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Groups > comp.lang.java.programmer > #10692

Re: Volatile keyword

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On 12/12/11 11:37 PM, Robert Klemme wrote:
> [...]Bottom line is that the JVM spec does not make
> any guarantees here (§17.7 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.

Similarly, 64-bit references (i.e. object references when running with 
the 64-bit JVM) are atomically written.  Of course, in that scenario 
it's a somewhat less interesting guarantee, since there's actually 
64-bit hardware there to support that (as opposed to the 64-bit double 
and long values on 32-bit systems).  But it exists as well.

>> In 32 bit, there is an assembler instruction cmpxchg8b designed to let
>> 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 is
> two operations i.e. not atomic.

There is no need to make that assumption provided the field involved is 
marked "volatile".

Pete

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Thread

Volatile keyword "raphfrk@gmail.com" <raphfrk@gmail.com> - 2011-12-12 04:51 -0800
  Re: Volatile keyword Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-12-12 14:31 +0000
    Re: Volatile keyword "raphfrk@gmail.com" <raphfrk@gmail.com> - 2011-12-12 06:52 -0800
  Re: Volatile keyword Lew <lewbloch@gmail.com> - 2011-12-12 06:34 -0800
  Re: Volatile keyword markspace <-@.> - 2011-12-12 10:00 -0800
    Re: Volatile keyword Tom Anderson <twic@urchin.earth.li> - 2011-12-12 18:41 +0000
      Re: Volatile keyword Patricia Shanahan <pats@acm.org> - 2011-12-12 11:43 -0800
    Re: Volatile keyword Roedy Green <see_website@mindprod.com.invalid> - 2011-12-12 11:15 -0800
      Re: Volatile keyword Robert Klemme <shortcutter@googlemail.com> - 2011-12-13 08:37 +0100
        Re: Volatile keyword Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2011-12-12 23:54 -0800
          Re: Volatile keyword Robert Klemme <shortcutter@googlemail.com> - 2011-12-14 19:39 +0100
            Re: Volatile keyword Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2011-12-14 10:59 -0800
            Re: Volatile keyword Patricia Shanahan <pats@acm.org> - 2011-12-14 11:00 -0800
              Re: Volatile keyword Robert Klemme <shortcutter@googlemail.com> - 2011-12-14 22:35 +0100
  Re: Volatile keyword Roedy Green <see_website@mindprod.com.invalid> - 2011-12-12 10:55 -0800

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