\section{Built-in Module \sectcode{select}}
\label{module-select}
\bimodindex{select}

This module provides access to the function \code{select} available in
most \UNIX{} versions.  It defines the following:

\setindexsubitem{(in module select)}
\begin{excdesc}{error}
The exception raised when an error occurs.  The accompanying value is
a pair containing the numeric error code from \code{errno} and the
corresponding string, as would be printed by the C function
\code{perror()}.
\end{excdesc}

\begin{funcdesc}{select}{iwtd\, owtd\, ewtd\optional{\, timeout}}
This is a straightforward interface to the \UNIX{} \code{select()}
system call.  The first three arguments are lists of `waitable
objects': either integers representing \UNIX{} file descriptors or
objects with a parameterless method named \code{fileno()} returning
such an integer.  The three lists of waitable objects are for input,
output and `exceptional conditions', respectively.  Empty lists are
allowed.  The optional \var{timeout} argument specifies a time-out as a
floating point number in seconds.  When the \var{timeout} argument
is omitted the function blocks until at least one file descriptor is
ready.  A time-out value of zero specifies a poll and never blocks.

The return value is a triple of lists of objects that are ready:
subsets of the first three arguments.  When the time-out is reached
without a file descriptor becoming ready, three empty lists are
returned.

Amongst the acceptable object types in the lists are Python file
objects (e.g. \code{sys.stdin}, or objects returned by \code{open()}
or \code{posix.popen()}), socket objects returned by
\code{socket.socket()}, and the module \code{stdwin} which happens to
define a function \code{fileno()} for just this purpose.  You may
also define a \dfn{wrapper} class yourself, as long as it has an
appropriate \code{fileno()} method (that really returns a \UNIX{} file
descriptor, not just a random integer).
\end{funcdesc}
\ttindex{socket}
\ttindex{stdwin}