On localhost how do I pick a free port number

Question

I m trying to play with inter-process communication and since I could not figure out how to use named pipes under Windows I thought I ll use network sockets  Everything happens locally  The server is able to launch slaves in a separate process and listens on some port  The slaves do their work and submit the result to the master  How do I figure out which port is available  I assume I cannot listen on port 80 or 21  I m using Python  if that cuts the choices down

User · Answer

You can listen on whatever port you want; generally, user applications should listen to ports 1024 and above (through 65535). The main thing if you have a variable number of listeners is to allocate a range to your app - say 20000-21000, and CATCH EXCEPTIONS. That is how you will know if a port is unusable (used by another process, in other words) on your computer.

However, in your case, you shouldn't have a problem using a single hard-coded port for your listener, as long as you print an error message if the bind fails.

Note also that most of your sockets (for the slaves) do not need to be explicitly bound to specific port numbers - only sockets that wait for incoming connections (like your master here) will need to be made a listener and bound to a port. If a port is not specified for a socket before it is used, the OS will assign a useable port to the socket. When the master wants to respond to a slave that sends it data, the address of the sender is accessible when the listener receives data.

I presume you will be using UDP for this?

User · Answer

If you only need to find a free port for later use  here is a snippet similar to a previous answer  but shorter  using socketserver   import socketserver  with socketserver TCPServer   localhost   0   None  as s      free port   s server address 1    Note that the port is not guaranteed to remain free  so you may need to put this snippet and the code using it in a loop

User · Answer

In my experience  just pick a relatively high number  between 1024-65535  that you think is unlikely to be used by anything else  For example  port   8080 and   5555 are ones that I routinely use  Just pick a port number like this as opposed to just making the code randomly select it and then having to find the port number later is much easier for me  For example  in my current ChatBot project  port   8080

User · Answer

For the sake of snippet of what the guys have explained above   import socket from contextlib import closing  def find free port        with closing socket socket socket AF INET  socket SOCK STREAM   as s          s bind      0           s setsockopt socket SOL SOCKET  socket SO REUSEADDR  1          return s getsockname   1

User · Answer

Bind the socket to port 0  A random free port from 1024 to 65535 will be selected  You may retrieve the selected port with getsockname   right after bind

User · Answer

Do not bind to a specific port  Instead  bind to port 0  sock bind      0    The OS will then pick an available port for you   You can get the port that was chosen using sock getsockname   1   and pass it on to the slaves so that they can connect back

[python] On localhost, how do I pick a free port number?

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