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Documentation Index

Fetch the complete documentation index at: https://mintlify.com/fltk/fltk/llms.txt

Use this file to discover all available pages before exploring further.

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Introduction to FLTK

The Fast Light Tool Kit (FLTK) is a cross-platform C++ GUI toolkit for UNIX®/Linux® (X11 or Wayland), Microsoft® Windows®, and macOS®. FLTK provides modern GUI functionality without bloat and supports 3D graphics via OpenGL® and its built-in GLUT emulation. Originally developed by Bill Spitzak, FLTK is currently maintained by a small group of developers across the world with a central repository on GitHub.

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Get started

Quick start

Build your first FLTK application in under 5 minutes

Installation

Install FLTK on Linux, Windows, or macOS

Hello world

Step-by-step guide to creating your first window

API reference

Complete API documentation for all FLTK classes

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Why FLTK?

FLTK stands out as a GUI toolkit for developers who value efficiency, simplicity, and cross-platform compatibility.

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Cross-platform native support

Write once, compile anywhere. FLTK runs natively on:
  • Linux/Unix: X11 or Wayland backends with Cairo and Pango support
  • Windows: Native Win32 API with GDI+ antialiasing
  • macOS: Native Cocoa framework for both Intel and Apple Silicon

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Lightweight and fast

FLTK’s core design philosophy prioritizes minimal overhead:
  • Small binary size - applications start in milliseconds
  • Low memory footprint ideal for embedded systems
  • Direct rendering without unnecessary abstraction layers
  • Single header inclusion model for faster compilation

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Mature and stable

FLTK has been in active development since 1998:
  • Battle-tested in production environments worldwide
  • Stable API with careful consideration for backwards compatibility
  • LGPL license with static linking exception - use freely in commercial software
  • Active community and responsive maintainers

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Modern C++ features

FLTK 1.5 embraces modern C++ while maintaining simplicity:
  • C++11 standard requirement enables cleaner, safer code
  • Standard library integration with std::string support
  • Smart pointer compatibility for memory management
  • Template-based event handling

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Key features

FLTK includes over 70 built-in widgets:
  • Standard controls: buttons, sliders, input fields, menus
  • Advanced widgets: browser, table, tree, tabs, wizard
  • Text editing: full-featured editor with syntax highlighting support
  • File choosers: native file dialogs on all platforms
  • Custom drawing: create your own widgets with simple draw() override
Built-in OpenGL support with GLUT emulation:
  • Fl_Gl_Window class for 3D graphics
  • Hardware-accelerated rendering
  • Cross-platform OpenGL context management
  • Compatible with modern OpenGL 3+ shaders
Comprehensive image handling capabilities:
  • Built-in support: PNG, JPEG, BMP, XPM, XBM
  • SVG rendering and creation with built-in library
  • Animated GIF playback and control
  • Image manipulation: scaling, rotation, color adjustment
Full Unicode and internationalization support:
  • UTF-8 text rendering throughout the toolkit
  • Pango integration for complex scripts (Arabic, Hindi, Thai)
  • Right-to-left text support
  • Multiple font support including TrueType on all platforms
Interactive interface designer included:
  • Visual layout editor with drag-and-drop
  • Code generation for C++ classes
  • Live preview of interface designs
  • Integration with CMake and build systems

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Architecture overview

FLTK uses a simple, event-driven architecture: 
#include <FL/Fl.H>
#include <FL/Fl_Window.H>
#include <FL/Fl_Button.H>

void button_callback(Fl_Widget *widget, void *data) {
  // Handle button click
}

int main(int argc, char **argv) {
  Fl_Window *window = new Fl_Window(320, 240, "My App");
  Fl_Button *button = new Fl_Button(20, 20, 80, 25, "Click");
  button->callback(button_callback);
  window->end();
  window->show(argc, argv);
  return Fl::run();  // Enter event loop
}
Key concepts:
  • Window hierarchy: Windows contain widgets in parent-child relationships
  • Event loop: Fl::run() processes events until all windows close
  • Callbacks: Functions triggered by user interactions
  • Automatic layout: Widgets manage their own drawing and event handling

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What’s new in FLTK 1.5

FLTK 1.5 requires C++11 and uses CMake exclusively for building.
Major improvements in version 1.5:
  • C++11 baseline: Modern language features throughout the API
  • CMake only: Simplified, consistent build system across platforms
  • Enhanced Wayland support: First-class support for Wayland on Linux
  • Improved text rendering: Better Pango integration and Unicode handling
  • Pen/stylus support: Pressure-sensitive input on supporting hardware
  • Better HiDPI: Automatic scaling on high-resolution displays

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Community and support

Get help and connect with other FLTK developers:

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License

FLTK is free software under the GNU Library General Public License (LGPL) with exceptions that permit static linking. This means you can:
  • Use FLTK in commercial applications
  • Statically link FLTK into proprietary software
  • Distribute FLTK-based applications under any license
  • Modify FLTK source (modifications must remain LGPL)
The LGPL with static linking exception makes FLTK ideal for commercial software development while keeping the toolkit itself open source.

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Next steps

Install FLTK

Set up your development environment

Build your first app

Create a working application in 5 minutes