How to make spline using C

Splines requires far too much code to be answered here. Typically, if you do it from scratch, you will need code to work with matrix and polynomials or do it by hand. You will also have to select which contraints you want to apply to your spline.

In my case, I hace done it in C# using code found on the web for polynomial and matrix handling (well in fact, it was a simplified function to solve matrix in a tridiagonal form) and a mathematical book explaining spline and information found on the Internet was really helpful.