Re: Question on volatile functions and GCC

[Jeffrey Walton <noloader@gmail.com>]

On Mon, Mar 4, 2013 at 6:46 PM, Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
> On 4 March 2013 23:40, Jeffrey Walton wrote:
>> Hi All,
>>
>> I was looking at some slides on OpenSSL and secure memory wiping using
>> volatile (Slide 36 at
>> http://www.slideshare.net/guanzhi/crypto-with-openssl).
>>
>> I believe GCC's interpretation of the use for 'volatile' is memory
>> mapped hardware. I think Ian stated it for me some time ago when I was
>> trying to understand different interpretations among compilers. If
>> volatile is for memory mapped hardware, why does GCC compile the
>> following:
>>
>> volatile void clean_memory(volatile void* dest, size_t len)
>> {
>>   volatile unsigned char* p;
>>   for(p = (volatile unsigned char*)dest; len; dest[--len] = 0)
>>     ;;
>> }
>
> This doesn't compile, it dereferences void.  Did you mean p[--len] ?
Yes, my bad. Sorry - copy[paste was not available because the slide
was an umage.

>> How does a function become a 'volatile' memory mapped object related
>> to hardware?
>
> The function isn't volatile, the return type is. Qualifying void as
> volatile is meaningless, but allowed by the C grammar.
:)

Jeff