Appendix F. Operating System Issues

Table of Contents
X-Specific Interface
WIN32-Specific Interface

This appendix describes the X and WIN32 specific interfaces in FLTK.

X-Specific Interface

#include <FL/x.H>

On X you can include this file to access FLTK's X-specific functions. Be warned that some of the structures and calls in it are subject to change in future version of FLTK. Try to avoid doing this so your code is portable.

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 with the fl_xevent variable.

The argument is 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. In this case the argument is non-zero (for example FL_SHORTCUT).

extern XEvent *fl_xvent

The most recent X event.

extern ulong fl_event_time

This is the time stamp from the most recent X event that reported it (not all 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 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 true if FLTK understood the event (if the window does not belong to FLTK and the add_handler() functions all ignore it this returns 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.

extern XFontStruct *fl_xfont

Points at 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.

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 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 calcualte 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 structure 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() now owns it and will 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() {

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(char *)

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 the icon using a bitmap compiled with your application use:

#include "icon.xbm"

Pixmap p = XCreateBitmapFromData(fl_display, DefaultRootWindow(fl_display),
                                 icon_bits, icon_width, icon_height);

window->icon((char *)p);