Re: [Boost-users] shared_ptr and weak_ptr concurrency

Subject: Re: [Boost-users] shared_ptr and weak_ptr concurrency
From: Rajeev Rao (rbsrao79_at_[hidden])
Date: 2009-09-02 01:18:42

Thanks for the response.   What if I place accesses to the global pointer within  critical sections (indicated by mutex.lock()-unlock()) ?  I've enclosed the dominant portions of the code in while loops.  Does this make it thread safe ?
<code>
//initializion Thread (runs before any other thread): shared_ptr< MyClass > global_ptr(createNewObject()) ;
//WriterThread :
while(true) {mutex.lock()global_ptr.reset( createNewObject())mutex.unlock();sleep (5) ;}

//ReaderThreads:
mutex.lock()weak_ptr<MyClass> local_weak_ptr (globally_ptr) ;mutex.lock()...while(true) {if(shared_ptr< MyClass > local_shared_ptr = local_weak_ptr.lock() ) { // using local_shared_ptr.} else {    // recreate weak ptr from global ?}} // end of while loop
</code>

From: Gottlob Frege <gottlobfrege_at_[hidden]>
Subject: Re: [Boost-users] shared_ptr and weak_ptr concurrency
To: boost-users_at_[hidden]
Date: Wednesday, September 2, 2009, 8:21 AM

On Mon, Aug 31, 2009 at 8:32 PM, Rajeev Rao <rbsrao79_at_[hidden]> wrote:

 This question may have been asked in other forms. However, from what I've been able to search (google) up, I could not get a crystal clear answer.  I'm essentially trying to use the solution provided in this page. 

http://onlamp.com/pub/a/onlamp/2006/05/04/smart-pointers.html?page=5. 
>From the boost documentation, it appears that this could be an  undefined operation as the global shared pointer is being read and written to by multiple threads. Is that correct ? 

In case the link fails  or someone wants more details, please read on. 
I've got a n reader threads and 1 writer
 thread. 
initializion Thread : shared_ptr< MyClass > global_ptr(createNewObject()) ;
WriterThread :global_ptr.reset( createNewObject())

ReaderThreads:weak_ptr<MyClass> local_weak_ptr (globally_ptr) ;...
if(shared_ptr< MyClass > local_shared_ptr = local_weak_ptr.lock() ) {
 // using local_shared_ptr.} else {    // recreate weak ptr from global ?}

thanks.
Rajeev

 I don't think that code is thread-safe.  You can modify the shared count safely between threads, but you can't modify the pointer itself.
Tony 

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