Crate fltk

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Rust bindings for the FLTK Graphical User Interface library.

The fltk crate is a cross-platform lightweight gui library which can be statically linked to produce small, self-contained (no external dependencies) and fast gui applications.

Why choose FLTK?

  • Lightweight. Small binary, around 1mb after stripping. Small memory footprint.
  • Speed. Fast to install, fast to build, fast at startup and fast at runtime.
  • Single executable. No DLLs to deploy.
  • Supports old architectures.
  • FLTK’s permissive license which allows static linking for closed-source applications.
  • Themeability (4 supported schemes: Base, GTK, Plastic and Gleam), and additional theming using fltk-theme.
  • Provides around 80 customizable widgets.
  • Has inbuilt image support.


Here is a list of software using FLTK. For software using fltk-rs, check here.

  • Link to the official FLTK repository.
  • Link to the official documentation.


Just add the following to your project’s Cargo.toml file:

fltk = "^1.2"

To use the latest changes in the repo:

fltk = { version = "^1.2", git = "" }

To use the bundled libs (available for x64 windows (msvc & gnu (msys2)), x64 linux & macos):

fltk = { version = "^1.2", features = ["fltk-bundled"] }

The library is automatically built and statically linked to your binary.

An example hello world application:

use fltk::{app, prelude::*, window::Window};
fn main() {
    let app = app::App::default();
    let mut wind = Window::new(100, 100, 400, 300, "Hello from rust");

Another example showing the basic callback functionality:

use fltk::{app, button::Button, frame::Frame, prelude::*, window::Window};
fn main() {
    let app = app::App::default();
    let mut wind = Window::new(100, 100, 400, 300, "Hello from rust");
    let mut frame = Frame::new(0, 0, 400, 200, "");
    let mut but = Button::new(160, 210, 80, 40, "Click me!");
    but.set_callback(move |_| frame.set_label("Hello World!")); // the closure capture is mutable borrow to our button;

Please check the examples directory for more examples. You will notice that all widgets are instantiated with a new() method, taking the x and y coordinates, the width and height of the widget, as well as a label which can be left blank if needed. Another way to initialize a widget is using the builder pattern: (The following buttons are equivalent)

use fltk::{button::Button, prelude::*};
let but1 = Button::new(10, 10, 80, 40, "Button 1");

let but2 = Button::default()
    .with_pos(10, 10)
    .with_size(80, 40)
    .with_label("Button 2");

An example of a counter showing use of the builder pattern:

use fltk::{app, button::Button, frame::Frame, prelude::*, window::Window};
fn main() {
    let app = app::App::default();
    let mut wind = Window::default()
        .with_size(160, 200)
    let mut frame = Frame::default()
        .with_size(100, 40)
    let mut but_inc = Button::default()
        .above_of(&frame, 0)
    let mut but_dec = Button::default()
        .below_of(&frame, 0)
    /* Event handling */;

Alternatively, you can use Pack, Flex (for flexbox layouts) or Grid:

use fltk::{app, button::Button, frame::Frame, group::Flex, prelude::*, window::Window};
fn main() {
    let app = app::App::default();
    let mut wind = Window::default().with_size(160, 200).with_label("Counter");
    let mut flex = Flex::default().with_size(120, 140).center_of_parent().column();
    let mut but_inc = Button::default().with_label("+");
    let mut frame = Frame::default().with_label("0");
    let mut but_dec = Button::default().with_label("-");


Events can be handled using the set_callback method (as above) or the available fltk::app::set_callback() free function, which will handle the default trigger of each widget(like clicks for buttons):

    /* previous hello world code */
    but.set_callback(move |_| frame.set_label("Hello World!"));
    another_but.set_callback(|this_button| this_button.set_label("Works"));;

Another way is to use message passing:

    /* previous counter code */
    let (s, r) = app::channel::<Message>();

    but_inc.emit(s, Message::Increment);
    but_dec.emit(s, Message::Decrement);

    while app.wait() {
        let label: i32 = frame.label().parse().unwrap();
        if let Some(msg) = r.recv() {
            match msg {
                Message::Increment => frame.set_label(&(label + 1).to_string()),
                Message::Decrement => frame.set_label(&(label - 1).to_string()),

For the remainder of the code, check the full example here.

