Re: [Boost-users] [Thread] What do I #include?

From: Khandelwal, Amit (amit.khandelwal_at_[hidden])
Date: 2008-07-11 09:31:45

Hmm...I don't know why you say that there will be a race condition. Let
say 2 threads are calling pop on the queue and another thread is calling
front(). Let us call these 3 thread p1, p2 (for pop) and f1 for front. 

As per my understanding - this could be one possible scenario.

P1 -- acquire the lock
P2 -- blocked on the lock
F1 -- blocked on the lock

P1 -- done and waiting on the lock
P2 -- still blocking 
F1 -- acquired the lock

So where is the race condition ?

 

-----Original Message-----
From: boost-users-bounces_at_[hidden]
[mailto:boost-users-bounces_at_[hidden]] On Behalf Of Anthony
Williams
Sent: Friday, July 11, 2008 9:14 AM
To: boost-users_at_[hidden]
Subject: Re: [Boost-users] [Thread] What do I #include?

"Khandelwal, Amit" <amit.khandelwal_at_[hidden]> writes:

> The way I use mutex is to protect one shared resource. Here is sample 
> code that I cooked up. I am not sure why you want to use a single 
> mutex to protect multiple resources. However, if your use case demands

> it than wrap those resources in a class and have a mutex as a member 
> variable of that class.

> template <typename T, typename SEQUENCE = std::deque<T> > class 
> tsqueue {
>
>         const T& front() {
>             boost::lock_guard<boost::mutex> lock(mutex);
>             std::cout << "Pop element: " << m_queue.front() << 
> std::endl;
>             return m_queue.front();
>         }
>
>         void pop() {
>             boost::lock_guard<boost::mutex> lock(mutex);
>             return m_queue.pop();
>         }

This code is not thread safe unless there is only one consumer. Though
each operation is safe, the front()/pop() model gives you a race
condition: some other thread may steal your element between the call to
front() and pop().

See
http://www.justsoftwaresolutions.co.uk/threading/implementing-a-thread-s
afe-queue-using-condition-variables.html
for a description of a thread-safe queue that uses a condition variable
to provide waiting and allows multiple consumers.

Anthony
-- 
Anthony Williams            | Just Software Solutions Ltd
Custom Software Development | http://www.justsoftwaresolutions.co.uk
Registered in England, Company Number 5478976.
Registered Office: 15 Carrallack Mews, St Just, Cornwall, TR19 7UL
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