josuttis.de - input streambuf

dieser Code:

class fdinbuf : public std::streambuf {
  protected:
    int fd;    // file descriptor
  protected:
    /* data buffer:
     * - at most, pbSize characters in putback area plus
     * - at most, bufSize characters in ordinary read buffer
     */
    static const int pbSize = 4;        // size of putback area
    static const int bufSize = 1024;    // size of the data buffer
    char buffer[bufSize+pbSize];        // data buffer

  public:
    /* constructor
     * - initialize file descriptor
     * - initialize empty data buffer
     * - no putback area
     * => force underflow()
     */
    fdinbuf (int _fd) : fd(_fd) {
        setg (buffer+pbSize,     // beginning of putback area
              buffer+pbSize,     // read position
              buffer+pbSize);    // end position
    }

  protected:
    // insert new characters into the buffer
    virtual int_type underflow () {
#ifndef _MSC_VER
        using std::memmove;
#endif

        // is read position before end of buffer?
        if (gptr() < egptr()) {
            return traits_type::to_int_type(*gptr());
        }

        /* process size of putback area
         * - use number of characters read
         * - but at most size of putback area
         */
        int numPutback;
        numPutback = gptr() - eback();
        if (numPutback > pbSize) {
            numPutback = pbSize;
        }

        /* copy up to pbSize characters previously read into
         * the putback area
         */
        memmove (buffer+(pbSize-numPutback), gptr()-numPutback,
                numPutback);

        // read at most bufSize new characters
        int num;
        num = read (fd, buffer+pbSize, bufSize);
        if (num <= 0) {
            // ERROR or EOF
            return EOF;
        }

        // reset buffer pointers
        setg (buffer+(pbSize-numPutback),   // beginning of putback area
              buffer+pbSize,                // read position
              buffer+pbSize+num);           // end of buffer

        // return next character
        return traits_type::to_int_type(*gptr());
    }
};

Kann mir jemand erklären, was es mit dieser putback-area auf sich hat?
Wieso schreibt er nicht einfach den Buffer von vorne bis hinten voll?
Steh irgendwie auf dem Schlauch (Hab selber eine Test-Implementation gemacht, in der ich kein "putback" verwendet hab und hat so weit ich gesehen hab prima funktioniert)