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.
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 has the same effect as registering the descriptor exactly
once.
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.
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
-- POLLIN for waiting input,
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 None, the call will
block until there is an event for this poll object.