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

Subject: Re: [boost] [Boost.smart_ptr] Thread Safety
From: Dave Abrahams (dave_at_[hidden])
Date: 2012-01-07 16:49:07

on Sat Jan 07 2012, "Thomas Jordan" <thomasjordan-AT-btinternet.com> wrote:

> Sorry, please ignore my previous post - finger trouble. The question is as follows:
> ------
>
> 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...

> // 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.

HTH,

-- 
Dave Abrahams
BoostPro Computing
http://www.boostpro.com