"print object on" and MI interface

"print object on" and MI interface

[André Pönitz]

Hi all.

I have a problem with accessing the dynamic type of a variable using
gdb's MI interface.

Given the following code

  struct base { virtual ~base() {}; int x; };

  struct derived : public base { };

  int main()
  {
      base *b = new derived;
      b->x = 1;
  }


I get using  "gdb -i mi" the following session (skipping the prompts
to improve readability)

~"GNU gdb 6.8-debian\n"
~"Copyright (C) 2008 Free Software Foundation, Inc.\n"
~"License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>\n"
~"This is free software: you are free to change and redistribute it.\n"
~"There is NO WARRANTY, to the extent permitted by law.  Type \"show copying\"\n"
~"and \"show warranty\" for details.\n"
~"This GDB was configured as \"x86_64-linux-gnu\"...\n"

set print object on
^done

break main.cpp:10
~"Breakpoint 1 at 0x4008ca: file /tmp/p/main.cpp, line 10.\n"
^done

run
~"Starting program: /tmp/p/p \n"
~"[Thread debugging using libthread_db enabled]\n"
~"[New Thread 0x2ac921fdcf20 (LWP 32536)]\n"
~"[Switching to Thread 0x2ac921fdcf20 (LWP 32536)]\n"
~"\n"
~"Breakpoint 1, main () at /tmp/p/main.cpp:10\n"
~"10\t}\n"
^done

p b
~"$1 = (derived *) 0x602780\n"
^done

-var-create b * b
^done,name="b",numchild="1",value="0x602780",type="base *"

-var-info-type b
^done,type="base *"

-symbol-type b
^error,msg="Undefined mi command: symbol-type (missing implementation)"


So the CLI seems to know the dynamic type of 'b', namely 'derived'. The MI
interface only seems to provide the static type, or warns about a missing
implementation. The "-symbol-type" command is documented at
http://sourceware.org/gdb/current/onlinedocs/gdb_25.html, though.

What could I do to convince the MI interface to give me the dynamic type, too?

Thank you for your help in advance,
André 

Re: "print object on" and MI interface

[Vladimir Prus]

André Pönitz wrote:

> 
> Hi all.
> 
> I have a problem with accessing the dynamic type of a variable using
> gdb's MI interface.
> 
> Given the following code
> 
>   struct base { virtual ~base() {}; int x; };
> 
>   struct derived : public base { };
> 
>   int main()
>   {
>       base *b = new derived;
>       b->x = 1;
>   }
> 
> 
> I get using  "gdb -i mi" the following session (skipping the prompts
> to improve readability)
> 
> ~"GNU gdb 6.8-debian\n"
> ~"Copyright (C) 2008 Free Software Foundation, Inc.\n"
> ~"License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>\n"
> ~"This is free software: you are free to change and redistribute it.\n"
> ~"There is NO WARRANTY, to the extent permitted by law.  Type \"show copying\"\n"
> ~"and \"show warranty\" for details.\n"
> ~"This GDB was configured as \"x86_64-linux-gnu\"...\n"
> 
> set print object on
> ^done
> 
> break main.cpp:10
> ~"Breakpoint 1 at 0x4008ca: file /tmp/p/main.cpp, line 10.\n"
> ^done
> 
> run
> ~"Starting program: /tmp/p/p \n"
> ~"[Thread debugging using libthread_db enabled]\n"
> ~"[New Thread 0x2ac921fdcf20 (LWP 32536)]\n"
> ~"[Switching to Thread 0x2ac921fdcf20 (LWP 32536)]\n"
> ~"\n"
> ~"Breakpoint 1, main () at /tmp/p/main.cpp:10\n"
> ~"10\t}\n"
> ^done
> 
> p b
> ~"$1 = (derived *) 0x602780\n"
> ^done
> 
> -var-create b * b
> ^done,name="b",numchild="1",value="0x602780",type="base *"
> 
> -var-info-type b
> ^done,type="base *"
> 
> -symbol-type b
> ^error,msg="Undefined mi command: symbol-type (missing implementation)"
> 
> 
> So the CLI seems to know the dynamic type of 'b', namely 'derived'. The MI
> interface only seems to provide the static type, or warns about a missing
> implementation. The "-symbol-type" command is documented at
> http://sourceware.org/gdb/current/onlinedocs/gdb_25.html, though.
> 
> What could I do to convince the MI interface to give me the dynamic type, too?

Nothing, it's not implemented yet. It's one of two high-priority issues in my
queue, however.

- Volodya