Danny Amor's Online Library/Danny Amor's Online Library - Volume 1.iso/html/faqs/faq/x-faq/part6

Text File | 1995-07-25 | 44.7 KB | 925 lines

Subject: comp.windows.x Frequently Asked Questions (FAQ) 6/6
Newsgroups: comp.windows.x,news.answers,comp.answers
From: [e-mail] (David B. Lewis)
Date: 23 Oct 1994 23:09:14 GMT

Archive-name: x-faq/part6
Last-modified: 1994/10/23

----------------------------------------------------------------------
Subject: 132)  Are callbacks guaranteed to be called in the order registered?

    Although some books demonstrate that the current implementation of Xt
happens to call callback procedures in the order in which they are registered,
the specification does not guarantee such a sequence, and supplemental
authoritative documents (i.e. the Asente/Swick volume) do say that the order is
undefined.  Because the callback list can be manipulated by both the widget and
the application, Xt cannot guarantee the order of execution.
    In general, the callback procedures should be thought of as operating
independently of one another and should not depend on side-effects of other
callbacks operating; if a seqence is needed, then the single callback to be
registered can explicitly call other functions necessary.

[4/92; thanks to [e-mail]]

----------------------------------------------------------------------
Subject: 133)  Why doesn't XtDestroyWidget() actually destroy the widget?

    XtDestroyWidget() operates in two passes, in order to avoid leaving
dangling data structures; the function-call marks the widget, which is not
actually destroyed until your program returns to its event-loop.

----------------------------------------------------------------------
Subject: 134)! How do I query the user synchronously using Xt?

    It is possible to have code which looks like this trivial callback,
which has a clear flow of control. The calls to AskUser() block until answer
is set to one of the valid values. If it is not a "yes" answer, the code drops
out of the callback and back to an event-processing loop:

    void quit(Widget w, XtPointer client, XtPointer call)
    {
        int             answer;
        answer = AskUser(w, "Really Quit?");
        if (RET_YES == answer)
            {
            answer = AskUser(w, "Are You Really Positive?");
            if (RET_YES == answer)
                exit(0);
                }
    }

    A more realistic example might ask whether to create a file or whether
to overwrite it.
    This is accomplished by entering a second event-processing loop and
waiting until the user answers the question; the answer is returned to the
calling function. That function AskUser() looks something like this, where the
Motif can be replaced with widget-set-specific code to create some sort of
dialog-box displaying the question string and buttons for "OK", "Cancel" and
"Help" or equivalents:

  int AskUser(w, string)
        Widget          w;
        char           *string;
  {
        int             answer=RET_NONE;    /* some not-used marker */
        Widget          dialog;            /* could cache&carry, but ...*/
        Arg             args[3];
        int             n = 0;
        XtAppContext    context;

        n=0;
        XtSetArg(args[n], XmNmessageString, XmStringCreateLtoR(string,
                XmSTRING_DEFAULT_CHARSET)); n++;
        XtSetArg(args[n], XmNdialogStyle, XmDIALOG_APPLICATION_MODAL); n++;
        dialog = XmCreateQuestionDialog(XtParent(w), string, args, n);
        XtAddCallback(dialog, XmNokCallback, response, &answer);
        XtAddCallback(dialog, XmNcancelCallback, response, &answer);
        XtAddCallback(dialog, XmNhelpCallback, response, &answer);
        XtManageChild(dialog);

        context = XtWidgetToApplicationContext(w);
        while ((RET_NONE == answer) || XtAppPending(context))
                XtAppProcessEvent (context, XtIMAll);
        XtDestroyWidget(dialog);  /* blow away the dialog box and shell */
        return answer;
  }

    The dialog supports three buttons, which are set to call the same
function when tickled by the user.  The variable answer is set when the user
finally selects one of those choices:

  void response(w, client, call)
        Widget          w;
        XtPointer client;
        XtPointer call;
  {
        int *answer = (int *) client;
        XmAnyCallbackStruct *reason = (XmAnyCallbackStruct *) call;
        switch (reason->reason) {
        case XmCR_OK:
                *answer = RET_YES;    /* some #define value */
                break;
        case XmCR_CANCEL:
                *answer = RET_NO;
        break;
        case XmCR_HELP:
                *answer = RET_HELP;
                break;
        default:
                return;
        }
}

and the code unwraps back to the point at which an answer was needed and
continues from there.

Note that modifications are needed to handle receiving WM_DELETE_WINDOW on
the window; possibly WM_DELETE_WINDOW can be handled by setting the "answer"
variable.

[Thanks to Dan Heller (now [e-mail]); note that the code in his book
caches the dialog but neglects to make sure that the callbacks point to the
current automatic "answer".]

