Re: how to use volatile key word?
Paavo Helde <myfirstname@osa.pri.ee> writes:
Juha Nieminen <nospam@thanks.invalid> wrote in
news:jse5si$c7s$1@speranza.aioe.org:
Paavo Helde <myfirstname@osa.pri.ee> wrote:
Volatile is meant to be used for accessing memory-mapped hardware, if
you are not writing device drivers or such you can just forget about
it. It is neither needed nor sufficient for portable thread
synchronisation.
'volatile' might not be sufficient for thread synchronization nor
atomicity, but it can certainly make a difference in a multithreaded
program.
In a program I had a case where I was just reading an integral from
one thread that was changed by another thread to signal a minor,
unimportant effect (namely something related to updating the UI). This
did not require full-fledged locking (which would have made it
needlessly expensive) because if the integral had a wrong value for a
split second, that wasn't really a catastrophical event.
Out of curiosity - did you measure how much the "needlessly expensive"
locking was slower than using a volatile? I'm asking because I find myself
often in urge to invent all kind of clever tricks to bypass proper locking.
This will be dependent on how the volatile is used. For example:
volatile bool Class::terminate_thread = false;
.
.
.
void
Class::Run(void *arg)
{
while (!terminate_thread) {
// do something
}
pthread_exit(NULL);
}
.
.
.
If terminate_thread is declared volatile (since its value can be changed by
another thread or a signal handler), the effect on code generation is minimal
(a load each time through the loop instead of using a register cached value), while
most synchronization mechanisms will require at least one function call.
Given that accesses to 'bool' datatypes are atomic on most modern architectures,
it is a common paradigm to use volatile on such variables.
scott
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