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

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

On Sun, 2022-05-08 at 09:16 +0100, Jonathan Wakely wrote:
> > gdb.set_convenience_variable('mgr', val['mgr'])
> > init = gdb.parse_and_eval('$mgr->initialized')
> > 
> > This will use the xmethod to evaluate the expression.
> 
> And then:
> 
> if init:
>     return gdb.parse_and_eval('*$mgr')

Unfortunately, this doesn't work :(.  I can't do it from the GDB
command line or python (I have tried both with the same results). 
Something about convenience variables doesn't play well with xmethods 
(or maybe this xmethod implementation specifically?)

Very simple test program:

  /* /tmp/sleep.cpp */
  #include <unistd.h>
  #include <memory>

  class Foo { public: int val = 0; };

  std::unique_ptr<Foo> foo;

  int main()
  {
      foo = std::make_unique<Foo>();
      sleep(100000);
      return 0;
  }

Compile it & run it:

  g++ -ggdb3 -o /tmp/sleep /tmp/sleep.cpp
  /tmp/sleep &

Now attach with GDB (probably you can just run it in GDB but I did it
like this so I could detach/reattach):

  gdb -p <pid>

Switch to main, set an convenience variable to "foo", then try to use
xmethods on it and no joy:

  (gdb) fr 3
  11          sleep(100000);

  (gdb) p foo
  $1 = std::unique_ptr<Foo> = {
    get() = 0x105aeb0
  }
  (gdb) p foo->val
  $2 = 0

  (gdb) set $xx = foo
  (gdb) p $xx
  $3 = std::unique_ptr<Foo> = {
    get() = 0x105aeb0
  }

  (gdb) p $xx->val
  Attempt to take address of value not located in memory.

Same behavior with GDB 10.2 and 12.1.