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diff --git a/Doc/library/select.rst b/Doc/library/select.rst new file mode 100644 index 0000000..f68a0da --- /dev/null +++ b/Doc/library/select.rst @@ -0,0 +1,141 @@ + +:mod:`select` --- Waiting for I/O completion +============================================ + +.. module:: select + :synopsis: Wait for I/O completion on multiple streams. + + +This module provides access to the :cfunc:`select` and :cfunc:`poll` functions +available in most operating systems. Note that on Windows, it only works for +sockets; on other operating systems, it also works for other file types (in +particular, on Unix, it works on pipes). It cannot be used on regular files to +determine whether a file has grown since it was last read. + +The module defines the following: + + +.. exception:: error + + The exception raised when an error occurs. The accompanying value is a pair + containing the numeric error code from :cdata:`errno` and the corresponding + string, as would be printed by the C function :cfunc:`perror`. + + +.. function:: poll() + + (Not supported by all operating systems.) Returns a polling object, which + supports registering and unregistering file descriptors, and then polling them + for I/O events; see section :ref:`poll-objects` below for the methods supported + by polling objects. + + +.. function:: select(iwtd, owtd, ewtd[, timeout]) + + This is a straightforward interface to the Unix :cfunc:`select` system call. + The first three arguments are sequences of 'waitable objects': either + integers representing file descriptors or objects with a parameterless method + named :meth:`fileno` returning such an integer. The three sequences of + waitable objects are for input, output and 'exceptional conditions', + respectively. Empty sequences are allowed, but acceptance of three empty + sequences is platform-dependent. (It is known to work on Unix but not on + Windows.) The optional *timeout* argument specifies a time-out as a floating + point number in seconds. When the *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. + + .. index:: + single: socket() (in module socket) + single: popen() (in module os) + + Among the acceptable object types in the sequences are Python file objects (e.g. + ``sys.stdin``, or objects returned by :func:`open` or :func:`os.popen`), socket + objects returned by :func:`socket.socket`. You may also define a :dfn:`wrapper` + class yourself, as long as it has an appropriate :meth:`fileno` method (that + really returns a file descriptor, not just a random integer). + + .. % + + .. note:: + + .. index:: single: WinSock + + File objects on Windows are not acceptable, but sockets are. On Windows, the + underlying :cfunc:`select` function is provided by the WinSock library, and does + not handle file descriptors that don't originate from WinSock. + + +.. _poll-objects: + +Polling Objects +--------------- + +The :cfunc:`poll` system call, supported on most Unix systems, provides better +scalability for network servers that service many, many clients at the same +time. :cfunc:`poll` scales better because the system call only requires listing +the file descriptors of interest, while :cfunc:`select` builds a bitmap, turns +on bits for the fds of interest, and then afterward the whole bitmap has to be +linearly scanned again. :cfunc:`select` is O(highest file descriptor), while +:cfunc:`poll` is O(number of file descriptors). + + +.. method:: poll.register(fd[, eventmask]) + + Register a file descriptor with the polling object. Future calls to the + :meth:`poll` method will then check whether the file descriptor has any pending + I/O events. *fd* can be either an integer, or an object with a :meth:`fileno` + method that returns an integer. File objects implement :meth:`fileno`, so they + can also be used as the argument. + + *eventmask* is an optional bitmask describing the type of events you want to + check for, and can be a combination of the constants :const:`POLLIN`, + :const:`POLLPRI`, and :const:`POLLOUT`, described in the table below. If not + specified, the default value used will check for all 3 types of events. + + +-------------------+------------------------------------------+ + | Constant | Meaning | + +===================+==========================================+ + | :const:`POLLIN` | There is data to read | + +-------------------+------------------------------------------+ + | :const:`POLLPRI` | There is urgent data to read | + +-------------------+------------------------------------------+ + | :const:`POLLOUT` | Ready for output: writing will not block | + +-------------------+------------------------------------------+ + | :const:`POLLERR` | Error condition of some sort | + +-------------------+------------------------------------------+ + | :const:`POLLHUP` | Hung up | + +-------------------+------------------------------------------+ + | :const:`POLLNVAL` | Invalid request: descriptor not open | + +-------------------+------------------------------------------+ + + Registering a file descriptor that's already registered is not an error, and has + the same effect as registering the descriptor exactly once. + + +.. method:: poll.unregister(fd) + + Remove a file descriptor being tracked by a polling object. Just like the + :meth:`register` method, *fd* can be an integer or an object with a + :meth:`fileno` method that returns an integer. + + Attempting to remove a file descriptor that was never registered causes a + :exc:`KeyError` exception to be raised. + + +.. method:: poll.poll([timeout]) + + Polls the set of registered file descriptors, and returns a possibly-empty list + containing ``(fd, event)`` 2-tuples for the descriptors that have events or + errors to report. *fd* is the file descriptor, and *event* is a bitmask with + bits set for the reported events for that descriptor --- :const:`POLLIN` for + waiting input, :const:`POLLOUT` to indicate that the descriptor can be written + to, and so forth. An empty list indicates that the call timed out and no file + descriptors had any events to report. If *timeout* is given, it specifies the + length of time in milliseconds which the system will wait for events before + returning. If *timeout* is omitted, negative, or :const:`None`, the call will + block until there is an event for this poll object. + |