### 2.1 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,2

    J0(5) = -1.775967713143382642e-01

The steps needed to compile this program are described in the following sections.

#### Footnotes

### (2)

The last few digits may vary slightly depending on the compiler and platform used—this is normal.