The functions supplied in the program library are spline and splint. In order to use cubic spline interpolation you need first to call

spline(double x[], double y[], int n, double yp1,  double yp2, double y2[])

This function takes as input $x[0,..,n - 1]$ and $y[0,..,n - 1]$ containing a tabulation $y_i = f(x_i)$ with $x_0 < x_1 < .. < x_{n - 1}$ together with the first derivatives of $f(x)$ at $x_0$ and $x_{n-1}$, respectively. Then the function returns $y2[0,..,n-1]$ which contains the second derivatives of $f(x_i)$ at each point $x_i$. $n$ is the number of points. This function provides the cubic spline interpolation for all subintervals and is called only once.