----------------------------------------------------------------------
Subject: 135)  How do I determine the name of an existing widget?
I have a widget ID and need to know what the name of that widget is.

    Users of R4 and later are best off using the XtName() function, which
will work on both widgets and non-widget objects.

    If you are still using R3, you can use this simple bit of code to do
what you want. Note that it depends on the widget's internal data structures
and is not necessarily portable to future versions of Xt, including R4.

    #include <X11/CoreP.h>
    #include <X11/Xresource.h>
    String XtName (widget)
    Widget widget;    /* WILL work with non-widget objects */
    {
    return XrmNameToString(widget->core.xrm_name);
    }

[7/90; modified with suggestion by Larry Rogers ([e-mail]) 9/91]

----------------------------------------------------------------------
Subject: 136)  Why do I get a BadDrawable error drawing to XtWindow(widget)?
I'm doing this in order to get a window into which I can do Xlib graphics
within my Xt-based program:

> canvas = XtCreateManagedWidget ( ...,widgetClass,...) /* drawing area */
> ...
> window = XtWindow(canvas);    /* get the window associated with the widget */
> ...
> XDrawLine (...,window,...);    /* produces error */

    The window associated with the widget is created as a part of the
realization of the widget.  Using a window id of None ("no window") could
create the error that you describe.  It is necessary to call XtRealizeWidget()
before attempting to use the window associated with a widget.
    Note that the window will be created after the XtRealizeWidget() call,
but that the server may not have actually mapped it yet, so you should also
wait for an Expose event on the window before drawing into it.

----------------------------------------------------------------------
Subject: 137)  Where can I get documentation on Xaw, the Athena widget set?

    Check ftp.x.org in /pub/R5untarred/mit/hardcopy for the originals of
documentation distributed with X11R5. In R6, see xc/doc/specs/Xaw or
xc/doc/hardcopy/Xaw.

----------------------------------------------------------------------
Subject: 138)  What's the difference between actions and callbacks?

Actions and callbacks may be closely tied; the user may click a mouse-button
in an object's window, causing an action procedure in that particular object
to be called. As part of its processing of the event, the action procedure
may inform the application via a callback registered on the object. However,
callbacks can be given for any reason, including some that don't arise as a
result of user action; and many actions don't result in any notification to
the application.

Callbacks generally are a means of interaction between the user interface
(UI) and some other piece of code interested in the "results"; the interested
party to which the data is communicated is usually the application's back-end
functions but may be another widget in a related part of the UI.  For
example, a text widget invokes a callback to say "the user just entered this
text string; never mind what I had to do to get it or what X events took
place."

In object-oriented programming terminology, callback lists are messages
defined by the widget class by which the widget istance notifies another
entity that something significant has happened to the widget.

Actions, however, constitute a widget's repertoire of internal i/o
behaviors.  Actions are not about results; actions are about "how", not
"what" gets done. The text widget may define a dozen or two actions which
define how the user can manipulate the text; the procedures for removing a
line of text or switching two words can be associated with particular X event
sequences (and in fact often rely on particular types of events).

Actions are (in OOP terminology) methods of the widget class by which the
widget responds to some external stimulus (one or more X events).

To avoid confusing yourself on the issue of actions vs. callbacks, try
thinking of actions defined by an application as methods *of the application*
-- applications may define actions, as well -- by which the application
responds to one or more X events (and happens to be handed an object handle
as part of the method argument list). Similarly, callback handlers registered
by an application with a widget can be thought of as methods of the
application which respond to messages from a widget or widgets.

[Thanks to Michael Johnson ([e-mail]) and to Kerry Kimbrough]

----------------------------------------------------------------------
Subject: 139)  How do I simulate a button press/release event for a widget?

    You can do this using XSendEvent(); it's likely that you're not setting
the window field in the event, which Xt needs in order to match to the widget
which should receive the event.
     If you're sending events to your own application, then you can use
XtDispatchEvent() instead. This is more efficient than XSendEvent() in that you
avoid a round-trip to the server.
    Depending on how well the widget was written, you may be able to call
its action procedures in order to get the effects you want.

[courtesy Mark A. Horstman ([e-mail]), 11/90]

----------------------------------------------------------------------
Subject: 140)  Can I make Xt or Xlib calls from a signal handler?

No. Xlib and Xt have no mutual exclusion for protecting critical sections. If
your signal handler makes such a call at the wrong time (which might be while
the function you are calling is already executing), it can leave the library
in an inconsistent state. Note that the ANSI C standard points out that
behavior of a signal handler is undefined if the signal handler calls any
function other than signal() itself, so this is not a problem specific to
Xlib and Xt; the POSIX specification mentions other functions which may be
called safely but it may not be assumed that these functions are called by
Xlib or Xt functions.

