 data has already been
read from the stream and the stream is not seekable. It is used in conjunction with <tt>ibytes</tt>.  In this case, <tt>initial</tt>
should be a pointer to a buffer containing the data read.</dd>
<dt><i>ibytes</i></dt>
<dd>Typically set to 0.  This parameter is useful if some data has already been
read from the stream and the stream is not seekable. In this case, <tt>ibytes</tt>
should contain the length (in bytes) of the buffer.  Used together with <tt>initial</tt>.</dd>
<dt><i>callb