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 · 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

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

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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