Skip to main content

FLTK

FLTK is a small, cross-platform C++ GUI toolkit. This builds it from source on Windows with the MSYS2 UCRT64 toolchain (see C++ for the toolchain setup), installs it to a prefix, then compiles a hello-world app against it with CMake.

Run everything from the MSYS2 UCRT64 shell.

Install the toolchain​

pacman -Syu       # update first; reopen the shell if it asks

pacman -S --needed \
  git \
  unzip \
  mingw-w64-ucrt-x86_64-toolchain \
  mingw-w64-ucrt-x86_64-cmake \
  mingw-w64-ucrt-x86_64-ninja \
  mingw-w64-ucrt-x86_64-zlib \
  mingw-w64-ucrt-x86_64-libpng \
  mingw-w64-ucrt-x86_64-libjpeg-turbo

zlib, libpng and libjpeg-turbo let FLTK read those image formats; drop them if you do not need image support.

Build FLTK from source​

Clone the repository and check out a release tag:

FLTK_VER=1.4.4
mkdir -p ~/fltk/src && cd ~/fltk/src
git clone https://github.com/fltk/fltk.git fltk
cd fltk
git checkout release-$FLTK_VER

Configure a static build, install it to a dedicated prefix, then build and install:

cmake -S . -B build-ucrt64-static -G Ninja \
  -D CMAKE_BUILD_TYPE=Debug \
  -D CMAKE_INSTALL_PREFIX=/opt/fltk-ucrt64-static \
  -D FLTK_BUILD_TEST=OFF

cmake --build   build-ucrt64-static
cmake --install build-ucrt64-static

Keep -D FLTK_BUILD_TEST=OFF. With the test programs enabled, the build can export broken library references (the demo executables get built, but the installed CMake targets your own project links against come out wrong). Turning the tests off leaves a clean, linkable install.

Verify the install exported its CMake config and libraries:

find /opt/fltk-ucrt64-static -name "FLTKConfig.cmake" 2>/dev/null
ls -1 /opt/fltk-ucrt64-static/lib

For shared libraries instead, configure into a separate build and prefix with FLTK_BUILD_SHARED_LIBS=ON, then build and install it the same way:

cmake -S . -B build-ucrt64-shared -G Ninja \
  -D CMAKE_BUILD_TYPE=Debug \
  -D CMAKE_INSTALL_PREFIX=/opt/fltk-ucrt64-shared \
  -D FLTK_BUILD_TEST=OFF \
  -D FLTK_BUILD_SHARED_LIBS=ON

cmake --build   build-ucrt64-shared
cmake --install build-ucrt64-shared

With a shared build the resulting .exe needs libfltk*.dll on the PATH at run time; the static build has no such dependency.

Build an app against FLTK​

Create a small project:

mkdir -p ~/fltk/src/fltk-hello
cd ~/fltk/src/fltk-hello

main.cpp — a window with a button that sets a label when clicked:

#include <FL/Fl.H>
#include <FL/Fl_Window.H>
#include <FL/Fl_Button.H>
#include <FL/Fl_Box.H>

static Fl_Box* g_text = nullptr;

static void on_click(Fl_Widget*, void*) {
    if (g_text) {
        g_text->label("Hello");
        g_text->redraw();
    }
}

int main() {
    Fl_Window win(360, 180, "FLTK Hello");

    Fl_Box text(20, 25, 320, 50, "");
    text.labelsize(28);
    g_text = &text;

    Fl_Button btn(130, 100, 100, 40, "Press");
    btn.callback(on_click);

    win.end();
    win.show();
    return Fl::run();
}

CMakeLists.txt — find the installed FLTK by its CMake config and link the fltk::fltk target:

cmake_minimum_required(VERSION 3.20)
project(fltk_hello_button LANGUAGES CXX)

set(CMAKE_CXX_STANDARD 17)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

find_package(FLTK CONFIG REQUIRED)

add_executable(fltk_hello main.cpp)
target_link_libraries(fltk_hello PRIVATE fltk::fltk)

Configure, build and run. CMAKE_PREFIX_PATH points CMake at the prefix you installed FLTK into:

cmake -S . -B build -G Ninja \
  -D CMAKE_BUILD_TYPE=Debug \
  -D CMAKE_PREFIX_PATH=/opt/fltk-ucrt64-static

cmake --build build
./build/fltk_hello.exe

To link the shared build instead, point CMAKE_PREFIX_PATH at /opt/fltk-ucrt64-shared.

Clean up​

rm -rf ~/fltk
rm -rf /opt/fltk-ucrt64-static /opt/fltk-ucrt64-shared