select
— 等待 I/O 完成
¶
此模块提供访问
select()
and
poll()
函数可用于大多数操作系统，
epoll()
可用于 Linux 2.5+ 和
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.
模块定义如下：
select.
error
¶
The exception raised when an error occurs. The accompanying value is a pair containing the numeric error code from
errno
and the corresponding string, as would be printed by the C function
perror()
.
select.
epoll
(
[
sizehint=-1
]
)
¶
(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.
2.6 版新增。
select.
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 轮询对象 below for the methods supported by polling objects.
select.
kqueue
(
)
¶
(Only supported on BSD.) Returns a kernel queue object; see section Kqueue 对象 below for the methods supported by kqueue objects.
2.6 版新增。
select.
kevent
(
ident
,
filter=KQ_FILTER_READ
,
flags=KQ_EV_ADD
,
fflags=0
,
data=0
,
udata=0
)
¶
(Only supported on BSD.) Returns a kernel event object; see section Kevent 对象 below for the methods supported by kevent objects.
2.6 版新增。
select.
select
(
rlist
,
wlist
,
xlist
[
,
timeout
]
)
¶
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:
rlist ：等待直到读取就绪
wlist ：等待直到写入就绪
xlist : wait for an “exceptional condition” (see the manual page for what your system considers such a condition)
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).
注意
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.
select.
PIPE_BUF
¶
Files reported as ready for writing by
select()
,
poll()
or similar interfaces in this module are guaranteed to not block on a write of up to
PIPE_BUF
bytes. This value is guaranteed by POSIX to be at least 512. Availability: Unix.
2.7 版新增。
http://linux.die.net/man/4/epoll
eventmask
常量
含义
EPOLLIN可用于读取
EPOLLOUT可用于写入
EPOLLPRIUrgent data for read
EPOLLERRError condition happened on the assoc. fd
EPOLLHUPHang up happened on the assoc. fd
EPOLLETSet Edge Trigger behavior, the default is Level Trigger behavior
EPOLLONESHOTSet one-shot behavior. After one event is pulled out, the fd is internally disabled
EPOLLRDNORM相当于
EPOLLIN
EPOLLRDBANDPriority data band can be read.
EPOLLWRNORM相当于
EPOLLOUT
EPOLLWRBANDPriority data may be written.
EPOLLMSGIgnored.
epoll.close( ) ¶Close the control file descriptor of the epoll object.
epoll.fileno( ) ¶Return the file descriptor number of the control fd.
epoll.fromfd( fd ) ¶Create an epoll object from a given file descriptor.
epoll.register( fd [ , eventmask ] ) ¶Register a fd descriptor with the epoll object.
注意
Registering a file descriptor that’s already registered raises an IOError – contrary to 轮询对象 ’s register.
epoll.修改( fd , eventmask ) ¶Modify a register file descriptor.
epoll.unregister( fd ) ¶Remove a registered file descriptor from the epoll object.
epoll.poll( [ timeout=-1 [ , maxevents=-1 ] ] ) ¶Wait for events. timeout in seconds (float)
16.1.2. 轮询对象 ¶
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, whileselect()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), whilepoll()is O(number of file descriptors).
poll.register( fd [ , eventmask ] ) ¶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 afileno()method that returns an integer. File objects implementfileno(), 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，和POLLOUT, described in the table below. If not specified, the default value used will check for all 3 types of events.
常量
含义
POLLINThere is data to read
POLLPRIThere is urgent data to read
POLLOUTReady for output: writing will not block
POLLERRError condition of some sort
POLLHUPHung up
POLLNVALInvalid 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.
poll.修改( fd , eventmask ) ¶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 anIOErrorexception with errnoENOENT要被引发。2.6 版新增。
poll.unregister( fd ) ¶Remove a file descriptor being tracked by a polling object. Just like the
register()方法， 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 a
KeyErrorexception to be raised.
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 —POLLINfor waiting input,POLLOUTto 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, orNone, the call will block until there is an event for this poll object.16.1.3. Kqueue 对象 ¶
kqueue.close( ) ¶Close the control file descriptor of the kqueue object.
kqueue.fileno( ) ¶Return the file descriptor number of the control fd.
kqueue.fromfd( fd ) ¶Create a kqueue object from a given file descriptor.
kqueue.控制( changelist , max_events [ , timeout ] ) → eventlist ¶Low level interface to kevent
changelist must be an iterable of kevent objects or
Nonemax_events must be 0 or a positive integer
timeout in seconds (floats possible); the default is
None, to wait forever16.1.4. Kevent 对象 ¶
https://www.freebsd.org/cgi/man.cgi?query=kqueue&sektion=2
kevent.ident¶Value used to identify the event. The interpretation depends on the filter but it’s usually the file descriptor. In the constructor ident can either be an int or an object with a fileno() function. kevent stores the integer internally.
kevent.filter¶Name of the kernel filter.
常量
含义
KQ_FILTER_READTakes a descriptor and returns whenever there is data available to read
KQ_FILTER_WRITETakes a descriptor and returns whenever there is data available to write
KQ_FILTER_AIOAIO requests
KQ_FILTER_VNODEReturns when one or more of the requested events watched in fflag occurs
KQ_FILTER_PROCWatch for events on a process id
KQ_FILTER_NETDEVWatch for events on a network device [not available on Mac OS X]
KQ_FILTER_SIGNALReturns whenever the watched signal is delivered to the process
KQ_FILTER_TIMEREstablishes an arbitrary timer
kevent.flags¶Filter action.
常量
含义
KQ_EV_ADDAdds or modifies an event
KQ_EV_DELETERemoves an event from the queue
KQ_EV_ENABLEPermitscontrol() to returns the event
KQ_EV_DISABLEDisablesevent
KQ_EV_ONESHOTRemoves event after first occurrence
KQ_EV_CLEARReset the state after an event is retrieved
KQ_EV_SYSFLAGSinternal event
KQ_EV_FLAG1internal event
KQ_EV_EOFFilter specific EOF condition
KQ_EV_ERRORSee return values
kevent.fflags¶Filter specific flags.
KQ_FILTER_READandKQ_FILTER_WRITEfilter flags:
常量
含义
KQ_NOTE_LOWATlow water mark of a socket buffer
KQ_FILTER_VNODEfilter flags:
常量
含义
KQ_NOTE_DELETEunlink() was called
KQ_NOTE_WRITEa write occurred
KQ_NOTE_EXTENDthe file was extended
KQ_NOTE_ATTRIBan attribute was changed
KQ_NOTE_LINKthe link count has changed
KQ_NOTE_RENAMEthe file was renamed
KQ_NOTE_REVOKEaccess to the file was revoked
KQ_FILTER_PROCfilter flags:
常量
含义
KQ_NOTE_EXITthe process has exited
KQ_NOTE_FORKthe process has called fork()
KQ_NOTE_EXECthe process has executed a new process
KQ_NOTE_PCTRLMASKinternal filter flag
KQ_NOTE_PDATAMASKinternal filter flag
KQ_NOTE_TRACKfollow a process across fork()
KQ_NOTE_CHILDreturned on the child process for NOTE_TRACK
KQ_NOTE_TRACKERRunable to attach to a child
KQ_FILTER_NETDEVfilter flags (not available on Mac OS X):
常量
含义
KQ_NOTE_LINKUPlink is up
KQ_NOTE_LINKDOWNlink is down
KQ_NOTE_LINKINVlink state is invalid
kevent.data¶Filter specific data.
kevent.udata¶User defined value.