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From: Alexander Terekhov <terekhov@web.de>
Newsgroups: comp.programming.threads
Subject: Re: Is volatile so bad?
Date: Mon, 02 Jan 2012 15:49:41 +0100
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Mark Adler wrote:
> 
> My parallel gzip, pigz (http://zlib.net/pigz/) is written in C, uses
> pthreads, and everything seems to work fine.  Until recently the word
> "volatile" did not appear in the code.
> 
> However I got an email from a user about a global variable (local to a
> single source file) that he said needed to be volatile.  The variable
> is a pointer to the head of a linked list of jobs, and access to it is
> locked by a related pthreads mutex.  The user said:
> 
> "Without volatile the HP C compiler is doing a prefetch on this
> variable before possess/enter_mutex and uses the prefetched old value
> from a processor register still after the mutex."
> 
> ("possess" is the name of a routine that calls pthread_mutex_lock.)
> 
> So I figured what the heck, it can't hurt too much so I put the
> volatile in there.  The intent is that pigz be portable over a wide
> range of operating systems, so I generally try to accommodate requests
> like this.

If your program is data-race-free then just file a bug report against HP
C (it is really hard to believe that HP C can't deal with locking
correctly).

For use of volatile, see also:

http://www.hpl.hp.com/personal/Hans_Boehm/c++mm/user-faq.html

regards,
alexander.