Re: [Boost-users] [thread] thread_specific_ptr is not initialized with NULL

Subject: Re: [Boost-users] [thread] thread_specific_ptr is not initialized with NULL
From: Anthony Williams (anthony.ajw_at_[hidden])
Date: 2009-06-07 16:52:37

"Michael Gopshtein" <mgopshtein_at_[hidden]> writes:

> Hi,
>
> As I understand from documentation, once thread_specific_ptr is
> created, first call to get() should return NULL in all
> threads. Actually sometimes it doesn't happen. Consider the following
> scenario:
>
> 1) thread T1 initializes some global tss object (by doing "new" on
> global thread_specific_ptr<>*)
> 2) thread T2 tries to read the value with get(), and gets null. It
> sets a correct value (unsigned int 42)
> 3) thread T1 deletes tss object, and allocates a new one instead.

This is now undefined behaviour. If you destroy a thread_specific_ptr
whilst any thread has a non-NULL value then you cannot rely on what
happens subsequently with respect to any thread that tries to use that
or a new thread_specific_ptr instance.

> 4) thread T2 tries to read the value from the new object
>
> So I expected that in (4) thread T2 will get NULL again. Actually,
> when new object created in (3) is created in same memory location, T2
> gets the original value in (4), and not NULL. When  new pointer
> allocated in (3) is different, T2 gets NULL as expected (verified
> using "placement new" to control location of the tss object).
>
> Is it a bug or inpropriate use pattern?

Inappropriate use pattern.

Anthony
-- 
Author of C++ Concurrency in Action | http://www.manning.com/williams
just::thread C++0x thread library   | http://www.stdthread.co.uk
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