Timer function to provide time in nano seconds using C++

I wish to calculate the time it took for an API to return a value. The time taken for such an action is in the space of nanoseconds. As the API is a C++ class/function, I am using the timer.h to calculate the same:

  #include <ctime>
  #include <iostream>

  using namespace std;

  int main(int argc, char** argv) {

      clock_t start;
      double diff;
      start = clock();
      diff = ( std::clock() - start ) / (double)CLOCKS_PER_SEC;
      cout<<"printf: "<< diff <<'\n';

      return 0;
  }

The above code gives the time in seconds. How do I get the same in nano seconds and with more precision?