Setting a global variable is one of the few permitted operations.  You can
work around the problem by setting a flag in the interrupt handler and later
checking it with a work procedure or a timer event which has previously been
added or by using a custom event loop.

R6 Xt has have support for signal handlers; there is a mechanism to set a
flag in a signal handler, and XtAppNextEvent will notice that the flag has
been set and call the associated callbacks.

Note: the article in The X Journal 1:4 and the example in the first edition
of O'Reilly & Associates' Volume 6 are in error.

[Thanks to Pete Ware ([e-mail]) and Donna Converse
([e-mail]), 5/92]

An alternate solution is to create a pipe and add the read side of the pipe
as an input event with XtAppAddInput; then write a byte to the write side of
the pipe with your signal handler (write is re-entrant). The callback for the
read side of the pipe reads the byte and does the actual processing that you
intended. You may want the byte to be the signal number unless your callback
handles only one kind.

[Thanks to Steve Kappel ([e-mail])]

----------------------------------------------------------------------
Subject: 141)  What are these "Xlib sequence lost" errors?

    You may see these errors if you issue Xlib requests from an Xlib error
handler, or, more likely, if you make calls which generate X requests to Xt or
Xlib from a signal handler, which you shouldn't be doing in any case.

----------------------------------------------------------------------
Subject: 142)  How can my Xt program handle socket, pipe, or file input?

    It's very common to need to write an Xt program that can accept input
both from a user via the X connection and from some other file descriptor, but
which operates efficiently and without blocking on either the X connection or
the other file descriptor.
    A solution is use XtAppAddInput(). After you open your file descriptor,
use XtAppAddInput() to register an input handler. The input handler will be
called every time there is something on the file descriptor requiring your
program's attention. Write the input handler like you would any other Xt
callback, so it does its work quickly and returns.  It is important to use only
non-blocking I/O system calls in your input handlers.
    Most input handlers read the file descriptor, although you can have an
input handler write or handle exception conditions if you wish.
    Be careful when you register an input handler to read from a disk file.
You will find that the function is called even when there isn't input pending.
XtAppAddInput() is actually working as it is supposed to. The input handler is
called whenever the file descriptor is READY to be read, not only when there is
new data to be read. A disk file (unlike a pipe or socket) is almost always
ready to be read, however, if only because you can spin back to the beginning
and read data you've read before.  The result is that your function will almost
always be called every time around XtAppMainLoop(). There is a way to get the
type of interaction you are expecting; add this line to the beginning of your
function to test whether there is new data:
         if (ioctl(fd, FIONREAD, &n) == -1 || n == 0) return;
But, because this is called frequently, your application is effectively in a
busy-wait; you may be better off not using XtAppAddInput() and instead setting
a timer and in the timer procedure checking the file for input.

[courtesy Dan Heller ([e-mail]), 8/90; [e-mail] 5/91;
Ollie Jones ([e-mail]) 6/92]

There are two alternatives: the simple one is to use XtAppAddTimeout instead
of XtAppAddInput and check for input occasionally; the more complex solution,
and perhaps the better one, is to popen or fork&exec a child which does
blocking reads on the file, relaying what it has read to your application via
a pipe or a socket. XtAppAddInput will work as expected on pipes and
sockets.

[Thanks to Kaleb Keithley ([e-mail]); 12/93]

----------------------------------------------------------------------
Subject: 143)  What's this R6 error: X Toolkit Error: NULL ArgVal in XtGetValues?

The application has a bug! A workaround is described in Section 3.4 of
the R6 release notes.  Here's the relevant excerpt:

   GetValuesBC
     Setting this variable to YES allows illegal XtGetValues requests with
     NULL ArgVal to usually succeed, as R5 did.  Some applications erro-
     neously rely on this behavior.  Support for this will be removed in a
     future release.

----------------------------------------------------------------------
Subject: 144)  Why do I get a BadMatch error when calling XGetImage?

The BadMatch error can occur if the specified rectangle goes off the edge of
the screen. If you don't want to catch the error and deal with it, you can take
the following steps to avoid the error:

1) Make a pixmap the same size as the rectangle you want to capture.
2) Clear the pixmap to background using XFillRectangle.
3) Use XCopyArea to copy the window to the pixmap.
4) If you get a NoExpose event, the copy was clean. Use XGetImage to grab the
image from the pixmap.
5) If you get one or more GraphicsExpose events, the copy wasn't clean, and
the x/y/width/height members of the GraphicsExpose event structures tell you
the parts of the pixmap which aren't good.
6) Get rid of the pixmap; it probably takes a lot of memory.

[10/92; thanks to Oliver Jones ([e-mail])]

----------------------------------------------------------------------
Subject: 145)  How can my application tell if it is being run under X?

