AVR-Chat

R E Purcella wrote:

> ... "the complete reference C++" by Herbert
> Schildt, third edition ... [says] ... "The
> modifier volatile tells the compiler that a 
> variable's value may be changed in ways not
> explicitly specified by the program."

You see, this isn't actually correct.  I think the author is trying to make a compromise between telling us what the volatile type qualifier (not "modifier", there's the first mistake) actually is and when to use it.  Sentences involving what you "tell the compiler" are always going to be tricky.  If you're "telling" the compiler anything with the volatile qualifier, you're telling it that accesses to the qualified object (not variable, second mistake) must be done in strict conformity with the abstract machine used to define the semantics of the C language in the specification.  You're not telling the compiler why you're asking for this (third mistake) and the "value may be changed in ways not explicitly specified by the program" is only one of the reasons you might want strict conformance (fourth mistake).

> My friend says "... you need to declare that
> variable as volatile, to tell the compiler
> to not use the optimized value ..."

What the heck is "the optimized value"?  This is nonsense.  If you think you understand it, then that's because you think you already understand volatile and you're able to map this gobbledygook onto your understanding.

> So I understand him to be saying Volatile
> tells the compiler 1) not to use an optimized
> value 2) not to assume that the value hasn't
> changed

This is a poor description of volatile for reasons presented above.

I have yet to come across a description of volatile that conforms to what is written in the standard, is complete, correct and is in all situations helpful to people who do not fully understand the terminology of the standard, compilation and software engineering in general.  I am starting to think that nobody would read such a description as it would necessarily be quite long and people's preference is for short over complete and correct.

You might think, for example, that Keil (part of ARM) could get this right.  But, they gave the job of writing this description of the volatile type qualifier:

http://www.keil.com/support/man/docs/c51/c51_le_volatile.htm

to someone who has a pretty good understanding of the C language but no understanding of hardware.  Can you spot the glaring problem?

Graham Davies
ECROS Technology
www.ecrostech.com