Standard C library namespace

[Szabolcs]

According to the standard, should the standard C functions be in the std
namespace?

So, which one is correct:

#include <cmath>
int main() { sqrt(10.0); }

or

#include <cmath>
int main() { std::sqrt(10.0); }

?

All compilers I tried accept both variants, but I'd like to know which
is the "official" one.

Szabolcs