Using XML_GetBuffer/XML_ParseBuffer

Hi,

I'm looking at integrating expat into my TCP server and I'd like to use 
avoid the extra copy by utilizing XML_GetBuffer and XML_ParseBuffer.  So 
I'll use the buffer returned from XML_GetBuffer as my TCP/IP receive buffer, 
passing the buffer to XML_ParseBuffer when some data is received.  So as the 
XML stream is received and processed, I'll need to know how much of the XML 
buffer is unused and where the unused index begins so that I can receiev 
data into the buffer without overwriting XML bytes left over from the prior 
TCP read.  So...are there any functions in the expat API that were intended 
to support this kind of design?

Thanks,
---James