Isn't the derivate relatively easy in C?
#include <stdio.h>
const double delta = 1.0e-6;
double cube(double x) { return x * x * x; }
double deriv(double (*f)(double), double x) { return (f(x+delta) - f(x)) / delta; }
int main()
{
printf("%f", deriv(&cube, 2));
return 0;
}