次のようにすると core を吐きます。

% ./ruby -e '
str = "abc"
width = 1000
pad = Object.new
class << pad; self end.send(:define_method, :to_str) {
  str.replace "x" * 100000
  "p"
}
p str.ljust(width, pad)
'
-e:9: [BUG] Segmentation fault
ruby 1.9.0 (2004-12-29) [i686-linux]

zsh: abort (core dumped)  ./ruby -e 
% gdb ruby core 
GNU gdb 6.1-debian
Copyright 2004 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "i386-linux"...Using host libthread_db library "/lib/libthread_db.so.1".

Core was generated by `./ruby -e 
str = "abc"
width = 1000
pad = Object.new
class << pad; self end.sen'.
Program terminated with signal 6, Aborted.
Reading symbols from /lib/libdl.so.2...done.
Loaded symbols for /lib/libdl.so.2
Reading symbols from /lib/libcrypt.so.1...done.
Loaded symbols for /lib/libcrypt.so.1
Reading symbols from /lib/libm.so.6...done.
Loaded symbols for /lib/libm.so.6
Reading symbols from /lib/libc.so.6...done.
Loaded symbols for /lib/libc.so.6
Reading symbols from /lib/ld-linux.so.2...done.
Loaded symbols for /lib/ld-linux.so.2
#0  0x4009e6b1 in kill () from /lib/libc.so.6
(gdb) bt
#0  0x4009e6b1 in kill () from /lib/libc.so.6
#1  0x4009e435 in raise () from /lib/libc.so.6
#2  0x4009f978 in abort () from /lib/libc.so.6
#3  0x080d59b6 in rb_bug (fmt=0x0) at error.c:214
#4  0x080b2972 in sigsegv (sig=11) at signal.c:446
#5  <signal handler called>
#6  0x400eca1f in memcpy () from /lib/libc.so.6
#7  0x080bc0c8 in rb_str_justify (argc=135653720, argv=0x186a0, str=1075607636, jflag=108 'l') at string.c:4537
#8  0x080bc2e8 in rb_str_ljust (argc=100000, argv=0x186a0, str=100000) at string.c:4579
#9  0x0806ae3a in call_cfunc (func=0x80bc2c0 <rb_str_ljust>, recv=1075607636, len=135721256, argc=16882, argv=0xbfffeaa8)
    at eval.c:5418
#10 0x0805eab2 in rb_call0 (klass=1075668916, recv=1075607636, id=4537, oid=100000, argc=2, argv=0xbfffeaa8, 
    body=0x401d5ef4, nosuper=0) at eval.c:5559
#11 0x0805ed78 in rb_call (klass=1075668916, recv=1075607636, mid=4537, argc=2, argv=0xbfffeaa8, scope=0) at eval.c:5781
#12 0x0805989a in rb_eval (self=1075673536, n=0x186a0) at ruby.h:635
#13 0x08059bcb in rb_eval (self=1075673536, n=0x186a0) at ruby.h:664
#14 0x08055e6d in ruby_exec_internal () at eval.c:1470
#15 0x08055e86 in ruby_exec () at eval.c:1488
#16 0x08055ed0 in ruby_run () at eval.c:1505
#17 0x08053ed5 in main (argc=100000, argv=0x186a0, envp=0xbffff8e4) at main.c:38
(gdb) 
-- 
[田中 哲][たなか あきら][Tanaka Akira]