Re: [boost] [Boost.smart_ptr] Thread Safety

Subject: Re: [boost] [Boost.smart_ptr] Thread Safety
From: Thomas Jordan (thomasjordan_at_[hidden])
Date: 2012-01-08 08:20:41

Message: 7
Date: Sat, 07 Jan 2012 16:49:07 -0500
From: Dave Abrahams <dave_at_[hidden]>
To: boost_at_[hidden]
Subject: Re: [boost] [Boost.smart_ptr] Thread Safety
Message-ID: <m24nw7p4j0.fsf_at_[hidden]>
Content-Type: text/plain

>on Sat Jan 07 2012, "Thomas Jordan" <thomasjordan-AT-btinternet.com> wrote:
>> Hi,
>> Is shared_array completely thread-safe, including
>> destruction/reference counting? From the code in
>> /boost/detail/sp_counted_base_pt.hpp it looks like it probably is but
>> according to
>>
>> http://www.boost.org/doc/libs/1_33_1/libs/smart_ptr/shared_ptr.htm#ThreadSafety

>I can't reach that (very ancient) page right now, but...

The documentation for this is example is the same in the latest v1.48 
release.

>> // thread A
>> p3 = p2; // reads p2, writes p3
>>
>> // thread B
>> // p2 goes out of scope: undefined, the destructor is considered a "write 
>> access"
>>
>> shared_ptr isn't completely thread-safe for simultaneous read and
>> write/destruct access? The same would presumbly apply to shared_array?
>> Maybe I've missed something, are there compile-time configuration
>> options to determine level of safety?

>shared_ptr<T> is almost exactly as threadsafe as T* is.  In this case,
>thread A has a reference to an object on B's stack.  That object is
>called p2.  If thread A tries to use that object's value while the
>object is being destroyed, yeah, that's a race condition.

>The way to avoid this situation is for thread B to pass p2 to thread A
>by value, thus creating p2a, which thread A can read without concern.
>Now there's no race condition.  Note: this change does /not/ cause the T
>object held by p2 to be copied.

Thanks, I think my confusion arose from my inferring that the P2 in thread A 
in this example was shorthand for P2a, i.e., a copy of the actual P2 in 
thread B.
Makes sense as per your interpretation.

Regards,
T.