    A number of programs offer X modes but otherwise run in a straight
character-only mode. The easiest way for an application to determine that it is
running on an X display is to attempt to open a connection to the X server:

    display = XOpenDisplay(display_name);
    if (display)
        { do X stuff }
    else
        { do curses or something else }
where display_name is either the string specified on the command-line following
-display, by convention, or otherwise is (char*)NULL [in which case
XOpenDisplay uses the value of $DISPLAY, if set].

This is superior to simply checking for the existence a -display command-line
argument or checking for $DISPLAY set in the environment, neither of which is
adequate. [5/91]

----------------------------------------------------------------------
Subject: 146)  How do I make a "busy cursor" while my application is computing?
Is it necessary to call XDefineCursor() for every window in my application?

    The easiest thing to do is to create a single InputOnly window that
is as large as the largest possible screen; make it a child of your toplevel
window (which must be realized) and it will be clipped to that window, so it
won't affect any other application. (It needs to be as big as the largest
possible screen in case the user enlarges the window while it is busy or
moves elsewhere within a virtual desktop.) Substitute "toplevel" with your
top-most widget here (similar code should work for Xlib-only applications;
just use your top Window):

     unsigned long valuemask;
     XSetWindowAttributes attributes;

     /* Ignore device events while the busy cursor is displayed. */
     valuemask = CWDontPropagate | CWCursor;
     attributes.do_not_propagate_mask =  (KeyPressMask | KeyReleaseMask |
         ButtonPressMask | ButtonReleaseMask | PointerMotionMask);
     attributes.cursor = XCreateFontCursor(XtDisplay(toplevel), XC_watch);

     /* The window will be as big as the display screen, and clipped by
        its own parent window, so we never have to worry about resizing */
     XCreateWindow(XtDisplay(toplevel), XtWindow(toplevel), 0, 0,
         65535, 65535, (unsigned int) 0, 0, InputOnly,
         CopyFromParent, valuemask, &attributes);

where the maximum size above could be replaced by the real size of the screen,
particularly to avoid servers which have problems with windows larger than
32767.

When you want to use this busy cursor, map and raise this window; to go back to
normal, unmap it. This will automatically keep you from getting extra mouse
events; depending on precisely how the window manager works, it may or may not
have a similar effect on keystrokes as well.

In addition, note also that most of the Xaw widgets support an XtNcursor
resource which can be temporarily reset, should you merely wish to change the
cursor without blocking pointer events.

[thanks to Andrew Wason ([e-mail]), Dan Heller
(now [e-mail]), and [e-mail]; 11/90,5/91]

----------------------------------------------------------------------
Subject: 147)  How do I fork without hanging my parent X program?

    An X-based application which spawns off other Unix processes which
continue to run after it is closed typically does not vanish until all of its
children are terminated; the children inherit from the parent the open X
connection to the display.
    What you need to do is fork; then, immediately, in the child process,
        close (ConnectionNumber(XtDisplay(widget)));
to close the file-descriptor in the display information. After this do your
exec. You will then be able to exit the parent.
    Alternatively, before exec'ing make this call, which causes the file
descriptor to be closed on exec.
        (void) fcntl(ConnectionNumber(XDisplay), F_SETFD, 1);

[Thanks to Janet Anstett ([e-mail]), Gordon Freedman
([e-mail]); 2/91. Greg Holmberg ([e-mail]), 3/93.]

----------------------------------------------------------------------
Subject: 148)  Why doesn't anything appear when I run this simple program?

> ...
> the_window = XCreateSimpleWindow(the_display,
>      root_window,size_hints.x,size_hints.y,
>      size_hints.width,size_hints.height,BORDER_WIDTH,
>      BlackPixel(the_display,the_screen),
>      WhitePixel(the_display,the_screen));
> ...
> XSelectInput(the_display,the_window,ExposureMask|ButtonPressMask|
>     ButtonReleaseMask);
> XMapWindow(the_display,the_window);
> ...
> XDrawLine(the_display,the_window,the_GC,5,5,100,100);
> ...

    You are right to map the window before drawing into it. However, the
window is not ready to be drawn into until it actually appears on the screen --
until your application receives an Expose event. Drawing done before that will
generally not appear. You'll see code like this in many programs; this code
would appear after the window was created and mapped:
  while (!done)
    {
      XNextEvent(the_display,&the_event);
      switch (the_event.type) {
    case Expose:     /* On expose events, redraw */
        XDrawLine(the_display,the_window,the_GC,5,5,100,100);
        break;
    ...
    }
    }

    Note that there is a second problem: some Xlib implementations don't
set up the default graphics context to have correct foreground/background
colors, so this program could previously include this code:
  ...
  the_GC_values.foreground=BlackPixel(the_display,the_screen);    /* e.g. */
  the_GC_values.background=WhitePixel(the_display,the_screen);    /* e.g. */
  the_GC = XCreateGC(the_display,the_window,
                GCForeground|GCBackground,&the_GC_values);
  ...

