FLTK 1.3.4
Operating System Issues

This appendix describes the operating system specific interfaces in FLTK:

Accessing the OS Interfaces

All programs that need to access the operating system specific interfaces must include the following header file:

#include <FL/x.H>

Despite the name, this header file will define the appropriate interface for your environment. The pages that follow describe the functionality that is provided for each operating system.


The interfaces provided by this header file may change radically in new FLTK releases. Use them only when an existing generic FLTK interface is not sufficient.

The UNIX (X11) Interface

The UNIX interface provides access to the X Window System state information and data structures.

Handling Other X Events

void Fl::add_handler(int (*f)(int))

Installs a function to parse unrecognized events. If FLTK cannot figure out what to do with an event, it calls each of these functions (most recent first) until one of them returns non-zero. If none of them returns non-zero then the event is ignored.
FLTK calls this for any X events it does not recognize, or X events with a window ID that FLTK does not recognize. You can look at the X event in the fl_xevent variable.
The argument is the FLTK event type that was not handled, or zero for unrecognized X events. These handlers are also called for global shortcuts and some other events that the widget they were passed to did not handle, for example FL_SHORTCUT.

extern XEvent *fl_xevent

This variable contains the most recent X event.

extern ulong fl_event_time

This variable contains the time stamp from the most recent X event that reported it; not all events do. Many X calls like cut and paste need this value.

Window fl_xid(const Fl_Window *)

Returns the XID for a window, or zero if not shown().

Fl_Window *fl_find(ulong xid)

Returns the Fl_Window that corresponds to the given XID, or NULL if not found. This function uses a cache so it is slightly faster than iterating through the windows yourself.

int fl_handle(const XEvent &)

This call allows you to supply the X events to FLTK, which may allow FLTK to cooperate with another toolkit or library. The return value is non-zero if FLTK understood the event. If the window does not belong to FLTK and the add_handler() functions all return 0, this function will return false.
Besides feeding events your code should call Fl::flush() periodically so that FLTK redraws its windows.
This function will call the callback functions. It will not return until they complete. In particular, if a callback pops up a modal window by calling fl_ask(), for instance, it will not return until the modal function returns.

Drawing using Xlib

The following global variables are set before Fl_Widget::draw() is called, or by Fl_Window::make_current():

extern Display *fl_display;
extern Window fl_window;
extern GC fl_gc;
extern int fl_screen;
extern XVisualInfo *fl_visual;
extern Colormap fl_colormap;

You must use them to produce Xlib calls. Don't attempt to change them. A typical X drawing call is written like this:

XDrawSomething(fl_display, fl_window, fl_gc, ...);

Other information such as the position or size of the X window can be found by looking at Fl_Window::current(), which returns a pointer to the Fl_Window being drawn.

unsigned long fl_xpixel(Fl_Color i)
unsigned long fl_xpixel(uchar r, uchar g, uchar b)

Returns the X pixel number used to draw the given FLTK color index or RGB color. This is the X pixel that fl_color() would use.

int fl_parse_color(const char* p, uchar& r, uchar& g, uchar& b)

Convert a name into the red, green, and blue values of a color by parsing the X11 color names. On other systems, fl_parse_color() can only convert names in hexadecimal encoding, for example #ff8083.

extern XFontStruct *fl_xfont

Points to the font selected by the most recent fl_font(). This is not necessarily the current font of fl_gc, which is not set until fl_draw() is called. If FLTK was compiled with Xft support, fl_xfont will usually be 0 and fl_xftfont will contain a pointer to the XftFont structure instead.

extern void *fl_xftfont

If FLTK was compiled with Xft support enabled, fl_xftfont points to the xft font selected by the most recent fl_font(). Otherwise it will be 0. fl_xftfont should be cast to XftFont*.

