cpython/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
                appearantly 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}