On 11/20/2010 12:24 AM, Patrick Horgan wrote:
In Scott Meyers' Effective C++ Item 4, Scott recommends using
something like:
T& getaccess()
{
static T localT;
return localT;
}
so that a static local is used instead of a static global to access
the global object and you can be sure it's initialized before used.
What happens in the presence of threading? What if two threads enter
getaccess and exit in different orders? I'm ignoring here any
threading issues that a T itself may have, and only thinking about the
local static initialization itself.
Would the local t get initialized more than once?
The language says that t would only be initialized the first time the
routine is called, so if the first one sleeps and a second one gets in
and out before the first thread gets rescheduled, can it return a
reference to localT before localT is initialized?
Patrick
This doesn't match Item 4 in my (older?) Meyers.
As you have presented this, the threads would have a race condition on
access to localT. I see only an initialization at compile time, so it
appears to be initialized prior to thread creation. In view of the
race, when threads modify the value, it's indeterminate whether a thread
will read the value set by itself or by another thread.
I don't know whether others might share my view that since gomp is part
of gcc, questions on OpenMP, or, possibly, std::thread, would be more on
topic than questions about some unspecified threading method.
--
Tim Prince
