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

[Paul Smith <paul@mad-scientist.net>]

On Sat, 2022-05-07 at 20:51 +0100, Jonathan Wakely wrote:
> On Sat, 7 May 2022 at 20:07, Paul Smith <paul@mad-scientist.net>
> wrote:
> GCC's 'make install' should do everything needed. That installs
> $prefix/lib64/libstdc++.so.6.0.30-gdb.py alongside the .so file, and
> gdb auto-loads that when debugging a process linked to the
> libstdc++.so.6.0.30 library. That python script imports the
> register_libstdcxx_printers function and runs it.
> 
> Maybe you're only linking statically to libstdc++.a?

Ah.  Yes I'm linking statically.

> Hmm, that's reminiscent of
> https://sourceware.org/bugzilla/show_bug.cgi?id=25234

I checked and when I first attach I do see:

  (gdb) show lang
  The current source language is "auto; currently c".

and things don't work, then after I change to a C++ frame I see:

  (gdb) show lang
  The current source language is "auto; currently c++".

and things work.

I discovered that if I add:

  set language c++

to my init, that it all works properly.  For my purposes this is a
sufficient workaround.

It's a bit strange (confusing) that the C++ pretty-printers work
without having to do that, but the C++ xmethods do not.

Also just a data point, my previous GDB (10.2) didn't require this:
when I attach with that version, GDB chose the auto language as "c++"
immediately.  I suppose it's worth a bugzilla report.

> All the logic to do that in Python is already present in the
> printers,

I figured so I'd hoped there was something here already.  I get what
you're saying of course.  Maybe I'll find some time to dig into this...
at some point...

> > (b) Ways to access the contents of containers like unique_ptr,
> > shared_ptr, etc. from python functions.  So if in my class I have
> > "std::unique_ptr<Foo> fooPtr" and in my python functions I have a
> > variable "fooPtr" which refers to this object, I would like a way
> > to retrieve a gdb.Value containing its pointer.
> 
> The UniquePtrGetWorker Xmethod already does that. You should be able
> to just do:
> 
>  py ptr = gdb.parse_and_eval('uniqptr.get()')

xmethods don't help me (IIUC) because I'm in the middle of some Python
function and the value I want to retrieve is in a Python variable, not
in a GDB variable, so I can't easily access it with parse_and_eval().

For instance in my examples here I'd have a python method:

  def find_obj(val):
      if val['mgr']['initialized']:
          return val['mgr']
      return val['otherMgr']

or whatever, but of course I can't do this because val['mgr'] is a
std::unique_ptr and I don't know how to dig out the object it points
to.  The above doesn't need to work as-is: something like:

  def find_obj(val):
      mgr = StdUnique(val['mgr']).get()
      if mgr['initialized']:
          return mgr
      return val['otherMgr']

or whatever would be fine.