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Date: Tue, 05 Mar 2013 09:46:00 -0000
From: Georg-Johann Lay <avr@gjlay.de>
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Subject: Re: Question on volatile functions and GCC
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Jeffrey Walton schrieb:
> 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)
>     ;;
> }
> 
> How does a function become a 'volatile' memory mapped object related
> to hardware?

volatile can be used to express that a function is noreturn like in the 
following example where only one call of f() is issued:

typedef void ft (void);
volatile ft f;

void foo (void)
{
     f();
     f();
}

or this example that throws
<stdin>: In function 'f':
<stdin>:4:1: warning: 'noreturn' function does return [enabled by default]

typedef void ft (void);
volatile ft f;

void f (void) {}

In your example, the volatile looks meaningless and like a typo.

Johann