1. Overview

Linux is merely a kernel, which is the core of an operating system. It doesn’t provide any GUI or tools of its own for interacting with the operating system. However, there are userland tools to get user interfaces, shells, and other utilities that work on top of the Linux kernel.

In this tutorial, we’ll explore two of the most popular GUI toolkits for Linux: GTK+ and Qt. We’ll learn how they differ from each other. Apart from that, we’ll also discuss notable applications that use these toolkits.

2. GUI Toolkits on Linux

GUI toolkits are libraries that provide the basic building blocks for creating graphical user interfaces (GUIs) on Linux. We can use them to develop everything from simple dialog boxes to complex applications.

The two most popular GUI toolkits on Linux are Qt and GTK+. Other popular GUI toolkits include wxWidgets, FLTK, and Tk. However, we’ll only focus on GTK+ and Qt.

2.1. GTK+

GTK+ is a free and open-source graphical user interface (GUI) toolkit for creating graphical user interfaces (GUIs), mainly for Linux and other Unix-like operating systems.

The toolkit is the basis for many popular Linux applications, such as the GNOME desktop environment and the GIMP image editor.

2.2. Qt

Qt is a GUI toolkit that we can use to develop applications for various platforms like Linux, Windows, and macOS. Not only that, but we can also develop graphical applications for embedded devices.

It’s used by the K Desktop Environment, the Qt Creator IDE, and Google Earth.

3. Differences Between GTK and Qt Applications

Qt is a more powerful and versatile toolkit than GTK. However, GTK is simpler to use and has a larger community of developers.

In this section, we’ll review the major differences between the two.

3.1. Licensing

The Qt toolkit is dual-licensed, meaning it can be used under either a free and open-source license (LGPL and GPL) or a commercial license. The main difference between the two licenses is that the LGPL (GNU Lesser General Public License) allows us to link Qt libraries to our own proprietary code. At the same time, the GPL (GNU Public License) requires that we should release under the GPL as well.

This means that if we use Qt under the LGPL, we can still sell our application without having to release the source code. However, if we use Qt under the GPL, we must release the source code for our application as well.

On the other hand, the GTK toolkit is licensed under the LGPL.

3.2. Underlying Architecture

Under the hood, GTK+ applications contain multiple layers of the GTK+ stack. These layers have different goals and functionality:

  • GObject is a generic object system that is used by all of the other layers in a GTK+ applications
  • GLib provides a number of utility functions, such as string manipulation, memory management, and threading
  • The GDK layer provides an abstraction layer for the underlying windowing system, which provides functions for drawing, event handling, and input
  • The GTK+ layer provides a set of graphical widgets

Similarly, a Qt application is designed to be modular. The Core component is necessary because it provides the foundation for Qt applications. It includes classes for objects, strings, files, and other basic types.

In addition, the Qt framework also has other components for different purposes:

  • Qt GUI component, which provides the widgets and other graphical elements
  • Qt Network for networking, such as sockets and HTTP clients
  • Qt SQL layer provides classes for working with databases
  • Qt Multimedia component provides classes for working with multimedia, such as audio and video

3.3. Implementation Languages and Bindings

GTK+ is implemented in the C programming language. However, it also has bindings for a variety of other programming languages, including Python (PyGTK), JavaScript, C++ (GTKmm), Vala, and D (GTK.D). In addition to these bindings, there are also bindings for a number of other programming languages, such as Go, Rust (gtk-rs), and Perl.

Conversely, the implementation language for Qt is C++. It comes with QML (Qt Modeling Language), which we can use to quickly develop widgets for an application. Moreover, Makefiles for Qt applications are generated through qmake.

Like GTK+, Qt also has bindings for other languages like Python (PySide and PyQt) and Java (QtJambi).

3.4. Platform Support

GTK+ applications can be deployed on Linux, macOS, Windows, and other Unix-like operating systems. Therefore, it’s predominantly a desktop-oriented GUI toolkit.

Similarly, Qt applications support the aforementioned platforms as well, but they also support Android, iOS, and embedded devices. So, Qt has the lead when it comes to platform support.

On another note, the newer versions of both GTK+ and Qt fully support the Wayland protocol.

3.5. Look and Feel

The look and feel of GTK+ applications seem very polished and consistent. It’s because GTK+ follows a very opinionated pattern when it comes to designing graphical widgets. By default, it makes use of the Adwaita design language. However, we can change the theme if we wish to.

In contrast, the design of Qt applications is very flexible. For instance, QML apps can look very different, depending on the designer’s preferences. Moreover, we can make them more visually appealing with some extra work.

3.6. Development Workflow

There is no dedicated IDE for GTK+ applications. However, we can choose an IDE of our choice that has good support for the programming language. Moreover, there is Glade, which is an official UI designer for GTK+.

In contrast, Qt Creator is the official IDE for developing Qt applications, which itself depends on the Qt framework. In addition, we can also use Qt Creator to develop GTK+ applications.

3.7. Notable Examples

There is a plethora of software written in both toolkits. For instance, the GNOME Project relies on GTK+, while the K Development Environment heavily relies on the Qt framework. Most of the application suite of KDE depends on the Qt framework.

GTK+ isn’t very famous in most commercial applications. Some notable GTK+-based applications include GIMP, gedit, GNOME Terminal, Ubuntu Unity, and Mozilla Firefox.

Qt has a well established foundation in commercial applications. For instance, programs like CryEngine, Autodesk Maya, Google Earth, and Adobe Elements use the Qt framework.

3.8. Summary

The following table compares the basic characteristics of the toolkits:

GTK Qt
License LGPL GPL, LGPL, commercial
Language C C++
Supported Platforms Linux, macOS, Windows Linux, macOS, Windows, Android, iOS
Development Environment Flexible Qt Creator, MSVS Extension, CLion
Notable Examples GNOME, GIMP, Budgie, Inkscape, Transmission, Cinelerra KDE, VLC, qBittorrent, FileZilla, Calibre, Wireshark, Krita, Maya, CryEngine

4. Conclusion

In this article, we explored the differences between GTK+ and Qt-based applications. We learned how these two popular GUI toolkits differ in architecture, licensing model, implementation language, and platform support.

Apart from that, we also discussed a few notable examples that use these toolkits.