network programming: how does s.accept() work?

[bockman at virgilio.it]

On 25 Feb, 09:51, 7stud <bbxx789_0... at yahoo.com> wrote:
> I have the following two identical clients
>
> #test1.py:-----------
> import socket
>
> s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
>
> host = 'localhost'
> port = 5052  #server port
>
> s.connect((host, port))
> print s.getsockname()
>
> response = []
> while 1:
>     piece = s.recv(1024)
>     if piece == '':
>         break
>
>     response.append(piece)
>
> #test3.py:----------------
> import socket
>
> s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
>
> host = 'localhost'
> port = 5052  #server port
>
> s.connect((host, port))
> print s.getsockname()
>
> response = []
> while 1:
>     piece = s.recv(1024)
>     if piece == '':
>         break
>
>     response.append(piece)
>
> and this basic server:
>
> #test2.py:--------------
> import socket
>
> s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
>
> host = ''
> port = 5052
>
> s.bind((host, port))
> s.listen(5)
>
> while 1:
>     newsock, client_addr = s.accept()
>     print "orignal socket:", s.getsockname()
>
>     print "new socket:", newsock.getsockname()
>     print "new socket:", newsock.getpeername()
>     print
>
> I started the server, and then I started the clients one by one.  I
> expected both clients to hang since they don't get notified that the
> server is done sending data, and I expected the server output to show
> that accept() created two new sockets.  But this is the output I got
> from the server:
>
> original socket: ('0.0.0.0', 5052)
> new socket, self: ('127.0.0.1', 5052)
> new socket, peer: ('127.0.0.1', 50816)
>
> original socket: ('0.0.0.0', 5052)
> new socket, self: ('127.0.0.1', 5052)
> new socket, peer: ('127.0.0.1', 50818)
>
> The first client I started generated this output:
>
> ('127.0.0.1', 50816)
>
> And when I ran the second client, the first client disconnected, and
> the second client produced this output:
>
> ('127.0.0.1', 50818)
>
> and then the second client hung.  I expected the server output to be
> something like this:
>
> original socket: ('127.0.0.1', 5052)
> new socket, self: ('127.0.0.1', 5053)
> new socket, peer: ('127.0.0.1', 50816)
>
> original socket: ('0.0.0.0', 5052)
> new socket, self: ('127.0.0.1', 5054)
> new socket, peer: ('127.0.0.1', 50818)
>
> And I expected both clients to hang.  Can someone explain how accept()
> works?

I guess (but I did not try it) that the problem is not accept(), that
should work as you expect,
but the fact that at the second connection your code actually throws
away the first connection
by reusing the same variables without storing the previous values.
This could make the Python
garbage collector to attempt freeing the socket object created with
the first connection, therefore
closing the connection.

If I'm right, your program should work as you expect if you for
instance collect in a list the sockets
returned by accept.

Ciao
----
FB