(FILECREATED "16-May-85 11:20:35" {ERIS}<LISPCORE>SOURCES>LFSETFILEPTRPATCH.;1
 2568   

      changes to:  (VARS LFSETFILEPTRPATCHCOMS))


(* Copyright (c) 1985 by Xerox Corporation. All rights reserved.)

(PRETTYCOMPRINT LFSETFILEPTRPATCHCOMS)

(RPAQQ LFSETFILEPTRPATCHCOMS
 ((FNS \LFReadOnePage)))
(DEFINEQ

(\LFReadOnePage

  [LAMBDA (stream streamPageNumber buffer lastStreamPage offset)
                                                             (* mjs "16-May-85 11:10")


          (* * Reads a single page from stream. Returns the number of bytes read.)




          (* * If asked to read a page which is off the end of the stream, it will zero the page. Odd though it may seem, 

	  reading off the end of the file is reasonable behavior for copybytes: buffer pages must come from somewhere, and 

	  copybytes may not have to write the whole page, and in general copybytes does not know whether a page is actually in

	  a file or off the end of it. Seems inefficient, but since reading past eof does not actually require disk access, 

	  its not that bad.)




          (* * Pilot level numbers pages starting from 0; so does stream, but stream does not count the leader page.)



    (
PROG
 ((vol (fetch
 (DLIONSTREAM VOLUME) of
 stream))
	   (file (fetch
 (DLIONSTREAM FILEDESC) of
 stream))
	   (filePageNumber (
ADD1
 streamPageNumber))
	   offset)
          (
RETURN
 (if
 (
AND
 (
LEQ
 streamPageNumber lastStreamPage)
			   (
LESSP
 filePageNumber (fetch
 (FileDescriptor size) of
 file)))
		      then
 

          (* * If page inside stream, then it has presumably already been written; read it in.)



			   (
\PFGetPage
 file filePageNumber (
\PFFindPageAddr
 file filePageNumber)
				       buffer)
			   (if
 (
EQ
 streamPageNumber lastStreamPage)
			       then
 

          (* * If this was the last page in the file, then fill in the trailing bytes with nulls.)



				    (
SETQ
 offset (fetch
 (DLIONSTREAM EOFFSET) of
 stream))
				    (if
 (
EQ
 offset 0)
					then
 (
SETQ
 offset BYTESPERPAGE)
				      else
 (for
 byte from
 offset to
 (
SUB1
 BYTESPERPAGE)
					      do
 (
\PUTBASEBYTE
 buffer byte 0)))
				    offset
			     else
 BYTESPERPAGE)
		    else
 

          (* * If this was outside the stream, clear the buffer.)



			 (
\ZEROPAGE
 (fetch
 (POINTER PAGE#) of
 buffer))
			 0])
)
(PUTPROPS LFSETFILEPTRPATCH COPYRIGHT ("Xerox Corporation" 1985))
(DECLARE: DONTCOPY
  (FILEMAP (NIL (309 2480 (\LFReadOnePage 319 . 2478)))))
STOP