Re: Printing to STDERR when %d is "Cannot access memory ..."

[Keith David Bershatsky]

Thank you, Eli, for taking a look at this thread.  The suggestion by Davis to 
deal with the STDERR message avoids the crash.

My STDERR printout (with the NULL font now being assigned a 0 value) is as 
follows:

    it->c (120021)
    char (ð)
    w->hscroll (0)
    hscl (false)
    it->hpos (5)
    it->vpos (0)
    it->current_x (55)
    new_x (55)
    font->space_width (0)
    it->lnum_pixel_width (55)
    it->lnum_width (3)
    it->first_visible_x (0)
    relative_x (55)
    it->face_id (38)
    foreground_face (magenta)
    background_face (black)
    rc (MOVE_X_REACHED)

I create the font pointer like this ....  It has worked for everything so far 
except this special character (U0001D4D5) on Emacs built --with-x.  I believe 
the face_id is correct in this example because the foreground (magenta) and 
background (black) are correctly returned.

  struct face *face = mc_lookup_face (it);
  struct font *font = face->font;
  Lisp_Object foreground_face = face->lface[LFACE_FOREGROUND_INDEX];
  Lisp_Object background_face = face->lface[LFACE_BACKGROUND_INDEX];

... using the following function:

struct face *
mc_lookup_face (struct it *it)
{
  if (!NILP (Vface_remapping_alist)
      && (it->face_id == DEFAULT_FACE_ID /* Qdefault */
          || it->face_id == MODE_LINE_FACE_ID /* Qmode_line */
          || it->face_id == MODE_LINE_INACTIVE_FACE_ID /* Qmode_line_inactive */
          || it->face_id == HEADER_LINE_FACE_ID /* Qheader_line */
          || it->face_id == TOOL_BAR_FACE_ID /* Qtool_bar */
          || it->face_id == FRINGE_FACE_ID /* Qfringe */
          || it->face_id == SCROLL_BAR_FACE_ID /* Qscroll_bar */
          || it->face_id == BORDER_FACE_ID /* Qborder */
          || it->face_id == CURSOR_FACE_ID /* Qcursor */
          || it->face_id == MOUSE_FACE_ID /* Qmouse */
          || it->face_id == MENU_FACE_ID /* Qmenu */
          || it->face_id == WINDOW_DIVIDER_FACE_ID /* Qwindow_divider */
          || it->face_id == WINDOW_DIVIDER_FIRST_PIXEL_FACE_ID /* 
Qwindow_divider_first_pixel */
          || it->face_id == WINDOW_DIVIDER_LAST_PIXEL_FACE_ID /* 
Qwindow_divider_last_pixel */
          || it->face_id == VERTICAL_BORDER_FACE_ID /* Qvertical_border */
          || it->face_id == INTERNAL_BORDER_FACE_ID)) /* Qinternal_border */
    return FACE_FROM_ID (it->f, lookup_basic_face (it->f, it->face_id));
      else
        return FACE_FROM_ID (it->f, it->face_id);
}

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DATE:  [06-13-2018 12:11:33] <13 Jun 2018 22:11:33 +0300>
FROM:  Eli Zaretskii <address@hidden>
> 
> > Date: Wed, 13 Jun 2018 11:37:09 -0700
> > From: Keith David Bershatsky <address@hidden>
> >
> > I am working on crosshairs (17684) and multiple fake cursors (22873) and 
> > created a little problem for myself in that a character not recognized by 
> > the font (U0001D4D5) [aka a pretty letter f] on a line all by itself does 
> > not have a font->space_width and causes Emacs to crash when printing said 
> > value to STDERR.
> >
> > GDB returns "Cannot access memory at address 0xa0" when printing the value 
> > of font->space_width.
> 
> This message means that 'font' is a NULL pointer, and that is your
> basic problem.  You need to find out why 'font' is NULL.