ref: 4bcd247b909898766d549c3990c230cbeb3d17a3
dir: /sys/src/cmd/python/Doc/lib/libxdrlib.tex/
\section{\module{xdrlib} --- Encode and decode XDR data} \declaremodule{standard}{xdrlib} \modulesynopsis{Encoders and decoders for the External Data Representation (XDR).} \index{XDR} \index{External Data Representation} The \module{xdrlib} module supports the External Data Representation Standard as described in \rfc{1014}, written by Sun Microsystems, Inc. June 1987. It supports most of the data types described in the RFC. The \module{xdrlib} module defines two classes, one for packing variables into XDR representation, and another for unpacking from XDR representation. There are also two exception classes. \begin{classdesc}{Packer}{} \class{Packer} is the class for packing data into XDR representation. The \class{Packer} class is instantiated with no arguments. \end{classdesc} \begin{classdesc}{Unpacker}{data} \code{Unpacker} is the complementary class which unpacks XDR data values from a string buffer. The input buffer is given as \var{data}. \end{classdesc} \begin{seealso} \seerfc{1014}{XDR: External Data Representation Standard}{This RFC defined the encoding of data which was XDR at the time this module was originally written. It has apparently been obsoleted by \rfc{1832}.} \seerfc{1832}{XDR: External Data Representation Standard}{Newer RFC that provides a revised definition of XDR.} \end{seealso} \subsection{Packer Objects \label{xdr-packer-objects}} \class{Packer} instances have the following methods: \begin{methoddesc}[Packer]{get_buffer}{} Returns the current pack buffer as a string. \end{methoddesc} \begin{methoddesc}[Packer]{reset}{} Resets the pack buffer to the empty string. \end{methoddesc} In general, you can pack any of the most common XDR data types by calling the appropriate \code{pack_\var{type}()} method. Each method takes a single argument, the value to pack. The following simple data type packing methods are supported: \method{pack_uint()}, \method{pack_int()}, \method{pack_enum()}, \method{pack_bool()}, \method{pack_uhyper()}, and \method{pack_hyper()}. \begin{methoddesc}[Packer]{pack_float}{value} Packs the single-precision floating point number \var{value}. \end{methoddesc} \begin{methoddesc}[Packer]{pack_double}{value} Packs the double-precision floating point number \var{value}. \end{methoddesc} The following methods support packing strings, bytes, and opaque data: \begin{methoddesc}[Packer]{pack_fstring}{n, s} Packs a fixed length string, \var{s}. \var{n} is the length of the string but it is \emph{not} packed into the data buffer. The string is padded with null bytes if necessary to guaranteed 4 byte alignment. \end{methoddesc} \begin{methoddesc}[Packer]{pack_fopaque}{n, data} Packs a fixed length opaque data stream, similarly to \method{pack_fstring()}. \end{methoddesc} \begin{methoddesc}[Packer]{pack_string}{s} Packs a variable length string, \var{s}. The length of the string is first packed as an unsigned integer, then the string data is packed with \method{pack_fstring()}. \end{methoddesc} \begin{methoddesc}[Packer]{pack_opaque}{data} Packs a variable length opaque data string, similarly to \method{pack_string()}. \end{methoddesc} \begin{methoddesc}[Packer]{pack_bytes}{bytes} Packs a variable length byte stream, similarly to \method{pack_string()}. \end{methoddesc} The following methods support packing arrays and lists: \begin{methoddesc}[Packer]{pack_list}{list, pack_item} Packs a \var{list} of homogeneous items. This method is useful for lists with an indeterminate size; i.e. the size is not available until the entire list has been walked. For each item in the list, an unsigned integer \code{1} is packed first, followed by the data value from the list. \var{pack_item} is the function that is called to pack the individual item. At the end of the list, an unsigned integer \code{0} is packed. For example, to pack a list of integers, the code might appear like this: \begin{verbatim} import xdrlib p = xdrlib.Packer() p.pack_list([1, 2, 3], p.pack_int) \end{verbatim} \end{methoddesc} \begin{methoddesc}[Packer]{pack_farray}{n, array, pack_item} Packs a fixed length list (\var{array}) of homogeneous items. \var{n} is the length of the list; it is \emph{not} packed into the buffer, but a \exception{ValueError} exception is raised if \code{len(\var{array})} is not