Note: the code uses BlackPixel and WhitePixel to avoid 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.

Also note that actually using BlackPixel and WhitePixel is usually the wrong
thing to do in a finished program, as it ignores the user's preference for
foreground and background.

And also note that you can run into the same situation in an Xt-based program
if you draw into the XtWindow(w) right after it has been realized; it may
not yet have appeared.

----------------------------------------------------------------------
Subject: 149)  What is the difference between a Screen and a screen?

    The 'Screen' is an Xlib structure which includes the information about
one of the monitors or virtual monitors which a single X display supports. A
server can support several independent screens. They are numbered unix:0.0,
unix:0.1, unix:0.2, etc; the 'screen' or 'screen_number' is the second digit --
the 0, 1, 2 which can be thought of as an index into the array of available
Screens on this particular Display connection.
    The macros which you can use to obtain information about the particular
Screen on which your application is running typically have two forms -- one
which takes a Screen and one with takes both the Display and the screen_number.
    In Xt-based programs, you typically use XtScreen(widget) to determine
the Screen on which your application is running, if it uses a single screen.
    (Part of the confusion may arise from the fact that some of the macros
which return characteristics of the Screen have "Display" in the names --
XDisplayWidth, XDisplayHeight, etc.)

----------------------------------------------------------------------
Subject: 150)  Can XGetWindowAttributes get a window's background pixel/pixmap?

    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. [courtesy Dave Lemke of NCD
and Stuart Marks of Sun]

    Note that the same applies to the border pixel/pixmap. This is a
(mis)feature of the protocol which allows the server is free to manipulate the
pixel/pixmap however it wants.  By not requiring the server to keep the
original pixel or pixmap, some (potentially a lot of) space can be saved.
[courtesy Jim Fulton, then of X Consortium]

----------------------------------------------------------------------
Subject: 151)  How do I create a transparent window?

    A completely transparent window is easy to get -- use an InputOnly
window. In order to create a window which is *mostly* transparent, you have
several choices:
    - the SHAPE extension first released with X11R4 offers an easy way to
make non-rectangular windows, so you can set the shape of the window to fit the
areas where the window should be nontransparent; however, not all servers
support the extension.
    - a machine-specific method of implementing transparent windows for
particular servers is to use an overlay plane supported by the hardware.  Note
that there is no X notion of a "transparent color index".
    - a generally portable solution is to use a large number of tiny
windows, but this makes operating on the application as a unit difficult.
    - a final answer is to consider whether you really need a transparent
window or if you would be satisfied with being able to overlay your application
window with information; if so, you can draw into separate bitplanes in colors
that will appear properly.

[thanks to der Mouse, [e-mail], 3/92; see also
The X Journal 1:4 for a more complete answer, including code samples for this
last option]

----------------------------------------------------------------------
Subject: 152)  Why doesn't GXxor produce mathematically-correct color values?

    When using GXxor you may expect that drawing with a value of black on a
background of black, for example, should produce white. However, the drawing
operation does not work on RGB values but on colormap indices. The color that
the resulting colormap index actually points to is undefined and visually
random unless you have actually filled it in yourself. [On many X servers Black
and White often 0/1 or 1/0; programs taking advantage of this mathematical
coincidence will break.]
    If you want to be combining colors with GXxor, then you should be
allocating a number of your own color cells and filling them with your chosen
pre-computed values.
    If you want to use GXxor simply to switch between two colors, then you
can take the shortcut of setting the background color in the GC (graphics
context) to 0 and the foreground color to a value such that when it draws over
red, say, the result is blue, and when it draws over blue the result is red.
This foreground value is itself the XOR of the colormap indices of red and
blue.

[Thanks to Chris Flatters ([e-mail]) and Ken Whaley
([e-mail]), 2/91]

----------------------------------------------------------------------
Subject: 153)  Why does every color I allocate show up as black?

    Make sure you're using 16 bits and not 8.  The red, green, and blue
fields of an XColor structure are scaled so that 0 is nothing and 65535 is
full-blast. If you forget to scale (using, for example, 0-255 for each color)
the XAllocColor function will perform correctly but the resulting color is
usually black.

[Thanks to Paul Asente, [e-mail], 7/91]

----------------------------------------------------------------------
Subject: 154)  Why do I get a protocol error when creating a cursor (sic)?

