Re: Threadsafe singletons

"kanze" <>
16 Aug 2006 18:14:50 -0400
Matthias Hofmann wrote:

"kanze" <> schrieb im Newsbeitrag

Fundamentally, it is almost the same issue here. The
problem is a compiler generated bool, which you, as the
user, don't have access to. It's not quite the same thing
as the static variables involved in, say, the stack walkback
of exception handling, but there is a basic principle
involved. If I write:

        static int const xxx = 42 ;
        // ...

no locking is required by the user. So I would expect the
same to hold for:

        static std::string const xxx( "doh" ) ;
        // ...

Where is the difference between these two functions with
regards to multithreading?

The first has static initialization, which occurs before program
execution begins (and thus before any thread can have been
started). In actual practice, on all of the systems I know, the
compiler will reserve 4 bytes for the int in the data segment,
and put the 42 in the corresponding 4 bytes of the executable;
"initialization" occurs when the process image is loaded from
disk. (Supposing you take the address of xxx, or do something
else which requires the actual variable to exist. Otherwise, of
course, the compiler just uses 42, without worrying about it.)

The second is dynamic initialization, which a non-trivial
constructor. The constructor will be called anytime execution
flow arrives at the statement, and the object is not already
constructed. As far as I know, there are only one technique
used to do this: an additional static bool variable which is set
after returning from the constructor. (Note that if the
constructor terminates because of an exception, the object is
not yet initialized, and the constructor will be called again
the next time the function is called.)

Of course, if a second thread enters the function while you are
executing the constructor of xxx...

James Kanze GABI Software
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