equal to \var{n}. As above, \var{pack_item} is the function used to pack each element. \end{methoddesc} \begin{methoddesc}[Packer]{pack_array}{list, pack_item} Packs a variable length \var{list} of homogeneous items. First, the length of the list is packed as an unsigned integer, then each element is packed as in \method{pack_farray()} above. \end{methoddesc} \subsection{Unpacker Objects \label{xdr-unpacker-objects}} The \class{Unpacker} class offers the following methods: \begin{methoddesc}[Unpacker]{reset}{data} Resets the string buffer with the given \var{data}. \end{methoddesc} \begin{methoddesc}[Unpacker]{get_position}{} Returns the current unpack position in the data buffer. \end{methoddesc} \begin{methoddesc}[Unpacker]{set_position}{position} Sets the data buffer unpack position to \var{position}. You should be careful about using \method{get_position()} and \method{set_position()}. \end{methoddesc} \begin{methoddesc}[Unpacker]{get_buffer}{} Returns the current unpack data buffer as a string. \end{methoddesc} \begin{methoddesc}[Unpacker]{done}{} Indicates unpack completion. Raises an \exception{Error} exception if all of the data has not been unpacked. \end{methoddesc} In addition, every data type that can be packed with a \class{Packer}, can be unpacked with an \class{Unpacker}. Unpacking methods are of the form \code{unpack_\var{type}()}, and take no arguments. They return the unpacked object. \begin{methoddesc}[Unpacker]{unpack_float}{} Unpacks a single-precision floating point number. \end{methoddesc} \begin{methoddesc}[Unpacker]{unpack_double}{} Unpacks a double-precision floating point number, similarly to \method{unpack_float()}. \end{methoddesc} In addition, the following methods unpack strings, bytes, and opaque data: \begin{methoddesc}[Unpacker]{unpack_fstring}{n} Unpacks and returns a fixed length string. \var{n} is the number of characters expected. Padding with null bytes to guaranteed 4 byte alignment is assumed. \end{methoddesc} \begin{methoddesc}[Unpacker]{unpack_fopaque}{n} Unpacks and returns a fixed length opaque data stream, similarly to \method{unpack_fstring()}. \end{methoddesc} \begin{methoddesc}[Unpacker]{unpack_string}{} Unpacks and returns a variable length string. The length of the string is first unpacked as an unsigned integer, then the string data is unpacked with \method{unpack_fstring()}. \end{methoddesc} \begin{methoddesc}[Unpacker]{unpack_opaque}{} Unpacks and returns a variable length opaque data string, similarly to \method{unpack_string()}. \end{methoddesc} \begin{methoddesc}[Unpacker]{unpack_bytes}{} Unpacks and returns a variable length byte stream, similarly to \method{unpack_string()}. \end{methoddesc} The following methods support unpacking arrays and lists: \begin{methoddesc}[Unpacker]{unpack_list}{unpack_item} Unpacks and returns a list of homogeneous items. The list is unpacked one element at a time by first unpacking an unsigned integer flag. If the flag is \code{1}, then the item is unpacked and appended to the list. A flag of \code{0} indicates the end of the list. \var{unpack_item} is the function that is called to unpack the items. \end{methoddesc} \begin{methoddesc}[Unpacker]{unpack_farray}{n, unpack_item} Unpacks and returns (as a list) a fixed length array of homogeneous items. \var{n} is number of list elements to expect in the buffer. As above, \var{unpack_item} is the function used to unpack each element. \end{methoddesc} \begin{methoddesc}[Unpacker]{unpack_array}{unpack_item} Unpacks and returns a variable length \var{list} of homogeneous items. First, the length of the list is unpacked as an unsigned integer, then each element is unpacked as in \method{unpack_farray()} above. \end{methoddesc} \subsection{Exceptions \label{xdr-exceptions}} Exceptions in this module are coded as class instances: \begin{excdesc}{Error} The base exception class. \exception{Error} has a single public data member \member{msg} containing the description of the error. \end{excdesc} \begin{excdesc}{ConversionError} Class derived from \exception{Error}. Contains no additional instance variables. \end{excdesc} Here is an example of how you would catch one of these exceptions: \begin{verbatim} import xdrlib p = xdrlib.Packer() try: p.pack_double(8.01) except xdrlib.ConversionError, instance: print 'packing the double failed:', instance.msg \end{verbatim}