Re: Help using the GDB C++ STL pretty-printers / xmethods

[Hannes Domani <ssbssa@yahoo.de>]

 Am Samstag, 7. Mai 2022, 03:23:46 MESZ hat Paul Smith <paul@mad-scientist.net> Folgendes geschrieben:

> Are there any docs or other information about how to use the GDB
> pretty-printers for C++ STL that come with GCC?
>
> I have them installed and they work for displaying data, but they don't
> work for accessing data.
>
> Just as one very simple example, is there a way to dereference a
> unique_ptr?  I see xmethods defined but nothing I've tried works.
> Suppose I have:
>
>   (gdb) p $mp->mgr
>   $6 = std::unique_ptr<class Mgr> = {
>     get() = 0x7f519a24e000
>   }
>
> (so you can see the pretty-printers are installed).  Now, how do I
> extract out this object pointer so I can see what's in it?  These don't
> work:
>
>   (gdb) p *$mp->mgr
>   One of the arguments you tried to pass to operator* could not be
>   converted to what the function wants.
>
>   (gdb) p $mp->mgr->initialized
>   One of the arguments you tried to pass to operator-> could not be
>   converted to what the function wants.
>
> It used to work in GCC 10.2 / GDB 10.1 to access the pointer directly
> if you knew, or deciphered, the internal structor of unique_ptr:
>
>   (gdb) p $mp->mgr._M_t._M_t._M_head_impl
>
> However, with GCC 11.3 / GDB 12.1 this no longer works: I get this
> error:
>
>   Request for member '_M_head_impl' is ambiguous in type
> 'std::tuple<Mgr*, std::default_delete<Mgr> >'.
>   Candidates are:
>     'std::default_delete<Mgr> std::_Head_base<1ul,
> std::default_delete<Mgr>, true>::_M_head_impl' (std::tuple<Mgr*,
> std::default_delete<Mgr> > -> std::_Tuple_impl<0ul, Mgr*,
> std::default_delete<Mgr> > -> std::_Tuple_impl<1ul,
> std::default_delete<Mgr> > -> std::_Head_base<1ul,
> std::default_delete<Mgr>, true>)
>     '<unnamed type> std::_Head_base<0ul, Mgr*, false>::_M_head_impl'
> (std::tuple<Mgr*, std::default_delete<Mgr> > -> std::_Tuple_impl<0ul,
> Mgr*, std::default_delete<Mgr> > -> std::_Head_base<0ul, Mgr*, false>)
>
> I have found no way to resolve this ambiguity to GDB's satisfaction.
>
> The only thing I've found that works is just to access the pointer
> value directly by cutting and pasting it with a cast:
>
>   (gdb) p *((Mgr*)0x7f519a24e000)
>   $8 = {
>     ...
>     initialized = true
>   }
>
>   (gdb) p ((Mgr*)0x7f519a24e000)->initialized
>   $9 = true
>
> Is that really what we have to do?

I'm assuming you installed the pretty printers with a call to
register_libstdcxx_printers() somewhere.
For access to C++ STL objects there is another set of gdb helpers,
called xmethods, which you need to install with a call to
register_libstdcxx_xmethods().


> Secondly, is there some interface that is defined by the libstdcxx.v6
> Python macros for GDB that people who are doing their own python
> scripting for their own C++ programs can take advantage of to avoid too
> much groveling through the depths of the C++ STL implementation?

Depends on what you want to do with it, but you can get access to the pretty
printers in gdb with a call to gdb.default_visualizer() [1].

[1] https://sourceware.org/gdb/onlinedocs/gdb/Pretty-Printing-API.html#index-gdb_002edefault_005fvisualizer


Hannes