This module provides access to theselect() andpoll() functionsavailable in most operating systems,devpoll() available onSolaris and derivatives,epoll() available on Linux 2.5+ andkqueue() 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 sinceit was last read.
The module defines the following:
(Only supported on Solaris and derivatives.) Returns a/dev/pollpolling object; see section/dev/poll Polling Objects below for themethods supported by devpoll objects.
devpoll() objects are linked to the number of filedescriptors allowed at the time of instantiation. If your programreduces this value,devpoll() will fail. If your programincreases this value,devpoll() may return anincomplete list of active file descriptors.
New in version 3.3.
(Only supported on Linux 2.5.44 and newer.) Return an edge polling object,which can be used as Edge or Level Triggered interface for I/Oevents.sizehint is deprecated and completely ignored.flags can be settoEPOLL_CLOEXEC, which causes the epoll descriptor to be closedautomatically whenos.execve() is called. See sectionEdge and Level Trigger Polling (epoll) Objects below for the methods supported by epolling objects.
Changed in version 3.3:Added theflags parameter.
(Not supported by all operating systems.) Returns a polling object, whichsupports registering and unregistering file descriptors, and then polling themfor I/O events; see sectionPolling Objects below for the methods supportedby polling objects.
(Only supported on BSD.) Returns a kernel queue object; see sectionKqueue Objects below for the methods supported by kqueue objects.
(Only supported on BSD.) Returns a kernel event object; see sectionKevent Objects below for the methods supported by kevent objects.
This is a straightforward interface to the Unixselect() system call.The first three arguments are sequences of ‘waitable objects’: eitherintegers representing file descriptors or objects with a parameterless methodnamedfileno() returning such an integer:
Empty sequences are allowed, but acceptance of three empty sequences isplatform-dependent. (It is known to work on Unix but not on Windows.) Theoptionaltimeout argument specifies a time-out as a floating point numberin seconds. When thetimeout argument is omitted the function blocks untilat least one file descriptor is ready. A time-out value of zero specifies apoll and never blocks.
The return value is a triple of lists of objects that are ready: subsets of thefirst three arguments. When the time-out is reached without a file descriptorbecoming ready, three empty lists are returned.
Among the acceptable object types in the sequences are Pythonfileobjects (e.g.sys.stdin, or objects returned byopen() oros.popen()), socket objects returned bysocket.socket(). You may also define awrapper class yourself,as long as it has an appropriatefileno() method (thatreally returns a file descriptor, not just a random integer).
Note
File objects on Windows are not acceptable, but sockets are. On Windows,the underlyingselect() function is provided by the WinSocklibrary, and does not handle file descriptors that don’t originate fromWinSock.
The minimum number of bytes which can be written without blocking to a pipewhen the pipe has been reported as ready for writing byselect(),poll() or another interface in this module. This doesn’t applyto other kind of file-like objects such as sockets.
This value is guaranteed by POSIX to be at least 512. Availability: Unix.
New in version 3.2.
Solaris and derivatives have/dev/poll. Whileselect() isO(highest file descriptor) andpoll() is O(number of filedescriptors),/dev/poll is O(active file descriptors).
/dev/poll behaviour is very close to the standardpoll()object.
Register a file descriptor with the polling object. Future calls to thepoll() method will then check whether the file descriptor has anypending I/O events.fd can be either an integer, or an object with afileno() method that returns an integer. File objectsimplementfileno(), so they can also be used as the argument.
eventmask is an optional bitmask describing the type of events you want tocheck for. The constants are the same that withpoll()object. The default value is a combination of the constantsPOLLIN,POLLPRI, andPOLLOUT.
Warning
Registering a file descriptor that’s already registered is not anerror, but the result is undefined. The appropiate action is tounregister or modify it first. This is an important differencecompared withpoll().
This method does anunregister() followed by aregister(). It is (a bit) more efficient that doing the sameexplicitly.
Remove a file descriptor being tracked by a polling object. Just like theregister() method,fd can be an integer or an object with afileno() method that returns an integer.
Attempting to remove a file descriptor that was never registered issafely ignored.
Polls the set of registered file descriptors, and returns a possibly-empty listcontaining(fd,event) 2-tuples for the descriptors that have events orerrors to report.fd is the file descriptor, andevent is a bitmask withbits set for the reported events for that descriptor —POLLIN forwaiting input,POLLOUT to indicate that the descriptor can be writtento, and so forth. An empty list indicates that the call timed out and no filedescriptors had any events to report. Iftimeout is given, it specifies thelength of time in milliseconds which the system will wait for events beforereturning. Iftimeout is omitted, -1, orNone, the call willblock until there is an event for this poll object.
http://linux.die.net/man/4/epoll
eventmask
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 isLevel Trigger behavior EPOLLONESHOT Set one-shot behavior. After one event ispulled out, the fd is internally disabled EPOLLRDNORM Equivalent toEPOLLIN EPOLLRDBAND Priority data band can be read. EPOLLWRNORM Equivalent toEPOLLOUT EPOLLWRBAND Priority data may be written. EPOLLMSG Ignored.
