Returns an error message string.
    Format
      #include  <string.h>
      char *hstrerror  (int errnum);

1  –  Arguments

 errnum
    An error number specifying a value of h_errno.

2  –  Description

    This function maps the error number specified by the errnum
    argument to a location-dependent error message string and returns
    a pointer to the string. The string pointed to by the return
    value cannot be modified by the program, but could be overwritten
    by subsequent calls to this function.

3  –  Return Values

    x                  A pointer to the generated message string.
    -1                 On error, errno might be set, but no return
                       value is reserved to indicate an error.

4  –  Errors

                       If the hstrerror() function fails, errno is
                       set to EINVAL, indicating the value of the
                       errnum argument is an invalid error number.