Luca's meaningless thoughts  

Debugging C++ with less pain

by Leandro Lucarella on 2010- 05- 14 23:52 (updated on 2010- 05- 14 23:52)
tagged c++, debug, en, gdb, python, stl - with 0 comment(s)

It turns out GDB 7.0+ can be extended through Python scripts, for instance, to add pretty-printers. And it turns out GCC 4.5 comes with some good pretty-printers for GDB.

Do you want to see the result of that combination?

$ cat -n p.cpp
     1
     2  #include <string>
     3  #include <vector>
     4  #include <map>
     5
     6  int main()
     7  {
     8          std::string s = "hello world";
     9          std::vector<std::string> v;
    10          v.push_back(s);
    11          v.push_back("nice");
    12          std::map<std::string, std::vector<std::string> > m;
    13          m[s] = v;
    14          v.push_back("yeah");
    15          m["lala"] = v;
    16          return 1;
    17  }
    18
$ g++ -g -o p p.cpp
$ gdb -q ./p
(gdb) break 16
Breakpoint 1 at 0x400f86: file p.cpp, line 16.
(gdb) run
Starting program: /tmp/p

Breakpoint 1, main () at p.cpp:16
16              return 1;
(gdb) print m
$1 = std::map with 2 elements = {
  ["hello world"] = std::vector of length 2, capacity 2 = {"hello world", "nice"},
  ["lala"] = std::vector of length 3, capacity 3 = {"hello world", "nice", "yeah"}
}
(gdb)

Nice, ugh?

The only missing step is configuration, because most distribution don't do the integration themselves yet (or don't have packages with the scripts).

Here are 3 quick steps to make it all work:

$ mkdir ~/.gdb # can be stored anywhere really
$ svn co svn://gcc.gnu.org/svn/gcc/trunk/libstdc++-v3/python ~/.gdb/python
$ cat << EOT > ~/.gdbinit
python
import sys
sys.path.insert(0, '/home/$HOME/.gdb/python')
from libstdcxx.v6.printers import register_libstdcxx_printers
register_libstdcxx_printers (None)
end
EOT

That's it!

If like to suffer once in a while you can get the raw values using /r:

(gdb) print /r m
$2 = {_M_t = {
    _M_impl = {<std::allocator<std::_Rb_tree_node<std::pair<std::basic_string<char, std::char_traits<char>,
std::allocator<char> > const, std::vector<std::basic_string<char, std::char_traits<char>, std::allocator<char> >,
std::allocator<std::basic_string<char, std::char_traits<char>, std::allocator<char> > > > > > >> =
{<__gnu_cxx::new_allocator<std::_Rb_tree_node<std::pair<std::basic_string<char, std::char_traits<char>, std::allocator<char>
> const, std::vector<std::basic_string<char, std::char_traits<char>, std::allocator<char> >,
std::allocator<std::basic_string<char, std::char_traits<char>, std::allocator<char> > > > > > >> = {<No data fields>}, <No
data fields>},
      _M_key_compare = {<std::binary_function<std::basic_string<char, std::char_traits<char>, std::allocator<char> >,
std::basic_string<char, std::char_traits<char>, std::allocator<char> >, bool>> = {<No data fields>}, <No data fields>},
_M_header = {
        _M_color = std::_S_red, _M_parent = 0x6070b0, _M_left = 0x6070b0,
        _M_right = 0x607190}, _M_node_count = 2}}}

Looks more familiar? I guess you won't miss it! =P