uninitialized memory, pretty printers and full backtraces

[Phil Muldoon <pmuldoon at redhat dot com>]

I was working on another pretty printing task, when I noted the 
behaviour below. I'm not sure how to fix it. Thought it worth discussing 
here.

The basic premise is that in a "bt full" condition, the pretty printers 
appear to try and grok (and pretty print) all local variables in a frame 
- even if they are not initialized at the time the "bt full" was 
requested.  I believe this is not so different from existing GDB 
behaviour.  But with the Python pretty printers, the problem  is 
magnified as the printers will attempt to read, and interpret assorted 
random memory addresses in an uninitialized class.

Take a simple C++ program:

#include <string>
#include <iostream>

using namespace std;

class foo
{
 std::string message;

public:
 foo (const char *c) :  message (c)
 {
 }

 void print()
 {
   cout << message << endl;
 }
};

main ()
{
 int i;
 i++;
 i++;
 foo test("hello world");
 test.print();
}


... and build it in the usual fashion and load into GDB:

./gdb ~/pretty_print_bug
<snips lots of GDB output>
(gdb) python 
execfile("/home/pmuldoon/pretty_printing_python_tree/archer/gdb/python/lib/gdb/libstdcxx/v6/printers.py")
(gdb) set print pretty on
(gdb) b main
(gdb) r
Starting program: /home/pmuldoon/pretty_print_bug
Breakpoint 1, main () at /home/pmuldoon/pretty_print_bug.cpp:26
26      i++;

As the the "test" instantiation of foo has not happened yet, a "bt full" 
will fail.

(gdb) bt full
#0  main () at /home/pmuldoon/pretty_print_bug.cpp:27
       i = 1
       test = {
         message = Traceback (most recent call last):
 File 
"/home/pmuldoon/pretty_printing_python_tree/archer/gdb/python/lib/gdb/libstdcxx/v6/printers.py", 
line 458, in to_string
   return self.val['_M_dataplus']['_M_p'].string(encoding)
RuntimeError: Input/output error

If you step  past the instantiation later:

(pg-gdb) n
29      foo test("hello world");
(pg-gdb) n
30      test.print();
(pg-gdb) bt full
#0  main () at /home/pmuldoon/pretty_print_bug.cpp:30
       i = 2
       test = {
         message = "hello world"
       }

Everything works. The "test" instantiation has been properly initialized 
and the pretty printers can grok the contents of the class. The printers 
fail in various ways with random "Cannot access memory at address 0x0" 
or the error above, or lots of others ;) The backtrace also seems to 
stop at that point (though I have not tested that in many scenarios.).  
Anyway, the errors are just covering up that pretty printers are trying 
to read and interpret garbage.

Regards

Phil