Changing the Display, Screen, or X Visual

FLTK uses only a single display, screen, X visual, and X colormap. This greatly simplifies its internal structure and makes it much smaller and faster. You can change which it uses by setting global variables before the first Fl_Window::show() is called. You may also want to call Fl::visual(), which is a portable interface to get a full color and/or double buffered visual.

int Fl::display(const char *)

Set which X display to use. This actually does putenv("DISPLAY=...") so that child programs will display on the same screen if called with exec(). This must be done before the display is opened. This call is provided under MacOS and WIN32 but it has no effect.

extern Display *fl_display

The open X display. This is needed as an argument to most Xlib calls. Don't attempt to change it! This is NULL before the display is opened.

void fl_open_display()

Opens the display. Does nothing if it is already open. This will make sure fl_display is non-zero. You should call this if you wish to do X calls and there is a chance that your code will be called before the first show() of a window.
This may call Fl::abort() if there is an error opening the display.

void fl_close_display()

This closes the X connection. You do not need to call this to exit, and in fact it is faster to not do so! It may be useful to call this if you want your program to continue without the X connection. You cannot open the display again, and probably cannot call any FLTK functions.

extern int fl_screen

Which screen number to use. This is set by fl_open_display() to the default screen. You can change it by setting this to a different value immediately afterwards. It can also be set by changing the last number in the Fl::display() string to "host:0.#".

extern XVisualInfo *fl_visual
extern Colormap fl_colormap

The visual and colormap that FLTK will use for all windows. These are set by fl_open_display() to the default visual and colormap. You can change them before calling show() on the first window. Typical code for changing the default visual is:
Fl::args(argc, argv); // do this first so $DISPLAY is set
fl_visual = find_a_good_visual(fl_display, fl_screen);
if (!fl_visual) Fl::abort("No good visual");
fl_colormap = make_a_colormap(fl_display, fl_visual->visual, fl_visual->depth);
// it is now ok to show() windows:
window->show(argc, argv);

Using a Subclass of Fl_Window for Special X Stuff

FLTK can manage an X window on a different screen, visual and/or colormap, you just can't use FLTK's drawing routines to draw into it. But you can write your own draw() method that uses Xlib (and/or OpenGL) calls only.

FLTK can also manage XID's provided by other libraries or programs, and call those libraries when the window needs to be redrawn.

To do this, you need to make a subclass of Fl_Window and override some of these virtual functions:

virtual void Fl_Window::show()

If the window is already shown() this must cause it to be raised, this can usually be done by calling Fl_Window::show(). If not shown() your implementation must call either Fl_X::set_xid() or Fl_X::make_xid().
An example:
void MyWindow::show() {
if (shown()) {Fl_Window::show(); return;} // you must do this!
fl_open_display(); // necessary if this is first window
// we only calculate the necessary visual colormap once:
static XVisualInfo *visual;
static Colormap colormap;
if (!visual) {
visual = figure_out_visual();
colormap = XCreateColormap(fl_display, RootWindow(fl_display,fl_screen),
vis->visual, AllocNone);
Fl_X::make_xid(this, visual, colormap);
Fl_X *Fl_X::set_xid(Fl_Window*, Window xid)
Allocate a hidden class called an Fl_X, put the XID into it, and set a pointer to it from the Fl_Window. This causes Fl_Window::shown() to return true.

void Fl_X::make_xid(Fl_Window*, XVisualInfo* = fl_visual, Colormap = fl_colormap)

This static method does the most onerous parts of creating an X window, including setting the label, resize limitations, etc. It then does Fl_X::set_xid() with this new window and maps the window.

virtual void Fl_Window::flush()

This virtual function is called by Fl::flush() to update the window. For FLTK's own windows it does this by setting the global variables fl_window and fl_gc and then calling the draw() method. For your own windows you might just want to put all the drawing code in here.
The X region that is a combination of all damage() calls done so far is in Fl_X::i(this)->region. If NULL then you should redraw the entire window. The undocumented function fl_clip_region(XRegion) will initialize the FLTK clip stack with a region or NULL for no clipping. You must set region to NULL afterwards as fl_clip_region() will own and delete it when done.
If damage() & FL_DAMAGE_EXPOSE then only X expose events have happened. This may be useful if you have an undamaged image (such as a backing buffer) around.
Here is a sample where an undamaged image is kept somewhere:
void MyWindow::flush() {
Fl_X::i(this)->region = 0;
if (damage() != 2) {... draw things into backing store ...}
... copy backing store to window ...

