--- Begin Message ---
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Subject: |
possibly uninitialized variables in update_window_fringes |
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Date: |
Thu, 10 Mar 2011 15:45:19 -0800 |
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User-agent: |
Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.13) 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
variables:
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
question.
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--- Begin Message ---
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Subject: |
Re: possibly uninitialized variables in update_window_fringes |
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Date: |
Thu, 10 Mar 2011 22:25:22 -0800 |
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User-agent: |
Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.2.14) Gecko/20110223 Thunderbird/3.1.8 |
On 03/10/2011 05:31 PM, YAMAMOTO Mitsuharu wrote:
> No problem. top_ind_rn is set to a non-negative value whenever
> row->indicate_bob_p is set.
Thanks for explaining that. I plan to add the following comment
to the code, just before the declarations of
top_row_ends_at_zv_p and bot_row_ends_at_zv_p:
/* top_ind_rn is set to a nonnegative value whenver
row->indicate_bob_p is set, so it's OK that top_row_ends_at_zv_p
is not initialized here. Similarly for bot_ind_rn,
row->indicate_eob_p and bot_row_ends_at_zv_p. */
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