mirror of https://github.com/python/cpython
130 lines
5.5 KiB
TeX
130 lines
5.5 KiB
TeX
\section{\module{select} ---
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Waiting for I/O completion}
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\declaremodule{builtin}{select}
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\modulesynopsis{Wait for I/O completion on multiple streams.}
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This module provides access to the \cfunction{select()}
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and \cfunction{poll()} functions
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available in most operating systems. Note that on Windows, it only
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works for sockets; on other operating systems, it also works for other
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file types (in particular, on \UNIX{}, it works on pipes). It cannot
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be used on regular files to determine whether a file has grown since
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it was last read.
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The module defines the following:
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\begin{excdesc}{error}
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The exception raised when an error occurs. The accompanying value is
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a pair containing the numeric error code from \cdata{errno} and the
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corresponding string, as would be printed by the \C{} function
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\cfunction{perror()}.
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\end{excdesc}
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\begin{funcdesc}{poll}{}
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(Not supported by all operating systems.) Returns a polling object,
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which supports registering and unregistering file descriptors, and
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then polling them for I/O events;
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see section~\ref{poll-objects} below for the methods supported by
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polling objects.
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\end{funcdesc}
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\begin{funcdesc}{select}{iwtd, owtd, ewtd\optional{, timeout}}
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This is a straightforward interface to the \UNIX{} \cfunction{select()}
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system call. The first three arguments are lists of `waitable
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objects': either integers representing \UNIX{} file descriptors or
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objects with a parameterless method named \method{fileno()} returning
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such an integer. The three lists of waitable objects are for input,
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output and `exceptional conditions', respectively. Empty lists are
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allowed. The optional \var{timeout} argument specifies a time-out as a
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floating point number in seconds. When the \var{timeout} argument
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is omitted the function blocks until at least one file descriptor is
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ready. A time-out value of zero specifies a poll and never blocks.
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The return value is a triple of lists of objects that are ready:
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subsets of the first three arguments. When the time-out is reached
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without a file descriptor becoming ready, three empty lists are
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returned.
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Amongst the acceptable object types in the lists are Python file
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objects (e.g. \code{sys.stdin}, or objects returned by
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\function{open()} or \function{os.popen()}), socket objects
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returned by \function{socket.socket()},%
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\withsubitem{(in module socket)}{\ttindex{socket()}}
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\withsubitem{(in module os)}{\ttindex{popen()}}
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and the module \module{stdwin}\refbimodindex{stdwin} which happens to
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define a function
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\function{fileno()}\withsubitem{(in module stdwin)}{\ttindex{fileno()}}
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for just this purpose. You may
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also define a \dfn{wrapper} class yourself, as long as it has an
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appropriate \method{fileno()} method (that really returns a \UNIX{}
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file descriptor, not just a random integer).
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\end{funcdesc}
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\subsection{Polling Objects
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\label{poll-objects}}
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The \cfunction{poll()} system call, supported on most Unix systems,
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provides better scalability for network servers that service many,
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many clients at the same time.
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\cfunction{poll()} scales better because the system call only
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requires listing the file descriptors of interest, while \cfunction{select()}
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builds a bitmap, turns on bits for the fds of interest, and then
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afterward the whole bitmap has to be linearly scanned again.
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\cfunction{select()} is O(highest file descriptor), while
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\cfunction{poll()} is O(number of file descriptors).
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\begin{methoddesc}{register}{fd\optional{, eventmask}}
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Register a file descriptor with the polling object. Future calls to
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the \method{poll()} method will then check whether the file descriptor
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has any pending I/O events. \var{fd} can be either an integer, or an
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object with a \method{fileno()} method that returns an integer. File
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objects implement
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\method{fileno()}, so they can also be used as the argument.
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\var{eventmask} is an optional bitmask describing the type of events you
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want to check for, and can be a combination of the constants
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\constant{POLLIN}, \constant{POLLPRI}, and \constant{POLLOUT},
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described in the table below. If not specified, the default value
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used will check for all 3 types of events.
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\begin{tableii}{l|l}{code}{Constant}{Meaning}
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\lineii{POLLIN}{There is data to read}
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\lineii{POLLPRI}{There is urgent data to read}
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\lineii{POLLOUT}{Ready for output: writing will not block}
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\lineii{POLLERR}{Error condition of some sort}
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\lineii{POLLHUP}{Hung up}
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\lineii{POLLNVAL}{Invalid request: descriptor not open}
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\end{tableii}
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Registering a file descriptor that's already registered is not an
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error, and has the same effect as registering the descriptor exactly
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once.
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\end{methoddesc}
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\begin{methoddesc}{unregister}{fd}
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Remove a file descriptor being tracked by a polling object. Just like
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the \method{register()} method, \var{fd} can be an integer or an
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object with a \method{fileno()} method that returns an integer.
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Attempting to remove a file descriptor that was never registered
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causes a \exception{KeyError} exception to be raised.
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\end{methoddesc}
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\begin{methoddesc}{poll}{\optional{timeout}}
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Polls the set of registered file descriptors, and returns a
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possibly-empty list containing \code{(\var{fd}, \var{event})} 2-tuples
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for the descriptors that have events or errors to report.
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\var{fd} is the file descriptor, and \var{event} is a bitmask
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with bits set for the reported events for that descriptor
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--- \constant{POLLIN} for waiting input,
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\constant{POLLOUT} to indicate that the descriptor can be written to, and
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so forth.
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An empty list indicates that the call timed out and no file
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descriptors had any events to report.
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\end{methoddesc}
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