X lib Programming

(lecture programs)

X server :  Controls the input/output resources of a host:

Display, Keyboard & Mouse.

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

May be different from  the X server's host.  

TCP/IP : used for communications between clients and 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., fast.cs.odu.edu) or Linux host (e.g. linux.cs.odu.edu) and to display the interface on your local window machine (e.g., 128.82.5.154):

1. Run the X server on your local window machine.

2. Use ssh to login to fast.cs.odu.edu or linux.cs.odu.edu

% setenv DISPLAY 128.82.5.154:0

% xterm

OR

click on Settings of the ssh button (3^rd from last)

click on Tunneling

check on the Tunnel X11 connections.

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);

}

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

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

:: The format for argv[1] is: host:0

Examples:

% who am i

cs476 pts/4 2011-09-01 14:36 (dhcp-154.cs.odu.edu)

% host dhcp-154.cs.odu.edu

dhcp-154.cs.odu.edu has address 128.82.5.154

% xcircles

OR

% xcircles 128.82.5.154:0

OR

% xcircles dhcp-154.cs.odu.edu :0

_____Create Root Window_____

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

:: 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);

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

window = XCreateSimpleWindow (display,root,0,0,200,200,2,fgcolor,bgcolor);

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

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

:: 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);

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

________Select Input Events________

XSelectInput (display, window, eventmask);

:: Ask the server to report the events specified by eventmask

XMapWindow (display,window);

:: 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);

:: This is "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;

:: Whenever an Expose event arrives, the window is cleared.

:: An expose event generated by  e.g., covering/uncovering window, closing/opening 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;

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

case KeyPress:
            exit(0);

:: Whenever any Key is pressed the program exits.

default:
fprintf(stderr,"Unexpected event:");  

:: Any other event is unexpected and should not happen.

Example 2: Drawing Lines xlines.c

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

:: odd clicks (1, 3, ...) draws a point

:: even clicks (2, 4, ...) draws line between current and previous mouse positions .

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;