[fpc-devel] volatile variables

[Sven Barth]

At beginning of June I've found the following link on the ReactOS 
mailing list when they were discussing about memory ordering and 
volatile as well: 
http://kernel.org/doc/Documentation/volatile-considered-harmful.txt

For those interested the following is the link to the starting 
discussion: 
http://www.reactos.org/archives/public/ros-dev/2011-June/014277.html (I 
hope the mail thread works in the web view as well as in my mail client)

Regards,
Sven

On 28.06.2011 13:14, Michael Schnell wrote:
> A similar discussion is going on in Lazarus-develop, but this obviously
> is a compiler question.
>
>
> In C, there is the volatile keyword that ensures that after the code
> sequence enters the next C instruction after that which modified this
> variable, another thread sees the correct state of the variable.
>
> This is ensured by not caching the value in registers and on SMP systems
> additionally by low level stuff like memory barriers.
>
> Of course this does not protect concurrent read-modify-write accesses
> (same need to be protected by low level atomic instructions or MUTEX and
> friend).
>
> For variables not defined as volatile, (e.g.) pthread_mutex (and similar
> stuff on Windows) can be used to protect them.
>
> AFAIK, in Pascal all Global (Static) variables are considered to be
> handled as volatile, so no problem here (other than handling all of them
> them as volatiles might decrease performance slightly).
>
> But what about variables on the heap ? If class instance variables or -
> say - a liked list done with records created by "new" are accessed by
> multiple threads, how can said consistency be enforced ?
>
> Even if you use a critical section to do the protection, this does not
> help if the compiler decides to cache the variable in a register while
> entering or exiting the critical section.
>
> But even without dedicated protection, a volatile variable should be
> able to be be monitored from another thread. Can this be enforced in
> Pascal for help variables ?
>
> -Michael
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