Re: Threads - When?

"James Kanze" <>
9 Jan 2007 19:46:52 -0500
Le Chaud Lapin wrote:

      [concerning a local static variable with a non-trivial

Well, as I have said before, when I look at that code, I never had any
illusions that it was not a global variable. The only thing I was
curious about was whether the compiler would initialize the global
variable before call to main() or wait until the first invocation of
the function. So I checked my TCPPPL, and saw that it was on first
invocation, which immediately lead to the question, "How does it know
it's the first time", which mean a global variable, which immediately
leads one to think, "Ok, I guess that works, but in a multi-threaded
program, it is going to be an issue."

That depends on the guarantees given by the compiler. IIRC, g++
simply puts the initialization in a pthread_once, and lets the
system take care of ensuring that it is only executed once. (At
least, that is the obvious implementation under Posix, where
you have a system primitive precisely for this sort of thing.)

But not one that is easily circumvented.

boost::once ? Under Posix, it maps directly to pthread_once,
but they've also implemented it under Windows.

Note that this does not mean that synchronization primitives go into the
core language, as in Java. It simply means that the definition of the
abstract machine acknowledges concurrent paths of execution. None of
your points about treating multiple threads as a high-level design issue
is invalidated by this.

Yes you're right. I definitely support discussing multi-threading
discussions in context of C++. Perhaps I was wrong to assume that
there are people who feel like the solution is to augment C++ with a
bunch of synchronization-related keywords, or worse, magically
ascertain the intent of the programmer in a complex, multi-threaded

Finally. (There probably are such people, but they're not
involved in the C++ standardization effort.)

James Kanze (GABI Software)
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