    You may have had this code working on a monochrome system by
coincidence.  Cursor pixmaps must always have a depth of 1; when you create
the cursor pixmap use the depth of 1 rather than the default depth of the
screen.

----------------------------------------------------------------------
Subject: 155)  Why can't my program get a standard colormap?
I have an image-processing program which uses XGetRGBColormap() to get the
standard colormap, but it doesn't work.

    XGetRGBColormap() when used with the property XA_RGB_DEFAULT_MAP does
not create a standard colormap -- it just returns one if one already exists.
Use xstdcmap or do what it does in order to create the standard colormap first.

[1/91; from der Mouse ([e-mail])]

----------------------------------------------------------------------
Subject: 156)  Why does the pixmap I copy to the screen show up as garbage?

    The initial contents of pixmaps are 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 a 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). [courtesy Dave Lemke of NCD and Stuart Marks of Sun]

----------------------------------------------------------------------
Subject: 157)  How can I most quickly send an image to the X server?

    The fastest mechanism may be to use an XImage and the shared-memory
extension to reduce the transmission time.
    The MIT-SHM code, documentation, and example client programs can be
found on the X11R5 source tape; many vendors also support the extension.
    If bandwidth is a problem, the X Image Extension has facilities for
transmitting compressed images.

----------------------------------------------------------------------
Subject: 158)  How do I check whether a window ID is valid?
My program has the ID of a window on a remote display. I want to check whether
the window exists before doing anything with it.

    Because X is asynchronous, there isn't a guarantee that the window
would still exist between the time that you got the ID and the time you sent an
event to the window or otherwise manipulated it. What you should do is send the
event without checking, but install an error handler to catch any BadWindow
errors, which would indicate that the window no longer exists. This scheme
will work except on the [rare] occasion that the original window has been
destroyed and its ID reallocated to another window.
    You can use this scheme to make a function which checks the validity
of a window; you can make this operation almost synchronous by calling
XSync() after the request, although there is still no guarantee that the
window will exist after the result (unless the sterver is grabbed). On the
whole, catching the error rather than pre-checking is preferable.

[courtesy Ken Lee (now [e-mail]), 4/90; 12/93]

----------------------------------------------------------------------
Subject: 159)  Can I have two applications draw to the same window?

    Yes. The X server assigns IDs to windows and other resources (actually,
the server assigns some bits, the client others), and any application that
knows the ID can manipulate the resource [almost any X server resource, except
for GCs and private color cells, can be shared].
    The problem you face is how to disseminate the window ID to multiple
applications. A simple way to handle this (and which solves the problem of the
applications' running on different machines) is in the first application to
create a specially-named property on the root-window and put the window ID into
it. The second application then retrieves the property, whose name it also
knows, and then can draw whatever it wants into the window.
    [Note: this scheme works iff there is only one instance of the first
application running, and the scheme is subject to the limitations mentioned
in the Question about using window IDs on remote displays.]
    Note also that you will still need to coordinate any higher-level
cooperation among your applications; you may find the Synchronization
extension in R6 useful for this.
    Note also that two processes can share a window but should not try to
use the same server connection. If one process is a child of the other, it
should close down the connection to the server and open its own connection.
    Note also that Display IDs and GC values describe addresses local
to an application and cannot be transmitted to another application; note also
that if you are using Xt you may not share widget IDs, which are local to the
client.
    Note also that several clients may draw to a window but for particular
X events such as button-presses only one client can receive the event.

[mostly courtesy Phil Karlton ([e-mail]) 6/90]

----------------------------------------------------------------------
Subject: 160)  Why can't my program work with tvtwm or swm?

    A number of applications, including xwd, xwininfo, and xsetroot, do not
handle the virtual root window which tvtwm and swm use; they typically return
the wrong child of root. A general solution is to add this code or to use it in
your own application where you would normally use RootWindow(dpy,screen):

/* Function Name: GetVRoot
 * Description: Gets the root window, even if it's a virtual root
 * Arguments: the display and the screen
 * Returns: the root window for the client
 */
#include <X11/Xatom.h>
Window GetVRoot(dpy, scr)
Display        *dpy;
int             scr;
{
Window          rootReturn, parentReturn, *children;
unsigned int    numChildren;
Window          root = RootWindow(dpy, scr);
Atom            __SWM_VROOT = None;
int             i;

  __SWM_VROOT = XInternAtom(dpy, "__SWM_VROOT", False);
  XQueryTree(dpy, root, &rootReturn, &parentReturn, &children, &numChildren);
  for (i = 0; i < numChildren; i++) {
    Atom            actual_type;
    int             actual_format;
    long            nitems, bytesafter;
    Window         *newRoot = NULL;

    if (XGetWindowProperty(dpy, children[i], __SWM_VROOT, 0, 1,
        False, XA_WINDOW, &actual_type, &actual_format, &nitems,
            &bytesafter, (unsigned char **) &newRoot) == Success && newRoot) {
            root = *newRoot;
            break;
        }
    }

    XFree((char *)children);
    return root;
}

[courtesy David Elliott ([e-mail]). Similar code is in ssetroot, a
version of xsetroot distributed with tvtwm. 2/91]

A header file by Andreas Stolcke of ICSI on ftp.x.org:contrib/vroot.h
functions similarly by providing macros for RootWindow and DefaultRootWindow;
code can include this header file first to run properly in the presence of a
virtual desktop.

