# Re: Am I or Alexandrescu wrong about singletons?

From:
Andrei Alexandrescu <SeeWebsiteForEmail@erdani.org>
Newsgroups:
comp.lang.c++.moderated
Date:
Thu, 18 Mar 2010 16:53:52 CST
Message-ID:
<hnth0c$prb$1@news.eternal-september.org>
On 03/17/2010 10:16 PM, Leigh Johnston wrote:

"Joshua Maurice" <joshuamaurice@gmail.com> wrote in message
<snip>

Well, for starters, why are you using the volatile keyword? Do you
think it's a portable threading construct? It's not. volatile in C and
C++ has nothing to do with threading. The C and C++ standards do not
By POSIX, volatile means nothing special for threading. (The Microsoft
compiler under certain versions does claim to make it like a mutex
acquire and release, but let's just ignore this bad form for now. Use
boost or ACE or some portable library if you need atomic functions, or
wait for the new C++ standard. At worst, wrap volatile yourself to not
litter your code with a not portable construct + usage.)

This makes me very confused. I've always been taught to use the volatile
keyword in front of variables that can be accessed from several threads.

I'm sorry that you were taught incorrectly. I suggest reading the
paper:
http://www.aristeia.com/Papers/DDJ_Jul_Aug_2004_revised.pdf
It's a much more thorough description of modern threading in C++, and
specifically the volatile keyword. Any programmer who uses threads
should read this paper and understand its implications.

This is a very common misconception. I would guess that it started
from poor teaching in schools and elsewhere. In the programming
courses I took in my college, I was taught a very simple and naive
threading model, just as most physicists are taught Newtonian
mechanics, except unlike the physicists and General Relativity and
quantum mechanics, I was never taught this naive threading model was
actually incorrect, that it's not how POSIX and WIN32 threads actually
work. I believe that most people who are taught threading are taught
this naive model first and never taught any better, and it's simply
perpetuated itself due to lack of understanding. This naive model is
part of the naive model of computers in general as executing source
code instructions faithfully, in order, without any changes. However,
in the real world, C++ (and other industry programming languages) have
the "as-if" rule which allows or is interpreted as allowing reordering
and changing of instructions of a single thread as long as the result
of that thread \in isolation\ is the same before and after the
modifications. This is the allowance for basically all optimizations.
To do otherwise in the presence of multiple threads would be \way\ too
great of a performance hit.

Coming full circle, volatile in C and C++ was not intended to
guarantee a global order across threads, no other standard such as
POSIX defines it as such (though visual studios compiler may), and
most compilers implement volatile as not guaranteeing a global order
in C or C++.

You are incorrect to claim that volatile as defined by the current C++
standard has no use in multi-threaded programming. Whilst volatile does not
guarantee atomicity nor memory ordering across multiple threads the fact
that it prevents the compiler from caching vales in registers is useful and
perhaps essential. From Modern C++ Design:

"The volatile qualifier applied to a type tells the compiler that values of
that type might be changed by multiple threads. Knowing this, the compiler
avoids some optimizations (such as keeping values in internal registers)
that would make multithreaded code run erratically".

Just because there is no mention of threads in the current C++ standard
does
not mean that the vast majority of C++ compilers don't implement
volatile in
such a way that it is useful in multi-threaded designs.

Obviously the volatile keyword may not cause a memory barrier
instruction to
be emitted but this is a side issue. The combination of a memory barrier
and volatile makes multi-threaded code work.

The goal when writing multi-threaded code though should be to minimize
shared state the result of which should mean very few volatile objects in

/Leigh

I apologize for my handling of the volatile qualifier stronger than I
should have in "Modern C++ Design". My understanding of the matters at
the time was incomplete; volatile was intended to deal with interrupts,
not threads, and in a very system-dependent manner.

You are essentially right that some compilers do implement volatile to
help with threads. As an example, GNU C and C++ ascribe thread-related
meaning to volatile, but only with asm:

http://www.ibiblio.org/gferg/ldp/GCC-Inline-Assembly-HOWTO.html#ss5.4

(I'm not sure whether GNU defines volatile as a qualifier to help with

But by and large that's not sufficient to make sure things do work, and
they will never work portably. Here's a good article on the topic:

entitled eloquently "Volatile: Almost Useless for Multi-Threaded
Programming". And here's another entitled even stronger 'Why the
"volatile" type class should not be used':

http://kernel.org/doc/Documentation/volatile-considered-harmful.txt

The presence of the volatile qualifier in Loki is at best helpful but
never a guarantee of correctness. I recommend Scott and my article on
the topic, which was mentioned earlier in this thread:

http://www.aristeia.com/Papers/DDJ_Jul_Aug_2004_revised.pdf

Bottom line: using volatile with threads is almost always a red herring.

Andrei

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