Subj : Re: 'volatile' Rules
To   : comp.programming.threads
From : Joe Seigh
Date : Tue Jun 07 2005 09:38 pm

Marcin 'Qrczak' Kowalczyk wrote:
> Joe Seigh <jseigh_01@xemaps.com> writes:
>>>In practice most programs using signals assume slightly more than
>>>that, even though it's not formally guaranteed by ISO C or POSIX.
>>>They assume that volatile variables which fit into machine word
>>>are accessed atomically wrt. a signal handler. I haven't heard of
>>>a platform where this is not true.
>>
>>They assume too much.  Volatile doesn't mean anything here.
> 
> 
> It ensures that the main program doesn't cache the variable in a
> register, so that its change by a signal handler will be seen
> immediately when the main program accesses it, and its change by the
> main program will be seen immediately if a signal arrives shortly
> after.

Until the signal handler exits, the main program doesn't see or do anything.
After the signal handler exits, the main program sees everything the signal
handler saw.

I don't know what "immediate" buys you.  The signal could occur just
before the store instruction even for volatile variables.  If you need
to coordiate something with a signal handler you should be using the
signal handling control functions to do that.

> 
> volatile variables are the only reliable means of communication of
> data between a signal handler and the main program.

I never depend on volatile for communicating with signal handlers.  I
use other mechanisms for correct memory visibility.



-- 
Joe Seigh

When you get lemons, you make lemonade.
When you get hardware, you make software.

.