InfoMagic Source Code 1993 July/THE_SOURCE_CODE_CD_ROM.iso/X/mit/doc/tutorials/answers.txt

Text File | 1991-08-26 | 11.3 KB | 225 lines

             Late Night's Top Ten X11 Questions

             Dave Lemke ([e-mail])
               Stuart Marks ([e-mail])

1.  Why doesn't my program draw anything?  I create the window and draw into
    it, but it's always blank.

    In order to get a window on the screen, you must map it first, using the
    XMapWindow() call.  Drawing into a window and then mapping it won't work
    in most cases.

1a. But I do map the window before drawing into it, and the window is still
    blank.

    The problem here is that if the window is a child of the root window,
    XMapWindow() doesn't actually map the window; it sends a message to the
    window manager asking it to map the window.  If you having drawing
    requests right after the XMapWindow() call, they will probably be
    processed before the window manager has actually mapped the window.  In
    order to get around this delayed mapping effect, you must wait for an
    Exposure event from the server.  This event indicates that your window is
    on the screen and ready to receive graphics.  In general, you should
    always draw in response to Exposure events.

2.  Why does my program display in inverse-video on some servers?

    You're assuming that 1 is black and 0 is white or vice-versa.  The
    relationship between pixels 0 and 1 and the colors black and white is
    implementation-dependent.  They may be reversed, or they may not even
    correspond to black and white at all.  For instance, an 8-bit grayscale
    server might choose to have 0 be black and 255 be white.  If you want to
    draw in black and white, set the foreground and background pixels in the
    GC using BlackPixel() and WhitePixel() macros instead of 0 and 1.  This
    will help ensure that your application is portable amongst different
    servers.

3.  Can I get the background pixel/pixmap of a window using
    GetWindowAttributes?

    No.  Once set, the background pixel or pixmap of a window cannot be
    re-read by clients.  The reason for this is that a client can create a
    pixmap, set it to be the background pixmap of a window, and then free the
    pixmap.  The window keeps this background, but the pixmap itself is
    destroyed.  If you're sure a window has a background pixel (not a
    pixmap), you can use XClearArea() to clear a region to the background
    color and then use XGetImage() to read back that pixel.  However, this
    action alters the contents of the window, and it suffers from race
    conditions with exposures.

4.  Why doesn't my program get keystrokes?  I select input for
    KeyPress events, but my program never gets them.  (Alternatively,
    my program works under uwm, but not under ...)

    The window manager controls how the input focus is transferred from one
    window to another.  In order to get keystrokes, your program must ask the
    window manager for the input focus.  To do this, you must set up what are
    called "hints" for the window manager.  If your applications is
    Xlib-based, you can use something like the following:

    XWMHints wmhints;
    ...
    wmhints.flags = InputHint;
    wmhints.input = True;
    XSetWMHints(dpy, window, &hints)

    If your application is based on the Xt Intrinsics, you can use something
    like the following:

    Arg arglist[MAXARGS];
    Cardinal nargs = (Cardinal) 0;
    ...
    XtSetArg(arglist[nargs++], XtNinput, TRUE);
    ...
    XtCreatePopupShell(...);

5.  I encountered a breakpoint when I was debugging a program, and now
    everything is hung.  I can still move the mouse around on the screen,
    but I can't type anything or get any mouse clicks to do anything.  HELP!

    The debugger stopped your program while it was in the middle of what is
    called a "grab".  There are various flavors of grabs, but the important
    effect of all of them is that the grabbing client gets serviced at the
    expense of other clients in the system.  This kind of feature is
    necessary for doing highly interactive things like pop-up menu tracking.

    The problem arises because the program you're debugging grabbed some
    server resource, such as the keyboard, and then hit a breakpoint.  The
    debugger is sharing the same resource with the debugged program, which
    has grabbed the keyboard away from the debugger.  You can't type
    debugger commands to continue the program, and thus you're stuck.

    To get around this problem, you might try to iconify the debugged
    program's window by using the mouse or the keyboard, whichever isn't
    grabbed.  When the window goes iconic, any grabs on that window will be
    deactivated by the server.

    If the debugged program is in the middle of a server grab, you're really
    stuck.  A server grab grabs *all* of the server's resources.  Thus, you
    have to attack the problem by going outside of the server.  Killing the
    debugged program and its debugger will release the grab and get you
    unstuck, but you obviously can't continue debugging.  You might also try
    making a special debugging-mode option to your program so that it doesn't
    grab the server while you're debugging it.  If you can't do that, your
    only choice is to debug the offending program from somewhere outside the
    server it's displaying on, e.g. another workstation or a terminal.  If
    you're running X11/NeWS from Sun, you can run a SunView shelltool on top
    of the server; it will be unaffected by server grabs.

6.  How do I do XOR drawing to work properly?  It doesn't work at all on some
    servers, and I get funny colors on other servers.

