5.2. Creating the Window   

The X library provides two window creation functions, XCreateSimpleWindow and XCreateWindow. Rather than using two different functions depending on what degree of control is desired, this course (and most references) will just use the more general XCreateWindow function. The syntax is given below. 

     #include <X11/Xlib.h>

     Display        *display;

     Window         parent_window;

     int       x, y;

     unsigned int   width, height;

     unsigned int   border_width;

     int       depth;

     unsigned int   class;

     Visual         *visual;

     unsigned long  valuemask;

     XSetWindowAttributes     *attributes;

     window = XCreateWindow (display, parent_window, x, y, width,

                            height, border_width, depth, class,

                            valuemask, attributes );

The function call will return the window identifier (or the constant None, if the call failed). The first parameter is the target display, as obtained from XOpenDisplay. The second parameter is the parent's id. Note that all windows are always clipped by their parent's boundaries. If this is the first window that we are creating then the parent window is the root window, and there is a macro, RootWindow, that will give us this window id.

The x and y parameters give the desired pixel location of the top left corner of this new window. However, the window manager has the option of arbitrarily changing where the window will go so view this as a request rather than an absolute value. Width and height are the size of the window in pixels. The border_width is the number of pixels wide that the window's border should be. If you are creating a child of the root window, the window manager may override the border width. In particular, on the Personal Iris the border has been known to disappear.

Depth is the number of bit planes available. This value may have been obtained from the DefaultDepth macro, or you can use the constant CopyFromParent. Class will take the form of a constant, either InputOutput, InputOnly or CopyFromParent. Most windows of course will be of the InputOutput class. The visual describes an abstract model for graphics hardware capabilities. Normally we will play safe and use CopyFromParent.

Valuemask and attributes are related. The attributes structure contains 15 fields related to the window. The valuemask variable is a bit mask, indicating which of the 15 fields you have actually specified. This allows us to specify only those attributes that we are interested in. The actual attributes structure is defined below. 

     typedef struct {

                    Pixmap         background_pixmap;

                    unsigned long  background_pixel;

                    Pixmap         border_pixmap;

                    unsigned long  border_pixel;

                    int       bit_gravity;

                    int       win_gravity;

                    int       backing_store;

                    unsigned long  backing_planes;

                    unsigned long  backing_pixel;

                    Bool      save_under;

                    long      event_mask;

                    long      do_not_propagate_mask;

                    Bool      override_redirect;

                    Colormap  colormap;

                    Cursor         cursor;

                    } XSetWindowAttributes;

We will only look at four of these attributes. 

  background_pixel -  the colour of the window background


  border_pixel -  the colour of the window border


  event_mask -  ask for window expose events.

    This gets round a potential problem.


  override_redirect -  allows you to override the window manager.

    To do so is normally considered unfriendly, and should

    only be used for short pop up screens or menus.

In practise we are likely to set these attributes using code such as that below. 

     #include <X11/Xlib.h>

     #include <X11/Xutil.h>


     Display             *display;

     int            screen;

     XSetWindowAttributes     attributes;

     unsigned long       attr_mask;


     attributes.background_pixel   = WhitePixel(display, screen);

     attributes.border_pixel       = BlackPixel(display, screen);

     attributes.event_mask         = ExposureMask;

     attributes.override_redirect  = False;


     attr_mask = CWBackPixel | CWBorderPixel | CWEventMask | CWOverrideRedirect;