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bug#8227: possibly uninitialized variables in update_window_fringes

From: Paul Eggert
Subject: bug#8227: possibly uninitialized variables in update_window_fringes
Date: Thu, 10 Mar 2011 15:45:19 -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

Severity: minor

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

The following code in the Emacs trunk src/fringe.c's
update_window_fringes function might be using uninitialized

  int top_row_ends_at_zv_p, bot_row_ends_at_zv_p;
  if (top_ind_rn >= 0)
      top_row_ends_at_zv_p = row->ends_at_zv_p;
  for (y = w->vscroll, rn = 0;
       y < yb && rn < nrows;
       y += row->height, rn++)
      if (WINDOW_LEFT_FRINGE_WIDTH (w) == 0)
      else if (row->left_user_fringe_bitmap != NO_FRINGE_BITMAP)
      else if ((!row->reversed_p && row->truncated_on_left_p)
               || (row->reversed_p && row->truncated_on_right_p))
      else if (row->indicate_bob_p && EQ (boundary_top, Qleft))
          left = ((row->indicate_eob_p && EQ (boundary_bot, Qleft))
                  ? LEFT_FRINGE (1, Qtop_bottom, top_row_ends_at_zv_p)
                  : LEFT_FRINGE (2, Qtop, 0));

The last assignment uses top_row_ends_at_zv_p, but it's not clear
from the previous tests that top_row_ends_at_zv_p must be initialized.
There is a similar issue with bot_row_ends_at_zv_p.

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 the two local variables 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 YAMAMOTO Mitsuharu, who committed the code in

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