[maemo-developers] Help debugging segfaults

[From: Eero Tamminen]

Hi,

ext Simon Pickering wrote:
> I don't have much experience debugging segfaults and would appreciate 
> any pointers. I've compiled giac/xcas for the N800, but get a segfault 
> when I try to run it (icas is the command line version).
> 
> The gdb output is as follows:
> 
> ------------------------------------------------------------------------------- 
> 
> Nokia-N800-51:/home/user# gdb ./icas
> GNU gdb 6.4
> Copyright 2005 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 "arm-linux-gnueabi"...Using host libthread_db 
> library "/lib/libthread_db.so.1".
> 
> (gdb) r
> Starting program: /home/user/icas
> 
> Program received signal SIGSEGV, Segmentation fault.
> 0x41851380 in std::string::compare () from /usr/lib/libstdc++.so.6
> (gdb) bt
> #0  0x41851380 in std::string::compare () from /usr/lib/libstdc++.so.6
> #1  0x0005fa28 in std::operator< <char, std::char_traits<char>, 
> std::allocator<char> > (__lhs=@0x10, __rhs=@0xbe9d5538) at 
> basic_string.h:2149
> #2  0x401e1260 in std::less<std::string>::operator() () from 
> /usr/lib/libgiac.so.0
> #3  0x401e17fc in std::_Rb_tree<std::string, std::pair<std::string 
> const, giac::gen>, std::_Select1st<std::pair<std::string const, 
> giac::gen> >, std::less<std::string>, 
> std::allocator<std::pair<std::string const, giac::gen> > 
>  >::insert_unique () from /usr/lib/libgiac.so.0
> #4  0x401e1378 in std::map<std::string, giac::gen, 
> std::less<std::string>, std::allocator<std::pair<std::string const, 
> giac::gen> > >::insert ()
>    from /usr/lib/libgiac.so.0
> #5  0x401e0cc4 in std::map<std::string, giac::gen, 
> std::less<std::string>, std::allocator<std::pair<std::string const, 
> giac::gen> > >::operator[] ()
>    from /usr/lib/libgiac.so.0
> #6  0x4053a6f4 in giac::lexer_functions_register () from 
> /usr/lib/libgiac.so.0
> #7  0x404b1354 in giac::unary_function_ptr::unary_function_ptr () from 
> /usr/lib/libgiac.so.0
> #8  0x40135a7c in giac::_resultant () from /usr/lib/libgiac.so.0
> #9  0x40136e88 in giac::_resultant () from /usr/lib/libgiac.so.0
> #10 0x4100b28c in _dl_rtld_di_serinfo () from /lib/ld-linux.so.3
> #11 0x00026980 in ?? ()
> #12 0x00026980 in ?? ()
> Previous frame identical to this frame (corrupt stack?)
> (gdb) q
> The program is running.  Exit anyway? (y or n) y
> ------------------------------------------------------------------------------- 

To get working backtrace on ARM you need to have either:
- debug symbols, or
- code compiled with -fno-omit-frame-pointer
for *every* binary/library you see in the backtrace, otherwise
Gdb reports bogus information for the backtrace.  In this case
the libs would be:
- /usr/lib/libstdc++.so.6
- /usr/lib/libgiac.so.0
- /lib/ld-linux.so.3 (I think this is wrong, but in general you
   almost always need at least minimal debug syms on C-library)

The reason is that without either debug symbols or framepointer Gdb
doesn't know how many args to skip when unwinding the stack. Even
the first function it shows is often wrong, because without debug
symbols Gdb can try to match it only to the nearest _exported_
function and the function might have been a static (non-exported)
one.


	- Eero