X lib Programming

(lecture programs)

Ø X server:  Controls the Input/Output Resources of a host:

Display, Keyboard and Mouse.

Ø X clients: Applications that runs at any host in the Internet:

May be different from  the X server's host.  

Ø TCP/IP: Is used for communications between the clients and the server:

The default port# for the X server is 6000.

Running X clients on remote hosts

To run an X client (e.g., xterm) on a remote Unix host (e.g., cash.cs.odu.edu) and display the interface on your local window machine (e.g., 128.82.5.88):

à Run the X server on your local window machine.

à Use ssh to login to cash.cs.odu.edu

à % setenv DISPLAY 128.82.5.88:0

à % xterm

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Examples of X lib Programs

Example 1: Drawing Circles xcircles.c

main(argc,argv)
int argc;
char **argv;
{
 Display *display;
    Window root, window;
    long fgcolor, bgcolor;
    int screen, pointx, pointy;

long eventmask =  ButtonPressMask | ExposureMask | KeyPressMask;

XEvent event;
    XGCValues gcval;
    GC draw;
    Colormap cmap;
    XColor color, ignore;

   char *colorname = "red";

int radious = 6;

The above are definitions that will be used throughout the program.

________________  Open Display _____________

if (!(display = XOpenDisplay (argv[1]))) {
        perror("XOpenDisplay");
        exit(1);
  }

à Opens a TCP connection to an X server running at the host specified by argv[1].

à If argv[1] is NULL, it contacts the server running at the DISPLAY machine.

à The format for argv[1] is: host:0

Examples:

% xcircles

% xcircles 128.82.5.88:0
% xcircles necromancer.cs.odu.edu:0

_________________  Create Root Window ______________

root = RootWindow (display, screen = DefaultScreen(display));

à In X every window must have a parent and this root is the parent of all other windows.

______________   Create Window ________________

fgcolor = BlackPixel (display,screen);
  bgcolor = WhitePixel (display,screen);

à Obtains the pixel values for the black and white colors.

window = XCreateSimpleWindow (display, root, 0,0, 200,200, 2,

fgcolor, bgcolor);

à Creates the application main window on display as child for root at position 0,0.

à The window size is 200x200 with border of 2 pixels.

à The window's foreground color is black and its background color is white.

_______________  Create Drawing Pen ______________

char *colorname = "red";

cmap = DefaultColormap (display, screen);
    XAllocNamedColor (display, cmap, colorname, &color, &ignore);
    fgcolor = color.pixel;
    gcval.foreground = fgcolor;
    gcval.background = bgcolor;
    draw = XCreateGC (display,window,GCForeground|GCBackground,&gcval);

à The above statements are used to create a "red" pen called draw

______________   Select Input Events _____________

XSelectInput (display, window, eventmask);

à Ask the server to report the events specified by eventmask

XMapWindow (display,window);

à Make the window visible on the screen.

______________   Handle Input Events __________

The following loop monitors and process the events sent by the X server

for (;;) {
XWindowEvent (display, window, eventmask, &event);

à This is a "blocking" call, i.e., the program will stop here until an event arrives from the X server.

switch (event.type) {

       case Expose:

XClearWindow (display,window);
            break;

à Whenever an Expose event arrives, the window is cleared. An expose event can be generated by  e.g., covering and uncovering the window, closing and opening the window.

case ButtonPress:

pointx = event.xbutton.x - radious;
      pointy = event.xbutton.y - radious;
XFillArc(display, window, draw, pointx, pointy,

2*radious, 2*radious,0, 360*64);
      break;

à Whenever any Button is Pressed a red point  is drawn at the x,y position where the event occurred.

case KeyPress:
            exit(0);

à Whenever any Key is pressed the program exits.

default:
            fprintf(stderr,"Unexpected event: %d\n",event.type);

à Any other event is unexpected and should not happen.

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Example 2: Drawing Lines xlines.c

The program  xlines.c is similar to xcircles.c  but it draws lines.

The user odd clicks (1, 3, ...) draws a point while the even clicks (2, 4, ...) draws lines between the current position and the previous position of the mouse.

Here is the code that achieves that:

case ButtonPress:
      if (FirstPt) {
        FirstPt=FALSE;
pointx = event.xbutton.x;
pointy = event.xbutton.y;
XDrawPoint (display,window,draw, pointx, pointy);
        break;

}
      else {
       FirstPt=TRUE;
XDrawLine (display,window,draw, pointx,pointy,

event.xbutton.x, event.xbutton.y);
       break;

à Odd clicks draw a point while even clicks draw a line between the previous mouse position and the current position.