts to enable more platforms in later FLTK versions.
FLTK 1.5 removed autotools (configure + Makefile) support in favor of exclusive use of CMake to generate the build files. FLTK 1.5 allows to use a basic set of C++ features from the C++11 standard.
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 included with several Linux distributions.
Here are some of the core features unique to FLTK:
Note: sizes given below are mostly from 32-bit systems and FLTK 1.1 or earlier, this list needs updates for current FLTK 1.5 (64-bit).
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!
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".
FLTK comes bundled with FLUID. FLUID, short for Fast Light User Interface Designer, is a graphical editor capable of generating C++ source code and header files ready for compilation. These files ultimately create the graphical user interface for an application.
The FLUID User Handbook is available at https://www.fltk.org/documentation.php . It can also be compiled from the FLTK source repository using the fluid_docs target in the CMake build environment.
Starting with version 1.4, the recommended FLTK build system was CMake. Since FLTK 1.5.0 CMake is the only available build system for FLTK.
CMake is a "Build System Generator" that can generate build environments for usage with Ninja, Make, and many more, for instance several IDE's. See file README.CMake.txt of the FLTK source tree for more information.
By default CMake will look for a C++ compiler named CC, c++, g++, or gcc. To use another compiler you can set the CXX and CC environment variables for the C++ and C compiler, respectively. The code to use depends on your shell, for instance:
Then execute CMake like
Note that you can also specify the compiler and build tools on the CMake command line or in the CMake GUI.
When CMake is done you can just run CMake again to build the FLTK library, fluid, fltk-options (setup tool), and all test programs, depending on the configuration.
To install the library, become root and type "cmake --install build". This will copy the "fluid" executable to "bindir", the header files to "includedir", and the library files to "libdir".
NOTE: This documentation section is currently under review. More up-to-date information for this release may be available in the files "README.Windows.txt" and "README.CMake.txt" and you should read these files to determine if there are changes that may be applicable to your build environment.
FLTK 1.5 is officially supported on Windows 10 (11) and later. We don't intend to remove the ability to build and run FLTK 1.5 programs on older Windows versions but the FLTK Team can't build and test on Windows systems that are no longer supported by Microsoft. Therefore, such older systems (including Windows 10, starting in Oct. 2025, when Microsoft does no longer support Windows 10) may work with FLTK or not.
FLTK currently supports the following development environments on the Windows platform:
Visual Studio 2015 Community or later versions use workspace and project files generated by CMake. Older versions and the commercial versions can be used as well, if they can open the project files generated by CMake. However, FLTK 1.5 and later requires newer C++ features (C++11 or higher) that may not be available in older versions of Visual Studio.
FLTK support of Visual Studio is limited to CMake's support for these Visual Studio versions. Be sure to get your service packs!
Since FLTK 1.5 the project files MUST be generated with CMake. Please read "README.txt", "README.CMake.txt", and "README.Windows.txt" for more information about this.
The Visual Studio project files can be used to build a DLL version of the FLTK library if CMake option 'FLTK_BUILD_SHARED_LIBS=ON' is set. Because of name mangling differences between PC compilers (even between different versions of Visual Studio) you can only use the DLL that is generated with the same compiler version that you built it with.
When compiling an application or DLL that uses the FLTK DLL with Visual Studio, you need to define the FL_DLL preprocessor symbol to get the correct linkage commands embedded within the FLTK header files.
New since FLTK 1.4.0:
If you build your application project with CMake and use the CMake target 'fltk::fltk-shared' to link your application, then 'FL_DLL' is defined automatically for you (by CMake Compile Definitions). If you use your own (hand-made) Visual Studio project you still need to define FL_DLL to compile all source files that use FLTK headers.
If using Cygwin with the Cygwin shell, or MinGW with the Msys shell, these build environments behave very much like a Unix or macOS build and the notes above can be applied. Build files are generated by CMake for the platform you are using. See README.CMake.txt for more info.
In general for a build using these tools, e.g. for the Msys shell with MinGW, it should suffice to "cd" into the directory where you have extracted the FLTK tarball and type:
This will build the FLTK libraries and they can then be utilised directly from the build location. NOTE: this may be simpler than "installing" them in many cases as different tool chains on Windows have different ideas about where the files should be "installed" to.
For example, if you "install" the libraries using Msys/MinGW with the following command
then Msys will "install" the libraries to where it thinks the path "/usr/local/" leads to. If you only ever build code from within the Msys environment this works well, but the actual "Windows path" these files are located in will be something like "C:\msys\1.0\local\lib", depending on where your Msys installation is rooted, which may not be useful to other tools.
If you want to install your built FLTK libraries in a non-standard location you may do:
Where the value passed to "CMAKE_INSTALL_PATH" is the path at which you would like FLTK to be installed. Depending on the build environment using standard Windows path syntax may work, or you need to use MinGW (or Cygwin) syntax.
FLTK is available on the internet in a bunch of locations:
To report a bug in FLTK, or for feature requests, please use https://www.fltk.org/bugs.php for information about where and how to post bugs, feature requests, or ask for help on using FLTK.
For general support and questions, please use the fltk.general newsgroup (see above, "NNTP Newsgroups") or the web interface to the newsgroups at https://www.fltk.org/newsgroups.php, or GitHub Discussions (see link above).
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