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bug#8215: possibly uninitialized variable lower_xoff in produce_glyphles

From: Paul Eggert
Subject: bug#8215: possibly uninitialized variable lower_xoff in produce_glyphless_glyph
Date: Wed, 09 Mar 2011 14:00:12 -0800
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20101209 Fedora/3.1.7-0.35.b3pre.fc14 Thunderbird/3.1.7

I found this problem by compiling Emacs with GCC's -Wuninitialized flag.

The following code in the Emacs trunk src/xdisp.c's
produce_glyphless_glyph function might be using an uninitialized

      if (base_width >= width)
          /* Align the upper to the left, the lower to the right.  */
          it->pixel_width = base_width;
          lower_xoff = base_width - 2 - metrics_lower.width;
          /* Center the shorter one.  */
          it->pixel_width = width;
          if (metrics_upper.width >= metrics_lower.width)
            lower_xoff = (width - metrics_lower.width) / 2;
            upper_xoff = (width - metrics_upper.width) / 2;
  if (it->glyph_row)
    append_glyphless_glyph (it, face_id, for_no_font, len,
                            upper_xoff, upper_yoff,
                            lower_xoff, lower_yoff);

The last call uses lower_xoff, but the last "else" does not initialize
lower_xoff.  The bug cannot occur if it->glyph_row is NULL, but I
don't see why that would necessarily be.  So I'm filing a bug report
so that someone who is more expert in this code can take a look at it.
In the meantime, I plan to work around the problem by initializing
lower_xoff to 0, with a FIXME explaining the situation: this shouldn't
introduce a bug, because at worst it will replace undefined behavior
with defined behavior.

I'm CC'ing this to Kenichi Handa, who committed the code in question.

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