bug#9990: valgrind warning in add_row_entry

[Dan Nicolaescu <dann@gnu.org>]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: Dan Nicolaescu <dann@gnu.org>
>> Date: Tue, 08 Nov 2011 09:27:06 -0500
>> 
>> 
>> valgrind ./temacs -Q gets this warning:
>> 
>> ==7487== Use of uninitialised value of size 8
>> ==7487==    at 0x4140F4: update_window (dispnew.c:4212)
>> ==7487==    by 0x414F32: update_window_tree (dispnew.c:3326)
>> ==7487==    by 0x414F0E: update_window_tree (dispnew.c:3324)
>> ==7487==    by 0x4181FD: update_frame (dispnew.c:3253)
>> ==7487==    by 0x443EDB: redisplay_internal (xdisp.c:13175)
>> ==7487==    by 0x4F6F47: read_char (keyboard.c:2443)
>> ==7487==    by 0x4F9406: read_key_sequence.constprop.14 (keyboard.c:9290)
>> ==7487==    by 0x4FB0D4: command_loop_1 (keyboard.c:1447)
>> ==7487==    by 0x560015: internal_condition_case (eval.c:1499)
>> ==7487==    by 0x4EE4AD: command_loop_2 (keyboard.c:1158)
>> ==7487==    by 0x55FEF7: internal_catch (eval.c:1256)
>> ==7487==    by 0x4EFA36: recursive_edit_1 (keyboard.c:1137)
>> ==7487== 
>> ==7487== 
>> ==7487== ---- Attach to debugger ? --- [Return/N/n/Y/y/C/c] ---- Y
>> 
>> The line in question is:
>> 
>> 4212      entry = row_table[i];
>> 
>> 
>> (gdb) p i
>> $1 = 0x157
>> (gdb) p row_table[i]
>> $2 = (struct row_entry *) 0x0
>> (gdb) p row_table_size
>> $3 = 0x193
>> 
>> Is it possible for the contents of row_table to be uninitialized?  Is this warning a false positive?
>
> row_table and row_table_size are static variables.  So at least in
> temacs they should be initialized to zero, by this code in
> scrolling_window:


>
>   n = desired_matrix->nrows;
>   n += current_matrix->nrows;
>   if (row_table_size < 3 * n)
>     {
>       ptrdiff_t size = next_almost_prime (3 * n);
>       row_table = xnrealloc (row_table, size, sizeof *row_table);
>       row_table_size = size;
>       memset (row_table, 0, size * sizeof *row_table);
>     }
>
> Because row_table_size is initially zero, the first call to
> scrolling_window will allocate row_table[] and zero it out.
>
> The only call to add_row_entry, the function where line 4212 belongs,
> is in the same scrolling_window, a few lines _below_ the above
> fragment that allocates and zeroes out row_table[].
>
> So I don't see how row_table[i] could be uninitialized for any i that
> is less than row_table_size.
>
> Does valgrind know that row_table_size is initially zero because it is
> static?

I think it should.

I got another (maybe) similar one.  For this one I had the option that
shows the location of uninitialized variable.  This happened after doing C-h H.

==4752== Conditional jump or move depends on uninitialised value(s)
==4752==    at 0x4137ED: update_window (dispnew.c:1276)
==4752==    by 0x414F02: update_window_tree (dispnew.c:3326)
==4752==    by 0x4181CD: update_frame (dispnew.c:3253)
==4752==    by 0x440E7B: redisplay_internal (xdisp.c:13175)
==4752==    by 0x4F0A87: read_char (keyboard.c:2443)
==4752==    by 0x4F2F46: read_key_sequence.constprop.14 (keyboard.c:9290)
==4752==    by 0x4F4C14: command_loop_1 (keyboard.c:1447)
==4752==    by 0x559B55: internal_condition_case (eval.c:1499)
==4752==    by 0x4E7FED: command_loop_2 (keyboard.c:1158)
==4752==    by 0x559A37: internal_catch (eval.c:1256)
==4752==    by 0x4E94EE: recursive_edit_1 (keyboard.c:1123)
==4752==    by 0x515CFB: read_minibuf (minibuf.c:677)
==4752==  Uninitialised value was created by a heap allocation
==4752==    at 0x4A0649D: malloc (vg_replace_malloc.c:236)
==4752==    by 0x5407CF: xrealloc (alloc.c:742)
==4752==    by 0x411001: adjust_glyph_matrix (dispnew.c:580)
==4752==    by 0x41148C: allocate_matrices_for_window_redisplay (dispnew.c:1838)
==4752==    by 0x4119DC: adjust_frame_glyphs (dispnew.c:2167)
==4752==    by 0x416BC9: adjust_glyphs (dispnew.c:1860)
==4752==    by 0x4686A7: Fdelete_other_windows_internal (window.c:2809)
==4752==    by 0x55B9FB: Ffuncall (eval.c:2977)
==4752==    by 0x593BE5: exec_byte_code (bytecode.c:785)
==4752==    by 0x55AE2A: eval_sub (eval.c:2328)
==4752==    by 0x559A37: internal_catch (eval.c:1256)
==4752==    by 0x594567: exec_byte_code (bytecode.c:966)
==4752== 


> and see what are the values of first_old, last_old, first_new, and
> last_new here, and whether the corresponding glyph rows look
> reasonable, including their hash values?  Or maybe just look at the
> row passed to add_row_entry.  You can display a given glyph_row
> structure with the pgrowx command in GDB (but it won't show the hash
> value, only how the row will look on the screen).  Another command is
> prowx.

I will do this when it happens again.