Re: Simple lock

From:

Paavo Helde <myfirstname@osa.pri.ee>

Newsgroups:

comp.lang.c++

Date:

Sat, 20 Mar 2010 01:47:05 -0500

Message-ID:

<Xns9D4159A518E65paavo256@216.196.109.131>

moerchendiser2k3 <googler.1.webmaster@spamgourmet.com> wrote in
news:dc9d7aba-9a5c-4f92-91b7-1bfced630eb6@q16g2000yqq.googlegroups.com:

Hi,
I have a common question about locks:
class SetPointer
{
private:
        void *ptr;
        MY_LOCK lock;
public:
        void SetPointer(void *p)
        {
                Lock(this->lock);
                this->ptr = p;
        }
        void *GetPointer()
        {
                Lock(this->lock);
                return this->ptr;
        }
};

Just a question, is this lock redundant, when the Pointer can be set/
get from different threads?

No, locking is needed. Even if pointer reading-writing is atomic on a given
platform, one still needs locking because otherwise it would not be certain
if and when other threads notice the change, especially on multicore
machines.

However, this code seems quite suspicious - is MY_LOCK a lock or a mutex?
If it is a lock, then when and how this will be unlocked? In C++ lock
classes typically use RAII and as such they normally cannot be part of the
same class whose data they protect.

If MY_LOCK is a misnamed mutex class and Lock is a lock class, then the
locking is not done correctly, as the Lock object is released immediately
and does not actually protect the pointer assignment.

Without further details it is impossible to say of course what we have
here.

hth
Paavo