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Re: Volatile keyword

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raphfrk@gmail.com <raphfrk@gmail.com> wrote:
> Does this make all read/writes happen in full.

That's how it is specified.

> For example, if one thread had
> volatile long a = 0xFFFFFFFF00000000L;
> <do stuff>
> a = ~a;
> If another thread read a, would it always give 0xFFFFFFFF00000000 or
> 0x00000000FFFFFFFF and not partial writes (say it updates 32 bits and
> then the next 32 bits).

That's how it is specified. :-)

> I understand that if two threads do a++, it might only increment
> twice, if they both read the old value at the same time.

"once," but I think you meant "once," anyway, with "only" in the 
context.

Even if more processes did "a = ~a;" concurrently on a volatile long a
with given initialization, then the assumption of always reading either
0xFFFFFFFF00000000 or 0x00000000FFFFFFFF but nothing else would still
be a rather safe bet. (i.e. betting only against hardware-flaws, or
severe bugs in the JVM implementation)  Of course, *which* one of these
values you'd see is not necessarily related to the total number of these
manipulations performed by all threads together (as in the a++ case).

> Effectively, does the CPU lock the entire 64-bits for the long when
> doing the write update, and so other threads either seen the updated
> value or the old value?

That's how it is specified. :-)

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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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