Re: [avr-libc-dev] Volatile not working?

[David Brown]

Ned Konz wrote:
> Ron wrote:
> > Hi Folks. I would have thought that with -Os the code below should work.
> >
> > static volatile char* Ptr;
> >    .
> >    .
> > static Send(char* p)
> > {
> >    if (p)
> >    {
> >       while (Ptr != NULL) {;}
> >       Ptr = p;
> >       UDR1 = *Ptr++;
> >    }
> > }
> >    .
> >    .
> > SIGNAL(SIG_UART1_TRANS)
> > {
> >    if (*Ptr == '\0')
> >       Ptr = NULL;
> >    else
> >       UDR1 = *Ptr++;
> > }
> >
> > However, the while loop becomes:
> >
> >        LDS    R24,0x018D
> >        LDS    R25,0x018E
> >        SBIW   R24,0x00
> >        BRNE   PC-0x01
> >
> > The code is as expected with -O0. Version is 3.4.6. Something I'm
> > missing?
>
> Probably.
>
> You didn't declare Ptr to be volatile.
>
> Instead you declared Ptr to *point to* a volatile char.
>
> So in the while loop (while you're comparing Ptr to NULL) there's no 
> reason to re-read Ptr because it's not volatile.

That's correct.  In particular,

"volatile char* ptr" parses as "(volatile char) *ptr".

What you want is "char * volatile ptr", meaning that the pointer itself 
is volatile, not the thing it points to.

A good rule is that a variable declaration should not consist of more 
than two parts (excluding "static" or "extern", and the variable 
itself).  Any time you need more complex types, use typedefs.  Thus:

typedef char *pchar;
volatile pchar Ptr;             // volatile pointer to a char

typedef volatile char vchar;
vchar *Ptr;                     // pointer to volatile char

typedef vchar *pvchar;
volatile pvchar Ptr;            // volatile pointer to volatile char

Then there is (almost!) no chance of getting things wrong.

mvh.,

David