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

[Eli Zaretskii <eliz@gnu.org>]

> Date:  Wed, 13 Jun 2018 12:34:14 -0700
> From:  Keith David Bershatsky <esq@lawlist.com>
> Cc:  emacs-devel@gnu.org,Davis Herring <herring@lanl.gov>
> 
> 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);
> }

I don't think face->font is guaranteed to be non-NULL.  You need to
check that explicitly, and you need some fallback for when it is NULL,
to serve as a substitute for font->space_width.