Callback structures are used to pass platform-specific information. The NPAnyCallbackStruct structure contains information required by the platformPrint field of the NPEmbedPrint structure during embedded mode printing.
During printing in embedded mode, the platformPrint field of the NPEmbedPrint structure points to an NPAnyCallbackStruct. This structure contains the file pointer to which the plug-in should write its Postscript data. At the time the plug-in is called, the browser has already opened the file and written Postscript for other parts of the page. When the plug-in is done, it should leave the file open, as the browser can continue to write additional Postscript data to the file.