(Note the possible race condition.)

----------------------------------------------------------------------
Subject: 161)  Can I rely on a server which offers backing store?

    You can assume only that the X server has the capability of doing
backing store and that it might do so and keep your application's visuals
up-to-date without your program's involvement; however, the X server can run
out of resources at any time, so you must be able to handle the exposure
events yourself. You cannot rely on a server which offers backing store to
maintain your windows' contents on your behalf.

----------------------------------------------------------------------
Subject: 162)  How do I catch the "close window" event to avoid "fatal IO error"?

    Several windows managers offer a function such as f.kill or f.delete
which sends a message to the application that it should delete its window;
this is usually interpreted as a shutdown message.
    The application needs to catch the WM_DELETE_WINDOW client message.
There is a good example in the xcalc sources in X11R5.
    Motif-based applications should in addition set the resource
XmNdeleteResponse on the top-level shell to XmDO_NOTHING, whether they are
using the Motif window manager or not.
    If the application doesn't handle this message the window manager may
wind up calling XKillClient, which disconnects the client from the display and
typically gives an Xlib error along the lines of "fatal IO error 32 (Broken
pipe)".

[Thanks to Kaleb Keithley, [e-mail]; 11/93]

----------------------------------------------------------------------
Subject: 163)  How do I keep a window from being resized by the user?

    Resizing the window is done through the window manager; window managers
can pay attention to the size hints your application places on the window, but
there is no guarantee that the window manager will listen. You can try setting
the minimum and maximum size hints to your target size and hope for the best.
    Note that you may wish to reconsider your justification for this
restriction.

----------------------------------------------------------------------
Subject: 164)  How do I keep a window in the foreground at all times?

    It's rather antisocial for an application to constantly raise itself
[e.g. by tracking VisibilityNotify events] so that it isn't overlapped --
imagine the conflict between two such programs running.
    The only sure way to have your window appear on the top of the stack
is to make the window override-redirect; this means that you are temporarily
assuming window-management duties while the window is up, so you want to do
this infrequently and then only for short periods of time (e.g. for popup
menus or other short parameter-setting windows).

[thanks to der Mouse ([e-mail]); 7/92]

----------------------------------------------------------------------
Subject: 165)  How do I make text and bitmaps blink in X?

    There is no easy way.  Unless you're willing to depend on some sort of
extension (as yet non-existent), you have to arrange for the blinking yourself,
either by redrawing the contents periodically or, if possible, by playing games
with the colormap and changing the color of the contents.

[Thanks to [e-mail] (der Mouse), 7/91]

----------------------------------------------------------------------
Subject: 166)  How do I get a double-click in Xlib?

    Users of Xt have the support of the translation manager to help
get notification of double-clicking.
    There is no good way to get only a double-click in Xlib, because the
protocol does not provide enough support to do double-clicks.  You have to do
client-side timeouts, unless the single-click action is such that you can defer
actually taking it until you next see an event from the server.  Thus, you
have to do timeouts, which means system-dependent code.  On most UNIXish
implementations, you can use XConnectionNumber to get the file descriptor of
the X connection and then use select() or something similar on that.
    Note that many user-interface references suggest that a double-click
be used to extend the action indicated by a single-click; if this is the case
in your interface then you can execute the first action and as a compromise
check the timestamp on the second event to determine whether it, too, should
be the single-click action or the double-click action.

[Thanks to [e-mail] (der Mouse), 4/93]

----------------------------------------------------------------------
Subject: 167)  How do I render rotated text?

    The X Logical Font Description was enhanced for R6 to allow embedding
a transformation matrix in certain fields of an XLFD name.  Thus arbitrary
rotation, scaling, shearing, etc. are possible.  To draw text along an
arbitrarily sloped line, open a font with the appropriate rotation
transformation and individually place and draw each character.  Drawing text
along a curve requires a different font for each character orientation
needed.  The overhead of opening so many fonts is somewhat mitigated by
another XLFD extension which allows you to ask for a subset of the
characters.  See section 4 of xc/doc/specs/XLFD/xlfd.tbl.ms in the R6
distribution.  Also see The X Resource, Issue Nine, p. 211, "New Font
Technology for X11R6," Nathan Meyers.  (Note: due to changes after
publication deadline, the information in the Meyers paper about the syntax of
character set subsetting is out of date.) These capabilities are also
available to an R5 X server using an R6 font server.

