Re: use of keyword volatile in multi-threaded C++ program

From:

red floyd <no.spam.here@example.com>

Newsgroups:

comp.lang.c++

Date:

Tue, 23 Sep 2008 22:57:08 -0700

Message-ID:

<_HkCk.889$pr6.870@flpi149.ffdc.sbc.com>

C++Liliput wrote:

It seems that the keyword "volatile" is used to make sure that threads
reading (or writing to) the same data should see a consistent picture
of the variable i.e. updates made to the common data should
immediately get reflected to other threads that are using it. However
I have never faced this issue in y multi-threaded code even though the
common variables are not declared volatile. So is this just a matter
of coincidence that I have had no runtime issues or is it that the
"volatile" keyword is required in some special situations that perhaps
my code does not encounter? I would love to know a demonstrable piece
of code that really shows the relevance of volatile variables in multi-
threaded environment.

volatile simply tells the compiler not to optimize out accesses to a
variable. It's very useful for things like memory-mapped hardware
registers.

Note that you can have a const volatile. One of my favorite
declarations from some real-life code I wrote was:

unsigned long const volatile * const HW_RDONLY_REG =
reinterpret_cast<unsigned long const volatile *>(
0xC0008020);

Volatile does not guarantee asynchronous concurrent access consistency.