FLTK 1.4.0
FLTK Basics

This chapter teaches you the basics of writing and compiling programs that use FLTK.

Writing Your First FLTK Program

All programs must include the file <FL/Fl.H>. This file should be included as the first FLTK header file. In addition the program must include a header file for each FLTK class it uses. Listing 1 shows a simple "Hello, World!" program that uses FLTK to display the window.

Listing 1 - "hello.cxx"
#include <FL/Fl.H>
#include <FL/Fl_Window.H>
#include <FL/Fl_Box.H>
int main(int argc, char **argv) {
Fl_Window *window = new Fl_Window(340, 180);
Fl_Box *box = new Fl_Box(20, 40, 300, 100, "Hello, World!");
window->show(argc, argv);
return Fl::run();
const Fl_Font FL_BOLD
add this to helvetica, courier, or times
Definition: Enumerations.H:1060
Draws a label with shadows behind the text.
Definition: Enumerations.H:845
const Fl_Font FL_ITALIC
add this to helvetica, courier, or times
Definition: Enumerations.H:1061
see figure Standard Box Types
Definition: Enumerations.H:631
Fl static class.
Fl_Window widget .
This widget simply draws its box, and possibly its label.
Definition: Fl_Box.H:32
void end()
Exactly the same as current(this->parent()).
Definition: Fl_Group.cxx:73
Fl_Font labelfont() const
Gets the font to use.
Definition: Fl_Widget.H:495
Fl_Boxtype box() const
Gets the box type of the widget.
Definition: Fl_Widget.H:370
Fl_Fontsize labelsize() const
Gets the font size in pixels.
Definition: Fl_Widget.H:510
Fl_Labeltype labeltype() const
Gets the label type.
Definition: Fl_Widget.H:464
This widget produces an actual window.
Definition: Fl_Window.H:55
void show() FL_OVERRIDE
Puts the window on the screen.
Definition: Fl_Window.cxx:539
static int run()
Calls Fl::wait()repeatedly as long as any windows are displayed.
Definition: Fl.cxx:604

After including the required header files, the program then creates a window. All following widgets will automatically be children of this window.

Fl_Window *window = new Fl_Window(340, 180);

Then we create a box with the "Hello, World!" string in it. FLTK automatically adds the new box to window, the current grouping widget.

Fl_Box *box = new Fl_Box(20, 40, 300, 100, "Hello, World!");

Next, we set the type of box and the font, size, and style of the label:

We tell FLTK that we will not add any more widgets to window.


Finally, we show the window and enter the FLTK event loop:

window->show(argc, argv);
return Fl::run();

The resulting program will display the "Hello, World!" window:

The Hello, World! Window

You can quit the program by closing the window or pressing the ESCape key.

Creating the Widgets

The widgets are created using the C++ new operator. For most widgets the arguments to the constructor are:

Fl_Widget(x, y, width, height, label)
Fl_Widget is the base class for all widgets in FLTK.
Definition: Fl_Widget.H:85

The x and y parameters determine where the widget or window is placed on the screen. In FLTK the top left corner of the window or screen is the origin (i.e. x = 0, y = 0) and the units are in pixels.

The width and height parameters determine the size of the widget or window in pixels. The maximum widget size is typically governed by the underlying window system or hardware.

label is a pointer to a character string to label the widget with or NULL. If not specified the label defaults to NULL. The label string must be in static storage such as a string constant because FLTK does not make a copy of it - it just uses the pointer.

Creating Widget Hierarchies

Widgets are commonly ordered into functional groups, which in turn may be grouped again, creating a hierarchy of widgets. FLTK makes it easy to fill groups by automatically adding all widgets that are created between a myGroup->begin() and myGroup->end(). In this example, myGroup would be the current group.

Newly created groups and their derived widgets implicitly call begin() in the constructor, effectively adding all subsequently created widgets to itself until end() is called.

Calling end() on one group widget transfers the "current group" property to the parent of that widget. Calling end() on a top level window (which has no parent) sets the current group to NULL.

Setting the current group to NULL will stop automatic hierarchies. New widgets can now be added manually using Fl_Group::add(...) and Fl_Group::insert(...).

Get/Set Methods

box->box(FL_UP_BOX) sets the type of box the Fl_Box draws, changing it from the default of FL_NO_BOX, which means that no box is drawn. In our "Hello, World!" example we use FL_UP_BOX, which means that a raised button border will be drawn around the widget. More details are available in the Box Types section.