virtual void Fl_Window::hide()

Destroy the window server copy of the window. Usually you will destroy contexts, pixmaps, or other resources used by the window, and then call Fl_Window::hide() to get rid of the main window identified by xid(). If you override this, you must also override the destructor as shown:
void MyWindow::hide() {
if (mypixmap) {
mypixmap = 0;
Fl_Window::hide(); // you must call this

virtual void Fl_Window::~Fl_Window()

Because of the way C++ works, if you override hide() you must override the destructor as well (otherwise only the base class hide() is called):
MyWindow::~MyWindow() {
Access to the Fl_X hidden class requires to #define FL_INTERNALS before compilation.

Setting the Icon of a Window

FLTK currently supports setting a window's icon before it is shown using the Fl_Window::icon() method.

void Fl_Window::icon(const void *)

Sets the icon for the window to the passed pointer. You will need to cast the icon Pixmap to a char* when calling this method. To set a monochrome icon using a bitmap compiled with your application use:
#include "icon.xbm"
fl_open_display(); // needed if display has not been previously opened
Pixmap p = XCreateBitmapFromData(fl_display, DefaultRootWindow(fl_display),
icon_bits, icon_width, icon_height);
window->icon((const void*)p);
To use a multi-colored icon, the XPM format and library should be used as follows:
#include <X11/xpm.h>
#include "icon.xpm"
fl_open_display(); // needed if display has not been previously opened
Pixmap p, mask;
XpmCreatePixmapFromData(fl_display, DefaultRootWindow(fl_display),
icon_xpm, &p, &mask, NULL);
window->icon((const void *)p);
When using the Xpm library, be sure to include it in the list of libraries that are used to link the application (usually "-lXpm").


You must call Fl_Window::show(int argc, char** argv) for the icon to be used. The Fl_Window::show() method does not bind the icon to the window.

X Resources

When the Fl_Window::show(int argc, char** argv) method is called, FLTK looks for the following X resources:

  • background - The default background color for widgets (color).
  • dndTextOps - The default setting for drag and drop text operations (boolean).
  • foreground - The default foreground (label) color for widgets (color).
  • scheme - The default scheme to use (string).
  • selectBackground - The default selection color for menus, etc. (color).
  • Text.background - The default background color for text fields (color).
  • tooltips - The default setting for tooltips (boolean).
  • visibleFocus - The default setting for visible keyboard focus on non-text widgets (boolean).

Resources associated with the first window's Fl_Window::xclass() string are queried first, or if no class has been specified then the class "fltk" is used (e.g. fltk.background). If no match is found, a global search is done (e.g. *background).

The Windows (WIN32) Interface

The Windows interface provides access to the WIN32 GDI state information and data structures.

Using filenames with non-ASCII characters

In FLTK, all strings, including filenames, are UTF-8 encoded. The utility functions fl_fopen() and fl_open() allow to open files potentially having non-ASCII names in a cross-platform fashion, whereas the standard fopen()/open() functions fail to do so.

Responding to WM_QUIT

FLTK will intercept WM_QUIT messages that are directed towards the thread that runs the main loop. These are converted to SIGTERM signals via raise(). This allows you to deal with outside termination requests with the same code on both Windows and UNIX systems. Other processes can send this message via PostThreadMessage() in order to request, rather than force your application to terminate.

Handling Other WIN32 Messages

By default a single WNDCLASSEX called "FLTK" is created. All Fl_Window 's are of this class unless you use Fl_Window::xclass(). The window class is created the first time Fl_Window::show() is called.

