[lldb-dev] lldb showing wrong type structure for virtual pointer type

Wed Feb 28 02:19:16 PST 2018

On 02/28/2018 11:31 AM, jonas echterhoff via lldb-dev wrote:

> I'm using lldb-900.0.64.
             ^^^^^^^^^^^^^^
             ??????????????
Latest official release is 5.0.1; also there are 6.0.0 (at -rc3, the next release)
and 7.0.0 (a.k.a SVN trunk). What's the 'version' output of your LLDB prompt?

> Unfortunately, I have not yet succeeded in coming up with a small, independent repro case which shows this problem.

IIUC this is it:

struct A {
   int id0;
   A () { id0 = 111; }
   virtual int f (int x) { return x + 1; }
   int g (int x) { return x + 11; }
};

struct B: A {
   int id1;
   B () { id1 = 222; }
   virtual int f (int x) { return x + 2; }
   int g (int x) { return x + 12; }
};

namespace S {
   struct AS {
     int id0;
     AS () { id0 = 333; }
     virtual int f (int x) { return x + 3; }
     int g (int x) { return x + 13; }
   };
   struct B: AS {
     int id1;
     B () { id1 = 444; }
     virtual int f (int x) { return x + 4; }
     int g (int x) { return x + 14; }
   };
}

int main (int argc, char *argv[])
{
   B obj1;
   S::B obj2;
   return
     obj1.f (argc) +
     obj2.f (argc) +
     obj1.g (argc) +
     obj2.g (argc);
}

And in gdb, it is:

$ gdb -q t-class2
Reading symbols from t-class2...done.
(gdb) b S::B::f
Breakpoint 1 at 0x400775: file t-class2.cc, line 25.
(gdb) b S::B::g
Breakpoint 2 at 0x400789: file t-class2.cc, line 26.
(gdb) r
Starting program: /home/dantipov/tmp/t-class2

Breakpoint 1, S::B::f (this=0x7fffffffdb50, x=1) at t-class2.cc:25
25	    virtual int f (int x) { return x + 4; }
(gdb) bt
#0  S::B::f (this=0x7fffffffdb50, x=1) at t-class2.cc:25
#1  0x0000000000400643 in main (argc=1, argv=0x7fffffffdc68) at t-class2.cc:36
(gdb) p this
$1 = (S::B * const) 0x7fffffffdb50
(gdb) p *this
$2 = {<S::AS> = {_vptr.AS = 0x400840 <vtable for S::B+16>, id0 = 333}, id1 = 444}
(gdb) c
Continuing.

Breakpoint 2, S::B::g (this=0x7fffffffdb50, x=1) at t-class2.cc:26
26	    int g (int x) { return x + 14; }
(gdb) bt
#0  S::B::g (this=0x7fffffffdb50, x=1) at t-class2.cc:26
#1  0x0000000000400669 in main (argc=1, argv=0x7fffffffdc68) at t-class2.cc:38
(gdb) p this
$3 = (S::B * const) 0x7fffffffdb50
(gdb) p *this
$4 = {<S::AS> = {_vptr.AS = 0x400840 <vtable for S::B+16>, id0 = 333}, id1 = 444}

E.g. in calls to obj2.f () and obj2.g (), 'this' is 0x7fffffffdb50, and the object
itself is {333, 444}.

With lldb, it is:

$ /home/dantipov/.local/llvm-6.0.0/bin/lldb t-class2
(lldb) target create "t-class2"
Current executable set to 't-class2' (x86_64).
(lldb) breakpoint set -n S::B::f
Breakpoint 1: where = t-class2`S::B::f(int) at t-class2.cc:25, address = 0x000000000040076a
(lldb) breakpoint set -n S::B::g
Breakpoint 2: where = t-class2`S::B::g(int) + 11 at t-class2.cc:26, address = 0x0000000000400789
(lldb) run
Process 5180 launched: '/home/dantipov/tmp/t-class2' (x86_64)
Process 5180 stopped
* thread #1, name = 't-class2', stop reason = breakpoint 1.1
     frame #0: 0x000000000040076a t-class2`S::B::f(this=0x00007fffffffdb50, x=1) at t-class2.cc:25
    22  	  struct B: AS {
    23  	    int id1;
    24  	    B () { id1 = 444; }
-> 25  	    virtual int f (int x) { return x + 4; }
    26  	    int g (int x) { return x + 14; }
    27  	  };
    28  	}
(lldb) bt
* thread #1, name = 't-class2', stop reason = breakpoint 1.1
   * frame #0: 0x000000000040076a t-class2`S::B::f(this=0x00007fffffffdb50, x=1) at t-class2.cc:25
     frame #1: 0x0000000000400643 t-class2`main(argc=1, argv=0x00007fffffffdc58) at t-class2.cc:36
     frame #2: 0x00007ffff712000a libc.so.6`__libc_start_main(main=(t-class2`main at t-class2.cc:31), argc=1, argv=0x00007fffffffdc58, init=<unavailable>, fini=<unavailable>, rtld_fini=<unavailable>, 
stack_end=0x00007fffffffdc48) at libc-start.c:308
     frame #3: 0x000000000040054a t-class2`_start + 42
(lldb) p this
(S::B *) $0 = 0x00007fffffffdb50
(lldb) p *this
(S::B) $1 = {
   S::AS = (id0 = 111)
   id1 = 222
}
(lldb) c
Process 5180 resuming
Process 5180 stopped
* thread #1, name = 't-class2', stop reason = breakpoint 2.1
     frame #0: 0x0000000000400789 t-class2`S::B::g(this=0x00007fffffffdb40, x=1) at t-class2.cc:26
    23  	    int id1;
    24  	    B () { id1 = 444; }
    25  	    virtual int f (int x) { return x + 4; }
-> 26  	    int g (int x) { return x + 14; }
    27  	  };
    28  	}
    29  	
(lldb) bt
* thread #1, name = 't-class2', stop reason = breakpoint 2.1
   * frame #0: 0x0000000000400789 t-class2`S::B::g(this=0x00007fffffffdb40, x=1) at t-class2.cc:26
     frame #1: 0x0000000000400669 t-class2`main(argc=1, argv=0x00007fffffffdc58) at t-class2.cc:38
     frame #2: 0x00007ffff712000a libc.so.6`__libc_start_main(main=(t-class2`main at t-class2.cc:31), argc=1, argv=0x00007fffffffdc58, init=<unavailable>, fini=<unavailable>, rtld_fini=<unavailable>, 
stack_end=0x00007fffffffdc48) at libc-start.c:308
     frame #3: 0x000000000040054a t-class2`_start + 42
(lldb) p this
(S::B *) $2 = 0x00007fffffffdb40
(lldb) p *this
(S::B) $3 = {
   S::AS = (id0 = 333)
   id1 = 444
}

Here 'this' is different between calls to obj2.f () and obj2.g () (0x00007fffffffdb50 vs.
0x00007fffffffdb40), and objects are shown as different as well - {111, 222} vs. {333, 444}.

Dmitry

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