Fork and exit in Python

Question

This code is supposed to try and start a server process and return. If the port was taken, it should say "couldn't bind to that port" and return. If the server started, it should print "Bound to port 51231" and return. But it does not return.

import socket
from multiprocessing import Process

def serverMainLoop(s,t):
    s.listen(5)
    while 1:
        pass # server goes here

host = ''
port = 51231
so = socket.socket(socket.AF_INET, socket.SOCK_STREAM)

try:
    so.bind((host,port))
    print "Bound to port %i"%port
    serverProcess = Process(target=serverMainloop, args=(so,60))
    serverProcess.start()
    sys.exit()
except socket.error, (value,message):
    if value==98:
        print "couldn't bind to that port"
    sys.exit()

Is there some switch I can throw that will make multiprocessing let me do this?

The code works with me as described by you: The first run hangs, if I run it twice, the second instance displays "couldn't bind to that port" and exits. So, what is your question? — theomega
– theomega, Commented Jul 28, 2010 at 18:12
I don't want the first one to hang, I want it to return and print the port it bound to, without killing the child process that is looping. — Nathan
– Nathan, Commented Jul 28, 2010 at 18:20

Community · Accepted Answer · 2017-05-23 10:30:04Z

4

Check this page, it describes how to use os.fork() and os._exit(1) to build a daemon which forks to background.

A prototype of what you perhaps want would be:

pid = os.fork()
if (pid == 0): # The first child.
   os.chdir("/")
   os.setsid()
   os.umask(0) 
   pid2 = os.fork() 
   if (pid2 == 0):  # Second child
     # YOUR CODE HERE
   else:
     sys.exit()    #First child exists
else:           # Parent Code
  sys.exit()   # Parent exists

For the reason why to fork twice see this Question (Short Version: It's needed to orphan the child and make it a child of the init-process)

edited May 23, 2017 at 10:30

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answered Jul 28, 2010 at 18:28

theomega

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2 Comments

theomega Over a year ago

stackoverflow.com/questions/881388/…

user48956 Over a year ago

NB. You might generally prefer os._exit at the end of the child process to avoid closing copies of object that the parent process owns (e.g. file descriptors)

Mark · Accepted Answer · 2010-07-28 18:28:49Z

1

To do what you describe, I wouldn't use multiprocessing, I'd look into writing a daemon.

answered Jul 28, 2010 at 18:28

Mark

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Comments

Marcelo Cantos · Accepted Answer · 2010-07-28 18:37:04Z

0

As an alternative to writing a daemon, just write your program as a console process (testing it as such), and use a service management application like supervisord to run it as a service. Supervisord also does much more that just run your program as a service. It will monitor, restart, log, report status and status changes, and even provide a minimal web interface to manage your process.

answered Jul 28, 2010 at 18:37

Marcelo Cantos

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1 Comment

Nathan Over a year ago

I would do this, but I need to log into a remote machine with an unknown configuration, upload the server, start it, and leave. This is being done from within a windows GUI application.

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