Re: MSDN volatile sample
Michal Nazarewicz <firstname.lastname@example.org> wrote in
George2 <email@example.com> writes:
In the MSDN volatile sample,
I do not understand what is the purpose of the sample. I have tried
remove the keyword volatile, and the result is the same. :-)
volatile means that given variable may change at any time and compiler
have no way of knowing that it may happen (no, it's not the definition
of volatile keyword but it helps understand it). Assume that compiler
knows that Sleep() function will never modify Sentinel -- in this case
might be replaced with
if there would be no volatile keyword. That's because without
keyword compiler could came to a conclusion that value of Sentinel
not change in body of the loop so if the condition was true at the
beginning it will be true all the time.
With volatile keyword compiler must check Sentinel's value since it no
longer knows that it's value was not changed (that's basically meaning
of that keyword).
In addition to the above, Microsoft added additional semantics to the
volatile keyword. From their msdn site:
Objects declared as volatile are not used in certain optimizations
because their values can change at any time. The system always reads the
current value of a volatile object at the point it is requested, even if
a previous instruction asked for a value from the same object. Also, the
value of the object is written immediately on assignment.
Also, when optimizing, the compiler must maintain ordering among
references to volatile objects as well as references to other global
objects. In particular,
A write to a volatile object (volatile write) has Release semantics; a
reference to a global or static object that occurs before a write to a
volatile object in the instruction sequence will occur before that
volatile write in the compiled binary.
A read of a volatile object (volatile read) has Acquire semantics; a
reference to a global or static object that occurs after a read of
volatile memory in the instruction sequence will occur after that
volatile read in the compiled binary.
This allows volatile objects to be used for memory locks and releases in
Hope that helps,