python - socket / setblocking

socket([family[, type[, proto]]]) -> socket object

Open a socket of the given type.  The family argument specifies the
address family; it defaults to AF_INET.  The type argument specifies
whether this is a stream (SOCK_STREAM, this is the default)
or datagram (SOCK_DGRAM) socket.  The protocol argument defaults to 0,
specifying the default protocol.  Keyword arguments are accepted.

A socket object represents one endpoint of a network connection.

Methods of socket objects (keyword arguments not allowed):

accept() -- accept a connection, returning new socket and client address
bind(addr) -- bind the socket to a local address
close() -- close the socket
connect(addr) -- connect the socket to a remote address
connect_ex(addr) -- connect, return an error code instead of an exception
dup() -- return a new socket object identical to the current one [*]
fileno() -- return underlying file descriptor
getpeername() -- return remote address [*]
getsockname() -- return local address
getsockopt(level, optname[, buflen]) -- get socket options
gettimeout() -- return timeout or None
listen(n) -- start listening for incoming connections
makefile([mode, [bufsize]]) -- return a file object for the socket [*]
recv(buflen[, flags]) -- receive data
recv_into(buffer[, nbytes[, flags]]) -- receive data (into a buffer)
recvfrom(buflen[, flags]) -- receive data and sender's address
recvfrom_into(buffer[, nbytes, [, flags])
-- receive data and sender's address (into a buffer)
sendall(data[, flags]) -- send all data
send(data[, flags]) -- send data, may not send all of it
sendto(data[, flags], addr) -- send data to a given address
setblocking(0 | 1) -- set or clear the blocking I/O flag
setsockopt(level, optname, value) -- set socket options
settimeout(None | float) -- set or clear the timeout
shutdown(how) -- shut down traffic in one or both directions

In network programming, it is common to hear about different kinds of I/O programming models such as blocking I/O, nonblocking I/O, and polling. The purpose of this exercise is to simply define what these terms mean with some simple socket examples.

(a) Blocking I/O

Blocking I/O means that operations such as accept(), recv() or send() will block (wait) until the requested operation can be completed. Consider the server you were working on in the last exercise:

>>> from socket import *
>>> s = socket(AF_INET, SOCK_STREAM)
>>> s.bind(("",15000))
>>> s.listen(5)
>>> c,a = s.accept()

You will notice that the accept() operation waits indefinitely until a new connection arrives. This is blocking I/O in action.

In IDLE (client), connect to your server to see it unblock and return.

>>> from socket import *
>>> s = socket(AF_INET, SOCK_STREAM)
>>> s.connect(("localhost",15000))
>>>

(b) Blocking I/O with a timeout

You can set a timeout using the settimeout() method of a socket. In your server, set a timeout on the client connection that you just received. Try to receive some data and just sit there and wait:

>>> c.settimeout(15)
>>> c.recv(8192)
... wait 15 seconds. See what happens ...

In production code, setting some kind of timeout is probably not a bad idea. A common failure mode for servers is the case where a client connects, but then never sends any data--holding the client connection open forever. With a timeout, the server can give up after a while and close the connection.

(c) Non-blocking I/O

Non-blocking I/O is a style of I/O handling where operations immediately raise an exception if the operation can't be completely immediately. On the server, try doing this with the client connection received in part (b):

>>> c.setblocking(False)
>>> c.recv(8192)
Traceback (most recent call last):
File "", line 1, in
socket.error: [Errno 35] Resource temporarily unavailable
>>>

You will notice that recv() immediately raises an exception. If you try it again, you'll get the same exception over and over.

In IDLE (the client), try sending a short message to the server:

>>> s.send("Hello?")
6
>>>

Try redoing the recv() operation. Notice that it works now. This is because the server received some data that it can now return.

>>> c.recv(8192)
'Hello?'
>>>

Although non-blocking I/O could be used to periodically check a socket for incoming data, it's extremely uncommon for it to be used in that manner. It is actually more common for non-blocking mode to be used when sending data. That might sound counterintuitive, but it allows the sender to continue with its data processing even if a socket's output buffers are temporarily full (instead of blocking, the sender continues to do other work).