Re: Need to use "volatile" for accessing variables between threads?
On Jul 27, 7:58 pm, "Bo Persson" <b...@gmb.dk> wrote:
Greg Herlihy wrote:
On Jul 27, 6:37 am, Richard Herring <junk@[127.0.0.1]> wrote:
3. Once you have that other mechanism in place, there's no
need to use volatile.
Not so. Declaring a variable "volatile" forces the compiler
to load that variable's value from memory each time that
value is needed. Otherwise - if there are no apparent write
operations to the variable within a given block of code -
then the compiler is very likely to store the variable's
value in a register once at the beginning of the block; and
then use the register-based value in subsequent operations
as that variable's current value.
I agree with Richard here. Once the "other mechanism" is in
place, presumably a mutex involving an OS call, the compiler
cannot cache values across the OS call - unless it first makes
sure that the OS is not using any callbacks previously set,
that just might affect our protected variable. Whether it is
volatile or not doesn't matter.
Just a nit, but it has nothing to do with callbacks or any other
particular feature. A compiler cannot cache values accross
certain OS calls because the OS specifications say that it
can't.
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