What does Python s socket recv return for non-blocking sockets if no data is received until a timeout occurs

Question

Basically  I ve read in several places that socket recv   will return whatever it can read  or an empty string signalling that the other side has shut down   the official docs don t even mention what it returns when the connection is shut down    great    This is all fine and dandy for blocking sockets  since we know that recv   only returns when there actually is something to receive  so when it returns an empty string  it MUST mean the other side has closed the connection  right   Okay  fine  but what happens when my socket is non-blocking   I have searched a bit  maybe not enough  who knows   and can t figure out how to tell when the other side has closed the connection using a non-blocking socket  There seems to be no method or attribute that tells us this  and comparing the return value of recv   to the empty string seems absolutely useless    is it just me having this problem   As a simple example  let s say my socket s timeout is set to 1 2342342  whatever non-negative number you like here  seconds and I call socket recv 1024   but the other side doesn t send anything during that 1 2342342 second period  The recv   call will return an empty string and I have no clue as to whether the connection is still standing or not

User · Accepted Answer

In the case of a non blocking socket that has no data available, recv will throw the socket.error exception and the value of the exception will have the errno of either EAGAIN or EWOULDBLOCK. Example:

import sys
import socket
import fcntl, os
import errno
from time import sleep

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect(('127.0.0.1',9999))
fcntl.fcntl(s, fcntl.F_SETFL, os.O_NONBLOCK)

while True:
    try:
        msg = s.recv(4096)
    except socket.error, e:
        err = e.args[0]
        if err == errno.EAGAIN or err == errno.EWOULDBLOCK:
            sleep(1)
            print 'No data available'
            continue
        else:
            # a "real" error occurred
            print e
            sys.exit(1)
    else:
        # got a message, do something :)

The situation is a little different in the case where you've enabled non-blocking behavior via a time out with socket.settimeout(n) or socket.setblocking(False). In this case a socket.error is stil raised, but in the case of a time out, the accompanying value of the exception is always a string set to 'timed out'. So, to handle this case you can do:

import sys
import socket
from time import sleep

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect(('127.0.0.1',9999))
s.settimeout(2)

while True:
    try:
        msg = s.recv(4096)
    except socket.timeout, e:
        err = e.args[0]
        # this next if/else is a bit redundant, but illustrates how the
        # timeout exception is setup
        if err == 'timed out':
            sleep(1)
            print 'recv timed out, retry later'
            continue
        else:
            print e
            sys.exit(1)
    except socket.error, e:
        # Something else happened, handle error, exit, etc.
        print e
        sys.exit(1)
    else:
        if len(msg) == 0:
            print 'orderly shutdown on server end'
            sys.exit(0)
        else:
            # got a message do something :)

As indicated in the comments, this is also a more portable solution since it doesn't depend on OS specific functionality to put the socket into non-blockng mode.

See recv(2) and python socket for more details.

User · Answer

Just to complete the existing answers  I d suggest using select instead of nonblocking sockets  The point is that nonblocking sockets complicate stuff  except perhaps sending   so I d say there is no reason to use them at all  If you regularly have the problem that your app is blocked waiting for IO  I would also consider doing the IO in a separate thread in the background

User · Answer

When you use recv in connection with select if the socket is ready to be read from but there is no data to read that means the client has closed the connection    Here is some code that handles this  also note the exception that is thrown when recv is called a second time in the while loop  If there is nothing left to read this exception will be thrown it doesn t mean the client has closed the connection    def listenToSockets self        while True           changed sockets   self currentSockets          ready to read  ready to write  in error   select select changed sockets          0 1           for s in ready to read               if s    self serverSocket                  self acceptNewConnection s              else                  self readDataFromSocket s    And the function that receives the data    def readDataFromSocket self  socket        data          buffer          try           while True              data   socket recv 4096               if not data                   break              buffer    data      except error   errorCode message              error 10035 is no data available  it is non-fatal         if errorCode    10035              print  socket error -    str errorCode         message       if data          print  received    buffer     else          print  disconnected

User · Answer

It is simple  if recv   returns 0 bytes  you will not receive any more data on this connection  Ever  You still might be able to send   It means that your non-blocking socket have to raise an exception  it might be system-dependent  if no data is available but the connection is still alive  the other end may send

[python] What does Python's socket.recv() return for non-blocking sockets if no data is received until a timeout occurs?

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