Re: how to avoid a memset() optimization

"Clayton Weaver" <cgweav@aol.com> writes:


>>int main()
>>{
>> char key[16];
>> strcpy(key, "whatever");
>> encrpts(key);
>> memset(key, 0, 16);
>>}
>
> if (!memset(key, 0, 16)) {
> return error;
> }
>
>(ie waste a jump; might even be more efficient if it still allows
>other optimizations that volatile would prevent).


This is not guaranteed to work. A compiler might easily know that
`memset(key, ...)' returns `key', and that the address of local variables
is never null, and might thus be able to optimize away both the if and
the memset.


Using volatile is better, because it provides more guarantees.


Unfortunately using volatile is not guaranteed to work either,
Using volatile guarantees that the contents of `key' will be
overwritten. But it doesn't prevent the compiler from having
alos stored the contents into other memory areas (e.g. the stack
frame used by encrpts()) which may not get cleared. However,
this failure mode is IMHO a lot less likely.
--
Fergus Henderson <fjh@cs.mu.oz.au> | "I have always known that the pursuit
The University of Melbourne | of excellence is a lethal habit"
WWW: <http://www.cs.mu.oz.au/~fjh> | -- the last words of T. S. Garp.

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