Thereafter, if you wish to make various interpolations, you need to call the function

splint(double x[], double y[], double y2a[], int n, double x, double *y)

which takes as input the tabulated values $x[0,..,n - 1]$ and $y[0,..,n - 1]$ and the output y2a[0,..,n - 1] from spline. It returns the value $y$ corresponding to the point $x$.