I'm not aware of any portable language that offers semantics that guarantee data is never rendered accessible to the outside world (I could well believe that current implementations don't, but I'm not aware of anything that specifies it). It always comes down to platform-specific APIs. But that's not a reason to not use C - unless you can suggest a better alternative?
Quite frankly, assembly currently is about it. And that's not the most portable thing out there, I know.
I think that C or C++ could, without too much effort, support semantics that would allow for this sort of thing. Something as simple as a "secure" keyword that could be applied to variables (where it means "leak as little as possible about this variable when it goes out of scope") or functions (where it means the same, but for the function itself and all locals of the function).