For custom event handling, the handle() method can be used:

    some_widget.handle(move |widget, ev: Event| {
        match ev {
            Event::Push => {
            /* other events to be handled */
            _ => false,

Handled or ignored events using the handle method should return true, unhandled events should return false. More examples are available in the examples directory.

For an alternative event handling mechanism using on_<event> methods, check the fltk-evented crate.


FLTK offers 4 application schemes:

  • Base
  • Gtk
  • Gleam
  • Plastic

(Additional theming can be found in the fltk-theme crate)

These can be set using the App::with_scheme() function.

let app = app::App::default().with_scheme(app::Scheme::Gleam);

Themes of individual widgets can be optionally modified using the provided methods in the WidgetExt trait, such as set_color(), set_label_font(), set_frame() etc:

    some_button.set_color(Color::Light1); //! You can use one of the provided colors in the fltk enums
    some_button.set_color(Color::from_rgb(255, 0, 0)); //! Or you can specify a color by rgb or hex/u32 value

For default application colors, fltk-rs provides app::background(), app::background2() and app::foreground(). You can also specify the default application selection/inactive colors, font, label size, frame type, scrollbar size, menu line-spacing. Additionally the fltk-theme crate offers some other predefined color maps (dark theme, tan etc) and widget themes which can be loaded into your application.


Rust (version > 1.45), CMake (version > 3.11), Git and a C++11 compiler need to be installed and in your PATH for a cross-platform build from source. Ninja is recommended, but not required. This crate also offers a bundled form of fltk on selected x86_64 platforms (Windows (msvc and gnu), MacOS, Linux), this can be enabled using the fltk-bundled feature flag as mentioned in the usage section (this requires curl and tar to download and unpack the bundled libraries).

  • Windows:
    • MSVC: Windows SDK
    • Gnu: No dependencies
  • MacOS: No dependencies.
  • Linux/BSD: X11 and OpenGL development headers need to be installed for development. The libraries themselves are normally available on linux distros with a graphical user interface.

For Debian-based GUI distributions, that means running:

$ sudo apt-get install libx11-dev libxext-dev libxft-dev libxinerama-dev libxcursor-dev libxrender-dev libxfixes-dev libpango1.0-dev libgl1-mesa-dev libglu1-mesa-dev

For RHEL-based GUI distributions, that means running:

$ sudo yum groupinstall "X Software Development" && yum install pango-devel libXinerama-devel

For Arch-based GUI distributions, that means running:

$ sudo pacman -S libx11 libxext libxft libxinerama libxcursor libxrender libxfixes pango cairo libgl mesa --needed

For Alpine linux:

$ apk add pango-dev fontconfig-dev libxinerama-dev libxfixes-dev libxcursor-dev mesa-gl

For NixOS (Linux distribution) this nix-shell environment can be used:

$ nix-shell --packages rustc cmake git gcc xorg.libXext xorg.libXft xorg.libXinerama xorg.libXcursor xorg.libXrender xorg.libXfixes libcerf pango cairo libGL mesa pkg-config


The following are the features offered by the crate:

  • use-ninja: Uses the ninja build system if available for a faster build, especially on Windows.
  • fltk-bundled: Support for bundled versions of cfltk and fltk on selected platforms (requires curl and tar)
  • no-pango: Build without pango support on Linux/BSD.
  • enable-glwindow: Support for drawing using OpenGL functions.
  • system-libpng: Uses the system libpng
  • system-libjpeg: Uses the system libjpeg
  • system-zlib: Uses the system zlib


please check the FAQ page for frequently asked questions, encountered issues, guides on deployment, and contribution.


Application related methods and functions

Browser widgets

Button widgets

Dialog widgets

Drawing primitives

Fltk defined enums: Color, Font, CallbackTrigger etc

Documentation for the examples

Basic fltk box/frame widget

Group widgets

Image types supported by fltk

Input widgets

mod macros;

Menu widgets

Miscellaneous widgets not fitting a certain group

Output widgets

All fltk widget traits and flt error types

Printing related functions

Widget surface to image functions

Table widgets

Text display widgets

Tree widgets

General utility functions

Valuator widgets

Basic empty widget

Window widgets


Defines a set of convenience functions for constructing and anchoring custom widgets. Usage: fltk::widget_extends!(CustomWidget, BaseWidget, member); It basically implements Deref and DerefMut on the custom widget, and adds the aforementioned methods.