Introduction
This tutorial looks at writing a simple X-Windows application that displays the text 'Hello World' in the middle of the frame window. When the window is re-sized, the text is re-positioned accordingly.
Throughout this tutorial, I shall be making references to the equivalent Microsoft Windows API routines, for those of you who are more familiar with that environment.
The X-Windows environment
One of the major differences between the Microsoft Windows and X-Windows systems is that X-Windows is client/server based. This has a significant impact on networked machines, as the application may be running on the server, yet displaying windows on a client machine. Seen from the users perspective, the application looks like it is running locally, when in actual fact it isn't. The downside to this approach is that response times from client to server and back to client are limited to the bandwidth of the network connection.
Accordingly, there are some X-Windows calls that may seen a little strange, but I will discuss them as we progress through the tutorial.
The code
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <X11/Xlib.h>
int main (int argc, char *argv[])
{
Display *display;
Visual *visual;
int depth;
int text_x;
int text_y;
XSetWindowAttributes frame_attributes;
Window frame_window;
XFontStruct *fontinfo;
XGCValues gr_values;
GC graphical_context;
XKeyEvent event;
char hello_string[] = "Hello World";
int hello_string_length = strlen(hello_string);
display = XOpenDisplay(NULL);
visual = DefaultVisual(display, 0);
depth = DefaultDepth(display, 0);
frame_attributes.background_pixel = XWhitePixel(display, 0);
/* create the application window */
frame_window = XCreateWindow(display, XRootWindow(display, 0),
0, 0, 400, 400, 5, depth,
InputOutput, visual, CWBackPixel,
&frame_attributes);
XStoreName(display, frame_window, "Hello World Example");
XSelectInput(display, frame_window, ExposureMask | StructureNotifyMask);
fontinfo = XLoadQueryFont(display, "10x20");
gr_values.font = fontinfo->fid;
gr_values.foreground = XBlackPixel(display, 0);
graphical_context = XCreateGC(display, frame_window,
GCFont+GCForeground, &gr_values);
XMapWindow(display, frame_window);
while ( 1 ) {
XNextEvent(display, (XEvent *)&event);
switch ( event.type ) {
case Expose:
{
XWindowAttributes window_attributes;
int font_direction, font_ascent, font_descent;
XCharStruct text_structure;
XTextExtents(fontinfo, hello_string, hello_string_length,
&font_direction, &font_ascent, &font_descent,
&text_structure);
XGetWindowAttributes(display, frame_window, &window_attributes);
text_x = (window_attributes.width - text_structure.width)/2;
text_y = (window_attributes.height -
(text_structure.ascent+text_structure.descent))/2;
XDrawString(display, frame_window, graphical_context,
text_x, text_y, hello_string, hello_string_length);
break;
}
default:
break;
}
}
return(0);
}
The code in detail
The first three lines of code open a connection to the X-Windows server and obtain a handle to the display, the visual type of display and the plane depth (Microsoft Windows uses a single plane display) of the display.
display = XOpenDisplay(NULL);
visual = DefaultVisual(display, 0);
depth = DefaultDepth(display, 0);
The next two lines sets the background colour for the window we are about to open and then opens the window. This is very similar to the Microsoft CreateWindow API call.
frame_attributes.background_pixel = XWhitePixel(display, 0);
/* create the application window */
frame_window = XCreateWindow(display, XRootWindow(display, 0),
0, 0, 400, 400, 5, depth,
InputOutput, visual, CWBackPixel,
&frame_attributes);
The next line sets the text in the title bar to 'Hello World Example' and is equivalent to the Microsoft SetWindowText API call.
XStoreName(display, frame_window, "Hello World Example");
The next line of code tells the X-Windows server the events that we are interested in for the window we have just created. Remember, because X-Windows is client/server based we have the ability to restrict the events that we receive for an individual window and thus reduce the amount of network traffic.
XSelectInput(display, frame_window, ExposureMask | StructureNotifyMask);
The next four lines create a suitable font for our 'Hello World' text, and create a device context suitable for the display and window on that display. These are equivalent to Microsoft CreateFont and GetDC API calls.
fontinfo = XLoadQueryFont(display, "10x20");
gr_values.font = fontinfo->fid;
gr_values.foreground = XBlackPixel(display, 0);
graphical_context = XCreateGC(display, frame_window, GCFont+GCForeground, &gr_values);
The next line of code actually displays the frame window on the display and is equivalent to the Microsoft ShowWindow API call.
XMapWindow(display, frame_window);
We then enter our event processing loop in a similar way we do for Microsoft Windows by entering a message processing loop.
The only event we are interested in is the Expose event.
Upon receipt of an expose event, we get the bounding rectangle size for the text 'Hello World' using the XTextExtents call (Microsoft GetTextExtentPoint32 API call); get the width and height of the window using the XGetWindowAttributes call (Microsoft GetWindowInfo API call) and finally draw the string using XDrawString call (Microsoft DrawText API call).
case Expose:
{
XWindowAttributes window_attributes;
int font_direction, font_ascent, font_descent;
XCharStruct text_structure;
XTextExtents(fontinfo, hello_string, hello_string_length,
&font_direction, &font_ascent, &font_descent,
&text_structure);
XGetWindowAttributes(display, frame_window, &window_attributes);
text_x = (window_attributes.width - text_structure.width)/2;
text_y = (window_attributes.height -
(text_structure.ascent+text_structure.descent))/2;
XDrawString(display, frame_window, graphical_context,
text_x, text_y, hello_string, hello_string_length);
break;
}
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