why so

why so

[Uladzislau Rezki]

Hello gdb list,

I am stuck with debugging C++ application that uses STL.
So, i have a simple code:

#include <iostream>

static int
test(std::string s)
{
    std::cout << s << std::endl;
    return 0;
}

int main(int argc, char **argv)
{
    std::string a = "hello";
	
    test(a);
    return 0;
}

while debugging *test* function, i try to print s variable and
i see following stauff, thought i expect to see "hello":

    (gdb) print s.c_str()
    $2 = 0xbf9486d8 "\024pR\b"

gdb version is:

    demon >>> gdb -v
    GNU gdb 6.8-debian
    Copyright (C) 2008 Free Software Foundation, Inc.
    License GPLv3+: GNU GPL version 3 or later
<http://gnu.org/licenses/gpl.html>
    This is free software: you are free to change and redistribute it.
    There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
    and "show warranty" for details.
    This GDB was configured as "i486-linux-gnu".
    demon >>>

thanks in advance.

--
Uladzislau Rezki

Re: why so

[Paul Pluzhnikov]

On Sun, Mar 8, 2009 at 6:12 AM, Uladzislau Rezki <urezki@gmail.com> wrote:

> I am stuck with debugging C++ application that uses STL.
> So, i have a simple code:
>
> #include <iostream>
>
> static int
> test(std::string s)

Note that passing strings by value is very inefficient.
You should probably pass 'const string &' instead, and this will
also cure the glitch you are observing.

> while debugging *test* function, i try to print s variable and
> i see following stauff, thought i expect to see "hello":
>
>    (gdb) print s.c_str()
>    $2 = 0xbf9486d8 "\024pR\b"

I can reproduce this using gcc-4.3.1, but not 4.2.2 or 4.4.4.
It's a GCC bug: it emits incorrect information for functions that
pass structs by value. I can't find it in GCC bugzilla though.

Cheers,
-- 
Paul Pluzhnikov

Re: why so

[Uladzislau Rezki]

On Sun, Mar 8, 2009 at 7:21 PM, Paul Pluzhnikov <ppluzhnikov@google.com> wrote:
> On Sun, Mar 8, 2009 at 6:12 AM, Uladzislau Rezki <urezki@gmail.com> wrote:
>
>> I am stuck with debugging C++ application that uses STL.
>> So, i have a simple code:
>>
>> #include <iostream>
>>
>> static int
>> test(std::string s)
>
> Note that passing strings by value is very inefficient.
> You should probably pass 'const string &' instead, and this will
> also cure the glitch you are observing.
>
If i pass "std::string &s', everything will work in my case and i see normal
string, this issue happens just in case 'std::string s'.

Thanks for your answer.

--
Uladzislau Rezki