From: David Masover <ninja@...>
Date: 2009-02-17T05:30:05+09:00
Subject: Re: Controlling a forked process

Michael Satterwhite wrote:
> David Masover wrote:
>
>   
>> Yes. The parent process should have the process ID of the child. With
>> that you can do things like:
>>
>> if (pid = fork)
>>   sleep 1
>>   Process.kill('TERM', pid)
>> else
>>   loop do
>>     puts 'KILL ME'
>>   end
>> end
>>     
>
> on the line
>      if (pid = fork)
>
> Is fork returning the pid of the spawned process or of the current (e.g. 
> parent) process?

It's returning the pid of the spawned process in the parent process, and 
nil in the spawned process. So in the code above, the 'if' clause is in 
the parent process, and the 'else' clause is in the spawned process.

> More to the point, assume that I spawn multiple 
> processes. How can I address each of them?

With multiple pids. Following my pattern above, you'd do:

if pid1 = fork
  if pid2 = fork
    # parent process
  else
    # inside pid2
  end
else
  # inside pid1
end

You could also create arbitrary process trees:

if child = fork
  # parent process
elsif grandchild = fork
  # child process
else
  # grandchild process
end

Now, Louise-Philippe has some more elegant syntax, where you can pass a 
block that will be executed in the spawned process. But the idea is the 
same. Also worth noting, my examples above are the same relatively 
low-level concept used everywhere else on Unix.

So, translating my example above:

child = fork do
  grandchild = fork do
    # grandchild process
  end
  # child process
end
# parent process