Blocking/Non-blocking

By default, TCP sockets are placed in a blocking mode. This means the control is not returned to your program until some specific operation is complete. For example, if you call theconnect()API, the connection blocks your program until the operation is complete. On many occasions, you don't want to keep your program waiting forever, either for a response from the server or for any error to stop the operation. For example, when you write a web browser client that connects to a web server, you should consider a stop functionality that can cancel the connection process in the middle of this operation. This can be achieved by placing the socket in the non-blocking mode.

How to do it...

Let us see what options are available under Python. In the non-blocking mode, if any call to API, for example,send()orrecv(), encounters any problem, an error will be raised. However, in the blocking mode, this will not stop the operation. We can create a normal TCP socket and experiment with both the blocking and non-blocking operations.

In order to manipulate the socket's blocking nature, we need to create a socket object first.

We can then callsetblocking(1)to set up blocking orsetblocking(0)to unset blocking. Finally, we bind the socket to a specific port and listen for incoming connections.

Listing 1.9 shows how the socket changes to blocking or non-blocking mode as follows:

import socket

def test_socket_modes():
    s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    s.setblocking(1)
    s.settimeout(0.5)
    s.bind(("127.0.0.1", 0))

    socket_address = s.getsockname()
    print "Trivial Server launched on socket: %s" %str(socket_address)
    while(1):
        s.listen(1)

if __name__ == '__main__':
    test_socket_modes()

If you run this, it will launch a trivial server that has the blocking mode enabled as shown in the following command:

$ python 1_9_socket_modes.py 
Trivial Server launched on socket: ('127.0.0.1', 51410)

How it works...

In this example, we enable blocking on a socket by setting the value1in thesetblocking()method. Similarly, you can unset the value0in this method to make it non-blocking.