You can probably combine FLTK with other libraries that make their own WIN32 window classes. The easiest way is to call Fl::wait(), as it will call DispatchMessage() for all messages to the other windows. If necessary you can let the other library take over as long as it calls DispatchMessage(), but you will have to arrange for the function Fl::flush() to be called regularly so that widgets are updated, timeouts are handled, and the idle functions are called.

extern MSG fl_msg

This variable contains the most recent message read by GetMessage(), which is called by Fl::wait(). This may not be the most recent message sent to an FLTK window, because silly WIN32 calls the handle procedures directly for some events (sigh).

void Fl::add_handler(int (*f)(int))

Installs a function to parse unrecognized messages sent to FLTK windows. If FLTK cannot figure out what to do with a message, it calls each of these functions (most recent first) until one of them returns non-zero. The argument passed to the functions is the FLTK event that was not handled or zero for unknown messages. If all the handlers return zero then FLTK calls DefWindowProc().

HWND fl_xid(const Fl_Window *)

Returns the window handle for a Fl_Window, or zero if not shown().

Fl_Window *fl_find(HWND xid)

Returns the Fl_Window that corresponds to the given window handle, or NULL if not found. This function uses a cache so it is slightly faster than iterating through the windows yourself.

Drawing Things Using the WIN32 GDI

When the virtual function Fl_Widget::draw() is called, FLTK stores all the extra arguments you need to make a proper GDI call in some global variables:

extern HINSTANCE fl_display;
extern HWND fl_window;
extern HDC fl_gc;
HPEN fl_pen();
HBRUSH fl_brush();

These global variables are set before Fl_Widget::draw() is called, or by Fl_Window::make_current(). You can refer to them when needed to produce GDI calls, but don't attempt to change them. The functions return GDI objects for the current color set by fl_color() and are created as needed and cached. A typical GDI drawing call is written like this:

DrawSomething(fl_gc, ..., fl_brush());

It may also be useful to refer to Fl_Window::current() to get the window's size or position.

Setting the Icon of a Window

FLTK currently supports setting a window's icon before it is shown using the Fl_Window::icon() method.

void Fl_Window::icon(const void *)

Sets the icon for the window to the passed pointer. You will need to cast the HICON handle to a char* when calling this method. To set the icon using an icon resource compiled with your application use:
window->icon((const void *)LoadIcon(fl_display, MAKEINTRESOURCE(IDI_ICON)));
You can also use the LoadImage() and related functions to load specific resolutions or create the icon from bitmap data.


You must call Fl_Window::show(int argc, char** argv) for the icon to be used. The Fl_Window::show() method does not bind the icon to the window.

How to Not Get a MSDOS Console Window

WIN32 has a really stupid mode switch stored in the executables that controls whether or not to make a console window.

To always get a console window you simply create a console application (the "/SUBSYSTEM:CONSOLE" option for the linker). For a GUI-only application create a WIN32 application (the "/SUBSYSTEM:WINDOWS" option for the linker).

FLTK includes a WinMain() function that calls the ANSI standard main() entry point for you. This function creates a console window when you use the debug version of the library.

WIN32 applications without a console cannot write to stdout or stderr, even if they are run from a console window. Any output is silently thrown away. Additionally, WIN32 applications are run in the background by the console, although you can use "start /wait program" to run them in the foreground.

Known WIN32 Bugs and Problems

The following is a list of known bugs and problems in the WIN32 version of FLTK:

  • If a program is deactivated, Fl::wait() does not return until it is activated again, even though many events are delivered to the program. This can cause idle background processes to stop unexpectedly. This also happens while the user is dragging or resizing windows or otherwise holding the mouse down. We were forced to remove most of the efficiency FLTK uses for redrawing in order to get windows to update while being moved. This is a design error in WIN32 and probably impossible to get around.
  • Fl_Gl_Window::can_do_overlay() returns true until the first time it attempts to draw an overlay, and then correctly returns whether or not there is overlay hardware.
  • SetCapture (used by Fl::grab()) doesn't work, and the main window title bar turns gray while menus are popped up.
  • Compilation with gcc 3.4.4 and -Os exposes an optimisation bug in gcc. The symptom is that when drawing filled circles only the perimeter is drawn. This can for instance be seen in the symbols demo. Other optimisation options such as -O2 and -O3 seem to work OK. More details can be found in STR#1656

The Apple OS X Interface

FLTK supports Apple OS X using the Apple Cocoa library. Older versions of MacOS are no longer supported.

