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

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From: Pip Cet <pipcet <at> gmail.com>
To: 41125 <at> debbugs.gnu.org
Subject: bug#41125: 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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