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

[Ned Konz]

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.

--
Ned Konz
address@hidden
http://bike-nomad.com