#include <stdio.h>
#include <X11/Xlib.h>
#include <X11/Xutil.h>
#include <X11/keysym.h>
#include <GL/gl.h>
#include <GL/glx.h>
#include <stdlib.h>
#include <ctype.h>
#include <unistd.h>

int main( int argc, char **argv )
{
    XVisualInfo *vinfo;
    XSetWindowAttributes swa;
    int glxAttrib[] = { GLX_RGBA, GLX_DOUBLEBUFFER,
                        GLX_RED_SIZE, 8, GLX_GREEN_SIZE, 8, GLX_BLUE_SIZE, 8,
                        None };
    Display *display;
    int screen;
    Window wnd;
    GLXContext ctx;
    Colormap cmap;
    GLubyte *pixels;

    display = XOpenDisplay( 0 );
    if( !display ) {
        fprintf( stderr, "glxoutput: Can't open display '%s'.\n",
                 getenv( "DISPLAY" ) ? getenv( "DISPLAY" ) : "" );
        return 0;
    }
    screen = DefaultScreen( display );

    vinfo = glXChooseVisual( display, screen, glxAttrib );
    if( !vinfo ) {
        fprintf( stderr, "glxoutput: No acceptable GL visual available.\n" );
        return 0;
    }
    fprintf( stderr, "glxoutput: Using visual 0x%x\n", (int) vinfo->visualid );

    cmap = XCreateColormap( display, XRootWindow( display, vinfo->screen ),
                            vinfo->visual, AllocNone );

    swa.colormap = cmap;
    swa.border_pixel = 0;

    wnd = XCreateWindow( display, XRootWindow( display, vinfo->screen ), 0, 0,
                         512, 512, 0, vinfo->depth,
                         InputOutput, vinfo->visual,
                         CWColormap|CWBorderPixel, &swa );
    XMapWindow( display, wnd );

    ctx = glXCreateContext( display, vinfo, 0, True );
    if( !ctx ) {
        fprintf( stderr, "glxoutput: Can't create OpenGL context.\n" );
        return 0;
    }

    if( !glXMakeCurrent( display, wnd, ctx ) ) {
        fprintf( stderr, "glxoutput: Unable to attach OpenGL to our window!\n");
        return 0;
    }

    pixels = malloc( 512 * 512 * 4 );
    glPixelZoom( 1.0, -1.0 );
    glDrawPixels( 512, 512, GL_BGRA, GL_UNSIGNED_BYTE, pixels );
    glXSwapBuffers( display, wnd );
}