Re: how to use volatile key word?

From:

Juha Nieminen <nospam@thanks.invalid>

Newsgroups:

comp.lang.c++

Date:

Wed, 27 Jun 2012 05:32:02 +0000 (UTC)

Message-ID:

<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.

However, 'volatile' had a major impact on that integral. Without it, the
other thread was not seeing the changes immediately, causing a noticeable
delay (it was a situation where the thread that was setting that integral
did it inside a loop, and the compiler was optimizing the updating of the
variable in such a way that the actual writing to the extern variable was
delayed to be after the loop). With 'volatile' the variable was always
updated immediately.