valgrind-users

[Valgrind-users] Memcheck does not report any error

[Damien R <dam...@gm...>]

Hi,

I am using valgrind 3.7.0 and with the following program, valgrind reports
no error.

#include <iostream>

struct Foo
{
  void print()
  {
    std::cout << "foo" << std::endl;
  }
};

int main()
{
  Foo * foo = new Foo;
  delete foo;
  foo->print();
  return 0;
}

Can someone explain why valgrind does not report any error?

Regards,
Damien R.

Re: [Valgrind-users] Memcheck does not report any error

[Matthias S. <zz...@ge...>]

On 29.09.2013 13:16, Damien R wrote:
> Hi,
>
> I am using valgrind 3.7.0 and with the following program, valgrind 
> reports no error.
>
> #include <iostream>
>
> struct Foo
> {
>   void print()
>   {
>     std::cout << "foo" << std::endl;
>   }
> };
>
> int main()
> {
>   Foo * foo = new Foo;
>   delete foo;
>   foo->print();
>   return 0;
> }
>
> Can someone explain why valgrind does not report any error?
>
Hi!

You call Foo::print() on a deleted instance of Foo, but this method does 
not access any data from inside Foo and is no virtual method.
So nothing happens, that valgrind should complain about.

Regards
Matthias

Re: [Valgrind-users] Memcheck does not report any error

[Philippe W. <phi...@sk...>]

On Sun, 2013-09-29 at 13:16 +0200, Damien R wrote:
> Hi,
> 
> 
> I am using valgrind 3.7.0 and with the following program, valgrind
> reports no error.
> 
> 
> #include <iostream>
> 
> struct Foo
> {
>   void print()
>   {
>     std::cout << "foo" << std::endl;
>   }
> };
> 
> int main()
> {
>   Foo * foo = new Foo;
>   delete foo;
>   foo->print();
>   return 0;
> }
> 
> 
> Can someone explain why valgrind does not report any error?
The compiler inserts a call to foo print, but this does not
imply a dereference of foo pointer. If you add int a
in Foo, and assigns a value to a inside print, then you get an error.

Philippe

Re: [Valgrind-users] Memcheck does not report any error

[Tom H. <to...@co...>]

On 29/09/13 12:16, Damien R wrote:

> I am using valgrind 3.7.0 and with the following program, valgrind
> reports no error.
>
> #include <iostream>
>
> struct Foo
> {
>    void print()
>    {
>      std::cout << "foo" << std::endl;
>    }
> };
>
> int main()
> {
>    Foo * foo = new Foo;
>    delete foo;
>    foo->print();
>    return 0;
> }
>
> Can someone explain why valgrind does not report any error?

Yes. Nothing in the "foo->print()" call actually accesses any memory 
that was part of the object.

The body of the function doesn't access any member variables, and it's 
not a virtual function so the vtable is not read, so there is no access 
to any freed memory.

Tom

-- 
Tom Hughes (to...@co...)
http://compton.nu/

Re: [Valgrind-users] Memcheck does not report any error

[Damien R <dam...@gm...>]

On 09/29/2013 02:22 PM, Tom Hughes wrote:
> Yes. Nothing in the "foo->print()" call actually accesses any memory
> that was part of the object.
>
> The body of the function doesn't access any member variables, and it's
> not a virtual function so the vtable is not read, so there is no
> access to any freed memory.
>
> Tom
>
Thanks to everyone who answered the question but I still do not
understand why foo->print() does work. For me foo is part of the object
because it is the object. Does it mean that the method print is
generated like:
void print(Foo *)
{
    std::cout << "foo" << std::endl;
}

If this is the case, can someone give me some resources about the memory
organisation of c++ and about class/methods generation?

Regards,
Damien R.

Re: [Valgrind-users] Memcheck does not report any error

[Tom H. <to...@co...>]

On 30/09/13 08:59, Damien R wrote:

> Thanks to everyone who answered the question but I still do not
> understand why foo->print() does work. For me foo is part of the object
> because it is the object. Does it mean that the method print is
> generated like:
> void print(Foo *)
> {
>      std::cout << "foo" << std::endl;
> }

Yes, basically that is exactly what happens. The function (with g++) is 
actually called _ZN3Foo5printEv but other than that you have the details 
exactly right.

> If this is the case, can someone give me some resources about the memory
> organisation of c++ and about class/methods generation?

Well strictly speaking those are implementation details which may vary 
from one compiler to another, or at least from one platform to another.

Tom

-- 
Tom Hughes (to...@co...)
http://compton.nu/