XLib application not redrawing unless resized

Question

So, I have an application in C++ that uses XLib. In it, i access the date and time using the ctime library, and in the Expose event, create a string from that and put it in the window, centered. My problem is that, The time only updates when it is resized, so for instance, the seconds do not change unless I continually resize the window. Here is the code:

#include <stdio.h>
#include <cstdio>
#include <stdlib.h>
#include <string.h>
#include <X11/Xlib.h>
#include <ctime>

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                    contents[80] = "                                                                               ";
  time_t                  rawtime;
  struct tm               *timeinfo;
  int                     contents_length = strlen(contents);

  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, 500, 50, 5, depth,
      InputOutput, visual, CWBackPixel,
      &frame_attributes);
  XStoreName(display, frame_window, "Fly Taskbar");
  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:
        {
          XClearWindow(display, frame_window);
          // Upkeep of interface usefulness.
          Window returned_root, returned_parent;
          Window* top_level_windows;
          unsigned int nwin = 0;
          XQueryTree(
              display,
              XDefaultRootWindow(display),
              &returned_root,
              &returned_parent,
              &top_level_windows,
              &nwin);

          time(&rawtime);
          timeinfo = localtime(&rawtime);
          strftime(contents, 80, "%a %d %b, %T %p", timeinfo);

          sprintf(contents, "%s [%d]", contents, nwin);


          XWindowAttributes window_attributes;
          int font_direction, font_ascent, font_descent;
          XCharStruct text_structure;
          XTextExtents(fontinfo, contents, contents_length,
              &font_direction, &font_ascent, &font_descent,
              &text_structure);
          XGetWindowAttributes(display, frame_window, &window_attributes);
          text_x = (window_attributes.width - text_structure.width) + 5;
          text_y = (window_attributes.height -
              (text_structure.ascent+text_structure.descent))/2;
          XDrawString(display, frame_window, graphical_context,
              text_x, text_y, contents, contents_length);
          break;
        }
      default:
        break;
    }
  }
  return 0;
}

In case any of you are wondering what purpose this is for, I am writing a window manager (I've actually gotten really far, like you can open apps, move, and resize them) and this is a tool I need for debugging purposes. Repo here

Answer 1

You need some event loop, e.g. calling poll(2) and handle delays. You should also call XFlush or XSync (because Xlib is buffering requests).

You might consider sending yourself some Expose event....

And you should study the source code of xclock