title: XMP metadata reference inside PDF
body: <p>I'm planning to develop an application to work with PDF metadata without any extern library, just accessing myself to the raw PDF.</p>
  
  <p>I've understood info dictionaries and how they are referenced by the <em>/Info</em> tag in the trailer. However, taking a look inside a PDF file with an hexadecimal editor I did not find any reference to the XMP object; it exists, but it is not referenced. (When I say "XMP metadata" I mean the whole file's metadata not individual objects' metadata).</p>
  
  <p>So, my <strong>question</strong> is: How is XMP metadata referenced inside a PDF file? How can an extern application retrieve XMP metadata if it is not referenced?</p>
  
  <p>I suppose that if it is not referenced it must be placed in some fixed location inside the file, but I'm not sure about it.</p>
  
  <p>Thanks in advance.</p>