Subj : Re: Can C++ local static objects be made thread safe?
To   : comp.programming.threads
From : David Schwartz
Date : Tue Feb 15 2005 10:47 am


"Alexander Terekhov" <terekhov@web.de> wrote in message 
news:4211CDC9.A5CF3C5F@web.de...

>> describe here.  Each thread has different memory mapped to the same
>> address (don't try to take the address of an __thread variable though).
>
> Why so?

    Because there are two different ways to implement __thread, and the 
semantics of taking the address of a thread local variable are different in 
the two cases.

    One implementation does the magic in the address taking. Each thread 
will get a different address for the thread local variable.

    Another implementation does the magic in the page mapping. Each thread 
gets the same address for the thread local variable, but a different page is 
mapped into that address in each thread.

    Unless you know for sure which implementation you have and are willing 
to break on a different implementation, or your are using the address only 
in a way which is safe under either implementation, you can get results 
other than you expected.

    DS

.