title: What does #define __REDIRECT_NTH do in unistd.h?
body: <p>GNU unistd.h has this bit of magic:</p>
  
  <pre><code>/* Move FD's file position to OFFSET bytes from the
     beginning of the file (if WHENCE is SEEK_SET),
     the current position (if WHENCE is SEEK_CUR),
     or the end of the file (if WHENCE is SEEK_END).
     Return the new file position.  */
  #ifndef __USE_FILE_OFFSET64
  extern __off_t lseek (int __fd, __off_t __offset, int __whence) __THROW;                                                                    
  #else
  # ifdef __REDIRECT_NTH
  extern __off64_t __REDIRECT_NTH (lseek,
                                (int __fd, __off64_t __offset, int __whence),
                                 lseek64);                                                                                                    
  # else
  #  define lseek lseek64
  # endif
  #endif
  #ifdef __USE_LARGEFILE64
  extern __off64_t lseek64 (int __fd, __off64_t __offset, int __whence) __THROW;                                                              
  #endif
  </code></pre>
  
  <p>What does __REDIRECT_NTH mean?</p>