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