From: Tim Pease <tim.pease@...>
Date: 2009-01-21T04:42:18+09:00
Subject: Re: Threads After Fork

On Jan 20, 2009, at 8:55 AM, James Gray wrote:

> Am I understanding this example right:
>
> #!/usr/bin/env ruby -wKU
>
> printer = Thread.new do
>  10.times do
>    sleep 1
>    puts "Thread running in #{Process.pid}..."
>  end
> end
>
> fork do
>  p [Process.pid, printer.status]
>  printer.join
> end
>
> p [Process.pid, printer.status]
> printer.join
> # >> [457, "sleep"]
> # >> [458, false]
> # >> Thread running in 457...
> # >> Thread running in 457...
> # >> Thread running in 457...
> # >> Thread running in 457...
> # >> Thread running in 457...
> # >> Thread running in 457...
> # >> Thread running in 457...
> # >> Thread running in 457...
> # >> Thread running in 457...
> # >> Thread running in 457...
>
> __END__
>
> Are all other threads stopped as part of a process fork?
>

 From the fork documentation ....

"The thread calling fork is the only thread in the created child  
process. fork doesn’t copy other threads."

And I modified your script slightly so things are a little more  
clear ...


#!/usr/bin/env ruby -wKU

p "parent: #{Process.pid}"

printer = Thread.new do
   10.times do
     sleep 1
     puts "Thread running in #{Process.pid}..."
   end
end

fork do
   p "child: #{Process.pid}"
   p [Process.pid, printer.status]
   printer.join
end

p [Process.pid, printer.status]
printer.join


# >> "parent: 9409"
# >> [9409, "sleep"]
# >> "child: 9410"
# >> [9410, false]
# >> Thread running in 9409...
# >> Thread running in 9409...
# >> Thread running in 9409...
# >> Thread running in 9409...
# >> Thread running in 9409...
# >> Thread running in 9409...
# >> Thread running in 9409...
# >> Thread running in 9409...
# >> Thread running in 9409...
# >> Thread running in 9409...


So when the child is forked, only the parent thread is copied over and  
the "printer" thread has been killed in the child. Hence, you get the  
"false" response for the status. And I'll stop being pedantic now ;)

Blessings,
TwP