### 3.5 Examples

Here is an example of some code which checks the return value of a function where an error might be reported,

    #include <stdio.h>
    #include <gsl/gsl_errno.h>
    #include <gsl/gsl_fft_complex.h>

    ...
      int status;
      size_t n = 37;

      gsl_set_error_handler_off();

      status = gsl_fft_complex_radix2_forward (data, stride, n);

      if (status) {
        if (status == GSL_EINVAL) {
           fprintf (stderr, "invalid argument, n=%d\n", n);
        } else {
           fprintf (stderr, "failed, gsl_errno=%d\n",
                            status);
        }
        exit (-1);
      }
    ...

The function `gsl_fft_complex_radix2` only accepts integer lengths which are a power of two. If the variable `n` is not a power of two then the call to the library function will return `GSL_EINVAL`, indicating that the length argument is invalid. The function call to `gsl_set_error_handler_off` stops the default error handler from aborting the program. The `else` clause catches any other possible errors.