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To: gdb@sources.redhat.com
From:  Vladimir Prus <ghost@cs.msu.su>
Subject:  Re: "info locals" -- is variable initialized
Date: Fri, 02 Sep 2005 09:57:00 -0000
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Ramana Radhakrishnan wrote:

> 
>> 
>> When entering this function, KDevelop asks gdb via "info locals" what
>> locals vars are there. But, gdb reports all variables, even though at the
>> function entry 'i2' is not initialized (or, from C++ point of view, not
>> even visible yet). For pretty priting 'i2', I have to evaluate
>> 'i2.prettyURL(0)', which will just crash. I can use "set unwindonsignal
>> on", but generally, calling methods on uninitialized object can damage
>> random memory.
> 
> GDB puts a breakpoint after the prologue of a function. Which means that
> all locals in scope have been created / allocated space for . Hence you
> would see i2 , right ?

Wrong, I think. Function progolue only allocates space for KURL, but does
not call the constructor.

Consider:
int main()
{
    int i = 10;
    std::vector<int> v2;
    KURL url = "http://boost.org";
}


And assembler of it:
0x080487f4 <main+0>:    push   %ebp
0x080487f5 <main+1>:    mov    %esp,%ebp
0x080487f7 <main+3>:    push   %ebx
0x080487f8 <main+4>:    sub    $0x84,%esp
0x080487fe <main+10>:   and    $0xfffffff0,%esp
0x08048801 <main+13>:   mov    $0x0,%eax
0x08048806 <main+18>:   sub    %eax,%esp

<gdb sets breakpoint on the following instruction>

0x08048808 <main+20>:   movl   $0xa,0xfffffff4(%ebp)

<initialization of vector starts here>

0x0804880f <main+27>:   lea    0xffffffc8(%ebp),%eax
0x08048812 <main+30>:   mov    %eax,(%esp)
0x08048815 <main+33>:   call   0x8048924 <allocator>
0x0804881a <main+38>:   lea    0xffffffc8(%ebp),%eax
0x0804881d <main+41>:   mov    %eax,0x4(%esp)
0x08048821 <main+45>:   lea    0xffffffd8(%ebp),%eax
0x08048824 <main+48>:   mov    %eax,(%esp)
0x08048827 <main+51>:   call   0x80489a0 <vector>
...........

and only here std::vector is initialized.

So, there's a window there variable does not exists according to C++, but
exists according to gdb. 

- Volodya