Control, Option, and Command Modifier Keys

FLTK maps the Mac 'control' key to FL_CTRL, the 'option' key to FL_ALT and the 'Apple' key to FL_META. Furthermore, FL_COMMAND designates the 'Apple' key on Mac OS X and the 'control' key on other platforms. Keyboard events return the key name in Fl::event_key() and the keystroke translation in Fl::event_text(). For example, typing Option-Y on a Mac US keyboard will set FL_ALT in Fl::event_state(), set Fl::event_key() to 'y' and return the Yen symbol in Fl::event_text().

Right Click simulation with Ctrl Click

The Apple HIG guidelines indicate applications should support 'Ctrl Click' to simulate 'Right Click' for e.g. context menus, so users with one-button mice and one-click trackpads can still access right-click features. However, paraphrasing Manolo's comment on the fltk.coredev newsgroup:
  • FLTK does /not/ support Ctrl-Click == Right Click itself because Mac OS X event processing doesn't support this at the system level: the system reports left-clicks with the ctrl modifier when the user ctrl-clicks, and OS X system preferences don't allow changing this behavior. Therefore, applications must handle simulation of Right Click with Ctrl Click in the application code.
Ian MacArthur provided the following handle() method code snippet showing an example of how to do this:
case FL_PUSH:
int btn = Fl::event_button();
#ifdef __APPLE__
int ev_state = Fl::event_state();
// Context menu can be called up in one of two ways: -
// 1 - right click, as normally used on Windows and Linux
// 2 - Ctrl + left click, as sometimes used on Mac
#ifdef __APPLE__
// On apple, check right click, and ctrl+left click
if ((btn == FL_RIGHT_MOUSE) || (ev_state == (FL_CTRL | FL_BUTTON1)))
// On other platforms, only check right click as ctrl+left is used for selections
if (btn == FL_RIGHT_MOUSE)
// Did we right click on the object?..
There is a thread about this subject on fltk.coredev (Aug 1-14, 2014) entitled "[RFC] Right click emulation for one button mouse on Mac".

Apple "Quit" Event

When the user presses Cmd-Q or requests a termination of the application, OS X will send a "Quit" Apple Event. FLTK handles this event by sending an FL_CLOSE event to all open windows. If all windows close, the application will terminate.

Apple "Open" Event

Whenever the user drops a file onto an application icon, OS X generates an Apple Event of the type "Open". You can have FLTK notify you of an Open event by calling the fl_open_callback function.

void fl_open_display()

Opens the display. Does nothing if it is already open. You should call this if you wish to do Cocoa or Quartz calls and there is a chance that your code will be called before the first show() of a window.

Window fl_xid(const Fl_Window *)

Returns the window reference for an Fl_Window, or NULL if the window has not been shown. This reference is a pointer to an instance of the subclass FLWindow of Cocoa's NSWindow class.

Fl_Window *fl_find(Window xid)

Returns the Fl_Window that corresponds to the given window reference, or NULL if not found.

void fl_mac_set_about( Fl_Callback *cb, void *user_data, int shortcut)

Attaches the callback cb to the "About myprog" item of the system application menu. cb will be called with NULL first argument and user_data second argument.

Fl_Sys_Menu_Bar class

The Fl_Sys_Menu_Bar class allows to build menu bars that, on Mac OS X, are placed in the system menu bar (at top-left of display), and, on other platforms, at a user-chosen location of a user-chosen window.

Setting the icon of an application

  • First, create a .icns file containing several copies of your icon of decreasing sizes. This can be done using the Preview application or the Icon Composer application available in "Graphics Tools for Xcode". To create a high resolution icon file, it is necessary to use the iconutil command-line utility.
  • Put your .icns file in the Resources subdirectory of your application bundle.
  • Add these two lines to the Info.plist file of your application bundle
    replacing foo by your application name. If you use Xcode, just add your .icns file to your application target.

