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, Jul 28 '10 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, Jul 28 '10 at 18:20

score 4 · Accepted Answer · edited May 23 '17 at 10:30

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 '17 at 10:30

Community

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answered Jul 28 '10 at 18:28

theomega

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http://stackoverflow.com/questions/881388/what-is-the-reason-for-performing-a-double-fork-when-creating-a-daemon – theomega Jul 28 '10 at 21:44
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) – user48956 Mar 09 '17 at 23:02

score 1 · Answer 2 · answered Jul 28 '10 at 18:28

1

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

answered Jul 28 '10 at 18:28

Mark

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score 0 · Answer 3 · answered Jul 28 '10 at 18:37

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 '10 at 18:37

Marcelo Cantos

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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. – Nathan Jul 28 '10 at 18:50

Fork and exit in Python

3 Answers3

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