## An Example Program

The following short program demonstrates the use of the library by computing the value of the Bessel function J_0(x) for x=5,

    #include <stdio.h>
    #include <gsl/gsl_sf_bessel.h>

    int
    main (void)
    {
      double x = 5.0;

      double y = gsl_sf_bessel_J0 (x);

      printf("J0(%g) = %.18e\n", x, y);

      return 0;
    }

The output is shown below, and should be correct to double-precision accuracy,

    J0(5) = -1.775967713143382920e-01

The steps needed to compile programs which use the library are described in the next chapter.