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: 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., fast.cs.odu.edu) or

Linux host (e.g. linux.cs.odu.edu) and to display the interface on your

local windows 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 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;

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;

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.

The user odd clicks (1, 3, ...) draws a point

while the even clicks (2, 4, ...) draws lines between the current mouse position and the previous mouse position.

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;

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