SIGSEGV

[Alex Schroeder]

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In GNU Emacs 21.2.92.1 (i686-pc-linux-gnu, X toolkit, Xaw3d scroll bars)
 of 2002-11-22 on confusibombus
Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: C
  locale-coding-system: nil
  default-enable-multibyte-characters: t

Please describe exactly what actions triggered the bug
and the precise symptoms of the bug:

I was using Emacs and the Emacs IRC Client (not yet included in
Emacs), and I was exchanging some Japanese text with other people on
the net.  I run Emacs under GDB.  So suddenly, I cannot remember what
I did exactly, Emacs is dead and in GDB I see this:

Program received signal SIGSEGV, Segmentation fault.
0x0805040d in increment_row_positions (row=0x9763950, delta=1, delta_bytes=1) 
at dispnew.c:1188
1188          if (BUFFERP (row->glyphs[area][i].object)
(gdb) 

Unfortunately, I know nothing of the display engine...

(gdb) l
1183      MATRIX_ROW_END_BYTEPOS (row) += delta_bytes;
1184
1185      /* Increment positions in glyphs.  */
1186      for (area = 0; area < LAST_AREA; ++area)
1187        for (i = 0; i < row->used[area]; ++i)
1188          if (BUFFERP (row->glyphs[area][i].object)
1189              && row->glyphs[area][i].charpos > 0)
1190            row->glyphs[area][i].charpos += delta;

So I skimmed through etc/DEBUG and found this:

    Note: It is not a good idea to try `pr' if you know that Emacs is
    in deep trouble: its stack smashed (e.g., if it encountered
    SIGSEGV due to stack overflow), or crucial data structures, such
    as `obarray', corrupted, etc.  In such cases, the Emacs subroutine
    called by `pr' might make more damage, like overwrite some data
    that is important for debugging the original problem.

Now, I did get a SIGSEGV, but I am not sure this means that I should
refrain from using pr.  Am I in "deep" trouble?  :)

Anyway, let me examine this loop, first.

(gdb) p area
$1 = 4159400
(gdb) p i
$2 = 150

Ok, then I tried to find the value for LAST_AREA.  I did not find it
with a grep through the sources, but I did not look too hard.  Then a
silly test:

(gdb) p LAST_AREA
$3 = LAST_AREA

Ok, time to check the loop boundaries:

(gdb) p row->used[area]
Cannot access memory at address 0x9f528b0

Perhaps this area value is too high?

The parameters:

(gdb) p row
$4 = (struct glyph_row *) 0x9763950
(gdb) p delta
$5 = 1
(gdb) p delta_bytes
$6 = 4

Anyway, I do not feel like keeping this gdb + emacs running forever,
since the computer stands in the same room where I sleep...  What can
I do now?

I am also on #emacs at irc.freenode.org as kensanata.  ;)

Alex.