From: Gary Wright <gwtmp01@...>
Date: 2009-04-25T09:52:46+09:00
Subject: [ruby-core:23307] Re: [Bug #1403] Process.daemon should do a double fork to avoid problems with controlling terminals


On Apr 24, 2009, at 7:15 PM, Nobuyoshi Nakada wrote:

> Hi,
>
> At Sat, 25 Apr 2009 08:06:36 +0900,
> Gary Wright wrote in [ruby-core:23305]:
>> Author: Gary Wright
>> Status: Open, Priority: Normal
>> ruby -v: ruby 1.9.0 (2008-07-25 revision 18217) [i686-darwin9]
>>
>> Standard practice when 'daemonizing' is to do a second fork
>> so that the final process is no longer a session leader and
>> therefore can't acquire a new controlling terminal.
>
> See Process.setsid.  Process.daemon implies it.

setsid is not sufficient.  That makes the child process a session  
leader, establishes a new process group, and breaks the association  
with the controlling terminal, but it doesn't prevent the process from  
acquiring a controlling terminal if it should happen to open a tty  
device (generally on system V based systems, BSD systems are different).

The second fork ensures that the process (the grandchild of the  
original process) is no longer a session leader and therefore can't  
acquire a controlling terminal by opening a tty device.

This is discussed in detail in Section 13.3 of Advanced Programming in  
the Unix Environment (2nd Ed).