Close the control file descriptor of the epoll object.
Return the file descriptor number of the control fd.
Create an epoll object from a given file descriptor.
Register a fd descriptor with the epoll object.
Modify a register file descriptor.
Remove a registered file descriptor from the epoll object.
Wait for events. timeout in seconds (float)
Thepoll() system call, supported on most Unix systems, provides betterscalability for network servers that service many, many clients at the sametime.poll() scales better because the system call only requires listingthe file descriptors of interest, whileselect() builds a bitmap, turnson bits for the fds of interest, and then afterward the whole bitmap has to belinearly scanned again.select() is O(highest file descriptor), whilepoll() is O(number of file descriptors).
Register a file descriptor with the polling object. Future calls to thepoll() method will then check whether the file descriptor has anypending I/O events.fd can be either an integer, or an object with afileno() method that returns an integer. File objectsimplementfileno(), so they can also be used as the argument.
eventmask is an optional bitmask describing the type of events you want tocheck for, and can be a combination of the constantsPOLLIN,POLLPRI, andPOLLOUT, described in the table below. If notspecified, the default value used will check for all 3 types of events.
| Constant | Meaning |
|---|---|
| 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 |
| POLLHUP | Hung up |
| POLLNVAL | Invalid request: descriptor not open |
Registering a file descriptor that’s already registered is not an error, and hasthe same effect as registering the descriptor exactly once.
Modifies an already registered fd. This has the same effect asregister(fd,eventmask). Attempting to modify a file descriptorthat was never registered causes anIOError exception with errnoENOENT to be raised.
Remove a file descriptor being tracked by a polling object. Just like theregister() method,fd can be an integer or an object with afileno() method that returns an integer.
Attempting to remove a file descriptor that was never registered causes aKeyError exception to be raised.
Polls the set of registered file descriptors, and returns a possibly-empty listcontaining(fd,event) 2-tuples for the descriptors that have events orerrors to report.fd is the file descriptor, andevent is a bitmask withbits set for the reported events for that descriptor —POLLIN forwaiting input,POLLOUT to indicate that the descriptor can be writtento, and so forth. An empty list indicates that the call timed out and no filedescriptors had any events to report. Iftimeout is given, it specifies thelength of time in milliseconds which the system will wait for events beforereturning. Iftimeout is omitted, negative, orNone, the call willblock until there is an event for this poll object.
Close the control file descriptor of the kqueue object.
Return the file descriptor number of the control fd.
Create a kqueue object from a given file descriptor.
Low level interface to kevent
http://www.freebsd.org/cgi/man.cgi?query=kqueue&sektion=2
Value used to identify the event. The interpretation depends on the filterbut it’s usually the file descriptor. In the constructor ident can eitherbe an int or an object with afileno() method. keventstores the integer internally.
Name of the kernel filter.
| Constant | Meaning |
|---|---|
| KQ_FILTER_READ | Takes a descriptor and returns wheneverthere is data available to read |
| KQ_FILTER_WRITE | Takes a descriptor and returns wheneverthere is data available to write |
| KQ_FILTER_AIO | AIO requests |
| KQ_FILTER_VNODE | Returns when one or more of the requestedevents watched infflag 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 isdelivered to the process |
| KQ_FILTER_TIMER | Establishes an arbitrary timer |
Filter action.
| Constant | Meaning |
|---|---|
| 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_DISABLE | Disablesevent |
| KQ_EV_ONESHOT | Removes event after first occurrence |
| KQ_EV_CLEAR | Reset the state after an event is retrieved |
| KQ_EV_SYSFLAGS | internal event |
| KQ_EV_FLAG1 | internal event |
| KQ_EV_EOF | Filter specific EOF condition |
| KQ_EV_ERROR | See return values |
Filter specific flags.
KQ_FILTER_READ andKQ_FILTER_WRITE filter flags:
| Constant | Meaning |
|---|---|
| KQ_NOTE_LOWAT | low water mark of a socket buffer |
KQ_FILTER_VNODE filter flags:
| Constant | Meaning |
|---|---|
| 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:
| Constant | Meaning |
|---|---|
| KQ_NOTE_EXIT | the process has exited |
| KQ_NOTE_FORK | the process has calledfork() |
| 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 acrossfork() |
| KQ_NOTE_CHILD | returned on the child process forNOTE_TRACK |
| KQ_NOTE_TRACKERR | unable to attach to a child |
KQ_FILTER_NETDEV filter flags (not available on Mac OS X):
| Constant | Meaning |
|---|---|
| KQ_NOTE_LINKUP | link is up |
| KQ_NOTE_LINKDOWN | link is down |
| KQ_NOTE_LINKINV | link state is invalid |
Filter specific data.
User defined value.
17.7.queue — A synchronized queue class
17.9.dummy_threading — Drop-in replacement for thethreading module
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