#include <iostream>
#include <iomanip>
#include <gsl/gsl_sf_bessel.h>

int main(void) {
  double x = 5.0;
  double y = gsl_sf_bessel_J0(x);
  printf("J0(%g) = %.18f\n", x, y);
  // or using iomanip:
  //cout<<fixed<<setprecision(18);
  //std::cout<<"J0("<<x<<") = "<<y<<endl;
}
// J0(5) = -0.177596771314338264