Drawing Things Using Quartz

All code inside Fl_Widget::draw() is expected to call Quartz drawing functions. The Quartz coordinate system is flipped to match FLTK's coordinate system. The origin for all drawing is in the top left corner of the enclosing Fl_Window. The global variable fl_gc (of type CGContextRef) is the appropriate Quartz 2D drawing environment. Include FL/x.H to declare the fl_gc variable.


All FLTK programs contain an application menu with, e.g., the About xxx, Hide xxx, and Quit xxx items. This menu can be internationalized/localized by any of two means.

  • using the Fl_Mac_App_Menu class.
  • using the standard Mac OS X localization procedure. Create a language-specific .lproj directory (e.g., German.lproj) in the Resources subdirectory of the application bundle. Create therein a Localizable.strings file that translates all menu items to this language. The German Localizable.strings file, for example, contains:
    "About %@" = "Über %@";
    "Print Front Window"="Frontfenster drucken";
    "Services" = "Dienste";
    "Hide %@"="%@ ausblenden";
    "Hide Others"="Andere ausblenden";
    "Show All"="Alle einblenden";
    "Quit %@"="%@ beenden";
    Set "Print Front Window" = ""; therein so the application menu doesn't show a "Print Front Window" item. To localize the application name itself, create a file InfoPlist.strings in each .lproj directory and put CFBundleName = "localized name"; in each such file.

OpenGL and 'retina' displays

It is possible to have OpenGL produce graphics at the high pixel resolution allowed by the so-called 'retina' displays present on recent Apple hardware. For this, call


before any Fl_Gl_Window is shown. Also, adapt your Fl_Gl_Window::draw() and Fl_Gl_Window::draw_overlay() methods replacing

glViewport(0, 0, w(), h());


glViewport(0, 0, pixel_w(), pixel_h());

making use of the Fl_Gl_Window::pixel_w() and Fl_Gl_Window::pixel_h() methods that return the width and height of the GL scene in pixels: if the Fl_Gl_Window is mapped on a retina display, these methods return twice as much as reported by Fl_Widget::w() and Fl_Widget::h(); if it's mapped on a regular display, they return the same values as w() and h(). These methods dynamically change their values if the window is moved into/out from a retina display. If Fl::use_high_res_GL(1) is not called, all Fl_Gl_Window 's are drawn at low resolution. These methods are synonyms of w() and h() on non-Mac OS X platforms, so the source code remains cross-platform.

The Fl_Gl_Window::pixels_per_unit() method is useful when the OpenGL code depends on the pixel dimension of the GL scene. This occurs, e.g., if a window's handle() method uses Fl::event_x() and Fl::event_y() whose returned values should be multiplied by Fl_Gl_Window::pixels_per_unit() to obtain the adequate pixel units. This method may also be useful, for example, to adjust the width of a line in a high resolution GL scene.


OS X double-buffers all windows automatically. On OS X, Fl_Window and Fl_Double_Window are handled internally in the same way.

Mac File System Specifics

Resource Forks

FLTK does not access the resource fork of an application. However, a minimal resource fork must be created for OS X applications. Starting with OS X 10.6, resource forks are no longer needed.

Caution (OS X 10.2 and older):

When using UNIX commands to copy or move executables, OS X will NOT copy any resource forks! For copying and moving use CpMac and MvMac respectively. For creating a tar archive, all executables need to be stripped from their Resource Fork before packing, e.g. "DeRez fluid > fluid.r". After unpacking the Resource Fork needs to be reattached, e.g. "Rez fluid.r -o fluid".

It is advisable to use the Finder for moving and copying and Mac archiving tools like Sit for distribution as they will handle the Resource Fork correctly.

Mac File Paths

FLTK uses UTF-8-encoded UNIX-style filenames and paths.

See Also
Mac OS X-specific symbols

[Prev] Forms Compatibility [Index] Migrating Code from FLTK 1.0 to 1.1 [Next]