cpython/Doc/libxdrlib.tex

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\section{Standard Module \sectcode{xdrlib}}
\label{module-xdrlib}
\stmodindex{xdrlib}
\index{XDR}
\index{External Data Representation}
\setindexsubitem{(in module xdrlib)}
The \code{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 \code{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.
\subsection{Packer Objects}
\code{Packer} is the class for packing data into XDR representation.
The \code{Packer} class is instantiated with no arguments.
\begin{funcdesc}{get_buffer}{}
Returns the current pack buffer as a string.
\end{funcdesc}
\begin{funcdesc}{reset}{}
Resets the pack buffer to the empty string.
\end{funcdesc}
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: \code{pack_uint()}, \code{pack_int()},
\code{pack_enum()}, \code{pack_bool()}, \code{pack_uhyper()},
and \code{pack_hyper()}.
\begin{funcdesc}{pack_float}{value}
Packs the single-precision floating point number \var{value}.
\end{funcdesc}
\begin{funcdesc}{pack_double}{value}
Packs the double-precision floating point number \var{value}.
\end{funcdesc}
The following methods support packing strings, bytes, and opaque data:
\begin{funcdesc}{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{funcdesc}
\begin{funcdesc}{pack_fopaque}{n, data}
Packs a fixed length opaque data stream, similarly to
\code{pack_fstring()}.
\end{funcdesc}
\begin{funcdesc}{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 \code{pack_fstring()}.
\end{funcdesc}
\begin{funcdesc}{pack_opaque}{data}
Packs a variable length opaque data string, similarly to
\code{pack_string()}.
\end{funcdesc}
\begin{funcdesc}{pack_bytes}{bytes}
Packs a variable length byte stream, similarly to \code{pack_string()}.
\end{funcdesc}
The following methods support packing arrays and lists:
\begin{funcdesc}{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.
\end{funcdesc}
\begin{funcdesc}{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 \code{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{funcdesc}
\begin{funcdesc}{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 \code{pack_farray()} above.
\end{funcdesc}
\subsection{Unpacker Objects}
\code{Unpacker} is the complementary class which unpacks XDR data
values from a string buffer, and has the following methods:
\begin{funcdesc}{__init__}{data}
Instantiates an \code{Unpacker} object with the string buffer
\var{data}.
\end{funcdesc}
\begin{funcdesc}{reset}{data}
Resets the string buffer with the given \var{data}.
\end{funcdesc}
\begin{funcdesc}{get_position}{}
Returns the current unpack position in the data buffer.
\end{funcdesc}
\begin{funcdesc}{set_position}{position}
Sets the data buffer unpack position to \var{position}. You should be
careful about using \code{get_position()} and \code{set_position()}.
\end{funcdesc}
\begin{funcdesc}{get_buffer}{}
Returns the current unpack data buffer as a string.
\end{funcdesc}
\begin{funcdesc}{done}{}
Indicates unpack completion. Raises an \code{xdrlib.Error} exception
if all of the data has not been unpacked.
\end{funcdesc}
In addition, every data type that can be packed with a \code{Packer},
can be unpacked with an \code{Unpacker}. Unpacking methods are of the
form \code{unpack_\var{type}()}, and take no arguments. They return the
unpacked object.
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\begin{funcdesc}{unpack_float}{}
Unpacks a single-precision floating point number.
\end{funcdesc}
\begin{funcdesc}{unpack_double}{}
Unpacks a double-precision floating point number, similarly to
\code{unpack_float()}.
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\end{funcdesc}
In addition, the following methods unpack strings, bytes, and opaque
data:
\begin{funcdesc}{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{funcdesc}
\begin{funcdesc}{unpack_fopaque}{n}
Unpacks and returns a fixed length opaque data stream, similarly to
\code{unpack_fstring()}.
\end{funcdesc}
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\begin{funcdesc}{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 \code{unpack_fstring()}.
\end{funcdesc}
\begin{funcdesc}{unpack_opaque}{}
Unpacks and returns a variable length opaque data string, similarly to
\code{unpack_string()}.
\end{funcdesc}
\begin{funcdesc}{unpack_bytes}{}
Unpacks and returns a variable length byte stream, similarly to
\code{unpack_string()}.
\end{funcdesc}
The following methods support unpacking arrays and lists:
\begin{funcdesc}{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{funcdesc}
\begin{funcdesc}{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{funcdesc}
\begin{funcdesc}{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 \code{unpack_farray()} above.
\end{funcdesc}
\subsection{Exceptions}
\nodename{Exceptions in xdrlib module}
Exceptions in this module are coded as class instances:
\begin{excdesc}{Error}
The base exception class. \code{Error} has a single public data
member \code{msg} containing the description of the error.
\end{excdesc}
\begin{excdesc}{ConversionError}
Class derived from \code{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}