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 a 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., localhost.cs.odu.edu):

Step 0: run the X server on your local window machine.

Then, You may do one of the following two alternatives:

Alternative 1:

Ø Use ssh to login to cash.cs.odu.edu

Ø % setenv DISPLAY localhost.cs.odu.edu:0

Ø % xterm

Alternative 2:

Ø start ssh

Ø click on Settings button (3^rd from last)

Ø check on Request tunnels only option and click OK, then click on that window and login to the remote host.

Ø click on New Terminal Window button (5^th from last) to get a new window and type: xterm

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Examples of X lib Programs

Drawing Points: xpoints.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";

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

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

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

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

Ø The format for argv[1] is: host:0

Examples: 128.82.4.67:0
                   isis.cs.odu.edu:0
localhost:0 (same as NULL).

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

Ø Creates a root window.

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

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

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

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

fgcolor, bgcolor);

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

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

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

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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);

Ø The above statements are used to create a "red" pen called draw
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XSelectInput (display, window, eventmask);

Ø Ask the server to report the events specified by eventmask
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XMapWindow (display,window);

Ø Make the window visible on the screen.

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

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

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

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switch (event.type) {

case Expose:

XClearWindow (display,window);
            break;

Ø 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.
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case ButtonPress:

XDrawPoint (display, window, draw,

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

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

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case KeyPress:
            exit(0);

Ø Whenever any Key is pressed the program exits.

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

Ø Any other event is unexpected and should not happen.

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Drawing Circles

The program xcircles.c is similar to  xpoints.c  but it draws  filled circles.

Here is the code that achieve that:

....
int radious = 6;
.....

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;

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Drawing Lines

The program xlines.c is similar to  xpoints.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;

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