How to tell if a connection is dead in python

Question

I want my python application to be able to tell when the socket on the other side has been dropped  Is there a method for this

User · Answer

If I m not mistaken this is usually handled via a timeout

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I translated the code sample in this blog post into Python  How to detect when the client closes the connection   and it works well for me   from ctypes import       CDLL  c int  POINTER  Structure  c void p  c size t      c short  c ssize t  c char  ARRAY       all      is remote alive     class pollfd Structure        fields                fd   c int             events   c short             revents   c short           MSG DONTWAIT   0x40 MSG PEEK   0x02  EPOLLIN   0x001 EPOLLPRI   0x002 EPOLLRDNORM   0x040  libc   CDLL None   recv   libc recv recv restype   c ssize t recv argtypes   c int  c void p  c size t  c int  poll   libc poll poll restype   c int poll argtypes   POINTER pollfd   c int  c int   class IsRemoteAlive     not needed  only for debugging     def   init   self  alive  msg           self alive   alive         self msg   msg      def   str   self           return self msg      def   repr   self           return  IsRemoteClosed  r  r      self alive  self msg       def   bool   self           return self alive   def is remote alive fd       fileno   getattr fd   fileno   None      if fileno is not None          if hasattr fileno     call                  fd   fileno           else              fd   fileno      p   pollfd fd fd  events EPOLLIN EPOLLPRI EPOLLRDNORM  revents 0      result   poll p  1  0      if not result          return IsRemoteAlive True   empty        buf   ARRAY c char  1        result   recv fd  buf  len buf   MSG DONTWAIT MSG PEEK      if result  gt  0          return IsRemoteAlive True   readable       elif result    0          return IsRemoteAlive False   closed       else          return IsRemoteAlive False   errored

User · Answer

Short answer      use a non-blocking recv    or a blocking recv     select   with a very   short timeout    Long answer   The way to handle socket connections is to read or write as you need to  and be prepared to handle connection errors   TCP distinguishes between 3 forms of  dropping  a connection  timeout  reset  close   Of these  the timeout can not really be detected  TCP might only tell you the time has not expired yet  But even if it told you that  the time might still expire right after   Also remember that using shutdown   either you or your peer  the other end of the connection  may close only the incoming byte stream  and keep the outgoing byte stream running  or close the outgoing stream and keep the incoming one running   So strictly speaking  you want to check if the read stream is closed  or if the write stream is closed  or if both are closed   Even if the connection was  dropped   you should still be able to read any data that is still in the network buffer  Only after the buffer is empty will you receive a disconnect from recv     Checking if the connection was dropped is like asking  what will I receive after reading all data that is currently buffered    To find that out  you just have to read all data that is currently bufferred   I can see how  reading all buffered data   to get to the end of it  might be a problem for some people  that still think of recv   as a blocking function  With a blocking recv     checking  for a read when the buffer is already empty will block  which defeats the purpose of  checking    In my opinion any function that is documented to potentially block the entire process indefinitely is a design flaw  but I guess it is still there for historical reasons  from when using a socket just like a regular file descriptor was a cool idea   What you can do is    set the socket to non-blocking mode  but than you get a system-depended error to indicate the receive buffer is empty  or the send buffer is full stick to blocking mode but set a very short socket timeout  This will allow you to  ping  or  check  the socket with recv    pretty much what you want to do use select   call or asyncore module with a very short timeout  Error reporting is still system-specific    For the write part of the problem  keeping the read buffers empty pretty much covers it  You will discover a connection  dropped  after a non-blocking read attempt  and you may choose to stop sending anything after a read returns a closed channel   I guess the only way to be sure your sent data has reached the other end  and is not still in the send buffer  is either    receive a proper response on the same socket for the exact message that you sent  Basically you are using the higher level protocol to provide confirmation  perform a successful shutdow   and close   on the socket   The python socket howto says send   will return 0 bytes written if channel is closed  You may use a non-blocking or a timeout socket send   and if it returns 0 you can no longer send data on that socket  But if it returns non-zero  you have already sent something  good luck with that     Also here I have not considered OOB  out-of-band  socket data here as a means to approach your problem  but I think OOB was not what you meant

User · Answer

It depends on what you mean by  dropped    For TCP sockets  if the other end closes the connection either through  close   or  the process terminating  you ll find out by reading an end of file  or getting a read error  usually the errno being set to whatever  connection reset by peer  is by your operating system    For python  you ll read a zero length string  or a socket error will be thrown when you try to read or write from the socket

User · Answer

From the link Jweede posted      exception socket timeout   This exception is raised when a timeout occurs on a socket which has had timeouts enabled via a prior call to settimeout    The accompanying value is a string whose value is currently always    timed out        Here are the demo server and client programs for the socket module from the python docs    Echo server program import socket  HOST                        Symbolic name meaning all available interfaces PORT   50007                Arbitrary non-privileged port s   socket socket socket AF INET  socket SOCK STREAM  s bind  HOST  PORT   s listen 1  conn  addr   s accept   print  Connected by   addr while 1      data   conn recv 1024      if not data  break     conn send data  conn close     And the client     Echo client program import socket  HOST    daring cwi nl       The remote host PORT   50007                The same port as used by the server s   socket socket socket AF INET  socket SOCK STREAM  s connect  HOST  PORT   s send  Hello  world   data   s recv 1024  s close   print  Received   repr data    On the docs example page I pulled these from  there are more complex examples that employ this idea  but here is the simple answer   Assuming you re writing the client program  just put all your code that uses the socket when it is at risk of being dropped  inside a try block     try      s connect  HOST  PORT       s send  Hello  World            except socket timeout        whatever you need to do when the connection is dropped

[python] How to tell if a connection is dead in python

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