This module provides access to the select and poll functions available in most operating systems, epoll available on Linux 2.5+ and kqueue available on most BSD. 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:
(Only supported on Linux 2.5.44 and newer.) Returns an edge polling object, which can be used as Edge or Level Triggered interface for I/O events; see section Edge and Level Trigger Polling (epoll) Objects below for the methods supported by epolling objects.
New in version 2.6.
(Only supported on BSD.) Returns a kernel queue object object; see section Kqueue Objects below for the methods supported by kqueue objects.
New in version 2.6.
(Only supported on BSD.) Returns a kernel event object object; see section Kevent Objects below for the methods supported by kqueue objects.
New in version 2.6.
This is a straightforward interface to the Unix select system call. The first three arguments are sequences of ‘waitable objects’: either integers representing file descriptors or objects with a parameterless method named fileno() returning such an integer:
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.
Among the acceptable object types in the sequences are Python file objects (e.g. sys.stdin, or objects returned by open() or os.popen()), socket objects returned by socket.socket(). You may also define a wrapper class yourself, as long as it has an appropriate fileno() method (that really returns a file descriptor, not just a random integer).
File objects on Windows are not acceptable, but sockets are. On Windows, the underlying select function is provided by the WinSock library, and does not handle file descriptors that don’t originate from WinSock.
Constant Meaning EPOLLIN Available for read EPOLLOUT Available for write EPOLLPRI Urgent data for read EPOLLERR Error condition happened on the assoc. fd EPOLLHUP Hang up happened on the assoc. fd EPOLLET Set Edge Trigger behavior, the default is Level Trigger behavior EPOLLONESHOT Set one-shot behavior. After one event is pulled out, the fd is internally disabled EPOLLRDNORM ??? EPOLLRDBAND ??? EPOLLWRNORM ??? EPOLLWRBAND ??? EPOLLMSG ???
The poll system call, supported on most Unix systems, provides better scalability for network servers that service many, many clients at the same time. poll scales better because the system call only requires listing the file descriptors of interest, while select builds a bitmap, turns on bits for the fds of interest, and then afterward the whole bitmap has to be linearly scanned again. select is O(highest file descriptor), while poll is O(number of file descriptors).
Register a file descriptor with the polling object. Future calls to the 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 fileno() method that returns an integer. File objects implement 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 POLLIN, POLLPRI, and POLLOUT, described in the table below. If not specified, the default value used will check for all 3 types of events.
|POLLIN||There is data to read|
|POLLPRI||There is urgent data to read|
|POLLOUT||Ready for output: writing will not block|
|POLLERR||Error condition of some sort|
|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.
Modifies an already registered fd. This has the same effect as register(fd, eventmask)(). Attempting to modify a file descriptor that was never registered causes an IOError exception with errno ENOENT to be raised.
New in version 2.6.
Remove a file descriptor being tracked by a polling object. Just like the register() method, fd can be an integer or an object with a fileno() method that returns an integer.
Attempting to remove a file descriptor that was never registered causes a KeyError exception to be raised.
Low level interface to kevent
Name of the kernel filter
|KQ_FILTER_READ||Takes a descriptor and returns whenever there is data available to read|
|KQ_FILTER_WRITE||Takes a descriptor and returns whenever there is data available to read|
|KQ_FILTER_VNODE||Returns when one or more of the requested events watched in fflag occurs|
|KQ_FILTER_PROC||Watch for events on a process id|
|KQ_FILTER_NETDEV||Watch for events on a network device [not available on Mac OS X]|
|KQ_FILTER_SIGNAL||Returns whenever the watched signal is delivered to the process|
|KQ_FILTER_TIMER||Establishes an arbitrary timer|
|KQ_EV_ADD||Adds or modifies an event|
|KQ_EV_DELETE||Removes an event from the queue|
|KQ_EV_ENABLE||Permitscontrol() to returns the event|
|KQ_EV_ONESHOT||Removes event after first occurrence|
|KQ_EV_CLEAR||Reset the state after an event is retrieved|
|KQ_EV_EOF||Filter specific EOF condition|
|KQ_EV_ERROR||See return values|
Filter specific flags
KQ_FILTER_READ and KQ_FILTER_WRITE filter flags
|KQ_NOTE_LOWAT||low water mark of a socket buffer|
KQ_FILTER_VNODE filter flags
|KQ_NOTE_DELETE||unlink() was called|
|KQ_NOTE_WRITE||a write occurred|
|KQ_NOTE_EXTEND||the file was extended|
|KQ_NOTE_ATTRIB||an attribute was changed|
|KQ_NOTE_LINK||the link count has changed|
|KQ_NOTE_RENAME||the file was renamed|
|KQ_NOTE_REVOKE||access to the file was revoked|
KQ_FILTER_PROC filter flags
|KQ_NOTE_EXIT||the process has exited|
|KQ_NOTE_FORK||the process has called fork()|
|KQ_NOTE_EXEC||the process has executed a new process|
|KQ_NOTE_PCTRLMASK||internal filter flag|
|KQ_NOTE_PDATAMASK||internal filter flag|
|KQ_NOTE_TRACK||follow a process across fork()|
|KQ_NOTE_CHILD||returned on the child process for NOTE_TRACK|
|KQ_NOTE_TRACKERR||unable to attach to a child|
KQ_FILTER_NETDEV filter flags [not available on Mac OS X]
|KQ_NOTE_LINKUP||link is up|
|KQ_NOTE_LINKDOWN||link is down|
|KQ_NOTE_LINKINV||link state is invalid|