Re: [RFC/PATCH] doc: volatile considered evil

Randy Dunlap
2007-05-08T23:09:34Z

On Tue, 8 May 2007 13:20:31 -0700 (PDT) David Rientjes wrote:

> On Tue, 8 May 2007, Krzysztof Halasa wrote:
>
> > > Since 'volatile' has two different semantics depending on the context in
> > > which it is used, this warning should be appended to include the fact that
> > > it is legitimate to use for inline assembly.
> >
> > I think it hasn't two semantics, it's like arguing that char has two
> > semantics.
> >
> > Volatile does one thing - prohibits C compiler from optimizing
> > accesses to the variable. Either with (volatile *) casts and with
> > volatile var;
> >
>
> What the meaning of an "access" to a volatile memory-mapped I/O port or a
> variable that can be asynchronously interrupted is implementation-defined.
>
> You're only citing qualified versions of objects.
>
> In an asm construct, if all your input operands are modified and specified
> as output operands as well, volatile must be added so that the entire
> construct is not optimized away. Additionally, it must be added if your
> construct modifies memory that is neither listed in inputs nor outputs to
> the construct so that it is known to have at least one side-effect. Then,
> the compiler cannot delete your construct if it is reachable because it
> may produce such side-effects.
>
> Thus, the warning that was proposed for addition to CodingStyle should be
> modified to explicitly state that the use of 'volatile' for asm constructs
> is perfectly legitimate and its use as a type qualifier for objects in
> code is inappropriate.

It's already there, isn't it? <quote from original:>

The only acceptable uses for "volatile" are:

- in _code_, i.e., for things like the definition of "readb()" etc, where we
use it to force a particular access.
- with inline asms
- on "jiffies", for stupid legacy reasons

</quote>

or are you saying that you want to subject/header/title modified also?

---
~Randy
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