Re: What does volatile guarantee?
Eric Sosman wrote in this thread on 2/12:
Before anybody whines^H^H^H^H^H^Hsuggests that making +=
atomic would be easy, let him ponder
volatile int a,b,c,...,z;
a += b += c+= ... += z;
Andreas Leitgeb wrote:
I'm not whining (nor suggesting) for += nor ++ being atomic,
but that particular argument does not extend to the "++"
operator.
Actually, it does. The particular argument is an extension of the observation
that += and ++ both represent a read followed by a write, with an increment or
addition in there somewhere. Those are inherently separate operations.
So, what to say to those whining^H^H^H^H^H^H^Hsuggesting
making just "++" atomic?
What Joshua Cranmer said, with "increment" added:
x++ properly involves a read, then a copy[, then an increment],
then a write, so it is not atomic.
and correspondingly for a pre-increment:
++x properly involves a read[, then an increment], then a write,
then a copy, so it is not atomic.
Also tell them that there already is an atomic version of that operation,
'AtomicInteger#getAndIncrement()', et al., to be happy that synchronization is
built in to the language, and to quit whining.
Not every operation can be atomic. Some actions involve too many steps to be
atomic. It is clear that a simple 'volatile' read or write can, and should,
be atomic. It is clear that a long sequence of steps, such as:
x = 0;
y = x * 3;
x = y / 17;
x = Math.sqrt( x );
should not be atomic without explicit synchronization. Therefore it is clear
that not all operations involving a mix of reads and writes should be atomic.
Since 'synchronized' or similar is needed to synchronize such multiple
operations, and 'synchronized' and similar are built into the platform, it
makes sense that those mechanisms should apply to all situations involving a
mix of reads and writes, rather than to draw some arbitrary and hard-to-manage
boundary between mixes that are atomic and those that are not.
Really, though, whining about the absence of a feature that is already
provided by other means is just stupidity and childish petulance. THE FEATURE
ALREADY EXISTS. You just can't use the ++ operator for it. Sheesh!
--
Lew