You could examine the boxtype by doing box->box(). FLTK uses method name overloading to make short names for get/set methods. A "set" method is always of the form "void name(type)", and a "get" method is always of the form "type name() const".

Redrawing After Changing Attributes

Almost all of the get/set pairs are very fast, short inline functions and thus very efficient. However, the "set" methods do not call redraw() - you have to call it yourself. This greatly reduces code size and execution time. The only common exceptions are value() which calls redraw() and label() which calls redraw_label() if necessary.


All widgets support labels. In the case of window widgets, the label is used for the label in the title bar. Our example program calls the labelfont(), labelsize(), and labeltype() methods.

The labelfont() method sets the typeface and style that is used for the label, which for this example we are using FL_BOLD and FL_ITALIC.

The labelsize() method sets the height of the font in pixels.

The labeltype() method sets the type of label. FLTK supports normal, embossed, and shadowed labels internally, and more types can be added as desired.

A complete list of all label options can be found in the section on Labels and Label Types.

Showing the Window

The show() method shows the widget or window. For windows you can also provide the command-line arguments to allow users to customize the appearance, size, and position of your windows.

The Main Event Loop

All FLTK applications (and most GUI applications in general) are based on a simple event processing model. User actions such as mouse movement, button clicks, and keyboard activity generate events that are sent to an application. The application may then ignore the events or respond to the user, typically by redrawing a button in the "down" position, adding the text to an input field, and so forth.

FLTK also supports idle, timer, and file pseudo-events that cause a function to be called when they occur. Idle functions are called when no user input is present and no timers or files need to be handled - in short, when the application is not doing anything. Idle callbacks are often used to update a 3D display or do other background processing.

Timer functions are called after a specific amount of time has expired. They can be used to pop up a progress dialog after a certain amount of time or do other things that need to happen at more-or-less regular intervals. FLTK timers are not 100% accurate, so they should not be used to measure time intervals, for example.

File functions are called when data is ready to read or write, or when an error condition occurs on a file. They are most often used to monitor network connections (sockets) for data-driven displays.

FLTK applications must periodically check (Fl::check()) or wait (Fl::wait()) for events or use the Fl::run() method to enter a standard event processing loop. Calling Fl::run() is equivalent to the following code:

while (Fl::wait());
static int wait()
Waits until "something happens" and then returns.
Definition: Fl.cxx:625

Fl::run() does not return until all of the windows under FLTK control are closed by the user or your program.

Naming Conventions

All public symbols in FLTK start with the characters 'F' and 'L':

  • Functions are either Fl::foo() or fl_foo().
  • Class and type names are capitalized: Fl_Foo.
  • All header files start with <FL/...>.

Header Files

The proper way to include FLTK header files is:

#include <FL/Fl_xyz.H>
Case is significant on many operating systems, and the C standard uses the forward slash (/) to separate directories. Do not use any of the following include lines:
#include <FL\Fl_xyz.H>
#include <fl/fl_xyz.h>
#include <Fl/fl_xyz.h>

Compiling Programs that Use FLTK

Since FLTK 1.4 CMake is the recommended build system. The details below show the "old" methods and reference information in case you like to write your build configuration manually (e.g. Makefiles, Visual Studio, other IDE's ...).

CMake can simplify this task substantially. For now, refer to README.CMake.txt for further information.

This section needs a major rework. Add a chapter "Building FLTK with CMake".

Compiling Programs with Standard Compilers

Under UNIX (and under Microsoft Windows when using the GNU development tools) you will probably need to tell the compiler where to find the header files. This is usually done using the -I option:

c++ -I/usr/local/include ...
You need a C++ compiler to build FLTK. The commands given in this chapter are examples using 'c++'. Please replace this command with the C++ compiler suitable for your system or use the fltk-config script as described below (this is recommended).

The fltk-config script included with FLTK can be used to get the compiler and the options that are required by your compiler:

fltk-config --cc
fltk-config --cxx

return the C and C++ compiler commands used to build FLTK.

c++ `fltk-config --cxxflags` ...

can be used to include the required compiler flags in the command line.

