Re: Volatile keyword

From:

Robert Klemme <shortcutter@googlemail.com>

Newsgroups:

comp.lang.java.programmer

Date:

Wed, 14 Dec 2011 19:39:30 +0100

Message-ID:

<9ks8r2Fld4U1@mid.individual.net>

On 12/13/2011 08:54 AM, Peter Duniho wrote:

On 12/12/11 11:37 PM, Robert Klemme wrote:

[...]Bottom line is that the JVM spec does not make
any guarantees here ( =A717.7 see Lew's reference).

I think it's worth being careful about "does not make any guarantees
here". The implication of 17.7 is that there _is_ in fact a guarantee,
for certain 64-bit fields marked as "volatile". Specifically, those
which are of the type "double" or "long". This is true even on 32-bit
systems.

And what guarantee is there? Can you unambiguously formulate that and
provide reference to the JLS so everybody can check it?

In 32 bit, there is an assembler instruction cmpxchg8b designed to le=

you implement a light weight atomic 64-bit read.

Since you do not know on what JVM your Java program will run when you
write it (or at least someone can choose to use a different JVM model)=

it is safer to code under the assumption that a long and double write =

two operations i.e. not atomic.

There is no need to make that assumption provided the field involved is=

marked "volatile".

I beg to differ - but I am eagerly waiting to be convinced otherwise.

Cheers

robert