// outline for LLSQ cubic polynomial approximation to some function
//
// compile: gcc -std=c11 -pedantic -Wall main.c llsq.c -lm

#include <stdio.h>
#include <math.h>
#include "llsq.h"

#define N 21

...

  double x[N], y[N], c[4]; // cubic polynomial has 4 coefficients

 // initialize x,y values
 //
 // generate x values in a linear scale from xmin to xmax
 //
 // ...

 // call llsq()
 //
  llsq( x, y, N, c, 4);

 // print x,y values
 // ...

 // evaluate and print polynomial values
 // ...