Similarly, when linking your application you will need to tell the compiler to use the FLTK library:

c++ ... -L/usr/local/lib -lfltk -lXext -lX11 ... -lm -ldl

Aside from the "fltk" library, there are also the following libraries

  • "fltk_forms" for the XForms compatibility classes (deprecated)
  • "fltk_gl" for the OpenGL and GLUT classes
  • "fltk_images" for the image file classes, Fl_Help_Dialog widget, and system icon support
  • "fltk_cairo" for optional integrated Cairo support.
The separate fltk_cairo library will likely be removed in FLTK 1.4.0 (this is work in progress).
The libraries are named "fltk.lib", "fltk_gl.lib", "fltk_forms.lib", "fltk_images.lib", and fltk_cairo.lib, respectively under Windows.

As before, the fltk-config script included with FLTK can be used to get the options that are required by your linker:

c++ ... `fltk-config --ldflags`

The forms, GL, and images libraries are included with the "--use-foo" options, as follows:

c++ ... `fltk-config --use-forms --ldflags`
c++ ... `fltk-config --use-gl --ldflags`
c++ ... `fltk-config --use-images --ldflags`
c++ ... `fltk-config --use-forms --use-gl --use-images --ldflags`
c++ ... `fltk-config --use-cairo --ldflags`

Finally, you can use the fltk-config script to compile a single source file as a FLTK program:

fltk-config --compile filename.cpp
fltk-config --use-forms --compile filename.cpp
fltk-config --use-gl --compile filename.cpp
fltk-config --use-images --compile filename.cpp
fltk-config --use-cairo --compile filename.cpp
fltk-config --use-forms --use-gl --use-images --compile filename.cpp

Any of these will create an executable named filename (or filename.exe under Windows).

'fltk-config --compile' accepts only a limited set of file extensions for C++ source files: '.cpp', '.cxx', '.cc', and '.C' (capital 'C').
fltk-config --help

displays all available options.

Compiling Programs with Makefiles

The previous section described how to use fltk-config to build a program consisting of a single source file from the command line, and this is very convenient for small test programs. But fltk-config can also be used to set the compiler and linker options as variables within a Makefile that can be used to build programs out of multiple source files:

CXX = $(shell fltk-config --cxx)
DEBUG = -g
CXXFLAGS = $(shell fltk-config --use-gl --use-images --cxxflags ) -I.
LDFLAGS = $(shell fltk-config --use-gl --use-images --ldflags )
LDSTATIC = $(shell fltk-config --use-gl --use-images --ldstaticflags )
TARGET = cube
OBJS = CubeMain.o CubeView.o CubeViewUI.o
SRCS = CubeMain.cxx CubeView.cxx CubeViewUI.cxx
.SUFFIXES: .o .cxx
%.o: %.cxx
$(CXX) $(CXXFLAGS) $(DEBUG) -c $<
all: $(TARGET)
CubeMain.o: CubeMain.cxx CubeViewUI.h
CubeView.o: CubeView.cxx CubeView.h CubeViewUI.h
CubeViewUI.o: CubeViewUI.cxx CubeView.h
clean: $(TARGET) $(OBJS)
rm -f *.o 2> /dev/null
rm -f $(TARGET) 2> /dev/null

Compiling Programs with Microsoft Visual C++

In Visual C++ you will need to tell the compiler where to find the FLTK header files. This can be done by selecting "Settings" from the "Project" menu and then changing the "Preprocessor" settings under the "C/C++" tab.

You will also need to add the following libraries to the Linker settings:

  • fltk.lib or fltkd.lib, the main FLTK library (postfix 'd' = Debug)
  • all FLTK libraries your program requires (fltk_gl, fltk_images, …)
  • additional libraries like libpng.lib, libjpeg.lib, etc.
  • the Windows Common Controls (comctl32.lib) and
  • the Windows Socket (ws2_32.lib) libraries.
There's a Linker setting "Additional Library Directories" or similar; the exact name depends on the Visual Studio version you're using. You can and should use this to simplify adding the libraries above. If you set this to the FLTK library path you can just use the library names and don't need to use the full paths to all libraries.

You must also define _WIN32 if the compiler doesn't do this. Currently all known Windows compilers define _WIN32 - unless you use Cygwin (that's correct, you must not define _WIN32 if you use Cygwin).

More information can be found in README.Windows.txt.

You can build your Microsoft Windows applications as Console or Desktop applications. If you want to use the standard C main() function as the entry point, FLTK includes a WinMain() function that will call your main() function for you.

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