    It depends on what you mean by "properly."  For the most part, when you
    do XOR drawing, you want to do some transient animation that won't
    disturb the contents of the window.  You can do this with XOR because the
    first XOR operation will change the contents to something different, and
    then the second XOR operation will restore the original contents.  The
    trick is to set up the GC source pixel so that the "something different"
    contrasts properly with the surroundings.

    Confusion arises because XOR drawing uses the GC foreground field as the
    source pixel for the XOR operation.  If you use your actual foreground
    pixel as the source for XOR, the result will probably be something off in
    a random section of whatever colormap is installed.  Unless you've set up
    and installed your own colormap, the color you end up with will be some
    unlikely value over which you don't have any control.

    What's often appropriate is to interchange the foreground color with the
    background color.  If your graphics has the concept of a foreground and a
    background, you can do XOR animation that switches foreground and
    background by setting the GC foreground pixel to the XOR of the
    foreground and background pixels.  In C, this would be

    XGCValues gcvalues;
    gcvalues.foreground = fgpixel ^ bgpixel;
    XChangeGC(dpy,gc,GCForeground,&gcvalues);

    For monochrome graphics, you can use

    gcvalues.foreground =
    BlackPixel(dpy,DefaultScreen(dpy))^WhitePixel(dpy,DefaultScreen(dpy));

7.  I create and draw into a pixmap, but when I copy it to the screen with
    XCopyArea, I get nothing but garbage.

    The initial contents of pixmaps is undefined.  This means that most
    servers will allocate the memory and leave around whatever happens to be
    there -- which is usually garbage.  You probably want to clear the pixmap
    first using XFillRectangle() with an alu function of GXcopy and a
    foreground pixel of whatever color you want as your background (or 0L
    if you are using the pixmap as a mask).

8.  My color program always fails on XAllocColorCells().  But it works on
    other servers.

    Your program is assuming that the default colormap for the screen is a
    dynamic one, that is, one out of which you can allocate nonshared color
    cells via XAllocColorCells().  Some servers have a static colormap as
    their default colormap, so XAllocColorCells() will fail.

    You have several alternatives.  One possibility is to change your program
    to use XAllocColor().  You won't be able to alter the color once you've
    allocated it, but you may not have needed to do that in the first place.
    You should use XAllocColor() in preference to XAllocColorCells() if you
    simply want to get colors on the screen.  The reason is that
    XAllocColor() allows the same colors to be shared amongst different
    clients.

    If you really need to allocate and change colormap cells, then you still
    need to use XAllocColorCells() or XAllocColorPlanes().  For these calls
    to work, you must search the server's visual list for a visual that
    supports dynamic colormaps.  Then, you must create a colormap against
    this visual and allocate colors out of this colormap.  If the server
    doesn't have any dynamic visuals, your program simply won't work on this
    server.  If you successfully create and use your own dynamic colormap,
    you'll also have to create a window using the same visual and set that
    window's colormap attribute to be your newly created colormap.  Don't
    forget to set up WM_COLORMAP_WINDOWS so the window manager can install
    your colormap.

9.  My program worked fine in R3, but fails in X11R4 with an error in
    ChangeWindowAttributes.  Also, my Andrew and Interviews code stopped
    working with the same error.

    A likely problem is the value given to a window's do_not_propagate mask.
    R3 allowed bogus values to be set, and early version of both Andrew and
    Interviews did just that.  X11R4 and several productized servers catch
    this error and complain.  The events that can be suppressed are:

    KeyPress, KeyRelease, ButtonPress, ButtonRelease, PointerMotion,
    ButtonMotion and Button[1-5]Motion

10. I set my windows to have BackingStore, yet they still get exposures.

    There are several things that can be going on here.  The most
    important thing to remember is that backing store is a hint.
    You may have asked for backing store on your window, but the server may
    not have enough memory to service your request.  If this is the case,
    the server will go ahead and create and window without backing store,
    and then send you an Exposure event to tell you to draw into it.

    Other behavior that could possibly be counterintuitive is that, if you
    create a window with BackingStore == Always, you will get an Exposure
    event as soon as you create the window.  The server does this to let you
    know that the initial contents of the window need to be filled.  It does
    this even before you have mapped the window.

    Some servers will throw away backing store if they start running out of
    memory.  A window may start out with backing store, but then have it
    thrown away by the server because of a memory shortage.  This window
    will then start getting Exposure events.

    Another possibility is a window manager with a 'Refresh' function.  Some
    window managers implement this function by doing a ClearArea with the
    Exposure flag set.  This will force the server the send an Exposure
    event, even if the window still has backing store.

    The bottom line is that clients must *always* be prepared to repaint
    their windows in response to Exposure events.  Backing store is a hint,
    and it is only useful to reduce the number of Exposures on the window;
    it doesn't guarantee that none will ever occur.  If you need to keep an
    image in the server, but not on the screen, use pixmaps.

[@(#)answers.txt 1.3 89/12/15]