Doc/lib/libchunk.tex

   1 \section{\module{chunk} ---
   2          Read IFF chunked data}
   3
   4 \declaremodule{standard}{chunk}
   5 \modulesynopsis{Module to read IFF chunks.}
   6 \moduleauthor{Sjoerd Mullender}{sjoerd@acm.org}
   7 \sectionauthor{Sjoerd Mullender}{sjoerd@acm.org}
   8
   9
  10
  11 This module provides an interface for reading files that use EA IFF 85
  12 chunks.\footnote{``EA IFF 85'' Standard for Interchange Format Files,
  13 Jerry Morrison, Electronic Arts, January 1985.}  This format is used
  14 in at least the Audio\index{Audio Interchange File
  15 Format}\index{AIFF}\index{AIFF-C} Interchange File Format
  16 (AIFF/AIFF-C) and the Real\index{Real Media File Format} Media File
  17 Format\index{RMFF} (RMFF).  The WAVE audio file format is closely
  18 related and can also be read using this module.
  19
  20 A chunk has the following structure:
  21
  22 \begin{tableiii}{c|c|l}{textrm}{Offset}{Length}{Contents}
  23   \lineiii{0}{4}{Chunk ID}
  24   \lineiii{4}{4}{Size of chunk in big-endian byte order, not including the
  25                  header}
  26   \lineiii{8}{\var{n}}{Data bytes, where \var{n} is the size given in
  27                        the preceding field}
  28   \lineiii{8 + \var{n}}{0 or 1}{Pad byte needed if \var{n} is odd and
  29                                 chunk alignment is used}
  30 \end{tableiii}
  31
  32 The ID is a 4-byte string which identifies the type of chunk.
  33
  34 The size field (a 32-bit value, encoded using big-endian byte order)
  35 gives the size of the chunk data, not including the 8-byte header.
  36
  37 Usually an IFF-type file consists of one or more chunks.  The proposed
  38 usage of the \class{Chunk} class defined here is to instantiate an
  39 instance at the start of each chunk and read from the instance until
  40 it reaches the end, after which a new instance can be instantiated.
  41 At the end of the file, creating a new instance will fail with a
  42 \exception{EOFError} exception.
  43
  44 \begin{classdesc}{Chunk}{file\optional{, align, bigendian, inclheader}}
  45 Class which represents a chunk.  The \var{file} argument is expected
  46 to be a file-like object.  An instance of this class is specifically
  47 allowed.  The only method that is needed is \method{read()}.  If the
  48 methods \method{seek()} and \method{tell()} are present and don't
  49 raise an exception, they are also used.  If these methods are present
  50 and raise an exception, they are expected to not have altered the
  51 object.  If the optional argument \var{align} is true, chunks are
  52 assumed to be aligned on 2-byte boundaries.  If \var{align} is
  53 false, no alignment is assumed.  The default value is true.  If the
  54 optional argument \var{bigendian} is false, the chunk size is assumed
  55 to be in little-endian order.  This is needed for WAVE audio files.
  56 The default value is true.  If the optional argument \var{inclheader}
  57 is true, the size given in the chunk header includes the size of the
  58 header.  The default value is false.
  59 \end{classdesc}
  60
  61 A \class{Chunk} object supports the following methods:
  62
  63 \begin{methoddesc}{getname}{}
  64 Returns the name (ID) of the chunk.  This is the first 4 bytes of the
  65 chunk.
  66 \end{methoddesc}
  67
  68 \begin{methoddesc}{getsize}{}
  69 Returns the size of the chunk.
  70 \end{methoddesc}
  71
  72 \begin{methoddesc}{close}{}
  73 Close and skip to the end of the chunk.  This does not close the
  74 underlying file.
  75 \end{methoddesc}
  76
  77 The remaining methods will raise \exception{IOError} if called after
  78 the \method{close()} method has been called.
  79
  80 \begin{methoddesc}{isatty}{}
  81 Returns \code{0}.
  82 \end{methoddesc}
  83
  84 \begin{methoddesc}{seek}{pos\optional{, whence}}
  85 Set the chunk's current position.  The \var{whence} argument is
  86 optional and defaults to \code{0} (absolute file positioning); other
  87 values are \code{1} (seek relative to the current position) and
  88 \code{2} (seek relative to the file's end).  There is no return value.
  89 If the underlying file does not allow seek, only forward seeks are
  90 allowed.
  91 \end{methoddesc}
  92
  93 \begin{methoddesc}{tell}{}
  94 Return the current position into the chunk.
  95 \end{methoddesc}
  96
  97 \begin{methoddesc}{read}{\optional{size}}
  98 Read at most \var{size} bytes from the chunk (less if the read hits
  99 the end of the chunk before obtaining \var{size} bytes).  If the
 100 \var{size} argument is negative or omitted, read all data until the
 101 end of the chunk.  The bytes are returned as a string object.  An
 102 empty string is returned when the end of the chunk is encountered
 103 immediately.
 104 \end{methoddesc}
 105
 106 \begin{methoddesc}{skip}{}
 107 Skip to the end of the chunk.  All further calls to \method{read()}
 108 for the chunk will return \code{''}.  If you are not interested in the
 109 contents of the chunk, this method should be called so that the file
 110 points to the start of the next chunk.
 111 \end{methoddesc}