GNU bug report logs - #41125
28.0.50; Fwindow_text_pixel_size uses FETCH_CHAR (charpos)

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Pip Cet <pipcet <at> gmail.com>
Subject: bug#41125: closed (Re: bug#41125: 28.0.50; Fwindow_text_pixel_size
 uses FETCH_CHAR (charpos))
Date: Fri, 08 May 2020 10:39:01 +0000

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Your bug report

#41125: 28.0.50; Fwindow_text_pixel_size uses FETCH_CHAR (charpos)

which was filed against the emacs package, has been closed.

The explanation is attached below, along with your original report.
If you require more details, please reply to 41125 <at> debbugs.gnu.org.

-- 
41125: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=41125
GNU Bug Tracking System
Contact help-debbugs <at> gnu.org with problems

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From: Eli Zaretskii <eliz <at> gnu.org>
To: martin rudalics <rudalics <at> gmx.at>
Cc: 41125-done <at> debbugs.gnu.org, pipcet <at> gmail.com
Subject: Re: bug#41125: 28.0.50; Fwindow_text_pixel_size uses FETCH_CHAR
 (charpos)
Date: Fri, 08 May 2020 13:37:42 +0300

> Cc: 41125 <at> debbugs.gnu.org
> From: martin rudalics <rudalics <at> gmx.at>
> Date: Fri, 8 May 2020 09:18:27 +0200
> 
>  >> which cannot possibly be correct: FETCH_CHAR takes a byte position,
>  >> not a character position, but BEGV and ZV are measured in characters.
>  >
>  > Ouch!  Thanks.  Does the patch below look good?
> 
> It does.  Thanks for fixing yet another of my blunders in this function.
> And thanks to Pip for catching it.

Thanks, I pushed the fix to the emacs-27 branch.

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From: Pip Cet <pipcet <at> gmail.com>
To: bug-gnu-emacs <at> gnu.org
Subject: 28.0.50; Fwindow_text_pixel_size uses FETCH_CHAR (charpos)
Date: Thu, 7 May 2020 11:55:07 +0000

Fwindow_text_pixel_size contains this code:

      start = pos = BEGV;
      while ((pos++ < ZV) && (c = FETCH_CHAR (pos))
         && (c == ' ' || c == '\t' || c == '\n' || c == '\r'))
    start = pos;
      while ((pos-- > BEGV) && (c = FETCH_CHAR (pos)) && (c == ' ' ||
c == '\t'))
    start = pos;

which cannot possibly be correct: FETCH_CHAR takes a byte position,
not a character position, but BEGV and ZV are measured in characters.

(I'm familiarizing myself with the xdisp.c code, partly by replacing
bytepos/charpos pairs of variables with a combined pos_t type
variable. I think that's a good idea partly because it would prevent
precisely this kind of bug.)

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