    If you are not using R6, your only choice, if you want to stay within
the core X protocol, is to render the text into a pixmap, read it back via
XGetImage(), rotate it "by hand" with whatever matrices you want, and put it
back to the server via XPutImage(); more specifically:

    Your only choice, if you want to stay within the core X protocol, is to
render the text into a pixmap, read it back via XGetImage(), rotate it "by
hand" with whatever matrices you want, and put it back to the server via
XPutImage(); more specifically:
    1) create a bitmap B and write your text to it.
    2) create an XYBitmap image I from B (via XGetImage).
    3) create an XYBitmap Image I2 big enough to handle the transformation.
    4) for each x,y in I2, I2(x,y) = I(a,b) where
        a = x * cos(theta) - y * sin(theta)
        b = x * sin(theta) + y * cos(theta)
    5) render I2
    Note that you should be careful how you implement this not to lose
bits; an algorithm based on shear transformations may in fact be better.
    The high-level server-extensions and graphics packages available for X
also permit rendering of rotated text: Display PostScript, PEX, PHiGS, and GKS,
although most are not capable of arbitrary rotation and probably do not use the
same fonts that would be found on a printer.
    In addition, if you have enough access to the server to install a font
on it, you can create a font which consists of letters rotated at some
predefined angle. Your application can then itself figure out placement of each
glyph.

[courtesy der Mouse ([e-mail]), Eric Taylor
([e-mail]), and Ken Lee (now [e-mail]), 11/90;
Liam Quin ([e-mail]), 12/90; Dave Wiggins ([e-mail]), 5/94.]

InterViews (C++ UI toolkit, in the X contrib software) has support for
rendering rotated fonts in X.  It could be one source of example code.
[Brian R. Smith ([e-mail]), 3/91]

Another possibility is to use the Hershey Fonts; they are stroke-rendered and
can be used by X by converting them into XDrawLine requests.
[[e-mail], 10/91]

The xrotfont program by Alan Richardson ([e-mail]) (posted to
comp.sources.x July 14 1992) paints a rotated font by implementing the method
above and by using an outline (Hershey) font.

The xvertext package by Alan Richardson ([e-mail]) is a set of
functions to facilitate the writing of text at any angle.  It is on ftp.x.org
as contrib/xvertext.5.0.shar.Z.

O'Reilly's X Resource issue 3 includes information from HP about
modifications to the X fonts server which provide for rotated and scaled
text.  The modifications are on ftp.x.org in contrib/hp_xlfd_enhancements.

Bristol Technology's XPrinter product has extensions to Xlib to rotate text.
Send email to [e-mail] for more details.

----------------------------------------------------------------------
Subject: 168)  Why doesn't my multi-threaded X program work (sic) ?

Support in Xlib and Xt for multi-threaded X programs is included in X11R6.
See the documentation for XInitThreads, XtToolkitThreadInitialize, section
2.7 of the Xlib specification, section 7.12 of the Xt specification, and the
article "Multi-Threaded Xlib," The X Resource, Issue 5, by Stephen Gildea.
The following discussion applies only to pre-R6 libraries:

You cannot use non-thread aware, non-reentrant libraries with threads.

If you must do this, you have only one choice: call the functions from the
initial thread only.

Why opening windows from other threads causes protocol errors can be
explained easily: you are accessing shared resources (the display
structure, the connection to the display, static data in the Xlib) from
a number of threads at the same time, without using any form of
exclusive access control.

[Thanks to [e-mail] (Casper H.S. Dik)]

----------------------------------------------------------------------
Subject: 169)  What is the X Registry? (How do I reserve names?)

    There are places in the X Toolkit, in applications, and in the X
protocol that define and use string names. The context is such that conflicts
are possible if different components use the same name for different things.
    The X Consortium maintains a registry of names in these domains:
orgainization names, selection names, selection targets, resource types,
application classes, and class extension record types; and several others.
    The list as of April 1994 is in the file xc/registry in the R6
distribution. The current Registry is also available by sending "send docs
registry" to the xstuff mail server.
    To register names (first come, first served) or to ask questions send
to [e-mail]; be sure to include a postal address for confirmation.

[11/90; condensed from Asente/Swick Appendix H; 1/94]
----------------------------------------------------------------------

David B. Lewis                     [e-mail]

        "Just the FAQs, ma'am." -- Joe Friday

--
David B. Lewis            Temporarily at but not speaking for ICS
day: [e-mail]        evening: [e-mail]