/* From: The X Window System, Programming and Applications with Xt
  *       OSF/Motif Edition, by Douglas Young, Prentice Hall, 1990
  *       Example described on pages:   149-151 */

#include <X11/Intrinsic.h>
#include <X11/StringDefs.h>
#include <Xm/Xm.h>
#include <Xm/Label.h>
#include <time.h>
#include <varargs.h>
#include <stdio.h>

void update_time(Widget w, XtIntervalId id);
void xs_wprintf();

main(int argc, char *argv[])
{
   Widget          toplevel, clock;
   /* Create the widgets. */
   toplevel = XtInitialize(argv[0], "Clock", NULL, 0, &argc, argv);
   clock = XtCreateManagedWidget("face", 
                                 xmLabelWidgetClass,
                                 toplevel, NULL, 0);
   /* Get the initial time. */
   update_time(clock, 0);
   XtRealizeWidget(toplevel);
   XtMainLoop();
}

void update_time(Widget w, XtIntervalId id)
{
    time_t   tloc, rounded_tloc, next_minute;
   /* Ask System for the time. */
    time(&tloc);
    /* Convert the time to a string and display it,
     * after rounding it down to the last minute. */
    rounded_tloc = tloc / 60 * 60;
    xs_wprintf(w, "%s", ctime(&rounded_tloc));
    /* Adjust the time to reflect the time till the next round minute. */
    next_minute = (60 - tloc % 60) * 1000;
    /* The Intrinsics removes timeouts when they occur,
     * so put ourselves back. */
    XtAddTimeOut((unsigned long)next_minute,
                 (XtTimerCallbackProc)update_time, w);
}

/* xs_wprintf: fprintf-like function for XmLabel widgets
 *         From: The X Window System, Programming and Applications with Xt
 *         OSF/Motif Edition by Douglas Young, Prentice Hall, 1990 */
void xs_wprintf(va_alist)
  va_dcl
{
  Widget    w;
  char     *format;
  va_list   args;
  char      str[1000];  /* DANGER: Fixed buffer size */
  Arg       wargs[1];
  XmString  xmstr;
  /* Init the variable length args list. */
  va_start(args);
  /* Extract the destination widget.
   * Make sure it is a subclass of XmLabel. */
  w = va_arg(args, Widget);
  if(!XtIsSubclass(w, xmLabelWidgetClass))
     XtError("xs_wprintf() requires a Label Widget");
  /* Extract the format to be used. */
  format = va_arg(args, char *);
  /* Use vsprintf to format the string to be displayed in the
   * XmLabel widget, then convert it to a compound string  */
  vsprintf(str, format, args);
  xmstr =  XmStringLtoRCreate(str, XmSTRING_DEFAULT_CHARSET);

  XtSetArg(wargs[0], XmNlabelString, xmstr);
  XtSetValues(w, wargs, 1);     

  va_end(args);
} /* end xs_wprintf */