Why is my log in the std namespace?

In the code below, I define a trivial log function. In main I try not to call it; I call std::log. Nevertheless, my own log is called; and I see "log!" on screen. Does anyone know why? I use G++ 4.7 and clang++ 3.2.

#include <iostream>
#include <cmath>

double log(const double x) { std::cout << "log!\n"; return x; }

int main(int argc, char *argv[])
{
  std::log(3.14);
  return 0;
}